<- STD Index (1..100)
(also RFC 5343, RFC 5590, RFC 5591, RFC 6353)
[Note that this file is a concatenation of more than one RFC.]
Network Working Group J. Schoenwaelder
Request for Comments: 5343 Jacobs University Bremen
Updates: 3411 September 2008
Category: Standards Track
Simple Network Management Protocol (SNMP) Context EngineID Discovery
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
The Simple Network Management Protocol (SNMP) version three (SNMPv3)
requires that an application know the identifier (snmpEngineID) of
the remote SNMP protocol engine in order to retrieve or manipulate
objects maintained on the remote SNMP entity.
This document introduces a well-known localEngineID and a discovery
mechanism that can be used to learn the snmpEngineID of a remote SNMP
protocol engine. The proposed mechanism is independent of the
features provided by SNMP security models and may also be used by
other protocol interfaces providing access to managed objects.
This document updates RFC 3411.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Local EngineID . . . . . . . . . . . . . . . . . . . . . . 4
3.2. EngineID Discovery . . . . . . . . . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . . 7
Schoenwaelder Standards Track [Page 1]
RFC 5343 SNMP Context EngineID Discovery September 2008 1. Introduction To retrieve or manipulate management information using the third version of the Simple Network Management Protocol (SNMPv3) [RFC3410], it is necessary to know the identifier of the remote SNMP protocol engine, the so-called snmpEngineID [RFC3411]. While an appropriate snmpEngineID can in principle be configured on each management application for each SNMP agent, it is often desirable to discover the snmpEngineID automatically. This document introduces a discovery mechanism that can be used to learn the snmpEngineID of a remote SNMP protocol engine. The proposed mechanism is independent of the features provided by SNMP security models. The mechanism has been designed to coexist with discovery mechanisms that may exist in SNMP security models, such as the authoritative engine identifier discovery of the User-based Security Model (USM) of SNMP [RFC3414]. This document updates RFC 3411 [RFC3411] by clarifying the IANA rules for the maintenance of the SnmpEngineID format registry. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. 2. Background Within an administrative domain, an SNMP engine is uniquely identified by an snmpEngineID value [RFC3411]. An SNMP entity, which consists of an SNMP engine and several SNMP applications, may provide access to multiple contexts. An SNMP context is a collection of management information accessible by an SNMP entity. An item of management information may exist in more than one context and an SNMP entity potentially has access to many contexts [RFC3411]. A context is identified by the snmpEngineID value of the entity hosting the management information (also called a contextEngineID) and a context name that identifies the specific context (also called a contextName). To identify an individual item of management information within an administrative domain, a four tuple is used consisting of 1. a contextEngineID, 2. a contextName, Schoenwaelder Standards Track [Page 2]
RFC 5343 SNMP Context EngineID Discovery September 2008 3. an object type, and 4. its instance identification. The last two elements are encoded in an object identifier (OID) value. The contextName is a character string (following the SnmpAdminString textual convention of the SNMP-FRAMEWORK-MIB [RFC3411]) while the contextEngineID is an octet string constructed according to the rules defined as part of the SnmpEngineID textual convention of the SNMP-FRAMEWORK-MIB [RFC3411]. The SNMP protocol operations and the protocol data units (PDUs) operate on OIDs and thus deal with object types and instances [RFC3416]. The SNMP architecture [RFC3411] introduces the concept of a scopedPDU as a data structure containing a contextEngineID, a contextName, and a PDU. The SNMP version 3 (SNMPv3) message format uses ScopedPDUs to exchange management information [RFC3412]. Within the SNMP framework, contextEngineIDs serve as end-to-end identifiers. This becomes important in situations where SNMP proxies are deployed to translate between protocol versions or to cross middleboxes such as network address translators. In addition, snmpEngineIDs separate the identification of an SNMP engine from the transport addresses used to communicate with an SNMP engine. This property can be used to correlate management information easily, even in situations where multiple different transports were used to retrieve the information or where transport addresses can change dynamically. To retrieve data from an SNMPv3 agent, it is necessary to know the appropriate contextEngineID. The User-based Security Model (USM) of SNMPv3 provides a mechanism to discover the snmpEngineID of the remote SNMP engine, since this is needed for security processing reasons. The discovered snmpEngineID can subsequently be used as a contextEngineID in a ScopedPDU to access management information local to the remote SNMP engine. Other security models, such as the Transport Security Model (TSM) [TSM], lack such a procedure and may use the discovery mechanism defined in this memo. 3. Procedure The proposed discovery mechanism consists of two parts, namely (i) the definition of a special well-known snmpEngineID value, called the localEngineID, which always refers to a local default context, and (ii) the definition of a procedure to acquire the snmpEngineID scalar of the SNMP-FRAMEWORK-MIB [RFC3411] using the special well-known local localEngineID value. Schoenwaelder Standards Track [Page 3]
RFC 5343 SNMP Context EngineID Discovery September 2008
3.1. Local EngineID
An SNMP command responder implementing this specification MUST
register their pduTypes using the localEngineID snmpEngineID value
(defined below) by invoking the registerContextEngineID() Abstract
Service Interface (ASI) defined in RFC 3412 [RFC3412]. This
registration is done in addition to the normal registration under the
SNMP engine's snmpEngineID. This is consistent with the SNMPv3
specifications since they explicitly allow registration of multiple
engineIDs and multiple pduTypes [RFC3412].
The SnmpEngineID textual convention [RFC3411] defines that an
snmpEngineID value MUST be between 5 and 32 octets long. This
specification proposes to use the variable length format 3) of the
SnmpEngineID textual convention and to allocate the reserved, unused
format value 6, using the enterprise ID 0 for the localEngineID. An
ASN.1 definition for localEngineID would look like this:
localEngineID OCTET STRING ::= '8000000006'H
The localEngineID value always provides access to the default context
of an SNMP engine. Note that the localEngineID value is intended to
be used as a special value for the contextEngineID field in the
ScopedPDU. It MUST NOT be used as a value to identify an SNMP
engine; that is, this value MUST NOT be used in the snmpEngineID.0
scalar [RFC3418] or in the msgAuthoritativeEngineID field in the
securityParameters of the User-based Security Model (USM) [RFC3414].
3.2. EngineID Discovery
Discovery of the snmpEngineID is done by sending a Read Class
protocol operation (see Section 2.8 of [RFC3411]) to retrieve the
snmpEngineID scalar using the localEngineID defined above as a
contextEngineID value. Implementations SHOULD only perform this
discovery step when it is needed. In particular, if security models
are used that already discover the remote snmpEngineID (such as USM),
then no further discovery is necessary. The same is true in
situations where the application already knows a suitable
snmpEngineID value.
The procedure to discover the snmpEngineID of a remote SNMP engine
can be described as follows:
1. Check whether a suitable contextEngineID value is already known.
If yes, use the provided contextEngineID value and stop the
discovery procedure.
Schoenwaelder Standards Track [Page 4]
RFC 5343 SNMP Context EngineID Discovery September 2008
2. Check whether the selected security model supports discovery of
the remote snmpEngineID (e.g., USM with its discovery mechanism).
If yes, let the security model perform the discovery. If the
remote snmpEngineID value has been successfully determined,
assign it to the contextEngineID and stop the discovery
procedure.
3. Send a Read Class operation to the remote SNMP engine using the
localEngineID value as the contextEngineID in order to retrieve
the scalar snmpEngineID.0 of the SNMP-FRAMEWORK-MIB [RFC3411].
If successful, set the contextEngineID to the retrieved value and
stop the discovery procedure.
4. Return an error indication that a suitable contextEngineID could
not be discovered.
The procedure outlined above is an example and can be modified to
retrieve more variables in step 3, such as the sysObjectID.0 scalar
or the snmpSetSerialNo.0 scalar of the SNMPv2-MIB [RFC3418].
4. IANA Considerations
RFC 3411 requested that IANA create a registry for SnmpEngineID
formats. However, RFC 3411 did not ask IANA to record the initial
assignments made by RFC 3411 nor did RFC 3411 spell out the precise
allocation rules. To address this issue, the following rules are
hereby established.
IANA maintains a registry for SnmpEngineID formats. The first four
octets of an SnmpEngineID carry an enterprise number, while the fifth
octet in a variable length SnmpEngineID value, called the format
octet, indicates how the following octets are formed. The following
format values were allocated in [RFC3411]:
Format Description References
------- ----------- ----------
0 reserved, unused [RFC3411]
1 IPv4 address [RFC3411]
2 IPv6 address [RFC3411]
3 MAC address [RFC3411]
4 administratively assigned text [RFC3411]
5 administratively assigned octets [RFC3411]
6-127 reserved, unused [RFC3411]
128-255 enterprise specific [RFC3411]
IANA can assign new format values out of the originally assigned and
reserved number space 1-127. For new assignments in this number
Schoenwaelder Standards Track [Page 5]
RFC 5343 SNMP Context EngineID Discovery September 2008
space, a specification is required as per [RFC5226]. The number
space 128-255 is enterprise specific and is not controlled by IANA.
Per this document, IANA has made the following assignment:
Format Description References
------- ----------- ----------
6 local engine [RFC5343]
5. Security Considerations
SNMP version 3 (SNMPv3) provides cryptographic security to protect
devices from unauthorized access. This specification recommends use
of the security services provided by SNMPv3. In particular, it is
RECOMMENDED to protect the discovery exchange.
An snmpEngineID can contain information such as a device's MAC
address, IPv4 address, IPv6 address, or administratively assigned
text. An attacker located behind a router / firewall / network
address translator may not be able to obtain this information
directly, and he therefore might discover snmpEngineID values in
order to obtain this kind of device information.
In many environments, making snmpEngineID values accessible via a
security level of noAuthNoPriv will benefit legitimate tools that try
to algorithmically determine some basic information about a device.
For this reason, the default View-based Access Control Model (VACM)
configuration in Appendix A of RFC 3415 [RFC3415] gives noAuthNoPriv
read access to the snmpEngineID. Furthermore, the USM discovery
mechanism defined in RFC 3414 [RFC3414] uses unprotected messages and
reveals snmpEngineID values.
In highly secure environments, snmpEngineID values can be protected
by using the discovery mechanism described in this document together
with a security model that does not exchange cleartext SNMP messages,
such as the Transport Security Model (TSM) [TSM].
The isAccessAllowed() abstract service primitive of the SNMP access
control subsystem does not take the contextEngineID into account when
checking access rights [RFC3411]. As a consequence, it is not
possible to define a special view for context engineID discovery. A
request with a localEngineID is thus treated like a request with the
correct snmpEngineID by the access control subsystem. This is inline
with the SNMPv3 design where the authenticated identity is the
securityName (together with the securityModel and securityLevel
information), and transport addresses or knowledge of contextEngineID
values do not impact the access-control decision.
Schoenwaelder Standards Track [Page 6]
RFC 5343 SNMP Context EngineID Discovery September 2008
6. Acknowledgments
Dave Perkins suggested the introduction of a "local" contextEngineID
during the interim meeting of the ISMS (Integrated Security Model for
SNMP) working group in Boston, 2006. Joe Fernandez, David
Harrington, Dan Romascanu, and Bert Wijnen provided helpful review
and feedback, which helped to improve this document.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3411] Harrington, D., Presuhn, R., and B. Wijnen, "An
Architecture for Describing Simple Network Management
Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
December 2002.
[RFC3412] Case, J., Harrington, D., Presuhn, R., and B. Wijnen,
"Message Processing and Dispatching for the Simple Network
Management Protocol (SNMP)", STD 62, RFC 3412,
December 2002.
[RFC3414] Blumenthal, U. and B. Wijnen, "User-based Security Model
(USM) for version 3 of the Simple Network Management
Protocol (SNMPv3)", STD 62, RFC 3414, December 2002.
[RFC3416] Presuhn, R., "Version 2 of the Protocol Operations for the
Simple Network Management Protocol (SNMP)", STD 62,
RFC 3416, December 2002.
[RFC3418] Presuhn, R., "Management Information Base (MIB) for the
Simple Network Management Protocol (SNMP)", STD 62,
RFC 3418, December 2002.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
7.2. Informative References
[RFC3410] Case, J., Mundy, R., Partain, D., and B. Stewart,
"Introduction and Applicability Statements for Internet-
Standard Management Framework", RFC 3410, December 2002.
Schoenwaelder Standards Track [Page 7]
RFC 5343 SNMP Context EngineID Discovery September 2008
[RFC3415] Wijnen, B., Presuhn, R., and K. McCloghrie, "View-based
Access Control Model (VACM) for the Simple Network
Management Protocol (SNMP)", STD 62, RFC 3415,
December 2002.
[TSM] Harrington, D., "Transport Security Model for SNMP", Work
in Progress, July 2008.
Author's Address
Juergen Schoenwaelder
Jacobs University Bremen
Campus Ring 1
28725 Bremen
Germany
Phone: +49 421 200-3587
EMail: j.schoenwaelder@jacobs-university.de
Schoenwaelder Standards Track [Page 8]
RFC 5343 SNMP Context EngineID Discovery September 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Schoenwaelder Standards Track [Page 9]
=========================================================================
Network Working Group D. Harrington
Request for Comments: 5590 Huawei Technologies (USA)
Updates: 3411, 3412, 3414, 3417 J. Schoenwaelder
Category: Standards Track Jacobs University Bremen
June 2009
Transport Subsystem for the Simple Network Management Protocol (SNMP)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Abstract
This document defines a Transport Subsystem, extending the Simple
Network Management Protocol (SNMP) architecture defined in RFC 3411.
This document defines a subsystem to contain Transport Models that is
comparable to other subsystems in the RFC 3411 architecture. As work
is being done to expand the transports to include secure transports,
such as the Secure Shell (SSH) Protocol and Transport Layer Security
Harrington & Schoenwaelder Standards Track [Page 1]
RFC 5590 SNMP Transport Subsystem June 2009
(TLS), using a subsystem will enable consistent design and modularity
of such Transport Models. This document identifies and describes
some key aspects that need to be considered for any Transport Model
for SNMP.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. The Internet-Standard Management Framework . . . . . . . . 3
1.2. Conventions . . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Where This Extension Fits . . . . . . . . . . . . . . . . 4
2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Requirements of a Transport Model . . . . . . . . . . . . . . 7
3.1. Message Security Requirements . . . . . . . . . . . . . . 7
3.1.1. Security Protocol Requirements . . . . . . . . . . . . 7
3.2. SNMP Requirements . . . . . . . . . . . . . . . . . . . . 8
3.2.1. Architectural Modularity Requirements . . . . . . . . 8
3.2.2. Access Control Requirements . . . . . . . . . . . . . 11
3.2.3. Security Parameter Passing Requirements . . . . . . . 12
3.2.4. Separation of Authentication and Authorization . . . . 12
3.3. Session Requirements . . . . . . . . . . . . . . . . . . . 13
3.3.1. No SNMP Sessions . . . . . . . . . . . . . . . . . . . 13
3.3.2. Session Establishment Requirements . . . . . . . . . . 14
3.3.3. Session Maintenance Requirements . . . . . . . . . . . 15
3.3.4. Message Security versus Session Security . . . . . . . 15
4. Scenario Diagrams and the Transport Subsystem . . . . . . . . 16
5. Cached Information and References . . . . . . . . . . . . . . 17
5.1. securityStateReference . . . . . . . . . . . . . . . . . . 17
5.2. tmStateReference . . . . . . . . . . . . . . . . . . . . . 17
5.2.1. Transport Information . . . . . . . . . . . . . . . . 18
5.2.2. securityName . . . . . . . . . . . . . . . . . . . . . 19
5.2.3. securityLevel . . . . . . . . . . . . . . . . . . . . 20
5.2.4. Session Information . . . . . . . . . . . . . . . . . 20
6. Abstract Service Interfaces . . . . . . . . . . . . . . . . . 21
6.1. sendMessage ASI . . . . . . . . . . . . . . . . . . . . . 21
6.2. Changes to RFC 3411 Outgoing ASIs . . . . . . . . . . . . 22
6.2.1. Message Processing Subsystem Primitives . . . . . . . 22
6.2.2. Security Subsystem Primitives . . . . . . . . . . . . 23
6.3. The receiveMessage ASI . . . . . . . . . . . . . . . . . . 24
6.4. Changes to RFC 3411 Incoming ASIs . . . . . . . . . . . . 25
6.4.1. Message Processing Subsystem Primitive . . . . . . . . 25
6.4.2. Security Subsystem Primitive . . . . . . . . . . . . . 26
7. Security Considerations . . . . . . . . . . . . . . . . . . . 27
7.1. Coexistence, Security Parameters, and Access Control . . . 27
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 29
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
10.1. Normative References . . . . . . . . . . . . . . . . . . . 30
Harrington & Schoenwaelder Standards Track [Page 2]
RFC 5590 SNMP Transport Subsystem June 2009
10.2. Informative References . . . . . . . . . . . . . . . . . . 30
Appendix A. Why tmStateReference? . . . . . . . . . . . . . . . . 32
A.1. Define an Abstract Service Interface . . . . . . . . . . . 32
A.2. Using an Encapsulating Header . . . . . . . . . . . . . . 32
A.3. Modifying Existing Fields in an SNMP Message . . . . . . . 32
A.4. Using a Cache . . . . . . . . . . . . . . . . . . . . . . 33
1. Introduction
This document defines a Transport Subsystem, extending the Simple
Network Management Protocol (SNMP) architecture defined in [RFC3411].
This document identifies and describes some key aspects that need to
be considered for any Transport Model for SNMP.
1.1. The Internet-Standard Management Framework
For a detailed overview of the documents that describe the current
Internet-Standard Management Framework, please refer to Section 7 of
RFC 3410 [RFC3410].
1.2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Lowercase versions of the keywords should be read as in normal
English. They will usually, but not always, be used in a context
that relates to compatibility with the RFC 3411 architecture or the
subsystem defined here but that might have no impact on on-the-wire
compatibility. These terms are used as guidance for designers of
proposed IETF models to make the designs compatible with RFC 3411
subsystems and Abstract Service Interfaces (ASIs). Implementers are
free to implement differently. Some usages of these lowercase terms
are simply normal English usage.
For consistency with SNMP-related specifications, this document
favors terminology as defined in STD 62, rather than favoring
terminology that is consistent with non-SNMP specifications that use
different variations of the same terminology. This is consistent
with the IESG decision to not require the SNMPv3 terminology be
modified to match the usage of other non-SNMP specifications when
SNMPv3 was advanced to Full Standard.
This document discusses an extension to the modular RFC 3411
architecture; this is not a protocol document. An architectural
"MUST" is a really sharp constraint; to allow for the evolution of
technology and to not unnecessarily constrain future models, often a
Harrington & Schoenwaelder Standards Track [Page 3]
RFC 5590 SNMP Transport Subsystem June 2009 "SHOULD" or a "should" is more appropriate than a "MUST" in an architecture. Future models MAY express tighter requirements for their own model-specific processing. 1.3. Where This Extension Fits It is expected that readers of this document will have read RFCs 3410 and 3411, and have a general understanding of the functionality defined in RFCs 3412-3418. The "Transport Subsystem" is an additional component for the SNMP Engine depicted in RFC 3411, Section 3.1. The following diagram depicts its place in the RFC 3411 architecture. +-------------------------------------------------------------------+ | SNMP entity | | | | +-------------------------------------------------------------+ | | | SNMP engine (identified by snmpEngineID) | | | | | | | | +------------+ | | | | | Transport | | | | | | Subsystem | | | | | +------------+ | | | | | | | | +------------+ +------------+ +-----------+ +-----------+ | | | | | Dispatcher | | Message | | Security | | Access | | | | | | | | Processing | | Subsystem | | Control | | | | | | | | Subsystem | | | | Subsystem | | | | | +------------+ +------------+ +-----------+ +-----------+ | | | +-------------------------------------------------------------+ | | | | +-------------------------------------------------------------+ | | | Application(s) | | | | | | | | +-------------+ +--------------+ +--------------+ | | | | | Command | | Notification | | Proxy | | | | | | Generator | | Receiver | | Forwarder | | | | | +-------------+ +--------------+ +--------------+ | | | | | | | | +-------------+ +--------------+ +--------------+ | | | | | Command | | Notification | | Other | | | | | | Responder | | Originator | | | | | | | +-------------+ +--------------+ +--------------+ | | | +-------------------------------------------------------------+ | | | +-------------------------------------------------------------------+ Harrington & Schoenwaelder Standards Track [Page 4]
RFC 5590 SNMP Transport Subsystem June 2009 The transport mappings defined in RFC 3417 do not provide lower-layer security functionality, and thus do not provide transport-specific security parameters. This document updates RFC 3411 and RFC 3417 by defining an architectural extension and modifying the ASIs that transport mappings (hereafter called "Transport Models") can use to pass transport-specific security parameters to other subsystems, including transport-specific security parameters that are translated into the transport-independent securityName and securityLevel parameters. The Transport Security Model [RFC5591] and the Secure Shell Transport Model [RFC5592] utilize the Transport Subsystem. The Transport Security Model is an alternative to the existing SNMPv1 Security Model [RFC3584], the SNMPv2c Security Model [RFC3584], and the User- based Security Model [RFC3414]. The Secure Shell Transport Model is an alternative to existing transport mappings as described in [RFC3417]. 2. Motivation Just as there are multiple ways to secure one's home or business, in a continuum of alternatives, there are multiple ways to secure a network management protocol. Let's consider three general approaches. In the first approach, an individual could sit on his front porch waiting for intruders. In the second approach, he could hire an employee, schedule the employee, position the employee to guard what he wants protected, hire a second guard to cover if the first gets sick, and so on. In the third approach, he could hire a security company, tell them what he wants protected, and leave the details to them. Considerations of hiring and training employees, positioning and scheduling the guards, arranging for cover, etc., are the responsibility of the security company. The individual therefore achieves the desired security, with significantly less effort on his part except for identifying requirements and verifying the quality of service being provided. The User-based Security Model (USM) as defined in [RFC3414] largely uses the first approach -- it provides its own security. It utilizes existing mechanisms (e.g., SHA), but provides all the coordination. USM provides for the authentication of a principal, message encryption, data integrity checking, timeliness checking, etc. USM was designed to be independent of other existing security infrastructures. USM therefore uses a separate principal and key management infrastructure. Operators have reported that deploying another principal and key management infrastructure in order to use Harrington & Schoenwaelder Standards Track [Page 5]
RFC 5590 SNMP Transport Subsystem June 2009 SNMPv3 is a deterrent to deploying SNMPv3. It is possible to use external mechanisms to handle the distribution of keys for use by USM. The more important issue is that operators wanted to leverage existing user management infrastructures that were not specific to SNMP. A USM-compliant architecture might combine the authentication mechanism with an external mechanism, such as RADIUS [RFC2865], to provide the authentication service. Similarly, it might be possible to utilize an external protocol to encrypt a message, to check timeliness, to check data integrity, etc. However, this corresponds to the second approach -- requiring the coordination of a number of differently subcontracted services. Building solid security between the various services is difficult, and there is a significant potential for gaps in security. An alternative approach might be to utilize one or more lower-layer security mechanisms to provide the message-oriented security services required. These would include authentication of the sender, encryption, timeliness checking, and data integrity checking. This corresponds to the third approach described above. There are a number of IETF standards available or in development to address these problems through security layers at the transport layer or application layer, among them are TLS [RFC5246], Simple Authentication and Security Layer (SASL) [RFC4422], and SSH [RFC4251] From an operational perspective, it is highly desirable to use security mechanisms that can unify the administrative security management for SNMPv3, command line interfaces (CLIs), and other management interfaces. The use of security services provided by lower layers is the approach commonly used for the CLI, and is also the approach being proposed for other network management protocols, such as syslog [RFC5424] and NETCONF [RFC4741]. This document defines a Transport Subsystem extension to the RFC 3411 architecture that is based on the third approach. This extension specifies how other lower-layer protocols with common security infrastructures can be used underneath the SNMP protocol and the desired goal of unified administrative security can be met. This extension allows security to be provided by an external protocol connected to the SNMP engine through an SNMP Transport Model [RFC3417]. Such a Transport Model would then enable the use of existing security mechanisms, such as TLS [RFC5246] or SSH [RFC4251], within the RFC 3411 architecture. Harrington & Schoenwaelder Standards Track [Page 6]
RFC 5590 SNMP Transport Subsystem June 2009 There are a number of Internet security protocols and mechanisms that are in widespread use. Many of them try to provide a generic infrastructure to be used by many different application-layer protocols. The motivation behind the Transport Subsystem is to leverage these protocols where it seems useful. There are a number of challenges to be addressed to map the security provided by a secure transport into the SNMP architecture so that SNMP continues to provide interoperability with existing implementations. These challenges are described in detail in this document. For some key issues, design choices are described that might be made to provide a workable solution that meets operational requirements and fits into the SNMP architecture defined in [RFC3411]. 3. Requirements of a Transport Model 3.1. Message Security Requirements Transport security protocols SHOULD provide protection against the following message-oriented threats: 1. modification of information 2. masquerade 3. message stream modification 4. disclosure These threats are described in Section 1.4 of [RFC3411]. The security requirements outlined there do not require protection against denial of service or traffic analysis; however, transport security protocols should not make those threats significantly worse. 3.1.1. Security Protocol Requirements There are a number of standard protocols that could be proposed as possible solutions within the Transport Subsystem. Some factors should be considered when selecting a protocol. Using a protocol in a manner for which it was not designed has numerous problems. The advertised security characteristics of a protocol might depend on it being used as designed; when used in other ways, it might not deliver the expected security characteristics. It is recommended that any proposed model include a description of the applicability of the Transport Model. Harrington & Schoenwaelder Standards Track [Page 7]
RFC 5590 SNMP Transport Subsystem June 2009 A Transport Model SHOULD NOT require modifications to the underlying protocol. Modifying the protocol might change its security characteristics in ways that could impact other existing usages. If a change is necessary, the change SHOULD be an extension that has no impact on the existing usages. Any Transport Model specification should include a description of potential impact on other usages of the protocol. Since multiple Transport Models can exist simultaneously within the Transport Subsystem, Transport Models MUST be able to coexist with each other. 3.2. SNMP Requirements 3.2.1. Architectural Modularity Requirements SNMP version 3 (SNMPv3) is based on a modular architecture (defined in Section 3 of [RFC3411]) to allow the evolution of the SNMP protocol standards over time and to minimize the side effects between subsystems when changes are made. The RFC 3411 architecture includes a Message Processing Subsystem for permitting different message versions to be handled by a single engine, a Security Subsystem for enabling different methods of providing security services, Applications to support different types of Application processors, and an Access Control Subsystem for allowing multiple approaches to access control. The RFC 3411 architecture does not include a subsystem for Transport Models, despite the fact there are multiple transport mappings already defined for SNMP [RFC3417]. This document describes a Transport Subsystem that is compatible with the RFC 3411 architecture. As work is being done to use secure transports such as SSH and TLS, using a subsystem will enable consistent design and modularity of such Transport Models. The design of this Transport Subsystem accepts the goals of the RFC 3411 architecture that are defined in Section 1.5 of [RFC3411]. This Transport Subsystem uses a modular design that permits Transport Models (which might or might not be security-aware) to be "plugged into" the RFC 3411 architecture. Such Transport Models would be independent of other modular SNMP components as much as possible. This design also permits Transport Models to be advanced through the standards process independently of other Transport Models. The following diagram depicts the SNMPv3 architecture, including the new Transport Subsystem defined in this document and a new Transport Security Model defined in [RFC5591]. Harrington & Schoenwaelder Standards Track [Page 8]
RFC 5590 SNMP Transport Subsystem June 2009
+------------------------------+
| Network |
+------------------------------+
^ ^ ^
| | |
v v v
+-------------------------------------------------------------------+
| +--------------------------------------------------+ |
| | Transport Subsystem | |
| | +-----+ +-----+ +-----+ +-----+ +-------+ | |
| | | UDP | | TCP | | SSH | | TLS | . . . | other | | |
| | +-----+ +-----+ +-----+ +-----+ +-------+ | |
| +--------------------------------------------------+ |
| ^ |
| | |
| Dispatcher v |
| +-------------------+ +---------------------+ +----------------+ |
| | Transport | | Message Processing | | Security | |
| | Dispatch | | Subsystem | | Subsystem | |
| | | | +------------+ | | +------------+ | |
| | | | +->| v1MP |<--->| | USM | | |
| | | | | +------------+ | | +------------+ | |
| | | | | +------------+ | | +------------+ | |
| | | | +->| v2cMP |<--->| | Transport | | |
| | Message | | | +------------+ | | | Security | | |
| | Dispatch <--------->| +------------+ | | | Model | | |
| | | | +->| v3MP |<--->| +------------+ | |
| | | | | +------------+ | | +------------+ | |
| | PDU Dispatch | | | +------------+ | | | Other | | |
| +-------------------+ | +->| otherMP |<--->| | Model(s) | | |
| ^ | +------------+ | | +------------+ | |
| | +---------------------+ +----------------+ |
| v |
| +-------+-------------------------+---------------+ |
| ^ ^ ^ |
| | | | |
| v v v |
| +-------------+ +---------+ +--------------+ +-------------+ |
| | COMMAND | | ACCESS | | NOTIFICATION | | PROXY | |
| | RESPONDER |<->| CONTROL |<->| ORIGINATOR | | FORWARDER | |
| | Application | | | | Applications | | Application | |
| +-------------+ +---------+ +--------------+ +-------------+ |
| ^ ^ |
| | | |
| v v |
| +----------------------------------------------+ |
| | MIB instrumentation | SNMP entity |
+-------------------------------------------------------------------+
Harrington & Schoenwaelder Standards Track [Page 9]
RFC 5590 SNMP Transport Subsystem June 2009 3.2.1.1. Changes to the RFC 3411 Architecture The RFC 3411 architecture and the Security Subsystem assume that a Security Model is called by a Message Processing Model and will perform multiple security functions within the Security Subsystem. A Transport Model that supports a secure transport protocol might perform similar security functions within the Transport Subsystem, including the translation of transport-security parameters to/from Security-Model-independent parameters. To accommodate this, an implementation-specific cache of transport- specific information will be described (not shown), and the data flows on this path will be extended to pass Security-Model- independent values. This document amends some of the ASIs defined in RFC 3411; these changes are covered in Section 6 of this document. New Security Models might be defined that understand how to work with these modified ASIs and the transport-information cache. One such Security Model, the Transport Security Model, is defined in [RFC5591]. 3.2.1.2. Changes to RFC 3411 Processing The introduction of secure transports affects the responsibilities and order of processing within the RFC 3411 architecture. While the steps are the same, they might occur in a different order, and might be done by different subsystems. With the existing RFC 3411 architecture, security processing starts when the Message Processing Model decodes portions of the encoded message to extract parameters that identify which Security Model MUST handle the security-related tasks. A secure transport performs those security functions on the message, before the message is decoded. Some of these functions might then be repeated by the selected Security Model. 3.2.1.3. Passing Information between SNMP Engines A secure Transport Model will establish an authenticated and possibly encrypted tunnel between the Transport Models of two SNMP engines. After a transport-layer tunnel is established, then SNMP messages can be sent through the tunnel from one SNMP engine to the other. While the Community Security Models [RFC3584] and the User-based Security Model establish a security association for each SNMP message, newer Transport Models MAY support sending multiple SNMP messages through the same tunnel to amortize the costs of establishing a security association. Harrington & Schoenwaelder Standards Track [Page 10]
RFC 5590 SNMP Transport Subsystem June 2009 3.2.2. Access Control Requirements RFC 3411 made some design decisions related to the support of an Access Control Subsystem. These include establishing and passing in a model-independent manner the securityModel, securityName, and securityLevel parameters, and separating message authentication from data-access authorization. 3.2.2.1. securityName and securityLevel Mapping SNMP data-access controls are expected to work on the basis of who can perform what operations on which subsets of data, and based on the security services that will be provided to secure the data in transit. The securityModel and securityLevel parameters establish the protections for transit -- whether authentication and privacy services will be or have been applied to the message. The securityName is a model-independent identifier of the security "principal". A Security Model plays a role in security that goes beyond protecting the message -- it provides a mapping between the Security-Model- specific principal for an incoming message to a Security-Model independent securityName that can be used for subsequent processing, such as for access control. The securityName is mapped from a mechanism-specific identity, and this mapping must be done for incoming messages by the Security Model before it passes securityName to the Message Processing Model via the processIncoming ASI. A Security Model is also responsible to specify, via the securityLevel parameter, whether incoming messages have been authenticated and encrypted, and to ensure that outgoing messages are authenticated and encrypted based on the value of securityLevel. A Transport Model MAY provide suggested values for securityName and securityLevel. A Security Model might have multiple sources for determining the principal and desired security services, and a particular Security Model might or might not utilize the values proposed by a Transport Model when deciding the value of securityName and securityLevel. Documents defining a new transport domain MUST define a prefix that MAY be prepended to all securityNames passed by the Security Model. The prefix MUST include one to four US-ASCII alpha-numeric characters, not including a ":" (US-ASCII 0x3a) character. If a prefix is used, a securityName is constructed by concatenating the prefix and a ":" (US-ASCII 0x3a) character, followed by a non-empty identity in an snmpAdminString-compatible format. The prefix can be used by SNMP Applications to distinguish "alice" authenticated by SSH Harrington & Schoenwaelder Standards Track [Page 11]
RFC 5590 SNMP Transport Subsystem June 2009 from "alice" authenticated by TLS. Transport domains and their corresponding prefixes are coordinated via the IANA registry "SNMP Transport Domains". 3.2.3. Security Parameter Passing Requirements A Message Processing Model might unpack SNMP-specific security parameters from an incoming message before calling a specific Security Model to handle the security-related processing of the message. When using a secure Transport Model, some security parameters might be extracted from the transport layer by the Transport Model before the message is passed to the Message Processing Subsystem. This document describes a cache mechanism (see Section 5) into which the Transport Model puts information about the transport and security parameters applied to a transport connection or an incoming message; a Security Model might extract that information from the cache. A tmStateReference is passed as an extra parameter in the ASIs between the Transport Subsystem and the Message Processing and Security Subsystems in order to identify the relevant cache. This approach of passing a model-independent reference is consistent with the securityStateReference cache already being passed around in the RFC 3411 ASIs. 3.2.4. Separation of Authentication and Authorization The RFC 3411 architecture defines a separation of authentication and the authorization to access and/or modify MIB data. A set of model- independent parameters (securityModel, securityName, and securityLevel) are passed between the Security Subsystem, the Applications, and the Access Control Subsystem. This separation was a deliberate decision of the SNMPv3 WG, in order to allow support for authentication protocols that do not provide data-access authorization capabilities, and in order to support data- access authorization schemes, such as the View-based access Control Model (VACM), that do not perform their own authentication. A Message Processing Model determines which Security Model is used, either based on the message version (e.g., SNMPv1 and SNMPv2c) or possibly by a value specified in the message (e.g., msgSecurityModel field in SNMPv3). The Security Model makes the decision which securityName and securityLevel values are passed as model-independent parameters to an Application, which then passes them via the isAccessAllowed ASI to the Access Control Subsystem. Harrington & Schoenwaelder Standards Track [Page 12]
RFC 5590 SNMP Transport Subsystem June 2009 An Access Control Model performs the mapping from the model- independent security parameters to a policy within the Access Control Model that is Access-Control-Model-dependent. A Transport Model does not know which Security Model will be used for an incoming message, and so cannot know how the securityName and securityLevel parameters will be determined. It can propose an authenticated identity (via the tmSecurityName field), but there is no guarantee that this value will be used by the Security Model. For example, non-transport-aware Security Models will typically determine the securityName (and securityLevel) based on the contents of the SNMP message itself. Such Security Models will simply not know that the tmStateReference cache exists. Further, even if the Transport Model can influence the choice of securityName, it cannot directly determine the authorization allowed to this identity. If two different Transport Models each authenticate a transport principal that are then both mapped to the same securityName, then these two identities will typically be afforded exactly the same authorization by the Access Control Model. The only way for the Access Control Model to differentiate between identities based on the underlying Transport Model would be for such transport-authenticated identities to be mapped to distinct securityNames. How and if this is done is Security-Model-dependent. 3.3. Session Requirements Some secure transports have a notion of sessions, while other secure transports provide channels or other session-like mechanisms. Throughout this document, the term "session" is used in a broad sense to cover transport sessions, transport channels, and other transport- layer, session-like mechanisms. Transport-layer sessions that can secure multiple SNMP messages within the lifetime of the session are considered desirable because the cost of authentication can be amortized over potentially many transactions. How a transport session is actually established, opened, closed, or maintained is specific to a particular Transport Model. To reduce redundancy, this document describes aspects that are expected to be common to all Transport Model sessions. 3.3.1. No SNMP Sessions The architecture defined in [RFC3411] and the Transport Subsystem defined in this document do not support SNMP sessions or include a session selector in the Abstract Service Interfaces. Harrington & Schoenwaelder Standards Track [Page 13]
RFC 5590 SNMP Transport Subsystem June 2009 The Transport Subsystem might support transport sessions. However, the Transport Subsystem does not have access to the pduType (i.e., the SNMP operation type), and so cannot select a given transport session for particular types of traffic. Certain parameters of the Abstract Service Interfaces might be used to guide the selection of an appropriate transport session to use for a given request by an Application. The transportDomain and transportAddress identify the transport connection to a remote network node. Elements of the transport address (such as the port number) might be used by an Application to send a particular PDU type to a particular transport address. For example, the SNMP-TARGET-MIB and SNMP-NOTIFICATION-MIB [RFC3413] are used to configure notification originators with the destination port to which SNMPv2-Trap PDUs or Inform PDUs are to be sent, but the Transport Subsystem never looks inside the PDU. The securityName identifies which security principal to communicate with at that address (e.g., different Network Management System (NMS) applications), and the securityLevel might permit selection of different sets of security properties for different purposes (e.g., encrypted SET vs. non-encrypted GET operations). However, because the handling of transport sessions is specific to each Transport Model, some Transport Models MAY restrict selecting a particular transport session. A user application might use a unique combination of transportDomain, transportAddress, securityModel, securityName, and securityLevel to try to force the selection of a given transport session. This usage is NOT RECOMMENDED because it is not guaranteed to be interoperable across implementations and across models. Implementations SHOULD be able to maintain some reasonable number of concurrent transport sessions, and MAY provide non-standard internal mechanisms to select transport sessions. 3.3.2. Session Establishment Requirements SNMP Applications provide the transportDomain, transportAddress, securityName, and securityLevel to be used to create a new session. If the Transport Model cannot provide at least the requested level of security, the Transport Model should discard the message and should notify the Dispatcher that establishing a session and sending the message failed. Similarly, if the session cannot be established, then the message should be discarded and the Dispatcher notified. Harrington & Schoenwaelder Standards Track [Page 14]
RFC 5590 SNMP Transport Subsystem June 2009 Transport session establishment might require provisioning authentication credentials at an engine, either statically or dynamically. How this is done is dependent on the Transport Model and the implementation. 3.3.3. Session Maintenance Requirements A Transport Model can tear down sessions as needed. It might be necessary for some implementations to tear down sessions as the result of resource constraints, for example. The decision to tear down a session is implementation-dependent. How an implementation determines that an operation has completed is implementation-dependent. While it is possible to tear down each transport session after processing for each message has completed, this is not recommended for performance reasons. The elements of procedure describe when cached information can be discarded, and the timing of cache cleanup might have security implications, but cache memory management is an implementation issue. If a Transport Model defines MIB module objects to maintain session state information, then the Transport Model MUST define what happens to the objects when a related session is torn down, since this will impact the interoperability of the MIB module. 3.3.4. Message Security versus Session Security A Transport Model session is associated with state information that is maintained for its lifetime. This state information allows for the application of various security services to multiple messages. Cryptographic keys associated with the transport session SHOULD be used to provide authentication, integrity checking, and encryption services, as needed, for data that is communicated during the session. The cryptographic protocols used to establish keys for a Transport Model session SHOULD ensure that fresh new session keys are generated for each session. This would ensure that a cross-session replay attack would be unsuccessful; that is, an attacker could not take a message observed on one session and successfully replay it on another session. A good security protocol would also protect against replay attacks within a session; that is, an attacker could not take a message observed on a session and successfully replay it later in the same session. One approach would be to use sequence information within the protocol, allowing the participants to detect if messages were replayed or reordered within a session. Harrington & Schoenwaelder Standards Track [Page 15]
RFC 5590 SNMP Transport Subsystem June 2009 If a secure transport session is closed between the time a request message is received and the corresponding response message is sent, then the response message SHOULD be discarded, even if a new session has been established. The SNMPv3 WG decided that this should be a "SHOULD" architecturally, and it is a Security-Model-specific decision whether to REQUIRE this. The architecture does not mandate this requirement in order to allow for future Security Models where this might make sense; however, not requiring this could lead to added complexity and security vulnerabilities, so most Security Models SHOULD require this. SNMPv3 was designed to support multiple levels of security, selectable on a per-message basis by an SNMP Application, because, for example, there is not much value in using encryption for a command generator to poll for potentially non-sensitive performance data on thousands of interfaces every ten minutes; such encryption might add significant overhead to processing of the messages. Some Transport Models might support only specific authentication and encryption services, such as requiring all messages to be carried using both authentication and encryption, regardless of the security level requested by an SNMP Application. A Transport Model MAY upgrade the security level requested by a transport-aware Security Model, i.e., noAuthNoPriv and authNoPriv might be sent over an authenticated and encrypted session. A Transport Model MUST NOT downgrade the security level requested by a transport-aware Security Model, and SHOULD discard any message where this would occur. This is a SHOULD rather than a MUST only to permit the potential development of models that can perform error-handling in a manner that is less severe than discarding the message. However, any model that does not discard the message in this circumstance should have a clear justification for why not discarding will not create a security vulnerability. 4. Scenario Diagrams and the Transport Subsystem Sections 4.6.1 and 4.6.2 of RFC 3411 provide scenario diagrams to illustrate how an outgoing message is created and how an incoming message is processed. RFC 3411 does not define ASIs for the "Send SNMP Request Message to Network", "Receive SNMP Response Message from Network", "Receive SNMP Message from Network" and "Send SNMP message to Network" arrows in these diagrams. This document defines two ASIs corresponding to these arrows: a sendMessage ASI to send SNMP messages to the network and a receiveMessage ASI to receive SNMP messages from the network. These ASIs are used for all SNMP messages, regardless of pduType. Harrington & Schoenwaelder Standards Track [Page 16]
RFC 5590 SNMP Transport Subsystem June 2009 5. Cached Information and References When performing SNMP processing, there are two levels of state information that might need to be retained: the immediate state linking a request-response pair and a potentially longer-term state relating to transport and security. The RFC 3411 architecture uses caches to maintain the short-term message state, and uses references in the ASIs to pass this information between subsystems. This document defines the requirements for a cache to handle additional short-term message state and longer-term transport state information, using a tmStateReference parameter to pass this information between subsystems. To simplify the elements of procedure, the release of state information is not always explicitly specified. As a general rule, if state information is available when a message being processed gets discarded, the state related to that message should also be discarded. If state information is available when a relationship between engines is severed, such as the closing of a transport session, the state information for that relationship should also be discarded. Since the contents of a cache are meaningful only within an implementation, and not on-the-wire, the format of the cache is implementation-specific. 5.1. securityStateReference The securityStateReference parameter is defined in RFC 3411. Its primary purpose is to provide a mapping between a request and the corresponding response. This cache is not accessible to Transport Models, and an entry is typically only retained for the lifetime of a request-response pair of messages. 5.2. tmStateReference For each transport session, information about the transport security is stored in a tmState cache or datastore that is referenced by a tmStateReference. The tmStateReference parameter is used to pass model-specific and mechanism-specific parameters between the Transport Subsystem and transport-aware Security Models. In general, when necessary, the tmState is populated by the Security Model for outgoing messages and by the Transport Model for incoming messages. However, in both cases, the model populating the tmState Harrington & Schoenwaelder Standards Track [Page 17]
RFC 5590 SNMP Transport Subsystem June 2009
might have incomplete information, and the missing information might
be populated by the other model when the information becomes
available.
The tmState might contain both long-term and short-term information.
The session information typically remains valid for the duration of
the transport session, might be used for several messages, and might
be stored in a local configuration datastore. Some information has a
shorter lifespan, such as tmSameSecurity and
tmRequestedSecurityLevel, which are associated with a specific
message.
Since this cache is only used within an implementation, and not on-
the-wire, the precise contents and format of the cache are
implementation-dependent. For architectural modularity between
Transport Models and transport-aware Security Models, a fully-defined
tmState MUST conceptually include at least the following fields:
tmTransportDomain
tmTransportAddress
tmSecurityName
tmRequestedSecurityLevel
tmTransportSecurityLevel
tmSameSecurity
tmSessionID
The details of these fields are described in the following
subsections.
5.2.1. Transport Information
Information about the source of an incoming SNMP message is passed up
from the Transport Subsystem as far as the Message Processing
Subsystem. However, these parameters are not included in the
processIncomingMsg ASI defined in RFC 3411; hence, this information
is not directly available to the Security Model.
A transport-aware Security Model might wish to take account of the
transport protocol and originating address when authenticating the
request and setting up the authorization parameters. It is therefore
Harrington & Schoenwaelder Standards Track [Page 18]
RFC 5590 SNMP Transport Subsystem June 2009
necessary for the Transport Model to include this information in the
tmStateReference cache so that it is accessible to the Security
Model.
o tmTransportDomain: the transport protocol (and hence the Transport
Model) used to receive the incoming message.
o tmTransportAddress: the source of the incoming message.
The ASIs used for processing an outgoing message all include explicit
transportDomain and transportAddress parameters. The values within
the securityStateReference cache might override these parameters for
outgoing messages.
5.2.2. securityName
There are actually three distinct "identities" that can be identified
during the processing of an SNMP request over a secure transport:
o transport principal: the transport-authenticated identity on whose
behalf the secure transport connection was (or should be)
established. This value is transport-, mechanism-, and
implementation-specific, and is only used within a given Transport
Model.
o tmSecurityName: a human-readable name (in snmpAdminString format)
representing this transport identity. This value is transport-
and implementation-specific, and is only used (directly) by the
Transport and Security Models.
o securityName: a human-readable name (in snmpAdminString format)
representing the SNMP principal in a model-independent manner.
This value is used directly by SNMP Applications, the Access
Control Subsystem, the Message Processing Subsystem, and the
Security Subsystem.
The transport principal might or might not be the same as the
tmSecurityName. Similarly, the tmSecurityName might or might not be
the same as the securityName as seen by the Application and Access
Control Subsystems. In particular, a non-transport-aware Security
Model will ignore tmSecurityName completely when determining the SNMP
securityName.
However, it is important that the mapping between the transport
principal and the SNMP securityName (for transport-aware Security
Models) is consistent and predictable in order to allow configuration
of suitable access control and the establishment of transport
connections.
Harrington & Schoenwaelder Standards Track [Page 19]
RFC 5590 SNMP Transport Subsystem June 2009
5.2.3. securityLevel
There are two distinct issues relating to security level as applied
to secure transports. For clarity, these are handled by separate
fields in the tmStateReference cache:
o tmTransportSecurityLevel: an indication from the Transport Model
of the level of security offered by this session. The Security
Model can use this to ensure that incoming messages were suitably
protected before acting on them.
o tmRequestedSecurityLevel: an indication from the Security Model of
the level of security required to be provided by the transport
protocol. The Transport Model can use this to ensure that
outgoing messages will not be sent over an insufficiently secure
session.
5.2.4. Session Information
For security reasons, if a secure transport session is closed between
the time a request message is received and the corresponding response
message is sent, then the response message SHOULD be discarded, even
if a new session has been established. The SNMPv3 WG decided that
this should be a "SHOULD" architecturally, and it is a Security-
Model-specific decision whether to REQUIRE this.
o tmSameSecurity: this flag is used by a transport-aware Security
Model to indicate whether the Transport Model MUST enforce this
restriction.
o tmSessionID: in order to verify whether the session has changed,
the Transport Model must be able to compare the session used to
receive the original request with the one to be used to send the
response. This typically needs some form of session identifier.
This value is only ever used by the Transport Model, so the format
and interpretation of this field are model-specific and
implementation-dependent.
When processing an outgoing message, if tmSameSecurity is true, then
the tmSessionID MUST match the current transport session; otherwise,
the message MUST be discarded and the Dispatcher notified that
sending the message failed.
Harrington & Schoenwaelder Standards Track [Page 20]
RFC 5590 SNMP Transport Subsystem June 2009 6. Abstract Service Interfaces Abstract service interfaces have been defined by RFC 3411 to describe the conceptual data flows between the various subsystems within an SNMP entity and to help keep the subsystems independent of each other except for the common parameters. This document introduces a couple of new ASIs to define the interface between the Transport and Dispatcher Subsystems; it also extends some of the ASIs defined in RFC 3411 to include transport-related information. This document follows the example of RFC 3411 regarding the release of state information and regarding error indications. 1) The release of state information is not always explicitly specified in a Transport Model. As a general rule, if state information is available when a message gets discarded, the message- state information should also be released, and if state information is available when a session is closed, the session-state information should also be released. Keeping sensitive security information longer than necessary might introduce potential vulnerabilities to an implementation. 2)An error indication in statusInformation will typically include the Object Identifier (OID) and value for an incremented error counter. This might be accompanied by values for contextEngineID and contextName for this counter, a value for securityLevel, and the appropriate state reference if the information is available at the point where the error is detected. 6.1. sendMessage ASI The sendMessage ASI is used to pass a message from the Dispatcher to the appropriate Transport Model for sending. The sendMessageASI defined in this document replaces the text "Send SNMP Request Message to Network" that appears in the diagram in Section 4.6.1 of RFC 3411 and the text "Send SNMP Message to Network" that appears in Section 4.6.2 of RFC 3411. If present and valid, the tmStateReference refers to a cache containing Transport-Model-specific parameters for the transport and transport security. How a tmStateReference is determined to be present and valid is implementation-dependent. How the information in the cache is used is Transport-Model-dependent and implementation- dependent. Harrington & Schoenwaelder Standards Track [Page 21]
RFC 5590 SNMP Transport Subsystem June 2009
This might sound underspecified, but a Transport Model might be
something like SNMP over UDP over IPv6, where no security is
provided, so it might have no mechanisms for utilizing a
tmStateReference cache.
statusInformation =
sendMessage(
IN destTransportDomain -- transport domain to be used
IN destTransportAddress -- transport address to be used
IN outgoingMessage -- the message to send
IN outgoingMessageLength -- its length
IN tmStateReference -- reference to transport state
)
6.2. Changes to RFC 3411 Outgoing ASIs
Additional parameters have been added to the ASIs defined in RFC 3411
that are concerned with communication between the Dispatcher and
Message Processing Subsystems, and between the Message Processing and
Security Subsystems.
6.2.1. Message Processing Subsystem Primitives
A tmStateReference parameter has been added as an OUT parameter to
the prepareOutgoingMessage and prepareResponseMessage ASIs. This is
passed from the Message Processing Subsystem to the Dispatcher, and
from there to the Transport Subsystem.
How or if the Message Processing Subsystem modifies or utilizes the
contents of the cache is Message-Processing-Model specific.
statusInformation = -- success or errorIndication
prepareOutgoingMessage(
IN transportDomain -- transport domain to be used
IN transportAddress -- transport address to be used
IN messageProcessingModel -- typically, SNMP version
IN securityModel -- Security Model to use
IN securityName -- on behalf of this principal
IN securityLevel -- Level of Security requested
IN contextEngineID -- data from/at this entity
IN contextName -- data from/in this context
IN pduVersion -- the version of the PDU
IN PDU -- SNMP Protocol Data Unit
IN expectResponse -- TRUE or FALSE
IN sendPduHandle -- the handle for matching
incoming responses
Harrington & Schoenwaelder Standards Track [Page 22]
RFC 5590 SNMP Transport Subsystem June 2009
OUT destTransportDomain -- destination transport domain
OUT destTransportAddress -- destination transport address
OUT outgoingMessage -- the message to send
OUT outgoingMessageLength -- its length
OUT tmStateReference -- (NEW) reference to transport state
)
statusInformation = -- success or errorIndication
prepareResponseMessage(
IN messageProcessingModel -- typically, SNMP version
IN securityModel -- Security Model to use
IN securityName -- on behalf of this principal
IN securityLevel -- Level of Security requested
IN contextEngineID -- data from/at this entity
IN contextName -- data from/in this context
IN pduVersion -- the version of the PDU
IN PDU -- SNMP Protocol Data Unit
IN maxSizeResponseScopedPDU -- maximum size able to accept
IN stateReference -- reference to state information
-- as presented with the request
IN statusInformation -- success or errorIndication
-- error counter OID/value if error
OUT destTransportDomain -- destination transport domain
OUT destTransportAddress -- destination transport address
OUT outgoingMessage -- the message to send
OUT outgoingMessageLength -- its length
OUT tmStateReference -- (NEW) reference to transport state
)
6.2.2. Security Subsystem Primitives
transportDomain and transportAddress parameters have been added as IN
parameters to the generateRequestMsg and generateResponseMsg ASIs,
and a tmStateReference parameter has been added as an OUT parameter.
The transportDomain and transportAddress parameters will have been
passed into the Message Processing Subsystem from the Dispatcher and
are passed on to the Security Subsystem. The tmStateReference
parameter will be passed from the Security Subsystem back to the
Message Processing Subsystem, and on to the Dispatcher and Transport
Subsystems.
If a cache exists for a session identifiable from the
tmTransportDomain, tmTransportAddress, tmSecurityName, and requested
securityLevel, then a transport-aware Security Model might create a
tmStateReference parameter to this cache and pass that as an OUT
parameter.
Harrington & Schoenwaelder Standards Track [Page 23]
RFC 5590 SNMP Transport Subsystem June 2009
statusInformation =
generateRequestMsg(
IN transportDomain -- (NEW) destination transport domain
IN transportAddress -- (NEW) destination transport address
IN messageProcessingModel -- typically, SNMP version
IN globalData -- message header, admin data
IN maxMessageSize -- of the sending SNMP entity
IN securityModel -- for the outgoing message
IN securityEngineID -- authoritative SNMP entity
IN securityName -- on behalf of this principal
IN securityLevel -- Level of Security requested
IN scopedPDU -- message (plaintext) payload
OUT securityParameters -- filled in by Security Module
OUT wholeMsg -- complete generated message
OUT wholeMsgLength -- length of generated message
OUT tmStateReference -- (NEW) reference to transport state
)
statusInformation =
generateResponseMsg(
IN transportDomain -- (NEW) destination transport domain
IN transportAddress -- (NEW) destination transport address
IN messageProcessingModel -- Message Processing Model
IN globalData -- msgGlobalData
IN maxMessageSize -- from msgMaxSize
IN securityModel -- as determined by MPM
IN securityEngineID -- the value of snmpEngineID
IN securityName -- on behalf of this principal
IN securityLevel -- for the outgoing message
IN scopedPDU -- as provided by MPM
IN securityStateReference -- as provided by MPM
OUT securityParameters -- filled in by Security Module
OUT wholeMsg -- complete generated message
OUT wholeMsgLength -- length of generated message
OUT tmStateReference -- (NEW) reference to transport state
)
6.3. The receiveMessage ASI
The receiveMessage ASI is used to pass a message from the Transport
Subsystem to the Dispatcher. The receiveMessage ASI replaces the
text "Receive SNMP Response Message from Network" that appears in the
diagram in Section 4.6.1 of RFC 3411 and the text "Receive SNMP
Message from Network" from Section 4.6.2 of RFC3411.
When a message is received on a given transport session, if a cache
does not already exist for that session, the Transport Model might
create one, referenced by tmStateReference. The contents of this
Harrington & Schoenwaelder Standards Track [Page 24]
RFC 5590 SNMP Transport Subsystem June 2009
cache are discussed in Section 5. How this information is determined
is implementation- and Transport-Model-specific.
"Might create one" might sound underspecified, but a Transport Model
might be something like SNMP over UDP over IPv6, where transport
security is not provided, so it might not create a cache.
The Transport Model does not know the securityModel for an incoming
message; this will be determined by the Message Processing Model in a
Message-Processing-Model-dependent manner.
statusInformation =
receiveMessage(
IN transportDomain -- origin transport domain
IN transportAddress -- origin transport address
IN incomingMessage -- the message received
IN incomingMessageLength -- its length
IN tmStateReference -- reference to transport state
)
6.4. Changes to RFC 3411 Incoming ASIs
The tmStateReference parameter has also been added to some of the
incoming ASIs defined in RFC 3411. How or if a Message Processing
Model or Security Model uses tmStateReference is message-processing-
and Security-Model-specific.
This might sound underspecified, but a Message Processing Model might
have access to all the information from the cache and from the
message. The Message Processing Model might determine that the USM
Security Model is specified in an SNMPv3 message header; the USM
Security Model has no need of values in the tmStateReference cache to
authenticate and secure the SNMP message, but an Application might
have specified to use a secure transport such as that provided by the
SSH Transport Model to send the message to its destination.
6.4.1. Message Processing Subsystem Primitive
The tmStateReference parameter of prepareDataElements is passed from
the Dispatcher to the Message Processing Subsystem. How or if the
Message Processing Subsystem modifies or utilizes the contents of the
cache is Message-Processing-Model-specific.
result = -- SUCCESS or errorIndication
prepareDataElements(
IN transportDomain -- origin transport domain
IN transportAddress -- origin transport address
IN wholeMsg -- as received from the network
Harrington & Schoenwaelder Standards Track [Page 25]
RFC 5590 SNMP Transport Subsystem June 2009
IN wholeMsgLength -- as received from the network
IN tmStateReference -- (NEW) from the Transport Model
OUT messageProcessingModel -- typically, SNMP version
OUT securityModel -- Security Model to use
OUT securityName -- on behalf of this principal
OUT securityLevel -- Level of Security requested
OUT contextEngineID -- data from/at this entity
OUT contextName -- data from/in this context
OUT pduVersion -- the version of the PDU
OUT PDU -- SNMP Protocol Data Unit
OUT pduType -- SNMP PDU type
OUT sendPduHandle -- handle for matched request
OUT maxSizeResponseScopedPDU -- maximum size sender can accept
OUT statusInformation -- success or errorIndication
-- error counter OID/value if error
OUT stateReference -- reference to state information
-- to be used for possible Response
)
6.4.2. Security Subsystem Primitive
The processIncomingMessage ASI passes tmStateReference from the
Message Processing Subsystem to the Security Subsystem.
If tmStateReference is present and valid, an appropriate Security
Model might utilize the information in the cache. How or if the
Security Subsystem utilizes the information in the cache is Security-
Model-specific.
statusInformation = -- errorIndication or success
-- error counter OID/value if error
processIncomingMsg(
IN messageProcessingModel -- typically, SNMP version
IN maxMessageSize -- of the sending SNMP entity
IN securityParameters -- for the received message
IN securityModel -- for the received message
IN securityLevel -- Level of Security
IN wholeMsg -- as received on the wire
IN wholeMsgLength -- length as received on the wire
IN tmStateReference -- (NEW) from the Transport Model
OUT securityEngineID -- authoritative SNMP entity
OUT securityName -- identification of the principal
OUT scopedPDU, -- message (plaintext) payload
OUT maxSizeResponseScopedPDU -- maximum size sender can handle
OUT securityStateReference -- reference to security state
-- information, needed for response
)
Harrington & Schoenwaelder Standards Track [Page 26]
RFC 5590 SNMP Transport Subsystem June 2009 7. Security Considerations This document defines an architectural approach that permits SNMP to utilize transport-layer security services. Each proposed Transport Model should discuss the security considerations of that Transport Model. It is considered desirable by some industry segments that SNMP Transport Models utilize transport-layer security that addresses perfect forward secrecy at least for encryption keys. Perfect forward secrecy guarantees that compromise of long-term secret keys does not result in disclosure of past session keys. Each proposed Transport Model should include a discussion in its security considerations of whether perfect forward secrecy is appropriate for that Transport Model. The denial-of-service characteristics of various Transport Models and security protocols will vary and should be evaluated when determining the applicability of a Transport Model to a particular deployment situation. Since the cache will contain security-related parameters, implementers SHOULD store this information (in memory or in persistent storage) in a manner to protect it from unauthorized disclosure and/or modification. Care must be taken to ensure that an SNMP engine is sending packets out over a transport using credentials that are legal for that engine to use on behalf of that user. Otherwise, an engine that has multiple transports open might be "tricked" into sending a message through the wrong transport. A Security Model might have multiple sources from which to define the securityName and securityLevel. The use of a secure Transport Model does not imply that the securityName and securityLevel chosen by the Security Model represent the transport-authenticated identity or the transport-provided security services. The securityModel, securityName, and securityLevel parameters are a related set, and an administrator should understand how the specified securityModel selects the corresponding securityName and securityLevel. 7.1. Coexistence, Security Parameters, and Access Control In the RFC 3411 architecture, the Message Processing Model makes the decision about which Security Model to use. The architectural change described by this document does not alter that. Harrington & Schoenwaelder Standards Track [Page 27]
RFC 5590 SNMP Transport Subsystem June 2009
The architecture change described by this document does, however,
allow SNMP to support two different approaches to security --
message-driven security and transport-driven security. With message-
driven security, SNMP provides its own security and passes security
parameters within the SNMP message; with transport-driven security,
SNMP depends on an external entity to provide security during
transport by "wrapping" the SNMP message.
Using a non-transport-aware Security Model with a secure Transport
Model is NOT RECOMMENDED for the following reasons.
Security Models defined before the Transport Security Model (i.e.,
SNMPv1, SNMPv2c, and USM) do not support transport-based security and
only have access to the security parameters contained within the SNMP
message. They do not know about the security parameters associated
with a secure transport. As a result, the Access Control Subsystem
bases its decisions on the security parameters extracted from the
SNMP message, not on transport-based security parameters.
Implications of combining older Security Models with Secure Transport
Models are known. The securityName used for access control decisions
is based on the message-driven identity, which might be
unauthenticated, and not on the transport-driven, authenticated
identity:
o An SNMPv1 message will always be paired with an SNMPv1 Security
Model (per RFC 3584), regardless of the transport mapping or
Transport Model used, and access controls will be based on the
unauthenticated community name.
o An SNMPv2c message will always be paired with an SNMPv2c Security
Model (per RFC 3584), regardless of the transport mapping or
Transport Model used, and access controls will be based on the
unauthenticated community name.
o An SNMPv3 message will always be paired with the securityModel
specified in the msgSecurityParameters field of the message (per
RFC 3412), regardless of the transport mapping or Transport Model
used. If the SNMPv3 message specifies the User-based Security
Model (USM) with noAuthNoPriv, then the access controls will be
based on the unauthenticated USM user.
o For outgoing messages, if a Secure Transport Model is selected in
combination with a Security Model that does not populate a
tmStateReference, the Secure Transport Model SHOULD detect the
lack of a valid tmStateReference and fail.
Harrington & Schoenwaelder Standards Track [Page 28]
RFC 5590 SNMP Transport Subsystem June 2009 In times of network stress, a Secure Transport Model might not work properly if its underlying security mechanisms (e.g., Network Time Protocol (NTP) or Authentication, Authorization, and Accounting (AAA) protocols or certificate authorities) are not reachable. The User- based Security Model was explicitly designed to not depend upon external network services, and provides its own security services. It is RECOMMENDED that operators provision authPriv USM as a fallback mechanism to supplement any Security Model or Transport Model that has external dependencies, so that secure SNMP communications can continue when the external network service is not available. 8. IANA Considerations IANA has created a new registry in the Simple Network Management Protocol (SNMP) Number Spaces. The new registry is called "SNMP Transport Domains". This registry contains US-ASCII alpha-numeric strings of one to four characters to identify prefixes for corresponding SNMP transport domains. Each transport domain MUST have an OID assignment under snmpDomains [RFC2578]. Values are to be assigned via [RFC5226] "Specification Required". The registry has been populated with the following initial entries: Registry Name: SNMP Transport Domains Reference: [RFC2578] [RFC3417] [RFC5590] Registration Procedures: Specification Required Each domain is assigned a MIB-defined OID under snmpDomains Prefix snmpDomains Reference ------- ----------------------------- --------- udp snmpUDPDomain [RFC3417] [RFC5590] clns snmpCLNSDomain [RFC3417] [RFC5590] cons snmpCONSDomain [RFC3417] [RFC5590] ddp snmpDDPDomain [RFC3417] [RFC5590] ipx snmpIPXDomain [RFC3417] [RFC5590] prxy rfc1157Domain [RFC3417] [RFC5590] 9. Acknowledgments The Integrated Security for SNMP WG would like to thank the following people for their contributions to the process. The authors of submitted Security Model proposals: Chris Elliot, Wes Hardaker, David Harrington, Keith McCloghrie, Kaushik Narayan, David Perkins, Joseph Salowey, and Juergen Schoenwaelder. The members of the Protocol Evaluation Team: Uri Blumenthal, Lakshminath Dondeti, Randy Presuhn, and Eric Rescorla. Harrington & Schoenwaelder Standards Track [Page 29]
RFC 5590 SNMP Transport Subsystem June 2009
WG members who performed detailed reviews: Wes Hardaker, Jeffrey
Hutzelman, Tom Petch, Dave Shield, and Bert Wijnen.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2578] McCloghrie, K., Ed., Perkins, D., Ed., and J.
Schoenwaelder, Ed., "Structure of Management Information
Version 2 (SMIv2)", STD 58, RFC 2578, April 1999.
[RFC3411] Harrington, D., Presuhn, R., and B. Wijnen, "An
Architecture for Describing Simple Network Management
Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
December 2002.
[RFC3412] Case, J., Harrington, D., Presuhn, R., and B. Wijnen,
"Message Processing and Dispatching for the Simple Network
Management Protocol (SNMP)", STD 62, RFC 3412,
December 2002.
[RFC3413] Levi, D., Meyer, P., and B. Stewart, "Simple Network
Management Protocol (SNMP) Applications", STD 62,
RFC 3413, December 2002.
[RFC3414] Blumenthal, U. and B. Wijnen, "User-based Security Model
(USM) for version 3 of the Simple Network Management
Protocol (SNMPv3)", STD 62, RFC 3414, December 2002.
[RFC3417] Presuhn, R., "Transport Mappings for the Simple Network
Management Protocol (SNMP)", STD 62, RFC 3417,
December 2002.
10.2. Informative References
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, June 2000.
[RFC3410] Case, J., Mundy, R., Partain, D., and B. Stewart,
"Introduction and Applicability Statements for Internet-
Standard Management Framework", RFC 3410, December 2002.
Harrington & Schoenwaelder Standards Track [Page 30]
RFC 5590 SNMP Transport Subsystem June 2009
[RFC3584] Frye, R., Levi, D., Routhier, S., and B. Wijnen,
"Coexistence between Version 1, Version 2, and Version 3
of the Internet-standard Network Management Framework",
BCP 74, RFC 3584, August 2003.
[RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, January 2006.
[RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and
Security Layer (SASL)", RFC 4422, June 2006.
[RFC4741] Enns, R., "NETCONF Configuration Protocol", RFC 4741,
December 2006.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5424] Gerhards, R., "The Syslog Protocol", RFC 5424, March 2009.
[RFC5591] Harrington, D. and W. Hardaker, "Transport Security Model
for the Simple Network Management Protocol (SNMP)",
RFC 5591, June 2009.
[RFC5592] Harrington, D., Salowey, J., and W. Hardaker, "Secure
Shell Transport Model for the Simple Network Management
Protocol (SNMP)", RFC 5592, June 2009.
Harrington & Schoenwaelder Standards Track [Page 31]
RFC 5590 SNMP Transport Subsystem June 2009
Appendix A. Why tmStateReference?
This appendix considers why a cache-based approach was selected for
passing parameters.
There are four approaches that could be used for passing information
between the Transport Model and a Security Model.
1. One could define an ASI to supplement the existing ASIs.
2. One could add a header to encapsulate the SNMP message.
3. One could utilize fields already defined in the existing SNMPv3
message.
4. One could pass the information in an implementation-specific
cache or via a MIB module.
A.1. Define an Abstract Service Interface
Abstract Service Interfaces (ASIs) are defined by a set of primitives
that specify the services provided and the abstract data elements
that are to be passed when the services are invoked. Defining
additional ASIs to pass the security and transport information from
the Transport Subsystem to the Security Subsystem has the advantage
of being consistent with existing RFC 3411/3412 practice; it also
helps to ensure that any Transport Model proposals pass the necessary
data and do not cause side effects by creating model-specific
dependencies between itself and models or subsystems other than those
that are clearly defined by an ASI.
A.2. Using an Encapsulating Header
A header could encapsulate the SNMP message to pass necessary
information from the Transport Model to the Dispatcher and then to a
Message Processing Model. The message header would be included in
the wholeMessage ASI parameter and would be removed by a
corresponding Message Processing Model. This would imply the (one
and only) Message Dispatcher would need to be modified to determine
which SNMP message version was involved, and a new Message Processing
Model would need to be developed that knew how to extract the header
from the message and pass it to the Security Model.
A.3. Modifying Existing Fields in an SNMP Message
[RFC3412] defines the SNMPv3 message, which contains fields to pass
security-related parameters. The Transport Subsystem could use these
fields in an SNMPv3 message (or comparable fields in other message
Harrington & Schoenwaelder Standards Track [Page 32]
RFC 5590 SNMP Transport Subsystem June 2009 formats) to pass information between Transport Models in different SNMP engines and to pass information between a Transport Model and a corresponding Message Processing Model. If the fields in an incoming SNMPv3 message are changed by the Transport Model before passing it to the Security Model, then the Transport Model will need to decode the ASN.1 message, modify the fields, and re-encode the message in ASN.1 before passing the message on to the Message Dispatcher or to the transport layer. This would require an intimate knowledge of the message format and message versions in order for the Transport Model to know which fields could be modified. This would seriously violate the modularity of the architecture. A.4. Using a Cache This document describes a cache into which the Transport Model (TM) puts information about the security applied to an incoming message; a Security Model can extract that information from the cache. Given that there might be multiple TM security caches, a tmStateReference is passed as an extra parameter in the ASIs between the Transport Subsystem and the Security Subsystem so that the Security Model knows which cache of information to consult. This approach does create dependencies between a specific Transport Model and a corresponding specific Security Model. However, the approach of passing a model-independent reference to a model- dependent cache is consistent with the securityStateReference already being passed around in the RFC 3411 ASIs. Harrington & Schoenwaelder Standards Track [Page 33]
RFC 5590 SNMP Transport Subsystem June 2009 Authors' Addresses David Harrington Huawei Technologies (USA) 1700 Alma Dr. Suite 100 Plano, TX 75075 USA Phone: +1 603 436 8634 EMail: ietfdbh@comcast.net Juergen Schoenwaelder Jacobs University Bremen Campus Ring 1 28725 Bremen Germany Phone: +49 421 200-3587 EMail: j.schoenwaelder@jacobs-university.de Harrington & Schoenwaelder Standards Track [Page 34]
=========================================================================
Network Working Group D. Harrington
Request for Comments: 5591 Huawei Technologies (USA)
Category: Standards Track W. Hardaker
Cobham Analytic Solutions
June 2009
Transport Security Model for the
Simple Network Management Protocol (SNMP)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Harrington & Hardaker Standards Track [Page 1]
RFC 5591 Transport Security Model for SNMP June 2009
Abstract
This memo describes a Transport Security Model for the Simple Network
Management Protocol (SNMP).
This memo also defines a portion of the Management Information Base
(MIB) for monitoring and managing the Transport Security Model for
SNMP.
Table of Contents
1. Introduction ....................................................3
1.1. The Internet-Standard Management Framework .................3
1.2. Conventions ................................................3
1.3. Modularity .................................................4
1.4. Motivation .................................................5
1.5. Constraints ................................................5
2. How the Transport Security Model Fits in the Architecture .......6
2.1. Security Capabilities of this Model ........................6
2.1.1. Threats .............................................6
2.1.2. Security Levels .....................................7
2.2. Transport Sessions .........................................7
2.3. Coexistence ................................................7
2.3.1. Coexistence with Message Processing Models ..........7
2.3.2. Coexistence with Other Security Models ..............8
2.3.3. Coexistence with Transport Models ...................8
3. Cached Information and References ...............................8
3.1. Transport Security Model Cached Information ................9
3.1.1. securityStateReference ..............................9
3.1.2. tmStateReference ....................................9
3.1.3. Prefixes and securityNames ..........................9
4. Processing an Outgoing Message .................................10
4.1. Security Processing for an Outgoing Message ...............10
4.2. Elements of Procedure for Outgoing Messages ...............11
5. Processing an Incoming SNMP Message ............................12
5.1. Security Processing for an Incoming Message ...............12
5.2. Elements of Procedure for Incoming Messages ...............13
6. MIB Module Overview ............................................14
6.1. Structure of the MIB Module ...............................14
6.1.1. The snmpTsmStats Subtree ...........................14
6.1.2. The snmpTsmConfiguration Subtree ...................14
6.2. Relationship to Other MIB Modules .........................14
6.2.1. MIB Modules Required for IMPORTS ...................15
7. MIB Module Definition ..........................................15
8. Security Considerations ........................................20
8.1. MIB Module Security .......................................20
9. IANA Considerations ............................................21
10. Acknowledgments ...............................................22
Harrington & Hardaker Standards Track [Page 2]
RFC 5591 Transport Security Model for SNMP June 2009
11. References ....................................................22
11.1. Normative References .....................................22
11.2. Informative References ...................................23
Appendix A. Notification Tables Configuration ....................24
A.1. Transport Security Model Processing for Notifications .....25
Appendix B. Processing Differences between USM and Secure
Transport ............................................26
B.1. USM and the RFC 3411 Architecture .........................26
B.2. Transport Subsystem and the RFC 3411 Architecture .........27
1. Introduction
This memo describes a Transport Security Model for the Simple Network
Management Protocol for use with secure Transport Models in the
Transport Subsystem [RFC5590].
This memo also defines a portion of the Management Information Base
(MIB) for monitoring and managing the Transport Security Model for
SNMP.
It is important to understand the SNMP architecture and the
terminology of the architecture to understand where the Transport
Security Model described in this memo fits into the architecture and
interacts with other subsystems and models within the architecture.
It is expected that readers will have also read and understood
[RFC3411], [RFC3412], [RFC3413], and [RFC3418].
1.1. The Internet-Standard Management Framework
For a detailed overview of the documents that describe the current
Internet-Standard Management Framework, please refer to section 7 of
RFC 3410 [RFC3410].
Managed objects are accessed via a virtual information store, termed
the Management Information Base or MIB. MIB objects are generally
accessed through the Simple Network Management Protocol (SNMP).
Objects in the MIB are defined using the mechanisms defined in the
Structure of Management Information (SMI). This memo specifies a MIB
module that is compliant to the SMIv2, which is described in STD 58,
RFC 2578 [RFC2578], STD 58, RFC 2579 [RFC2579] and STD 58, RFC 2580
[RFC2580].
1.2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Harrington & Hardaker Standards Track [Page 3]
RFC 5591 Transport Security Model for SNMP June 2009 Lowercase versions of the keywords should be read as in normal English. They will usually, but not always, be used in a context that relates to compatibility with the RFC 3411 architecture or the subsystem defined here but that might have no impact on on-the-wire compatibility. These terms are used as guidance for designers of proposed IETF models to make the designs compatible with RFC 3411 subsystems and Abstract Service Interfaces (ASIs). Implementers are free to implement differently. Some usages of these lowercase terms are simply normal English usage. For consistency with SNMP-related specifications, this document favors terminology as defined in STD 62, rather than favoring terminology that is consistent with non-SNMP specifications that use different variations of the same terminology. This is consistent with the IESG decision to not require the SNMPv3 terminology be modified to match the usage of other non-SNMP specifications when SNMPv3 was advanced to Full Standard. Authentication in this document typically refers to the English meaning of "serving to prove the authenticity of" the message, not data source authentication or peer identity authentication. The terms "manager" and "agent" are not used in this document because, in the RFC 3411 architecture, all SNMP entities have the capability of acting as manager, agent, or both depending on the SNMP applications included in the engine. Where distinction is needed, the application names of command generator, command responder, notification originator, notification receiver, and proxy forwarder are used. See "Simple Network Management Protocol (SNMP) Applications" [RFC3413] for further information. While security protocols frequently refer to a user, the terminology used in [RFC3411] and in this memo is "principal". A principal is the "who" on whose behalf services are provided or processing takes place. A principal can be, among other things, an individual acting in a particular role, a set of individuals each acting in a particular role, an application or a set of applications, or a combination of these within an administrative domain. 1.3. Modularity The reader is expected to have read and understood the description of the SNMP architecture, as defined in [RFC3411], and the architecture extension specified in "Transport Subsystem for the Simple Network Management Protocol (SNMP)" [RFC5590], which enables the use of external "lower-layer transport" protocols to provide message Harrington & Hardaker Standards Track [Page 4]
RFC 5591 Transport Security Model for SNMP June 2009
security. Transport Models are tied into the SNMP architecture
through the Transport Subsystem. The Transport Security Model is
designed to work with such lower-layer, secure Transport Models.
In keeping with the RFC 3411 design decisions to use self-contained
documents, this memo includes the elements of procedure plus
associated MIB objects that are needed for processing the Transport
Security Model for SNMP. These MIB objects SHOULD NOT be referenced
in other documents. This allows the Transport Security Model to be
designed and documented as independent and self-contained, having no
direct impact on other modules. It also allows this module to be
upgraded and supplemented as the need arises, and to move along the
standards track on different time-lines from other modules.
This modularity of specification is not meant to be interpreted as
imposing any specific requirements on implementation.
1.4. Motivation
This memo describes a Security Model to make use of Transport Models
that use lower-layer, secure transports and existing and commonly
deployed security infrastructures. This Security Model is designed
to meet the security and operational needs of network administrators,
maximize usability in operational environments to achieve high
deployment success, and at the same time minimize implementation and
deployment costs to minimize the time until deployment is possible.
1.5. Constraints
The design of this SNMP Security Model is also influenced by the
following constraints:
1. In times of network stress, the security protocol and its
underlying security mechanisms SHOULD NOT depend solely upon the
ready availability of other network services (e.g., Network Time
Protocol (NTP) or Authentication, Authorization, and Accounting
(AAA) protocols).
2. When the network is not under stress, the Security Model and its
underlying security mechanisms MAY depend upon the ready
availability of other network services.
3. It might not be possible for the Security Model to determine when
the network is under stress.
4. A Security Model SHOULD NOT require changes to the SNMP
architecture.
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RFC 5591 Transport Security Model for SNMP June 2009
5. A Security Model SHOULD NOT require changes to the underlying
security protocol.
2. How the Transport Security Model Fits in the Architecture
The Transport Security Model is designed to fit into the RFC 3411
architecture as a Security Model in the Security Subsystem and to
utilize the services of a secure Transport Model.
For incoming messages, a secure Transport Model will pass a
tmStateReference cache, described in [RFC5590]. To maintain RFC 3411
modularity, the Transport Model will not know which securityModel
will process the incoming message; the Message Processing Model will
determine this. If the Transport Security Model is used with a non-
secure Transport Model, then the cache will not exist or will not be
populated with security parameters, which will cause the Transport
Security Model to return an error (see Section 5.2).
The Transport Security Model will create the securityName and
securityLevel to be passed to applications, and will verify that the
tmTransportSecurityLevel reported by the Transport Model is at least
as strong as the securityLevel requested by the Message Processing
Model.
For outgoing messages, the Transport Security Model will create a
tmStateReference cache (or use an existing one), and will pass the
tmStateReference to the specified Transport Model.
2.1. Security Capabilities of this Model
2.1.1. Threats
The Transport Security Model is compatible with the RFC 3411
architecture and provides protection against the threats identified
by the RFC 3411 architecture. However, the Transport Security Model
does not provide security mechanisms such as authentication and
encryption itself. Which threats are addressed and how they are
mitigated depends on the Transport Model used. To avoid creating
potential security vulnerabilities, operators should configure their
system so this Security Model is always used with a Transport Model
that provides appropriate security, where "appropriate" for a
particular deployment is an administrative decision.
Harrington & Hardaker Standards Track [Page 6]
RFC 5591 Transport Security Model for SNMP June 2009
2.1.2. Security Levels
The RFC 3411 architecture recognizes three levels of security:
- without authentication and without privacy (noAuthNoPriv)
- with authentication but without privacy (authNoPriv)
- with authentication and with privacy (authPriv)
The model-independent securityLevel parameter is used to request
specific levels of security for outgoing messages and to assert that
specific levels of security were applied during the transport and
processing of incoming messages.
The transport-layer algorithms used to provide security should not be
exposed to the Transport Security Model, as the Transport Security
Model has no mechanisms by which it can test whether an assertion
made by a Transport Model is accurate.
The Transport Security Model trusts that the underlying secure
transport connection has been properly configured to support security
characteristics at least as strong as reported in
tmTransportSecurityLevel.
2.2. Transport Sessions
The Transport Security Model does not work with transport sessions
directly. Instead the transport-related state is associated with a
unique combination of transportDomain, transportAddress,
securityName, and securityLevel, and is referenced via the
tmStateReference parameter. How and if this is mapped to a
particular transport or channel is the responsibility of the
Transport Subsystem.
2.3. Coexistence
In the RFC 3411 architecture, a Message Processing Model determines
which Security Model SHALL be called. As of this writing, IANA has
registered four Message Processing Models (SNMPv1, SNMPv2c, SNMPv2u/
SNMPv2*, and SNMPv3) and three other Security Models (SNMPv1,
SNMPv2c, and the User-based Security Model).
2.3.1. Coexistence with Message Processing Models
The SNMPv1 and SNMPv2c message processing described in BCP 74
[RFC3584] always selects the SNMPv1(1) and SNMPv2c(2) Security
Models. Since there is no mechanism defined in RFC 3584 to select an
Harrington & Hardaker Standards Track [Page 7]
RFC 5591 Transport Security Model for SNMP June 2009 alternative Security Model, SNMPv1 and SNMPv2c messages cannot use the Transport Security Model. Messages might still be able to be conveyed over a secure transport protocol, but the Transport Security Model will not be invoked. The SNMPv2u/SNMPv2* Message Processing Model is an historic artifact for which there is no existing IETF specification. The SNMPv3 message processing defined in [RFC3412] extracts the securityModel from the msgSecurityModel field of an incoming SNMPv3Message. When this value is transportSecurityModel(4), security processing is directed to the Transport Security Model. For an outgoing message to be secured using the Transport Security Model, the application MUST specify a securityModel parameter value of transportSecurityModel(4) in the sendPdu Abstract Service Interface (ASI). 2.3.2. Coexistence with Other Security Models The Transport Security Model uses its own MIB module for processing to maintain independence from other Security Models. This allows the Transport Security Model to coexist with other Security Models, such as the User-based Security Model (USM) [RFC3414]. 2.3.3. Coexistence with Transport Models The Transport Security Model (TSM) MAY work with multiple Transport Models, but the RFC 3411 Abstract Service Interfaces (ASIs) do not carry a value for the Transport Model. The MIB module defined in this memo allows an administrator to configure whether or not TSM prepends a Transport Model prefix to the securityName. This will allow SNMP applications to consider Transport Model as a factor when making decisions, such as access control, notification generation, and proxy forwarding. To have SNMP properly utilize the security services coordinated by the Transport Security Model, this Security Model MUST only be used with Transport Models that know how to process a tmStateReference, such as the Secure Shell Transport Model [RFC5592]. 3. Cached Information and References When performing SNMP processing, there are two levels of state information that might need to be retained: the immediate state linking a request-response pair and a potentially longer-term state relating to transport and security. "Transport Subsystem for the Simple Network Management Protocol (SNMP)" [RFC5590] defines general requirements for caches and references. Harrington & Hardaker Standards Track [Page 8]
RFC 5591 Transport Security Model for SNMP June 2009 This document defines additional cache requirements related to the Transport Security Model. 3.1. Transport Security Model Cached Information The Transport Security Model has specific responsibilities regarding the cached information. 3.1.1. securityStateReference The Transport Security Model adds the tmStateReference received from the processIncomingMsg ASI to the securityStateReference. This tmStateReference can then be retrieved during the generateResponseMsg ASI so that it can be passed back to the Transport Model. 3.1.2. tmStateReference For outgoing messages, the Transport Security Model uses parameters provided by the SNMP application to look up or create a tmStateReference. For the Transport Security Model, the security parameters used for a response MUST be the same as those used for the corresponding request. This Security Model uses the tmStateReference stored as part of the securityStateReference when appropriate. For responses and reports, this Security Model sets the tmSameSecurity flag to true in the tmStateReference before passing it to a Transport Model. For incoming messages, the Transport Security Model uses parameters provided in the tmStateReference cache to establish a securityName, and to verify adequate security levels. 3.1.3. Prefixes and securityNames The SNMP-VIEW-BASED-ACM-MIB module [RFC3415], the SNMP-TARGET-MIB module [RFC3413], and other MIB modules contain objects to configure security parameters for use by applications such as access control, notification generation, and proxy forwarding. Transport domains and their corresponding prefixes are coordinated via the IANA registry "SNMP Transport Domains". If snmpTsmConfigurationUsePrefix is set to true, then all securityNames provided by, or provided to, the Transport Security Model MUST include a valid transport domain prefix. Harrington & Hardaker Standards Track [Page 9]
RFC 5591 Transport Security Model for SNMP June 2009
If snmpTsmConfigurationUsePrefix is set to false, then all
securityNames provided by, or provided to, the Transport Security
Model MUST NOT include a transport domain prefix.
The tmSecurityName in the tmStateReference stored as part of the
securityStateReference does not contain a prefix.
4. Processing an Outgoing Message
An error indication might return an Object Identifier (OID) and value
for an incremented counter, a value for securityLevel, values for
contextEngineID and contextName for the counter, and the
securityStateReference, if this information is available at the point
where the error is detected.
4.1. Security Processing for an Outgoing Message
This section describes the procedure followed by the Transport
Security Model.
The parameters needed for generating a message are supplied to the
Security Model by the Message Processing Model via the
generateRequestMsg() or the generateResponseMsg() ASI. The Transport
Subsystem architectural extension has added the transportDomain,
transportAddress, and tmStateReference parameters to the original RFC
3411 ASIs.
statusInformation = -- success or errorIndication
generateRequestMsg(
IN messageProcessingModel -- typically, SNMP version
IN globalData -- message header, admin data
IN maxMessageSize -- of the sending SNMP entity
IN transportDomain -- (NEW) specified by application
IN transportAddress -- (NEW) specified by application
IN securityModel -- for the outgoing message
IN securityEngineID -- authoritative SNMP entity
IN securityName -- on behalf of this principal
IN securityLevel -- Level of Security requested
IN scopedPDU -- message (plaintext) payload
OUT securityParameters -- filled in by Security Module
OUT wholeMsg -- complete generated message
OUT wholeMsgLength -- length of generated message
OUT tmStateReference -- (NEW) transport info
)
statusInformation = -- success or errorIndication
generateResponseMsg(
IN messageProcessingModel -- typically, SNMP version
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RFC 5591 Transport Security Model for SNMP June 2009
IN globalData -- message header, admin data
IN maxMessageSize -- of the sending SNMP entity
IN transportDomain -- (NEW) specified by application
IN transportAddress -- (NEW) specified by application
IN securityModel -- for the outgoing message
IN securityEngineID -- authoritative SNMP entity
IN securityName -- on behalf of this principal
IN securityLevel -- Level of Security requested
IN scopedPDU -- message (plaintext) payload
IN securityStateReference -- reference to security state
-- information from original
-- request
OUT securityParameters -- filled in by Security Module
OUT wholeMsg -- complete generated message
OUT wholeMsgLength -- length of generated message
OUT tmStateReference -- (NEW) transport info
)
4.2. Elements of Procedure for Outgoing Messages
1. If there is a securityStateReference (Response or Report
message), then this Security Model uses the cached information
rather than the information provided by the ASI. Extract the
tmStateReference from the securityStateReference cache. Set the
tmRequestedSecurityLevel to the value of the extracted
tmTransportSecurityLevel. Set the tmSameSecurity parameter in
the tmStateReference cache to true. The cachedSecurityData for
this message can now be discarded.
2. If there is no securityStateReference (e.g., a Request-type or
Notification message), then create a tmStateReference cache. Set
tmTransportDomain to the value of transportDomain,
tmTransportAddress to the value of transportAddress, and
tmRequestedSecurityLevel to the value of securityLevel.
(Implementers might optimize by pointing to saved copies of these
session-specific values.) Set the transaction-specific
tmSameSecurity parameter to false.
If the snmpTsmConfigurationUsePrefix object is set to false, then
set tmSecurityName to the value of securityName.
If the snmpTsmConfigurationUsePrefix object is set to true, then
use the transportDomain to look up the corresponding prefix.
(Since the securityStateReference stores the tmStateReference
with the tmSecurityName for the incoming message, and since
tmSecurityName never has a prefix, the prefix-stripping step only
occurs when we are not using the securityStateReference).
Harrington & Hardaker Standards Track [Page 11]
RFC 5591 Transport Security Model for SNMP June 2009
If the prefix lookup fails for any reason, then the
snmpTsmUnknownPrefixes counter is incremented, an error
indication is returned to the calling module, and message
processing stops.
If the lookup succeeds, but there is no prefix in the
securityName, or the prefix returned does not match the prefix
in the securityName, or the length of the prefix is less than
1 or greater than 4 US-ASCII alpha-numeric characters, then
the snmpTsmInvalidPrefixes counter is incremented, an error
indication is returned to the calling module, and message
processing stops.
Strip the transport-specific prefix and trailing ':' character
(US-ASCII 0x3a) from the securityName. Set tmSecurityName to
the value of securityName.
3. Set securityParameters to a zero-length OCTET STRING ('0400').
4. Combine the message parts into a wholeMsg and calculate
wholeMsgLength.
5. The wholeMsg, wholeMsgLength, securityParameters, and
tmStateReference are returned to the calling Message Processing
Model with the statusInformation set to success.
5. Processing an Incoming SNMP Message
An error indication might return an OID and value for an incremented
counter, a value for securityLevel, values for contextEngineID and
contextName for the counter, and the securityStateReference, if this
information is available at the point where the error is detected.
5.1. Security Processing for an Incoming Message
This section describes the procedure followed by the Transport
Security Model whenever it receives an incoming message from a
Message Processing Model. The ASI from a Message Processing Model to
the Security Subsystem for a received message is:
statusInformation = -- errorIndication or success
-- error counter OID/value if error
processIncomingMsg(
IN messageProcessingModel -- typically, SNMP version
IN maxMessageSize -- from the received message
IN securityParameters -- from the received message
IN securityModel -- from the received message
IN securityLevel -- from the received message
Harrington & Hardaker Standards Track [Page 12]
RFC 5591 Transport Security Model for SNMP June 2009
IN wholeMsg -- as received on the wire
IN wholeMsgLength -- length as received on the wire
IN tmStateReference -- (NEW) from the Transport Model
OUT securityEngineID -- authoritative SNMP entity
OUT securityName -- identification of the principal
OUT scopedPDU, -- message (plaintext) payload
OUT maxSizeResponseScopedPDU -- maximum size sender can handle
OUT securityStateReference -- reference to security state
) -- information, needed for response
5.2. Elements of Procedure for Incoming Messages
1. Set the securityEngineID to the local snmpEngineID.
2. If tmStateReference does not refer to a cache containing values
for tmTransportDomain, tmTransportAddress, tmSecurityName, and
tmTransportSecurityLevel, then the snmpTsmInvalidCaches counter
is incremented, an error indication is returned to the calling
module, and Security Model processing stops for this message.
3. Copy the tmSecurityName to securityName.
If the snmpTsmConfigurationUsePrefix object is set to true, then
use the tmTransportDomain to look up the corresponding prefix.
If the prefix lookup fails for any reason, then the
snmpTsmUnknownPrefixes counter is incremented, an error
indication is returned to the calling module, and message
processing stops.
If the lookup succeeds but the prefix length is less than 1 or
greater than 4 octets, then the snmpTsmInvalidPrefixes counter
is incremented, an error indication is returned to the calling
module, and message processing stops.
Set the securityName to be the concatenation of the prefix, a
':' character (US-ASCII 0x3a), and the tmSecurityName.
4. Compare the value of tmTransportSecurityLevel in the
tmStateReference cache to the value of the securityLevel
parameter passed in the processIncomingMsg ASI. If securityLevel
specifies privacy (Priv) and tmTransportSecurityLevel specifies
no privacy (noPriv), or if securityLevel specifies authentication
(auth) and tmTransportSecurityLevel specifies no authentication
(noAuth) was provided by the Transport Model, then the
snmpTsmInadequateSecurityLevels counter is incremented, an error
indication (unsupportedSecurityLevel) together with the OID and
Harrington & Hardaker Standards Track [Page 13]
RFC 5591 Transport Security Model for SNMP June 2009
value of the incremented counter is returned to the calling
module, and Transport Security Model processing stops for this
message.
5. The tmStateReference is cached as cachedSecurityData so that a
possible response to this message will use the same security
parameters. Then securityStateReference is set for subsequent
references to this cached data.
6. The scopedPDU component is extracted from the wholeMsg.
7. The maxSizeResponseScopedPDU is calculated. This is the maximum
size allowed for a scopedPDU for a possible Response message.
8. The statusInformation is set to success and a return is made to
the calling module passing back the OUT parameters as specified
in the processIncomingMsg ASI.
6. MIB Module Overview
This MIB module provides objects for use only by the Transport
Security Model. It defines a configuration scalar and related error
counters.
6.1. Structure of the MIB Module
Objects in this MIB module are arranged into subtrees. Each subtree
is organized as a set of related objects. The overall structure and
assignment of objects to their subtrees, and the intended purpose of
each subtree, is shown below.
6.1.1. The snmpTsmStats Subtree
This subtree contains error counters specific to the Transport
Security Model.
6.1.2. The snmpTsmConfiguration Subtree
This subtree contains a configuration object that enables
administrators to specify if they want a transport domain prefix
prepended to securityNames for use by applications.
6.2. Relationship to Other MIB Modules
Some management objects defined in other MIB modules are applicable
to an entity implementing the Transport Security Model. In
particular, it is assumed that an entity implementing the Transport
Security Model will implement the SNMP-FRAMEWORK-MIB [RFC3411], the
Harrington & Hardaker Standards Track [Page 14]
RFC 5591 Transport Security Model for SNMP June 2009
SNMP-TARGET-MIB [RFC3413], the SNMP-VIEW-BASED-ACM-MIB [RFC3415], and
the SNMPv2-MIB [RFC3418]. These are not needed to implement the
SNMP-TSM-MIB.
6.2.1. MIB Modules Required for IMPORTS
The following MIB module imports items from [RFC2578], [RFC2579], and
[RFC2580].
7. MIB Module Definition
SNMP-TSM-MIB DEFINITIONS ::= BEGIN
IMPORTS
MODULE-IDENTITY, OBJECT-TYPE,
mib-2, Counter32
FROM SNMPv2-SMI -- RFC2578
MODULE-COMPLIANCE, OBJECT-GROUP
FROM SNMPv2-CONF -- RFC2580
TruthValue
FROM SNMPv2-TC -- RFC2579
;
snmpTsmMIB MODULE-IDENTITY
LAST-UPDATED "200906090000Z"
ORGANIZATION "ISMS Working Group"
CONTACT-INFO "WG-EMail: isms@lists.ietf.org
Subscribe: isms-request@lists.ietf.org
Chairs:
Juergen Quittek
NEC Europe Ltd.
Network Laboratories
Kurfuersten-Anlage 36
69115 Heidelberg
Germany
+49 6221 90511-15
quittek@netlab.nec.de
Juergen Schoenwaelder
Jacobs University Bremen
Campus Ring 1
28725 Bremen
Germany
+49 421 200-3587
j.schoenwaelder@jacobs-university.de
Harrington & Hardaker Standards Track [Page 15]
RFC 5591 Transport Security Model for SNMP June 2009
Editor:
David Harrington
Huawei Technologies USA
1700 Alma Dr.
Plano TX 75075
USA
+1 603-436-8634
ietfdbh@comcast.net
Wes Hardaker
Cobham Analytic Solutions
P.O. Box 382
Davis, CA 95617
USA
+1 530 792 1913
ietf@hardakers.net
"
DESCRIPTION
"The Transport Security Model MIB.
In keeping with the RFC 3411 design decisions to use
self-contained documents, the RFC that contains the definition
of this MIB module also includes the elements of procedure
that are needed for processing the Transport Security Model
for SNMP. These MIB objects SHOULD NOT be modified via other
subsystems or models defined in other documents. This allows
the Transport Security Model for SNMP to be designed and
documented as independent and self-contained, having no direct
impact on other modules, and this allows this module to be
upgraded and supplemented as the need arises, and to move
along the standards track on different time-lines from other
modules.
Copyright (c) 2009 IETF Trust and the persons
identified as authors of the code. All rights reserved.
Redistribution and use in source and binary forms, with or
without modification, are permitted provided that the
following conditions are met:
- Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
- Redistributions in binary form must reproduce the above
copyright notice, this list of conditions and the following
disclaimer in the documentation and/or other materials
provided with the distribution.
Harrington & Hardaker Standards Track [Page 16]
RFC 5591 Transport Security Model for SNMP June 2009
- Neither the name of Internet Society, IETF or IETF Trust,
nor the names of specific contributors, may be used to endorse
or promote products derived from this software without
specific prior written permission.
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
CONTRIBUTORS 'AS IS' AND ANY EXPRESS OR IMPLIED WARRANTIES,
INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE,
EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
This version of this MIB module is part of RFC 5591;
see the RFC itself for full legal notices."
REVISION "200906090000Z"
DESCRIPTION "The initial version, published in RFC 5591."
::= { mib-2 190 }
-- ---------------------------------------------------------- --
-- subtrees in the SNMP-TSM-MIB
-- ---------------------------------------------------------- --
snmpTsmNotifications OBJECT IDENTIFIER ::= { snmpTsmMIB 0 }
snmpTsmMIBObjects OBJECT IDENTIFIER ::= { snmpTsmMIB 1 }
snmpTsmConformance OBJECT IDENTIFIER ::= { snmpTsmMIB 2 }
-- -------------------------------------------------------------
-- Objects
-- -------------------------------------------------------------
-- Statistics for the Transport Security Model
snmpTsmStats OBJECT IDENTIFIER ::= { snmpTsmMIBObjects 1 }
snmpTsmInvalidCaches OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The number of incoming messages dropped because the
Harrington & Hardaker Standards Track [Page 17]
RFC 5591 Transport Security Model for SNMP June 2009
tmStateReference referred to an invalid cache.
"
::= { snmpTsmStats 1 }
snmpTsmInadequateSecurityLevels OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The number of incoming messages dropped because
the securityLevel asserted by the Transport Model was
less than the securityLevel requested by the
application.
"
::= { snmpTsmStats 2 }
snmpTsmUnknownPrefixes OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The number of messages dropped because
snmpTsmConfigurationUsePrefix was set to true and
there is no known prefix for the specified transport
domain.
"
::= { snmpTsmStats 3 }
snmpTsmInvalidPrefixes OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The number of messages dropped because
the securityName associated with an outgoing message
did not contain a valid transport domain prefix.
"
::= { snmpTsmStats 4 }
-- -------------------------------------------------------------
-- Configuration
-- -------------------------------------------------------------
-- Configuration for the Transport Security Model
snmpTsmConfiguration OBJECT IDENTIFIER ::= { snmpTsmMIBObjects 2 }
snmpTsmConfigurationUsePrefix OBJECT-TYPE
SYNTAX TruthValue
MAX-ACCESS read-write
STATUS current
Harrington & Hardaker Standards Track [Page 18]
RFC 5591 Transport Security Model for SNMP June 2009
DESCRIPTION "If this object is set to true, then securityNames
passing to and from the application are expected to
contain a transport-domain-specific prefix. If this
object is set to true, then a domain-specific prefix
will be added by the TSM to the securityName for
incoming messages and removed from the securityName
when processing outgoing messages. Transport domains
and prefixes are maintained in a registry by IANA.
This object SHOULD persist across system reboots.
"
DEFVAL { false }
::= { snmpTsmConfiguration 1 }
-- -------------------------------------------------------------
-- snmpTsmMIB - Conformance Information
-- -------------------------------------------------------------
snmpTsmCompliances OBJECT IDENTIFIER ::= { snmpTsmConformance 1 }
snmpTsmGroups OBJECT IDENTIFIER ::= { snmpTsmConformance 2 }
-- -------------------------------------------------------------
-- Compliance statements
-- -------------------------------------------------------------
snmpTsmCompliance MODULE-COMPLIANCE
STATUS current
DESCRIPTION "The compliance statement for SNMP engines that support
the SNMP-TSM-MIB.
"
MODULE
MANDATORY-GROUPS { snmpTsmGroup }
::= { snmpTsmCompliances 1 }
-- -------------------------------------------------------------
-- Units of conformance
-- -------------------------------------------------------------
snmpTsmGroup OBJECT-GROUP
OBJECTS {
snmpTsmInvalidCaches,
snmpTsmInadequateSecurityLevels,
snmpTsmUnknownPrefixes,
snmpTsmInvalidPrefixes,
snmpTsmConfigurationUsePrefix
}
STATUS current
DESCRIPTION "A collection of objects for maintaining
information of an SNMP engine that implements
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RFC 5591 Transport Security Model for SNMP June 2009
the SNMP Transport Security Model.
"
::= { snmpTsmGroups 2 }
END
8. Security Considerations
This document describes a Security Model, compatible with the RFC
3411 architecture, that permits SNMP to utilize security services
provided through an SNMP Transport Model. The Transport Security
Model relies on Transport Models for mutual authentication, binding
of keys, confidentiality, and integrity.
The Transport Security Model relies on secure Transport Models to
provide an authenticated principal identifier and an assertion of
whether authentication and privacy are used during transport. This
Security Model SHOULD always be used with Transport Models that
provide adequate security, but "adequate security" is a configuration
and/or run-time decision of the operator or management application.
The security threats and how these threats are mitigated should be
covered in detail in the specifications of the Transport Models and
the underlying secure transports.
An authenticated principal identifier (securityName) is used in SNMP
applications for purposes such as access control, notification
generation, and proxy forwarding. This Security Model supports
multiple Transport Models. Operators might judge some transports to
be more secure than others, so this Security Model can be configured
to prepend a prefix to the securityName to indicate the Transport
Model used to authenticate the principal. Operators can use the
prefixed securityName when making application decisions about levels
of access.
8.1. MIB Module Security
There are a number of management objects defined in this MIB module
with a MAX-ACCESS clause of read-write and/or read-create. Such
objects may be considered sensitive or vulnerable in some network
environments. The support for SET operations in a non-secure
environment without proper protection can have a negative effect on
network operations. These are the tables and objects and their
sensitivity/vulnerability:
Harrington & Hardaker Standards Track [Page 20]
RFC 5591 Transport Security Model for SNMP June 2009
o The snmpTsmConfigurationUsePrefix object could be modified,
creating a denial of service or authorizing SNMP messages that
would not have previously been authorized by an Access Control
Model (e.g., the View-based Access Control Model (VACM)).
Some of the readable objects in this MIB module (i.e., objects with a
MAX-ACCESS other than not-accessible) may be considered sensitive or
vulnerable in some network environments. It is thus important to
control even GET and/or NOTIFY access to these objects and possibly
to even encrypt the values of these objects when sending them over
the network via SNMP. These are the tables and objects and their
sensitivity/vulnerability:
o All the counters in this module refer to configuration errors and
do not expose sensitive information.
SNMP versions prior to SNMPv3 did not include adequate security.
Even if the network itself is secure (for example by using IPsec),
even then, there is no control as to who on the secure network is
allowed to access and GET/SET (read/change/create/delete) the objects
in this MIB module.
It is RECOMMENDED that implementers consider the security features as
provided by the SNMPv3 framework (see [RFC3410], section 8),
including full support for the USM and Transport Security Model
cryptographic mechanisms (for authentication and privacy).
Further, deployment of SNMP versions prior to SNMPv3 is NOT
RECOMMENDED. Instead, it is RECOMMENDED to deploy SNMPv3 and to
enable cryptographic security. It is then a customer/operator
responsibility to ensure that the SNMP entity giving access to an
instance of this MIB module is properly configured to give access to
the objects only to those principals (users) that have legitimate
rights to indeed GET or SET (change/create/delete) them.
9. IANA Considerations
IANA has assigned:
1. An SMI number (190) with a prefix of mib-2 in the MIB module
registry for the MIB module in this document.
2. A value (4) to identify the Transport Security Model, in the
Security Models registry of the SNMP Number Spaces registry.
This results in the following table of values:
Harrington & Hardaker Standards Track [Page 21]
RFC 5591 Transport Security Model for SNMP June 2009
Value Description References
----- ----------- ----------
0 reserved for 'any' [RFC3411]
1 reserved for SNMPv1 [RFC3411]
2 reserved for SNMPv2c [RFC3411]
3 User-Based Security Model (USM) [RFC3411]
4 Transport Security Model (TSM) [RFC5591]
10. Acknowledgments
The editors would like to thank Jeffrey Hutzelman for sharing his SSH
insights and Dave Shield for an outstanding job wordsmithing the
existing document to improve organization and clarity.
Additionally, helpful document reviews were received from Juergen
Schoenwaelder.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2578] McCloghrie, K., Ed., Perkins, D., Ed., and J.
Schoenwaelder, Ed., "Structure of Management Information
Version 2 (SMIv2)", STD 58, RFC 2578, April 1999.
[RFC2579] McCloghrie, K., Ed., Perkins, D., Ed., and J.
Schoenwaelder, Ed., "Textual Conventions for SMIv2",
STD 58, RFC 2579, April 1999.
[RFC2580] McCloghrie, K., Perkins, D., and J. Schoenwaelder,
"Conformance Statements for SMIv2", STD 58, RFC 2580,
April 1999.
[RFC3411] Harrington, D., Presuhn, R., and B. Wijnen, "An
Architecture for Describing Simple Network Management
Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
December 2002.
[RFC3412] Case, J., Harrington, D., Presuhn, R., and B. Wijnen,
"Message Processing and Dispatching for the Simple Network
Management Protocol (SNMP)", STD 62, RFC 3412,
December 2002.
Harrington & Hardaker Standards Track [Page 22]
RFC 5591 Transport Security Model for SNMP June 2009
[RFC3413] Levi, D., Meyer, P., and B. Stewart, "Simple Network
Management Protocol (SNMP) Applications", STD 62,
RFC 3413, December 2002.
[RFC3414] Blumenthal, U. and B. Wijnen, "User-based Security Model
(USM) for version 3 of the Simple Network Management
Protocol (SNMPv3)", STD 62, RFC 3414, December 2002.
[RFC5590] Harrington, D. and J. Schoenwaelder, "Transport Subsystem
for the Simple Network Management Protocol (SNMP)",
RFC 5590, June 2009.
11.2. Informative References
[RFC3410] Case, J., Mundy, R., Partain, D., and B. Stewart,
"Introduction and Applicability Statements for Internet-
Standard Management Framework", RFC 3410, December 2002.
[RFC3415] Wijnen, B., Presuhn, R., and K. McCloghrie, "View-based
Access Control Model (VACM) for the Simple Network
Management Protocol (SNMP)", STD 62, RFC 3415,
December 2002.
[RFC3418] Presuhn, R., "Management Information Base (MIB) for the
Simple Network Management Protocol (SNMP)", STD 62,
RFC 3418, December 2002.
[RFC3584] Frye, R., Levi, D., Routhier, S., and B. Wijnen,
"Coexistence between Version 1, Version 2, and Version 3
of the Internet-standard Network Management Framework",
BCP 74, RFC 3584, August 2003.
[RFC5592] Harrington, D., Salowey, J., and W. Hardaker, "Secure
Shell Transport Model for the Simple Network Management
Protocol (SNMP)", RFC 5592, June 2009.
Harrington & Hardaker Standards Track [Page 23]
RFC 5591 Transport Security Model for SNMP June 2009
Appendix A. Notification Tables Configuration
The SNMP-TARGET-MIB and SNMP-NOTIFICATION-MIB [RFC3413] are used to
configure notification originators with the destinations to which
notifications should be sent.
Most of the configuration is Security-Model-independent and
Transport-Model-independent.
The values we will use in the examples for the five model-independent
security and transport parameters are:
transportDomain = snmpSSHDomain
transportAddress = 192.0.2.1:5162
securityModel = Transport Security Model
securityName = alice
securityLevel = authPriv
The following example will configure the notification originator to
send informs to a notification receiver at 192.0.2.1:5162 using the
securityName "alice". "alice" is the name for the recipient from the
standpoint of the notification originator and is used for processing
access controls before sending a notification.
The columns marked with an "*" are the items that are Security-Model-
specific or Transport-Model-specific.
The configuration for the "alice" settings in the SNMP-VIEW-BASED-
ACM-MIB objects are not shown here for brevity. First, we configure
which type of notification will be sent for this taglist (toCRTag).
In this example, we choose to send an Inform.
snmpNotifyTable row:
snmpNotifyName CRNotif
snmpNotifyTag toCRTag
snmpNotifyType inform
snmpNotifyStorageType nonVolatile
snmpNotifyColumnStatus createAndGo
Then we configure a transport address to which notifications
associated with this taglist will be sent, and we specify which
snmpTargetParamsEntry will be used (toCR) when sending to this
transport address.
Harrington & Hardaker Standards Track [Page 24]
RFC 5591 Transport Security Model for SNMP June 2009
snmpTargetAddrTable row:
snmpTargetAddrName toCRAddr
* snmpTargetAddrTDomain snmpSSHDomain
* snmpTargetAddrTAddress 192.0.2.1:5162
snmpTargetAddrTimeout 1500
snmpTargetAddrRetryCount 3
snmpTargetAddrTagList toCRTag
snmpTargetAddrParams toCR (MUST match below)
snmpTargetAddrStorageType nonVolatile
snmpTargetAddrColumnStatus createAndGo
Then we configure which principal at the host will receive the
notifications associated with this taglist. Here, we choose "alice",
who uses the Transport Security Model.
snmpTargetParamsTable row:
snmpTargetParamsName toCR
snmpTargetParamsMPModel SNMPv3
* snmpTargetParamsSecurityModel TransportSecurityModel
snmpTargetParamsSecurityName "alice"
snmpTargetParamsSecurityLevel authPriv
snmpTargetParamsStorageType nonVolatile
snmpTargetParamsRowStatus createAndGo
A.1. Transport Security Model Processing for Notifications
The Transport Security Model is called using the generateRequestMsg()
ASI, with the following parameters (those with an * are from the
above tables):
statusInformation = -- success or errorIndication
generateRequestMsg(
IN messageProcessingModel -- *snmpTargetParamsMPModel
IN globalData -- message header, admin data
IN maxMessageSize -- of the sending SNMP entity
IN transportDomain -- *snmpTargetAddrTDomain
IN transportAddress -- *snmpTargetAddrTAddress
IN securityModel -- *snmpTargetParamsSecurityModel
IN securityEngineID -- immaterial; TSM will ignore.
IN securityName -- snmpTargetParamsSecurityName
IN securityLevel -- *snmpTargetParamsSecurityLevel
IN scopedPDU -- message (plaintext) payload
OUT securityParameters -- filled in by Security Module
OUT wholeMsg -- complete generated message
OUT wholeMsgLength -- length of generated message
OUT tmStateReference -- reference to transport info
)
Harrington & Hardaker Standards Track [Page 25]
RFC 5591 Transport Security Model for SNMP June 2009
The Transport Security Model will determine the Transport Model based
on the snmpTargetAddrTDomain. The selected Transport Model will
select the appropriate transport connection using the
tmStateReference cache created from the values of
snmpTargetAddrTAddress, snmpTargetParamsSecurityName, and
snmpTargetParamsSecurityLevel.
Appendix B. Processing Differences between USM and Secure Transport
USM and secure transports differ in the processing order and
responsibilities within the RFC 3411 architecture. While the steps
are the same, they occur in a different order and might be done by
different subsystems. The following lists illustrate the difference
in the flow and the responsibility for different processing steps for
incoming messages when using USM and when using a secure transport.
(These lists are simplified for illustrative purposes, and do not
represent all details of processing. Transport Models MUST provide
the detailed elements of procedure.)
With USM, SNMPv1, and SNMPv2c Security Models, security processing
starts when the Message Processing Model decodes portions of the
ASN.1 message to extract header fields that are used to determine
which Security Model will process the message to perform
authentication, decryption, timeliness checking, integrity checking,
and translation of parameters to model-independent parameters. By
comparison, a secure transport performs those security functions on
the message, before the ASN.1 is decoded.
Step 6 cannot occur until after decryption occurs. Steps 6 and
beyond are the same for USM and a secure transport.
B.1. USM and the RFC 3411 Architecture
1) Decode the ASN.1 header (Message Processing Model).
2) Determine the SNMP Security Model and parameters (Message
Processing Model).
3) Verify securityLevel (Security Model).
4) Translate parameters to model-independent parameters (Security
Model).
5) Authenticate the principal, check message integrity and
timeliness, and decrypt the message (Security Model).
Harrington & Hardaker Standards Track [Page 26]
RFC 5591 Transport Security Model for SNMP June 2009
6) Determine the pduType in the decrypted portions (Message
Processing Model).
7) Pass on the decrypted portions with model-independent parameters.
B.2. Transport Subsystem and the RFC 3411 Architecture
1) Authenticate the principal, check integrity and timeliness of the
message, and decrypt the message (Transport Model).
2) Translate parameters to model-independent parameters (Transport
Model).
3) Decode the ASN.1 header (Message Processing Model).
4) Determine the SNMP Security Model and parameters (Message
Processing Model).
5) Verify securityLevel (Security Model).
6) Determine the pduType in the decrypted portions (Message
Processing Model).
7) Pass on the decrypted portions with model-independent security
parameters.
If a message is secured using a secure transport layer, then the
Transport Model will provide the translation from the authenticated
identity (e.g., an SSH user name) to a human-friendly identifier
(tmSecurityName) in step 2. The Security Model will provide a
mapping from that identifier to a model-independent securityName.
Harrington & Hardaker Standards Track [Page 27]
RFC 5591 Transport Security Model for SNMP June 2009 Authors' Addresses David Harrington Huawei Technologies (USA) 1700 Alma Dr. Suite 100 Plano, TX 75075 USA Phone: +1 603 436 8634 EMail: ietfdbh@comcast.net Wes Hardaker Cobham Analytic Solutions P.O. Box 382 Davis, CA 95617 US Phone: +1 530 792 1913 EMail: ietf@hardakers.net Harrington & Hardaker Standards Track [Page 28]
=========================================================================
Internet Engineering Task Force (IETF) W. Hardaker
Request for Comments: 6353 SPARTA, Inc.
Obsoletes: 5953 July 2011
Category: Standards Track
ISSN: 2070-1721
Transport Layer Security (TLS) Transport Model for
the Simple Network Management Protocol (SNMP)
Abstract
This document describes a Transport Model for the Simple Network
Management Protocol (SNMP), that uses either the Transport Layer
Security protocol or the Datagram Transport Layer Security (DTLS)
protocol. The TLS and DTLS protocols provide authentication and
privacy services for SNMP applications. This document describes how
the TLS Transport Model (TLSTM) implements the needed features of an
SNMP Transport Subsystem to make this protection possible in an
interoperable way.
This Transport Model is designed to meet the security and operational
needs of network administrators. It supports the sending of SNMP
messages over TLS/TCP and DTLS/UDP. The TLS mode can make use of
TCP's improved support for larger packet sizes and the DTLS mode
provides potentially superior operation in environments where a
connectionless (e.g., UDP) transport is preferred. Both TLS and DTLS
integrate well into existing public keying infrastructures.
This document also defines a portion of the Management Information
Base (MIB) for use with network management protocols. In particular,
it defines objects for managing the TLS Transport Model for SNMP.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6353.
Hardaker Standards Track [Page 1]
RFC 6353 TLS Transport Model for SNMP July 2011
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Conventions . . . . . . . . . . . . . . . . . . . . . . . 7
1.2. Changes Since RFC 5953 . . . . . . . . . . . . . . . . . . 8
2. The Transport Layer Security Protocol . . . . . . . . . . . . 8
3. How the TLSTM Fits into the Transport Subsystem . . . . . . . 8
3.1. Security Capabilities of This Model . . . . . . . . . . . 11
3.1.1. Threats . . . . . . . . . . . . . . . . . . . . . . . 11
3.1.2. Message Protection . . . . . . . . . . . . . . . . . . 12
3.1.3. (D)TLS Connections . . . . . . . . . . . . . . . . . . 13
3.2. Security Parameter Passing . . . . . . . . . . . . . . . . 14
3.3. Notifications and Proxy . . . . . . . . . . . . . . . . . 14
4. Elements of the Model . . . . . . . . . . . . . . . . . . . . 15
4.1. X.509 Certificates . . . . . . . . . . . . . . . . . . . . 15
4.1.1. Provisioning for the Certificate . . . . . . . . . . . 15
4.2. (D)TLS Usage . . . . . . . . . . . . . . . . . . . . . . . 17
4.3. SNMP Services . . . . . . . . . . . . . . . . . . . . . . 18
4.3.1. SNMP Services for an Outgoing Message . . . . . . . . 18
4.3.2. SNMP Services for an Incoming Message . . . . . . . . 19
Hardaker Standards Track [Page 2]
RFC 6353 TLS Transport Model for SNMP July 2011
4.4. Cached Information and References . . . . . . . . . . . . 20
4.4.1. TLS Transport Model Cached Information . . . . . . . . 20
4.4.1.1. tmSecurityName . . . . . . . . . . . . . . . . . . 20
4.4.1.2. tmSessionID . . . . . . . . . . . . . . . . . . . 21
4.4.1.3. Session State . . . . . . . . . . . . . . . . . . 21
5. Elements of Procedure . . . . . . . . . . . . . . . . . . . . 21
5.1. Procedures for an Incoming Message . . . . . . . . . . . . 21
5.1.1. DTLS over UDP Processing for Incoming Messages . . . . 22
5.1.2. Transport Processing for Incoming SNMP Messages . . . 23
5.2. Procedures for an Outgoing SNMP Message . . . . . . . . . 25
5.3. Establishing or Accepting a Session . . . . . . . . . . . 26
5.3.1. Establishing a Session as a Client . . . . . . . . . . 26
5.3.2. Accepting a Session as a Server . . . . . . . . . . . 28
5.4. Closing a Session . . . . . . . . . . . . . . . . . . . . 29
6. MIB Module Overview . . . . . . . . . . . . . . . . . . . . . 30
6.1. Structure of the MIB Module . . . . . . . . . . . . . . . 30
6.2. Textual Conventions . . . . . . . . . . . . . . . . . . . 30
6.3. Statistical Counters . . . . . . . . . . . . . . . . . . . 30
6.4. Configuration Tables . . . . . . . . . . . . . . . . . . . 30
6.4.1. Notifications . . . . . . . . . . . . . . . . . . . . 31
6.5. Relationship to Other MIB Modules . . . . . . . . . . . . 31
6.5.1. MIB Modules Required for IMPORTS . . . . . . . . . . . 31
7. MIB Module Definition . . . . . . . . . . . . . . . . . . . . 31
8. Operational Considerations . . . . . . . . . . . . . . . . . . 54
8.1. Sessions . . . . . . . . . . . . . . . . . . . . . . . . . 54
8.2. Notification Receiver Credential Selection . . . . . . . . 54
8.3. contextEngineID Discovery . . . . . . . . . . . . . . . . 55
8.4. Transport Considerations . . . . . . . . . . . . . . . . . 55
9. Security Considerations . . . . . . . . . . . . . . . . . . . 55
9.1. Certificates, Authentication, and Authorization . . . . . 55
9.2. (D)TLS Security Considerations . . . . . . . . . . . . . . 56
9.2.1. TLS Version Requirements . . . . . . . . . . . . . . . 56
9.2.2. Perfect Forward Secrecy . . . . . . . . . . . . . . . 57
9.3. Use with SNMPv1/SNMPv2c Messages . . . . . . . . . . . . . 57
9.4. MIB Module Security . . . . . . . . . . . . . . . . . . . 57
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 59
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 59
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 60
12.1. Normative References . . . . . . . . . . . . . . . . . . . 60
12.2. Informative References . . . . . . . . . . . . . . . . . . 61
Appendix A. Target and Notification Configuration Example . . . . 63
A.1. Configuring a Notification Originator . . . . . . . . . . 63
A.2. Configuring TLSTM to Utilize a Simple Derivation of
tmSecurityName . . . . . . . . . . . . . . . . . . . . . . 64
A.3. Configuring TLSTM to Utilize Table-Driven Certificate
Mapping . . . . . . . . . . . . . . . . . . . . . . . . . 64
Hardaker Standards Track [Page 3]
RFC 6353 TLS Transport Model for SNMP July 2011 1. Introduction It is important to understand the modular SNMPv3 architecture as defined by [RFC3411] and enhanced by the Transport Subsystem [RFC5590]. It is also important to understand the terminology of the SNMPv3 architecture in order to understand where the Transport Model described in this document fits into the architecture and how it interacts with the other architecture subsystems. For a detailed overview of the documents that describe the current Internet-Standard Management Framework, please refer to Section 7 of [RFC3410]. This document describes a Transport Model that makes use of the Transport Layer Security (TLS) [RFC5246] and the Datagram Transport Layer Security (DTLS) Protocol [RFC4347], within a Transport Subsystem [RFC5590]. DTLS is the datagram variant of the Transport Layer Security (TLS) protocol [RFC5246]. The Transport Model in this document is referred to as the Transport Layer Security Transport Model (TLSTM). TLS and DTLS take advantage of the X.509 public keying infrastructure [RFC5280]. While (D)TLS supports multiple authentication mechanisms, this document only discusses X.509 certificate-based authentication. Although other forms of authentication are possible, they are outside the scope of this specification. This transport model is designed to meet the security and operational needs of network administrators, operating in both environments where a connectionless (e.g., UDP) transport is preferred and in environments where large quantities of data need to be sent (e.g., over a TCP-based stream). Both TLS and DTLS integrate well into existing public keying infrastructures. This document supports sending of SNMP messages over TLS/TCP and DTLS/UDP. This document also defines a portion of the Management Information Base (MIB) for use with network management protocols. In particular, it defines objects for managing the TLS Transport Model for SNMP. Managed objects are accessed via a virtual information store, termed the Management Information Base or MIB. MIB objects are generally accessed through the Simple Network Management Protocol (SNMP). Objects in the MIB are defined using the mechanisms defined in the Structure of Management Information (SMI). This memo specifies a MIB module that is compliant to the SMIv2, which is described in STD 58: [RFC2578], [RFC2579], and [RFC2580]. Hardaker Standards Track [Page 4]
RFC 6353 TLS Transport Model for SNMP July 2011 The diagram shown below gives a conceptual overview of two SNMP entities communicating using the TLS Transport Model (shown as "TLSTM"). One entity contains a command responder and notification originator application, and the other a command generator and notification receiver application. It should be understood that this particular mix of application types is an example only and other combinations are equally valid. Note: this diagram shows the Transport Security Model (TSM) being used as the security model that is defined in [RFC5591]. Hardaker Standards Track [Page 5]
RFC 6353 TLS Transport Model for SNMP July 2011
+---------------------------------------------------------------------+
| Network |
+---------------------------------------------------------------------+
^ | ^ |
|Notifications |Commands |Commands |Notifications
+---|---------------------|-------+ +--|---------------|--------------+
| | V | | | V |
| +------------+ +------------+ | | +-----------+ +----------+ |
| | (D)TLS | | (D)TLS | | | | (D)TLS | | (D)TLS | |
| | (Client) | | (Server) | | | | (Client) | | (Server) | |
| +------------+ +------------+ | | +-----------+ +----------+ |
| ^ ^ | | ^ ^ |
| | | | | | | |
| +-------------+ | | +--------------+ |
| +-----|------------+ | | +-----|------------+ |
| | V | | | | V | |
| | +--------+ | +-----+ | | | +--------+ | +-----+ |
| | | TLS TM |<--------->|Cache| | | | | TLS TM |<--------->|Cache| |
| | +--------+ | +-----+ | | | +--------+ | +-----+ |
| |Transport Subsys. | ^ | | |Transport Subsys. | ^ |
| +------------------+ | | | +------------------+ | |
| ^ | | | ^ | |
| | +--+ | | | +--+ |
| v | | | V | |
| +-----+ +--------+ +-------+ | | | +-----+ +--------+ +-------+ | |
| | | |Message | |Securi.| | | | | | |Message | |Securi.| | |
| |Disp.| |Proc. | |Subsys.| | | | |Disp.| |Proc. | |Subsys.| | |
| | | |Subsys. | | | | | | | | |Subsys. | | | | |
| | | | | | | | | | | | | | | | | |
| | | | +----+ | | +---+ | | | | | | | +----+ | | +---+ | | |
| | <--->|v3MP|<--> |TSM|<--+ | | | <--->|v3MP|<--->|TSM|<--+ |
| | | | +----+ | | +---+ | | | | | | +----+ | | +---+ | |
| | | | | | | | | | | | | | | |
| +-----+ +--------+ +-------+ | | +-----+ +--------+ +-------+ |
| ^ | | ^ |
| | | | | |
| +-+------------+ | | +-+----------+ |
| | | | | | | |
| v v | | v V |
| +-------------+ +-------------+ | | +-------------+ +-------------+ |
| | COMMAND | | NOTIFICAT. | | | | COMMAND | | NOTIFICAT. | |
| | RESPONDER | | ORIGINATOR | | | | GENERATOR | | RECEIVER | |
| | application | | application | | | | application | | application | |
| +-------------+ +-------------+ | | +-------------+ +-------------+ |
| SNMP entity | | SNMP entity |
+---------------------------------+ +---------------------------------+
Hardaker Standards Track [Page 6]
RFC 6353 TLS Transport Model for SNMP July 2011 1.1. Conventions For consistency with SNMP-related specifications, this document favors terminology as defined in STD 62, rather than favoring terminology that is consistent with non-SNMP specifications. This is consistent with the IESG decision to not require the SNMPv3 terminology be modified to match the usage of other non-SNMP specifications when SNMPv3 was advanced to a Full Standard. "Authentication" in this document typically refers to the English meaning of "serving to prove the authenticity of" the message, not data source authentication or peer identity authentication. The terms "manager" and "agent" are not used in this document because, in the [RFC3411] architecture, all SNMP entities have the capability of acting as manager, agent, or both depending on the SNMP application types supported in the implementation. Where distinction is required, the application names of command generator, command responder, notification originator, notification receiver, and proxy forwarder are used. See "SNMP Applications" [RFC3413] for further information. Large portions of this document simultaneously refer to both TLS and DTLS when discussing TLSTM components that function equally with either protocol. "(D)TLS" is used in these places to indicate that the statement applies to either or both protocols as appropriate. When a distinction between the protocols is needed, they are referred to independently through the use of "TLS" or "DTLS". The Transport Model, however, is named "TLS Transport Model" and refers not to the TLS or DTLS protocol but to the specification in this document, which includes support for both TLS and DTLS. Throughout this document, the terms "client" and "server" are used to refer to the two ends of the (D)TLS transport connection. The client actively opens the (D)TLS connection, and the server passively listens for the incoming (D)TLS connection. An SNMP entity may act as a (D)TLS client or server or both, depending on the SNMP applications supported. The User-Based Security Model (USM) [RFC3414] is a mandatory-to- implement Security Model in STD 62. While (D)TLS and USM frequently refer to a user, the terminology preferred in RFC 3411 and in this memo is "principal". A principal is the "who" on whose behalf services are provided or processing takes place. A principal can be, among other things, an individual acting in a particular role; a set of individuals, with each acting in a particular role; an application or a set of applications, or a combination of these within an administrative domain. Hardaker Standards Track [Page 7]
RFC 6353 TLS Transport Model for SNMP July 2011 Throughout this document, the term "session" is used to refer to a secure association between two TLS Transport Models that permits the transmission of one or more SNMP messages within the lifetime of the session. The (D)TLS protocols also have an internal notion of a session and although these two concepts of a session are related, when the term "session" is used this document is referring to the TLSTM's specific session and not directly to the (D)TLS protocol's session. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 1.2. Changes Since RFC 5953 This document obsoletes [RFC5953]. Since the publication of RFC 5953, a few editorial errata have been noted. These errata are posted on the RFC Editor web site. These errors have been corrected in this document. This document updates the references to RFC 3490 (IDNA 2003) to [RFC5890] (IDNA 2008), because RFC 3490 was obsoleted by RFC 5890. References to RFC 1033 were replaced with references to [RFC1123]. Added informative reference to 5953. Updated MIB dates and revision date. 2. The Transport Layer Security Protocol (D)TLS provides authentication, data message integrity, and privacy at the transport layer (see [RFC4347]). The primary goals of the TLS Transport Model are to provide privacy, peer identity authentication, and data integrity between two communicating SNMP entities. The TLS and DTLS protocols provide a secure transport upon which the TLSTM is based. Please refer to [RFC5246] and [RFC4347] for complete descriptions of the protocols. 3. How the TLSTM Fits into the Transport Subsystem A transport model is a component of the Transport Subsystem. The TLS Transport Model thus fits between the underlying (D)TLS transport layer and the Message Dispatcher [RFC3411] component of the SNMP engine. Hardaker Standards Track [Page 8]
RFC 6353 TLS Transport Model for SNMP July 2011 The TLS Transport Model will establish a session between itself and the TLS Transport Model of another SNMP engine. The sending transport model passes unencrypted and unauthenticated messages from the Dispatcher to (D)TLS to be encrypted and authenticated, and the receiving transport model accepts decrypted and authenticated/ integrity-checked incoming messages from (D)TLS and passes them to the Dispatcher. After a TLS Transport Model session is established, SNMP messages can conceptually be sent through the session from one SNMP message Dispatcher to another SNMP Message Dispatcher. If multiple SNMP messages are needed to be passed between two SNMP applications they MAY be passed through the same session. A TLSTM implementation engine MAY choose to close the session to conserve resources. The TLS Transport Model of an SNMP engine will perform the translation between (D)TLS-specific security parameters and SNMP- specific, model-independent parameters. Hardaker Standards Track [Page 9]
RFC 6353 TLS Transport Model for SNMP July 2011
The diagram below depicts where the TLS Transport Model (shown as
"(D)TLS TM") fits into the architecture described in RFC 3411 and the
Transport Subsystem:
+------------------------------+
| Network |
+------------------------------+
^ ^ ^
| | |
v v v
+-------------------------------------------------------------------+
| +--------------------------------------------------+ |
| | Transport Subsystem | +--------+ |
| | +-----+ +-----+ +-------+ +-------+ | | | |
| | | UDP | | SSH | |(D)TLS | . . . | other |<--->| Cache | |
| | | | | TM | | TM | | | | | | |
| | +-----+ +-----+ +-------+ +-------+ | +--------+ |
| +--------------------------------------------------+ ^ |
| ^ | |
| | | |
| Dispatcher v | |
| +--------------+ +---------------------+ +----------------+ | |
| | Transport | | Message Processing | | Security | | |
| | Dispatch | | Subsystem | | Subsystem | | |
| | | | +------------+ | | +------------+ | | |
| | | | +->| v1MP |<--->| | USM | | | |
| | | | | +------------+ | | +------------+ | | |
| | | | | +------------+ | | +------------+ | | |
| | | | +->| v2cMP |<--->| | Transport | | | |
| | Message | | | +------------+ | | | Security |<--+ |
| | Dispatch <---->| +------------+ | | | Model | | |
| | | | +->| v3MP |<--->| +------------+ | |
| | | | | +------------+ | | +------------+ | |
| | PDU Dispatch | | | +------------+ | | | Other | | |
| +--------------+ | +->| otherMP |<--->| | Model(s) | | |
| ^ | +------------+ | | +------------+ | |
| | +---------------------+ +----------------+ |
| v |
| +-------+-------------------------+---------------+ |
| ^ ^ ^ |
| | | | |
| v v v |
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RFC 6353 TLS Transport Model for SNMP July 2011
| +-------------+ +---------+ +--------------+ +-------------+ |
| | COMMAND | | ACCESS | | NOTIFICATION | | PROXY | |
| | RESPONDER |<->| CONTROL |<->| ORIGINATOR | | FORWARDER | |
| | application | | | | applications | | application | |
| +-------------+ +---------+ +--------------+ +-------------+ |
| ^ ^ |
| | | |
| v v |
| +----------------------------------------------+ |
| | MIB instrumentation | SNMP entity |
+-------------------------------------------------------------------+
3.1. Security Capabilities of This Model
3.1.1. Threats
The TLS Transport Model provides protection against the threats
identified by the RFC 3411 architecture [RFC3411]:
1. Modification of Information - The modification threat is the
danger that an unauthorized entity may alter in-transit SNMP
messages generated on behalf of an authorized principal in such a
way as to effect unauthorized management operations, including
falsifying the value of an object.
(D)TLS provides verification that the content of each received
message has not been modified during its transmission through the
network, data has not been altered or destroyed in an
unauthorized manner, and data sequences have not been altered to
an extent greater than can occur non-maliciously.
2. Masquerade - The masquerade threat is the danger that management
operations unauthorized for a given principal may be attempted by
assuming the identity of another principal that has the
appropriate authorizations.
The TLSTM verifies the identity of the (D)TLS server through the
use of the (D)TLS protocol and X.509 certificates. A TLS
Transport Model implementation MUST support the authentication of
both the server and the client.
3. Message stream modification - The re-ordering, delay, or replay
of messages can and does occur through the natural operation of
many connectionless transport services. The message stream
modification threat is the danger that messages may be
maliciously re-ordered, delayed, or replayed to an extent that is
greater than can occur through the natural operation of
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RFC 6353 TLS Transport Model for SNMP July 2011
connectionless transport services, in order to effect
unauthorized management operations.
(D)TLS provides replay protection with a Message Authentication
Code (MAC) that includes a sequence number. Since UDP provides
no sequencing ability, DTLS uses a sliding window protocol with
the sequence number used for replay protection (see [RFC4347]).
4. Disclosure - The disclosure threat is the danger of eavesdropping
on the exchanges between SNMP engines.
(D)TLS provides protection against the disclosure of information
to unauthorized recipients or eavesdroppers by allowing for
encryption of all traffic between SNMP engines. A TLS Transport
Model implementation MUST support message encryption to protect
sensitive data from eavesdropping attacks.
5. Denial of Service - The RFC 3411 architecture [RFC3411] states
that denial-of-service (DoS) attacks need not be addressed by an
SNMP security protocol. However, connectionless transports (like
DTLS over UDP) are susceptible to a variety of DoS attacks
because they are more vulnerable to spoofed IP addresses. See
Section 4.2 for details on how the cookie mechanism is used.
Note, however, that this mechanism does not provide any defense
against DoS attacks mounted from valid IP addresses.
See Section 9 for more detail on the security considerations
associated with the TLSTM and these security threats.
3.1.2. Message Protection
The RFC 3411 architecture recognizes three levels of security:
o without authentication and without privacy (noAuthNoPriv)
o with authentication but without privacy (authNoPriv)
o with authentication and with privacy (authPriv)
The TLS Transport Model determines from (D)TLS the identity of the
authenticated principal, the transport type, and the transport
address associated with an incoming message. The TLS Transport Model
provides the identity and destination type and address to (D)TLS for
outgoing messages.
When an application requests a session for a message, it also
requests a security level for that session. The TLS Transport Model
MUST ensure that the (D)TLS connection provides security at least as
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RFC 6353 TLS Transport Model for SNMP July 2011 high as the requested level of security. How the security level is translated into the algorithms used to provide data integrity and privacy is implementation dependent. However, the NULL integrity and encryption algorithms MUST NOT be used to fulfill security level requests for authentication or privacy. Implementations MAY choose to force (D)TLS to only allow cipher_suites that provide both authentication and privacy to guarantee this assertion. If a suitable interface between the TLS Transport Model and the (D)TLS Handshake Protocol is implemented to allow the selection of security-level-dependent algorithms (for example, a security level to cipher_suites mapping table), then different security levels may be utilized by the application. The authentication, integrity, and privacy algorithms used by the (D)TLS Protocols may vary over time as the science of cryptography continues to evolve and the development of (D)TLS continues over time. Implementers are encouraged to plan for changes in operator trust of particular algorithms. Implementations SHOULD offer configuration settings for mapping algorithms to SNMPv3 security levels. 3.1.3. (D)TLS Connections (D)TLS connections are opened by the TLS Transport Model during the elements of procedure for an outgoing SNMP message. Since the sender of a message initiates the creation of a (D)TLS connection if needed, the (D)TLS connection will already exist for an incoming message. Implementations MAY choose to instantiate (D)TLS connections in anticipation of outgoing messages. This approach might be useful to ensure that a (D)TLS connection to a given target can be established before it becomes important to send a message over the (D)TLS connection. Of course, there is no guarantee that a pre-established session will still be valid when needed. DTLS connections, when used over UDP, are uniquely identified within the TLS Transport Model by the combination of transportDomain, transportAddress, tmSecurityName, and requestedSecurityLevel associated with each session. Each unique combination of these parameters MUST have a locally chosen unique tlstmSessionID for each active session. For further information, see Section 5. TLS over TCP sessions, on the other hand, do not require a unique pairing of address and port attributes since their lower-layer protocols (TCP) already provide adequate session framing. But they must still provide a unique tlstmSessionID for referencing the session. Hardaker Standards Track [Page 13]
RFC 6353 TLS Transport Model for SNMP July 2011 The tlstmSessionID MUST NOT change during the entire duration of the session from the TLSTM's perspective, and MUST uniquely identify a single session. As an implementation hint: note that the (D)TLS internal SessionID does not meet these requirements, since it can change over the life of the connection as seen by the TLSTM (for example, during renegotiation), and does not necessarily uniquely identify a TLSTM session (there can be multiple TLSTM sessions sharing the same D(TLS) internal SessionID). 3.2. Security Parameter Passing For the (D)TLS server-side, (D)TLS-specific security parameters (i.e., cipher_suites, X.509 certificate fields, IP addresses, and ports) are translated by the TLS Transport Model into security parameters for the TLS Transport Model and security model (e.g., tmSecurityLevel, tmSecurityName, transportDomain, transportAddress). The transport-related and (D)TLS-security-related information, including the authenticated identity, are stored in a cache referenced by tmStateReference. For the (D)TLS client side, the TLS Transport Model takes input provided by the Dispatcher in the sendMessage() Abstract Service Interface (ASI) and input from the tmStateReference cache. The (D)TLS Transport Model converts that information into suitable security parameters for (D)TLS and establishes sessions as needed. The elements of procedure in Section 5 discuss these concepts in much greater detail. 3.3. Notifications and Proxy (D)TLS connections may be initiated by (D)TLS clients on behalf of SNMP applications that initiate communications, such as command generators, notification originators, proxy forwarders. Command generators are frequently operated by a human, but notification originators and proxy forwarders are usually unmanned automated processes. The targets to whom notifications and proxied requests should be sent are typically determined and configured by a network administrator. The SNMP-TARGET-MIB module [RFC3413] contains objects for defining management targets, including transportDomain, transportAddress, securityName, securityModel, and securityLevel parameters, for notification originator, proxy forwarder, and SNMP-controllable command generator applications. Transport domains and transport addresses are configured in the snmpTargetAddrTable, and the securityModel, securityName, and securityLevel parameters are configured in the snmpTargetParamsTable. This document defines a MIB Hardaker Standards Track [Page 14]
RFC 6353 TLS Transport Model for SNMP July 2011 module that extends the SNMP-TARGET-MIB's snmpTargetParamsTable to specify a (D)TLS client-side certificate to use for the connection. When configuring a (D)TLS target, the snmpTargetAddrTDomain and snmpTargetAddrTAddress parameters in snmpTargetAddrTable SHOULD be set to the snmpTLSTCPDomain or snmpDTLSUDPDomain object and an appropriate snmpTLSAddress value. When used with the SNMPv3 message processing model, the snmpTargetParamsMPModel column of the snmpTargetParamsTable SHOULD be set to a value of 3. The snmpTargetParamsSecurityName SHOULD be set to an appropriate securityName value, and the snmpTlstmParamsClientFingerprint parameter of the snmpTlstmParamsTable SHOULD be set to a value that refers to a locally held certificate (and the corresponding private key) to be used. Other parameters, for example, cryptographic configuration such as which cipher_suites to use, must come from configuration mechanisms not defined in this document. The securityName defined in the snmpTargetParamsSecurityName column will be used by the access control model to authorize any notifications that need to be sent. 4. Elements of the Model This section contains definitions required to realize the (D)TLS Transport Model defined by this document. 4.1. X.509 Certificates (D)TLS can make use of X.509 certificates for authentication of both sides of the transport. This section discusses the use of X.509 certificates in the TLSTM. While (D)TLS supports multiple authentication mechanisms, this document only discusses X.509-certificate-based authentication; other forms of authentication are outside the scope of this specification. TLSTM implementations are REQUIRED to support X.509 certificates. 4.1.1. Provisioning for the Certificate Authentication using (D)TLS will require that SNMP entities have certificates, either signed by trusted Certification Authorities (CAs), or self signed. Furthermore, SNMP entities will most commonly need to be provisioned with root certificates that represent the list of trusted CAs that an SNMP entity can use for certificate verification. SNMP entities SHOULD also be provisioned with an X.509 certificate revocation mechanism which can be used to verify that a certificate has not been revoked. Trusted public keys from either CA certificates and/or self-signed certificates MUST be installed into Hardaker Standards Track [Page 15]
RFC 6353 TLS Transport Model for SNMP July 2011
the server through a trusted out-of-band mechanism and their
authenticity MUST be verified before access is granted.
Having received a certificate from a connecting TLSTM client, the
authenticated tmSecurityName of the principal is derived using the
snmpTlstmCertToTSNTable. This table allows mapping of incoming
connections to tmSecurityNames through defined transformations. The
transformations defined in the SNMP-TLS-TM-MIB include:
o Mapping a certificate's subjectAltName or CommonName components to
a tmSecurityName, or
o Mapping a certificate's fingerprint value to a directly specified
tmSecurityName
As an implementation hint: implementations may choose to discard any
connections for which no potential snmpTlstmCertToTSNTable mapping
exists before performing certificate verification to avoid expending
computational resources associated with certificate verification.
Deployments SHOULD map the "subjectAltName" component of X.509
certificates to the TLSTM specific tmSecurityNames. The
authenticated identity can be obtained by the TLS Transport Model by
extracting the subjectAltName(s) from the peer's certificate. The
receiving application will then have an appropriate tmSecurityName
for use by other SNMPv3 components like an access control model.
An example of this type of mapping setup can be found in Appendix A.
This tmSecurityName may be later translated from a TLSTM specific
tmSecurityName to an SNMP engine securityName by the security model.
A security model, like the TSM security model [RFC5591], may perform
an identity mapping or a more complex mapping to derive the
securityName from the tmSecurityName offered by the TLS Transport
Model.
The standard View-Based Access Control Model (VACM) access control
model constrains securityNames to be 32 octets or less in length. A
TLSTM generated tmSecurityName, possibly in combination with a
messaging or security model that increases the length of the
securityName, might cause the securityName length to exceed 32
octets. For example, a 32-octet tmSecurityName derived from an IPv6
address, paired with a TSM prefix, will generate a 36-octet
securityName. Such a securityName will not be able to be used with
standard VACM or TARGET MIB modules. Operators should be careful to
select algorithms and subjectAltNames to avoid this situation.
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RFC 6353 TLS Transport Model for SNMP July 2011
A pictorial view of the complete transformation process (using the
TSM security model for the example) is shown below:
+-------------+ +-------+ +-----+
| Certificate | | | | |
| Path | | TLSTM | tmSecurityName | TSM |
| Validation | --> | | ----------------->| |
+-------------+ +-------+ +-----+
|
| securityName
V
+-------------+
| application |
+-------------+
4.2. (D)TLS Usage
(D)TLS MUST negotiate a cipher_suite that uses X.509 certificates for
authentication, and MUST authenticate both the client and the server.
The mandatory-to-implement cipher_suite is specified in the TLS
specification [RFC5246].
TLSTM verifies the certificates when the connection is opened (see
Section 5.3). For this reason, TLS renegotiation with different
certificates MUST NOT be done. That is, implementations MUST either
disable renegotiation completely (RECOMMENDED), or they MUST present
the same certificate during renegotiation (and MUST verify that the
other end presented the same certificate).
For DTLS over UDP, each SNMP message MUST be placed in a single UDP
datagram; it MAY be split to multiple DTLS records. In other words,
if a single datagram contains multiple DTLS application_data records,
they are concatenated when received. The TLSTM implementation SHOULD
return an error if the SNMP message does not fit in the UDP datagram,
and thus cannot be sent.
For DTLS over UDP, the DTLS server implementation MUST support DTLS
cookies ([RFC4347] already requires that clients support DTLS
cookies). Implementations are not required to perform the cookie
exchange for every DTLS handshake; however, enabling it by default is
RECOMMENDED.
For DTLS, replay protection MUST be used.
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RFC 6353 TLS Transport Model for SNMP July 2011
4.3. SNMP Services
This section describes the services provided by the TLS Transport
Model with their inputs and outputs. The services are between the
Transport Model and the Dispatcher.
The services are described as primitives of an abstract service
interface (ASI) and the inputs and outputs are described as abstract
data elements as they are passed in these abstract service
primitives.
4.3.1. SNMP Services for an Outgoing Message
The Dispatcher passes the information to the TLS Transport Model
using the ASI defined in the Transport Subsystem:
statusInformation =
sendMessage(
IN destTransportDomain -- transport domain to be used
IN destTransportAddress -- transport address to be used
IN outgoingMessage -- the message to send
IN outgoingMessageLength -- its length
IN tmStateReference -- reference to transport state
)
The abstract data elements returned from or passed as parameters into
the abstract service primitives are as follows:
statusInformation: An indication of whether the sending of the
message was successful. If not, it is an indication of the
problem.
destTransportDomain: The transport domain for the associated
destTransportAddress. The Transport Model uses this parameter to
determine the transport type of the associated
destTransportAddress. This document specifies the
snmpTLSTCPDomain and the snmpDTLSUDPDomain transport domains.
destTransportAddress: The transport address of the destination TLS
Transport Model in a format specified by the SnmpTLSAddress
TEXTUAL-CONVENTION.
outgoingMessage: The outgoing message to send to (D)TLS for
encapsulation and transmission.
outgoingMessageLength: The length of the outgoingMessage.
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RFC 6353 TLS Transport Model for SNMP July 2011
tmStateReference: A reference used to pass model-specific and
mechanism-specific parameters between the Transport Subsystem and
transport-aware Security Models.
4.3.2. SNMP Services for an Incoming Message
The TLS Transport Model processes the received message from the
network using the (D)TLS service and then passes it to the Dispatcher
using the following ASI:
statusInformation =
receiveMessage(
IN transportDomain -- origin transport domain
IN transportAddress -- origin transport address
IN incomingMessage -- the message received
IN incomingMessageLength -- its length
IN tmStateReference -- reference to transport state
)
The abstract data elements returned from or passed as parameters into
the abstract service primitives are as follows:
statusInformation: An indication of whether the passing of the
message was successful. If not, it is an indication of the
problem.
transportDomain: The transport domain for the associated
transportAddress. This document specifies the snmpTLSTCPDomain
and the snmpDTLSUDPDomain transport domains.
transportAddress: The transport address of the source of the
received message in a format specified by the SnmpTLSAddress
TEXTUAL-CONVENTION.
incomingMessage: The whole SNMP message after being processed by
(D)TLS.
incomingMessageLength: The length of the incomingMessage.
tmStateReference: A reference used to pass model-specific and
mechanism-specific parameters between the Transport Subsystem and
transport-aware Security Models.
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RFC 6353 TLS Transport Model for SNMP July 2011 4.4. Cached Information and References When performing SNMP processing, there are two levels of state information that may need to be retained: the immediate state linking a request-response pair, and potentially longer-term state relating to transport and security. "Transport Subsystem for the Simple Network Management Protocol (SNMP)" [RFC5590] defines general requirements for caches and references. 4.4.1. TLS Transport Model Cached Information The TLS Transport Model has specific responsibilities regarding the cached information. See the Elements of Procedure in Section 5 for detailed processing instructions on the use of the tmStateReference fields by the TLS Transport Model. 4.4.1.1. tmSecurityName The tmSecurityName MUST be a human-readable name (in snmpAdminString format) representing the identity that has been set according to the procedures in Section 5. The tmSecurityName MUST be constant for all traffic passing through a single TLSTM session. Messages MUST NOT be sent through an existing (D)TLS connection that was established using a different tmSecurityName. On the (D)TLS server side of a connection, the tmSecurityName is derived using the procedures described in Section 5.3.2 and the SNMP- TLS-TM-MIB's snmpTlstmCertToTSNTable DESCRIPTION clause. On the (D)TLS client side of a connection, the tmSecurityName is presented to the TLS Transport Model by the security model through the tmStateReference. This tmSecurityName is typically a copy of or is derived from the securityName that was passed by application (possibly because of configuration specified in the SNMP-TARGET-MIB). The Security Model likely derived the tmSecurityName from the securityName presented to the Security Model by the application (possibly because of configuration specified in the SNMP-TARGET-MIB). Transport-Model-aware security models derive tmSecurityName from a securityName, possibly configured in MIB modules for notifications and access controls. Transport Models SHOULD use predictable tmSecurityNames so operators will know what to use when configuring MIB modules that use securityNames derived from tmSecurityNames. The TLSTM generates predictable tmSecurityNames based on the configuration found in the SNMP-TLS-TM-MIB's snmpTlstmCertToTSNTable and relies on the network operators to have configured this table appropriately. Hardaker Standards Track [Page 20]
RFC 6353 TLS Transport Model for SNMP July 2011 4.4.1.2. tmSessionID The tmSessionID MUST be recorded per message at the time of receipt. When tmSameSecurity is set, the recorded tmSessionID can be used to determine whether the (D)TLS connection available for sending a corresponding outgoing message is the same (D)TLS connection as was used when receiving the incoming message (e.g., a response to a request). 4.4.1.3. Session State The per-session state that is referenced by tmStateReference may be saved across multiple messages in a Local Configuration Datastore. Additional session/connection state information might also be stored in a Local Configuration Datastore. 5. Elements of Procedure Abstract service interfaces have been defined by [RFC3411] and further augmented by [RFC5590] to describe the conceptual data flows between the various subsystems within an SNMP entity. The TLSTM uses some of these conceptual data flows when communicating between subsystems. To simplify the elements of procedure, the release of state information is not always explicitly specified. As a general rule, if state information is available when a message gets discarded, the message-state information should also be released. If state information is available when a session is closed, the session state information should also be released. Sensitive information, like cryptographic keys, should be overwritten appropriately prior to being released. An error indication in statusInformation will typically include the Object Identifier (OID) and value for an incremented error counter. This may be accompanied by the requested securityLevel and the tmStateReference. Per-message context information is not accessible to Transport Models, so for the returned counter OID and value, contextEngine would be set to the local value of snmpEngineID and contextName to the default context for error counters. 5.1. Procedures for an Incoming Message This section describes the procedures followed by the (D)TLS Transport Model when it receives a (D)TLS protected packet. The required functionality is broken into two different sections. Hardaker Standards Track [Page 21]
RFC 6353 TLS Transport Model for SNMP July 2011
Section 5.1.1 describes the processing required for de-multiplexing
multiple DTLS connections, which is specifically needed for DTLS over
UDP sessions. It is assumed that TLS protocol implementations
already provide appropriate message demultiplexing.
Section 5.1.2 describes the transport processing required once the
(D)TLS processing has been completed. This will be needed for all
(D)TLS-based connections.
5.1.1. DTLS over UDP Processing for Incoming Messages
Demultiplexing of incoming packets into separate DTLS sessions MUST
be implemented. For connection-oriented transport protocols, such as
TCP, the transport protocol takes care of demultiplexing incoming
packets to the right connection. For DTLS over UDP, this
demultiplexing will either need to be done within the DTLS
implementation, if supported, or by the TLSTM implementation.
Like TCP, DTLS over UDP uses the four-tuple <source IP, destination
IP, source port, destination port> for identifying the connection
(and relevant DTLS connection state). This means that when
establishing a new session, implementations MUST use a different UDP
source port number for each active connection to a remote destination
IP-address/port-number combination to ensure the remote entity can
disambiguate between multiple connections.
If demultiplexing received UDP datagrams to DTLS connection state is
done by the TLSTM implementation (instead of the DTLS
implementation), the steps below describe one possible method to
accomplish this.
The important output results from the steps in this process are the
remote transport address, incomingMessage, incomingMessageLength, and
the tlstmSessionID.
1) The TLS Transport Model examines the raw UDP message, in an
implementation-dependent manner.
2) The TLS Transport Model queries the Local Configuration Datastore
(LCD) (see [RFC3411], Section 3.4.2) using the transport
parameters (source and destination IP addresses and ports) to
determine if a session already exists.
2a) If a matching entry in the LCD does not exist, then the UDP
packet is passed to the DTLS implementation for processing.
If the DTLS implementation decides to continue with the
connection and allocate state for it, it returns a new DTLS
connection handle (an implementation dependent detail). In
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RFC 6353 TLS Transport Model for SNMP July 2011
this case, TLSTM selects a new tlstmSessionId, and caches
this and the DTLS connection handle as a new entry in the
LCD (indexed by the transport parameters). If the DTLS
implementation returns an error or does not allocate
connection state (which can happen with the stateless cookie
exchange), processing stops.
2b) If a session does exist in the LCD, then its DTLS connection
handle (an implementation dependent detail) and its
tlstmSessionId is extracted from the LCD. The UDP packet
and the connection handle are passed to the DTLS
implementation. If the DTLS implementation returns success
but does not return an incomingMessage and an
incomingMessageLength, then processing stops (this is the
case when the UDP datagram contained DTLS handshake
messages, for example). If the DTLS implementation returns
an error, then processing stops.
3) Retrieve the incomingMessage and an incomingMessageLength from
DTLS. These results and the tlstmSessionID are used below in
Section 5.1.2 to complete the processing of the incoming message.
5.1.2. Transport Processing for Incoming SNMP Messages
The procedures in this section describe how the TLS Transport Model
should process messages that have already been properly extracted
from the (D)TLS stream. Note that care must be taken when processing
messages originating from either TLS or DTLS to ensure they're
complete and single. For example, multiple SNMP messages can be
passed through a single DTLS message and partial SNMP messages may be
received from a TLS stream. These steps describe the processing of a
singular SNMP message after it has been delivered from the (D)TLS
stream.
1) Determine the tlstmSessionID for the incoming message. The
tlstmSessionID MUST be a unique session identifier for this
(D)TLS connection. The contents and format of this identifier
are implementation dependent as long as it is unique to the
session. A session identifier MUST NOT be reused until all
references to it are no longer in use. The tmSessionID is equal
to the tlstmSessionID discussed in Section 5.1.1. tmSessionID
refers to the session identifier when stored in the
tmStateReference and tlstmSessionID refers to the session
identifier when stored in the LCD. They MUST always be equal
when processing a given session's traffic.
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RFC 6353 TLS Transport Model for SNMP July 2011
If this is the first message received through this session, and
the session does not have an assigned tlstmSessionID yet, then
the snmpTlstmSessionAccepts counter is incremented and a
tlstmSessionID for the session is created. This will only happen
on the server side of a connection because a client would have
already assigned a tlstmSessionID during the openSession()
invocation. Implementations may have performed the procedures
described in Section 5.3.2 prior to this point or they may
perform them now, but the procedures described in Section 5.3.2
MUST be performed before continuing beyond this point.
2) Create a tmStateReference cache for the subsequent reference and
assign the following values within it:
tmTransportDomain = snmpTLSTCPDomain or snmpDTLSUDPDomain as
appropriate.
tmTransportAddress = The address from which the message
originated.
tmSecurityLevel = The derived tmSecurityLevel for the session,
as discussed in Sections 3.1.2 and 5.3.
tmSecurityName = The derived tmSecurityName for the session as
discussed in Section 5.3. This value MUST be constant during
the lifetime of the session.
tmSessionID = The tlstmSessionID described in step 1 above.
3) The incomingMessage and incomingMessageLength are assigned values
from the (D)TLS processing.
4) The TLS Transport Model passes the transportDomain,
transportAddress, incomingMessage, and incomingMessageLength to
the Dispatcher using the receiveMessage ASI:
statusInformation =
receiveMessage(
IN transportDomain -- snmpTLSTCPDomain or snmpDTLSUDPDomain,
IN transportAddress -- address for the received message
IN incomingMessage -- the whole SNMP message from (D)TLS
IN incomingMessageLength -- the length of the SNMP message
IN tmStateReference -- transport info
)
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RFC 6353 TLS Transport Model for SNMP July 2011
5.2. Procedures for an Outgoing SNMP Message
The Dispatcher sends a message to the TLS Transport Model using the
following ASI:
statusInformation =
sendMessage(
IN destTransportDomain -- transport domain to be used
IN destTransportAddress -- transport address to be used
IN outgoingMessage -- the message to send
IN outgoingMessageLength -- its length
IN tmStateReference -- transport info
)
This section describes the procedure followed by the TLS Transport
Model whenever it is requested through this ASI to send a message.
1) If tmStateReference does not refer to a cache containing values
for tmTransportDomain, tmTransportAddress, tmSecurityName,
tmRequestedSecurityLevel, and tmSameSecurity, then increment the
snmpTlstmSessionInvalidCaches counter, discard the message, and
return the error indication in the statusInformation. Processing
of this message stops.
2) Extract the tmSessionID, tmTransportDomain, tmTransportAddress,
tmSecurityName, tmRequestedSecurityLevel, and tmSameSecurity
values from the tmStateReference. Note: the tmSessionID value
may be undefined if no session exists yet over which the message
can be sent.
3) If tmSameSecurity is true and tmSessionID is either undefined or
refers to a session that is no longer open, then increment the
snmpTlstmSessionNoSessions counter, discard the message, and
return the error indication in the statusInformation. Processing
of this message stops.
4) If tmSameSecurity is false and tmSessionID refers to a session
that is no longer available, then an implementation SHOULD open a
new session, using the openSession() ASI (described in greater
detail in step 5b). Instead of opening a new session an
implementation MAY return an snmpTlstmSessionNoSessions error to
the calling module and stop the processing of the message.
5) If tmSessionID is undefined, then use tmTransportDomain,
tmTransportAddress, tmSecurityName, and tmRequestedSecurityLevel
to see if there is a corresponding entry in the LCD suitable to
send the message over.
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RFC 6353 TLS Transport Model for SNMP July 2011
5a) If there is a corresponding LCD entry, then this session
will be used to send the message.
5b) If there is no corresponding LCD entry, then open a session
using the openSession() ASI (discussed further in
Section 5.3.1). Implementations MAY wish to offer message
buffering to prevent redundant openSession() calls for the
same cache entry. If an error is returned from
openSession(), then discard the message, discard the
tmStateReference, increment the snmpTlstmSessionOpenErrors,
return an error indication to the calling module, and stop
the processing of the message.
6) Using either the session indicated by the tmSessionID (if there
was one) or the session resulting from a previous step (4 or 5),
pass the outgoingMessage to (D)TLS for encapsulation and
transmission.
5.3. Establishing or Accepting a Session
Establishing a (D)TLS connection as either a client or a server
requires slightly different processing. The following two sections
describe the necessary processing steps.
5.3.1. Establishing a Session as a Client
The TLS Transport Model provides the following primitive for use by a
client to establish a new (D)TLS connection:
statusInformation = -- errorIndication or success
openSession(
IN tmStateReference -- transport information to be used
OUT tmStateReference -- transport information to be used
IN maxMessageSize -- of the sending SNMP entity
)
The following describes the procedure to follow when establishing an
SNMP over a (D)TLS connection between SNMP engines for exchanging
SNMP messages. This process is followed by any SNMP client's engine
when establishing a session for subsequent use.
This procedure MAY be done automatically for an SNMP application that
initiates a transaction, such as a command generator, a notification
originator, or a proxy forwarder.
1) The snmpTlstmSessionOpens counter is incremented.
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2) The client selects the appropriate certificate and cipher_suites
for the key agreement based on the tmSecurityName and the
tmRequestedSecurityLevel for the session. For sessions being
established as a result of an SNMP-TARGET-MIB based operation,
the certificate will potentially have been identified via the
snmpTlstmParamsTable mapping and the cipher_suites will have to
be taken from a system-wide or implementation-specific
configuration. If no row in the snmpTlstmParamsTable exists,
then implementations MAY choose to establish the connection using
a default client certificate available to the application.
Otherwise, the certificate and appropriate cipher_suites will
need to be passed to the openSession() ASI as supplemental
information or configured through an implementation-dependent
mechanism. It is also implementation-dependent and possibly
policy-dependent how tmRequestedSecurityLevel will be used to
influence the security capabilities provided by the (D)TLS
connection. However this is done, the security capabilities
provided by (D)TLS MUST be at least as high as the level of
security indicated by the tmRequestedSecurityLevel parameter.
The actual security level of the session is reported in the
tmStateReference cache as tmSecurityLevel. For (D)TLS to provide
strong authentication, each principal acting as a command
generator SHOULD have its own certificate.
3) Using the destTransportDomain and destTransportAddress values,
the client will initiate the (D)TLS handshake protocol to
establish session keys for message integrity and encryption.
If the attempt to establish a session is unsuccessful, then
snmpTlstmSessionOpenErrors is incremented, an error indication is
returned, and processing stops. If the session failed to open
because the presented server certificate was unknown or invalid,
then the snmpTlstmSessionUnknownServerCertificate or
snmpTlstmSessionInvalidServerCertificates MUST be incremented and
an snmpTlstmServerCertificateUnknown or
snmpTlstmServerInvalidCertificate notification SHOULD be sent as
appropriate. Reasons for server certificate invalidation
include, but are not limited to, cryptographic validation
failures and an unexpected presented certificate identity.
4) The (D)TLS client MUST then verify that the (D)TLS server's
presented certificate is the expected certificate. The (D)TLS
client MUST NOT transmit SNMP messages until the server
certificate has been authenticated, the client certificate has
been transmitted, and the TLS connection has been fully
established.
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RFC 6353 TLS Transport Model for SNMP July 2011
If the connection is being established from a configuration based
on SNMP-TARGET-MIB configuration, then the snmpTlstmAddrTable
DESCRIPTION clause describes how the verification is done (using
either a certificate fingerprint, or an identity authenticated
via certification path validation).
If the connection is being established for reasons other than
configuration found in the SNMP-TARGET-MIB, then configuration
and procedures outside the scope of this document should be
followed. Configuration mechanisms SHOULD be similar in nature
to those defined in the snmpTlstmAddrTable to ensure consistency
across management configuration systems. For example, a command-
line tool for generating SNMP GETs might support specifying
either the server's certificate fingerprint or the expected host
name as a command-line argument.
5) (D)TLS provides assurance that the authenticated identity has
been signed by a trusted configured Certification Authority. If
verification of the server's certificate fails in any way (for
example, because of failures in cryptographic verification or the
presented identity did not match the expected named entity), then
the session establishment MUST fail, and the
snmpTlstmSessionInvalidServerCertificates object is incremented.
If the session cannot be opened for any reason at all, including
cryptographic verification failures and snmpTlstmCertToTSNTable
lookup failures, then the snmpTlstmSessionOpenErrors counter is
incremented and processing stops.
6) The TLSTM-specific session identifier (tlstmSessionID) is set in
the tmSessionID of the tmStateReference passed to the TLS
Transport Model to indicate that the session has been established
successfully and to point to a specific (D)TLS connection for
future use. The tlstmSessionID is also stored in the LCD for
later lookup during processing of incoming messages
(Section 5.1.2).
5.3.2. Accepting a Session as a Server
A (D)TLS server should accept new session connections from any client
for which it is able to verify the client's credentials. This is
done by authenticating the client's presented certificate through a
certificate path validation process (e.g., [RFC5280]) or through
certificate fingerprint verification using fingerprints configured in
the snmpTlstmCertToTSNTable. Afterward, the server will determine
the identity of the remote entity using the following procedures.
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RFC 6353 TLS Transport Model for SNMP July 2011
The (D)TLS server identifies the authenticated identity from the
(D)TLS client's principal certificate using configuration information
from the snmpTlstmCertToTSNTable mapping table. The (D)TLS server
MUST request and expect a certificate from the client and MUST NOT
accept SNMP messages over the (D)TLS connection until the client has
sent a certificate and it has been authenticated. The resulting
derived tmSecurityName is recorded in the tmStateReference cache as
tmSecurityName. The details of the lookup process are fully
described in the DESCRIPTION clause of the snmpTlstmCertToTSNTable
MIB object. If any verification fails in any way (for example,
because of failures in cryptographic verification or because of the
lack of an appropriate row in the snmpTlstmCertToTSNTable), then the
session establishment MUST fail, and the
snmpTlstmSessionInvalidClientCertificates object is incremented. If
the session cannot be opened for any reason at all, including
cryptographic verification failures, then the
snmpTlstmSessionOpenErrors counter is incremented and processing
stops.
Servers that wish to support multiple principals at a particular port
SHOULD make use of a (D)TLS extension that allows server-side
principal selection like the Server Name Indication extension defined
in Section 3.1 of [RFC4366]. Supporting this will allow, for
example, sending notifications to a specific principal at a given TCP
or UDP port.
5.4. Closing a Session
The TLS Transport Model provides the following primitive to close a
session:
statusInformation =
closeSession(
IN tmSessionID -- session ID of the session to be closed
)
The following describes the procedure to follow to close a session
between a client and server. This process is followed by any SNMP
engine closing the corresponding SNMP session.
1) Increment either the snmpTlstmSessionClientCloses or the
snmpTlstmSessionServerCloses counter as appropriate.
2) Look up the session using the tmSessionID.
3) If there is no open session associated with the tmSessionID, then
closeSession processing is completed.
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RFC 6353 TLS Transport Model for SNMP July 2011
4) Have (D)TLS close the specified connection. This MUST include
sending a close_notify TLS Alert to inform the other side that
session cleanup may be performed.
6. MIB Module Overview
This MIB module provides management of the TLS Transport Model. It
defines needed textual conventions, statistical counters,
notifications, and configuration infrastructure necessary for session
establishment. Example usage of the configuration tables can be
found in Appendix A.
6.1. Structure of the MIB Module
Objects in this MIB module are arranged into subtrees. Each subtree
is organized as a set of related objects. The overall structure and
assignment of objects to their subtrees, and the intended purpose of
each subtree, is shown below.
6.2. Textual Conventions
Generic and Common Textual Conventions used in this module can be
found summarized at http://www.ops.ietf.org/mib-common-tcs.html.
This module defines the following new Textual Conventions:
o A new TransportAddress format for describing (D)TLS connection
addressing requirements.
o A certificate fingerprint allowing MIB module objects to
generically refer to a stored X.509 certificate using a
cryptographic hash as a reference pointer.
6.3. Statistical Counters
The SNMP-TLS-TM-MIB defines counters that provide network management
stations with information about session usage and potential errors
that a device may be experiencing.
6.4. Configuration Tables
The SNMP-TLS-TM-MIB defines configuration tables that an
administrator can use for configuring a device for sending and
receiving SNMP messages over (D)TLS. In particular, there are MIB
tables that extend the SNMP-TARGET-MIB for configuring (D)TLS
certificate usage and a MIB table for mapping incoming (D)TLS client
certificates to SNMPv3 tmSecurityNames.
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RFC 6353 TLS Transport Model for SNMP July 2011
6.4.1. Notifications
The SNMP-TLS-TM-MIB defines notifications to alert management
stations when a (D)TLS connection fails because a server's presented
certificate did not meet an expected value
(snmpTlstmServerCertificateUnknown) or because cryptographic
validation failed (snmpTlstmServerInvalidCertificate).
6.5. Relationship to Other MIB Modules
Some management objects defined in other MIB modules are applicable
to an entity implementing the TLS Transport Model. In particular, it
is assumed that an entity implementing the SNMP-TLS-TM-MIB will
implement the SNMPv2-MIB [RFC3418], the SNMP-FRAMEWORK-MIB [RFC3411],
the SNMP-TARGET-MIB [RFC3413], the SNMP-NOTIFICATION-MIB [RFC3413],
and the SNMP-VIEW-BASED-ACM-MIB [RFC3415].
The SNMP-TLS-TM-MIB module contained in this document is for managing
TLS Transport Model information.
6.5.1. MIB Modules Required for IMPORTS
The SNMP-TLS-TM-MIB module imports items from SNMPv2-SMI [RFC2578],
SNMPv2-TC [RFC2579], SNMP-FRAMEWORK-MIB [RFC3411], SNMP-TARGET-MIB
[RFC3413], and SNMPv2-CONF [RFC2580].
7. MIB Module Definition
SNMP-TLS-TM-MIB DEFINITIONS ::= BEGIN
IMPORTS
MODULE-IDENTITY, OBJECT-TYPE,
OBJECT-IDENTITY, mib-2, snmpDomains,
Counter32, Unsigned32, Gauge32, NOTIFICATION-TYPE
FROM SNMPv2-SMI -- RFC 2578 or any update thereof
TEXTUAL-CONVENTION, TimeStamp, RowStatus, StorageType,
AutonomousType
FROM SNMPv2-TC -- RFC 2579 or any update thereof
MODULE-COMPLIANCE, OBJECT-GROUP, NOTIFICATION-GROUP
FROM SNMPv2-CONF -- RFC 2580 or any update thereof
SnmpAdminString
FROM SNMP-FRAMEWORK-MIB -- RFC 3411 or any update thereof
snmpTargetParamsName, snmpTargetAddrName
FROM SNMP-TARGET-MIB -- RFC 3413 or any update thereof
;
snmpTlstmMIB MODULE-IDENTITY
LAST-UPDATED "201107190000Z"
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RFC 6353 TLS Transport Model for SNMP July 2011
ORGANIZATION "ISMS Working Group"
CONTACT-INFO "WG-EMail: isms@lists.ietf.org
Subscribe: isms-request@lists.ietf.org
Chairs:
Juergen Schoenwaelder
Jacobs University Bremen
Campus Ring 1
28725 Bremen
Germany
+49 421 200-3587
j.schoenwaelder@jacobs-university.de
Russ Mundy
SPARTA, Inc.
7110 Samuel Morse Drive
Columbia, MD 21046
USA
Editor:
Wes Hardaker
SPARTA, Inc.
P.O. Box 382
Davis, CA 95617
USA
ietf@hardakers.net
"
DESCRIPTION "
The TLS Transport Model MIB
Copyright (c) 2010-2011 IETF Trust and the persons identified
as authors of the code. All rights reserved.
Redistribution and use in source and binary forms, with or
without modification, is permitted pursuant to, and subject
to the license terms contained in, the Simplified BSD License
set forth in Section 4.c of the IETF Trust's Legal Provisions
Relating to IETF Documents
(http://trustee.ietf.org/license-info)."
REVISION "201107190000Z"
DESCRIPTION "This version of this MIB module is part of
RFC 6353; see the RFC itself for full legal
notices. The only change was to introduce
new wording to reflect require changes for
IDNA addresses in the SnmpTLSAddress TC."
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RFC 6353 TLS Transport Model for SNMP July 2011
REVISION "201005070000Z"
DESCRIPTION "This version of this MIB module is part of
RFC 5953; see the RFC itself for full legal
notices."
::= { mib-2 198 }
-- ************************************************
-- subtrees of the SNMP-TLS-TM-MIB
-- ************************************************
snmpTlstmNotifications OBJECT IDENTIFIER ::= { snmpTlstmMIB 0 }
snmpTlstmIdentities OBJECT IDENTIFIER ::= { snmpTlstmMIB 1 }
snmpTlstmObjects OBJECT IDENTIFIER ::= { snmpTlstmMIB 2 }
snmpTlstmConformance OBJECT IDENTIFIER ::= { snmpTlstmMIB 3 }
-- ************************************************
-- snmpTlstmObjects - Objects
-- ************************************************
snmpTLSTCPDomain OBJECT-IDENTITY
STATUS current
DESCRIPTION
"The SNMP over TLS via TCP transport domain. The
corresponding transport address is of type SnmpTLSAddress.
The securityName prefix to be associated with the
snmpTLSTCPDomain is 'tls'. This prefix may be used by
security models or other components to identify which secure
transport infrastructure authenticated a securityName."
REFERENCE
"RFC 2579: Textual Conventions for SMIv2"
::= { snmpDomains 8 }
snmpDTLSUDPDomain OBJECT-IDENTITY
STATUS current
DESCRIPTION
"The SNMP over DTLS via UDP transport domain. The
corresponding transport address is of type SnmpTLSAddress.
The securityName prefix to be associated with the
snmpDTLSUDPDomain is 'dtls'. This prefix may be used by
security models or other components to identify which secure
transport infrastructure authenticated a securityName."
REFERENCE
"RFC 2579: Textual Conventions for SMIv2"
::= { snmpDomains 9 }
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RFC 6353 TLS Transport Model for SNMP July 2011
SnmpTLSAddress ::= TEXTUAL-CONVENTION
DISPLAY-HINT "1a"
STATUS current
DESCRIPTION
"Represents an IPv4 address, an IPv6 address, or a
US-ASCII-encoded hostname and port number.
An IPv4 address must be in dotted decimal format followed by a
colon ':' (US-ASCII character 0x3A) and a decimal port number
in US-ASCII.
An IPv6 address must be a colon-separated format (as described
in RFC 5952), surrounded by square brackets ('[', US-ASCII
character 0x5B, and ']', US-ASCII character 0x5D), followed by
a colon ':' (US-ASCII character 0x3A) and a decimal port number
in US-ASCII.
A hostname is always in US-ASCII (as per RFC 1123);
internationalized hostnames are encoded as A-labels as specified
in RFC 5890. The hostname is followed by a
colon ':' (US-ASCII character 0x3A) and a decimal port number
in US-ASCII. The name SHOULD be fully qualified whenever
possible.
Values of this textual convention may not be directly usable
as transport-layer addressing information, and may require
run-time resolution. As such, applications that write them
must be prepared for handling errors if such values are not
supported, or cannot be resolved (if resolution occurs at the
time of the management operation).
The DESCRIPTION clause of TransportAddress objects that may
have SnmpTLSAddress values must fully describe how (and
when) such names are to be resolved to IP addresses and vice
versa.
This textual convention SHOULD NOT be used directly in object
definitions since it restricts addresses to a specific
format. However, if it is used, it MAY be used either on its
own or in conjunction with TransportAddressType or
TransportDomain as a pair.
When this textual convention is used as a syntax of an index
object, there may be issues with the limit of 128
sub-identifiers specified in SMIv2 (STD 58). It is RECOMMENDED
that all MIB documents using this textual convention make
explicit any limitations on index component lengths that
management software must observe. This may be done either by
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RFC 6353 TLS Transport Model for SNMP July 2011
including SIZE constraints on the index components or by
specifying applicable constraints in the conceptual row
DESCRIPTION clause or in the surrounding documentation."
REFERENCE
"RFC 1123: Requirements for Internet Hosts - Application and
Support
RFC 5890: Internationalized Domain Names for Applications (IDNA):
Definitions and Document Framework
RFC 5952: A Recommendation for IPv6 Address Text Representation
"
SYNTAX OCTET STRING (SIZE (1..255))
SnmpTLSFingerprint ::= TEXTUAL-CONVENTION
DISPLAY-HINT "1x:1x"
STATUS current
DESCRIPTION
"A fingerprint value that can be used to uniquely reference
other data of potentially arbitrary length.
An SnmpTLSFingerprint value is composed of a 1-octet hashing
algorithm identifier followed by the fingerprint value. The
octet value encoded is taken from the IANA TLS HashAlgorithm
Registry (RFC 5246). The remaining octets are filled using the
results of the hashing algorithm.
This TEXTUAL-CONVENTION allows for a zero-length (blank)
SnmpTLSFingerprint value for use in tables where the
fingerprint value may be optional. MIB definitions or
implementations may refuse to accept a zero-length value as
appropriate."
REFERENCE "RFC 5246: The Transport Layer
Security (TLS) Protocol Version 1.2
http://www.iana.org/assignments/tls-parameters/
"
SYNTAX OCTET STRING (SIZE (0..255))
-- Identities for use in the snmpTlstmCertToTSNTable
snmpTlstmCertToTSNMIdentities OBJECT IDENTIFIER
::= { snmpTlstmIdentities 1 }
snmpTlstmCertSpecified OBJECT-IDENTITY
STATUS current
DESCRIPTION "Directly specifies the tmSecurityName to be used for
this certificate. The value of the tmSecurityName
to use is specified in the snmpTlstmCertToTSNData
column. The snmpTlstmCertToTSNData column must
contain a non-zero length SnmpAdminString compliant
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RFC 6353 TLS Transport Model for SNMP July 2011
value or the mapping described in this row must be
considered a failure."
::= { snmpTlstmCertToTSNMIdentities 1 }
snmpTlstmCertSANRFC822Name OBJECT-IDENTITY
STATUS current
DESCRIPTION "Maps a subjectAltName's rfc822Name to a
tmSecurityName. The local part of the rfc822Name is
passed unaltered but the host-part of the name must
be passed in lowercase. This mapping results in a
1:1 correspondence between equivalent subjectAltName
rfc822Name values and tmSecurityName values except
that the host-part of the name MUST be passed in
lowercase.
Example rfc822Name Field: FooBar@Example.COM
is mapped to tmSecurityName: FooBar@example.com."
::= { snmpTlstmCertToTSNMIdentities 2 }
snmpTlstmCertSANDNSName OBJECT-IDENTITY
STATUS current
DESCRIPTION "Maps a subjectAltName's dNSName to a
tmSecurityName after first converting it to all
lowercase (RFC 5280 does not specify converting to
lowercase so this involves an extra step). This
mapping results in a 1:1 correspondence between
subjectAltName dNSName values and the tmSecurityName
values."
REFERENCE "RFC 5280 - Internet X.509 Public Key Infrastructure
Certificate and Certificate Revocation
List (CRL) Profile."
::= { snmpTlstmCertToTSNMIdentities 3 }
snmpTlstmCertSANIpAddress OBJECT-IDENTITY
STATUS current
DESCRIPTION "Maps a subjectAltName's iPAddress to a
tmSecurityName by transforming the binary encoded
address as follows:
1) for IPv4, the value is converted into a
decimal-dotted quad address (e.g., '192.0.2.1').
2) for IPv6 addresses, the value is converted into a
32-character all lowercase hexadecimal string
without any colon separators.
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RFC 6353 TLS Transport Model for SNMP July 2011
This mapping results in a 1:1 correspondence between
subjectAltName iPAddress values and the
tmSecurityName values.
The resulting length of an encoded IPv6 address is
the maximum length supported by the View-Based
Access Control Model (VACM). Using both the
Transport Security Model's support for transport
prefixes (see the SNMP-TSM-MIB's
snmpTsmConfigurationUsePrefix object for details)
will result in securityName lengths that exceed what
VACM can handle."
::= { snmpTlstmCertToTSNMIdentities 4 }
snmpTlstmCertSANAny OBJECT-IDENTITY
STATUS current
DESCRIPTION "Maps any of the following fields using the
corresponding mapping algorithms:
|------------+----------------------------|
| Type | Algorithm |
|------------+----------------------------|
| rfc822Name | snmpTlstmCertSANRFC822Name |
| dNSName | snmpTlstmCertSANDNSName |
| iPAddress | snmpTlstmCertSANIpAddress |
|------------+----------------------------|
The first matching subjectAltName value found in the
certificate of the above types MUST be used when
deriving the tmSecurityName. The mapping algorithm
specified in the 'Algorithm' column MUST be used to
derive the tmSecurityName.
This mapping results in a 1:1 correspondence between
subjectAltName values and tmSecurityName values. The
three sub-mapping algorithms produced by this
combined algorithm cannot produce conflicting
results between themselves."
::= { snmpTlstmCertToTSNMIdentities 5 }
snmpTlstmCertCommonName OBJECT-IDENTITY
STATUS current
DESCRIPTION "Maps a certificate's CommonName to a tmSecurityName
after converting it to a UTF-8 encoding. The usage
of CommonNames is deprecated and users are
encouraged to use subjectAltName mapping methods
instead. This mapping results in a 1:1
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RFC 6353 TLS Transport Model for SNMP July 2011
correspondence between certificate CommonName values
and tmSecurityName values."
::= { snmpTlstmCertToTSNMIdentities 6 }
-- The snmpTlstmSession Group
snmpTlstmSession OBJECT IDENTIFIER ::= { snmpTlstmObjects 1 }
snmpTlstmSessionOpens OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of times an openSession() request has been executed
as a (D)TLS client, regardless of whether it succeeded or
failed."
::= { snmpTlstmSession 1 }
snmpTlstmSessionClientCloses OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of times a closeSession() request has been
executed as a (D)TLS client, regardless of whether it
succeeded or failed."
::= { snmpTlstmSession 2 }
snmpTlstmSessionOpenErrors OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of times an openSession() request failed to open a
session as a (D)TLS client, for any reason."
::= { snmpTlstmSession 3 }
snmpTlstmSessionAccepts OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of times a (D)TLS server has accepted a new
connection from a client and has received at least one SNMP
message through it."
::= { snmpTlstmSession 4 }
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RFC 6353 TLS Transport Model for SNMP July 2011
snmpTlstmSessionServerCloses OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of times a closeSession() request has been
executed as a (D)TLS server, regardless of whether it
succeeded or failed."
::= { snmpTlstmSession 5 }
snmpTlstmSessionNoSessions OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of times an outgoing message was dropped because
the session associated with the passed tmStateReference was no
longer (or was never) available."
::= { snmpTlstmSession 6 }
snmpTlstmSessionInvalidClientCertificates OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of times an incoming session was not established
on a (D)TLS server because the presented client certificate
was invalid. Reasons for invalidation include, but are not
limited to, cryptographic validation failures or lack of a
suitable mapping row in the snmpTlstmCertToTSNTable."
::= { snmpTlstmSession 7 }
snmpTlstmSessionUnknownServerCertificate OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of times an outgoing session was not established
on a (D)TLS client because the server certificate presented
by an SNMP over (D)TLS server was invalid because no
configured fingerprint or Certification Authority (CA) was
acceptable to validate it.
This may result because there was no entry in the
snmpTlstmAddrTable or because no path could be found to a
known CA."
::= { snmpTlstmSession 8 }
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RFC 6353 TLS Transport Model for SNMP July 2011
snmpTlstmSessionInvalidServerCertificates OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of times an outgoing session was not established
on a (D)TLS client because the server certificate presented
by an SNMP over (D)TLS server could not be validated even if
the fingerprint or expected validation path was known. That
is, a cryptographic validation error occurred during
certificate validation processing.
Reasons for invalidation include, but are not
limited to, cryptographic validation failures."
::= { snmpTlstmSession 9 }
snmpTlstmSessionInvalidCaches OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of outgoing messages dropped because the
tmStateReference referred to an invalid cache."
::= { snmpTlstmSession 10 }
-- Configuration Objects
snmpTlstmConfig OBJECT IDENTIFIER ::= { snmpTlstmObjects 2 }
-- Certificate mapping
snmpTlstmCertificateMapping OBJECT IDENTIFIER ::= { snmpTlstmConfig 1 }
snmpTlstmCertToTSNCount OBJECT-TYPE
SYNTAX Gauge32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"A count of the number of entries in the
snmpTlstmCertToTSNTable."
::= { snmpTlstmCertificateMapping 1 }
snmpTlstmCertToTSNTableLastChanged OBJECT-TYPE
SYNTAX TimeStamp
MAX-ACCESS read-only
STATUS current
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RFC 6353 TLS Transport Model for SNMP July 2011
DESCRIPTION
"The value of sysUpTime.0 when the snmpTlstmCertToTSNTable was
last modified through any means, or 0 if it has not been
modified since the command responder was started."
::= { snmpTlstmCertificateMapping 2 }
snmpTlstmCertToTSNTable OBJECT-TYPE
SYNTAX SEQUENCE OF SnmpTlstmCertToTSNEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"This table is used by a (D)TLS server to map the (D)TLS
client's presented X.509 certificate to a tmSecurityName.
On an incoming (D)TLS/SNMP connection, the client's presented
certificate must either be validated based on an established
trust anchor, or it must directly match a fingerprint in this
table. This table does not provide any mechanisms for
configuring the trust anchors; the transfer of any needed
trusted certificates for path validation is expected to occur
through an out-of-band transfer.
Once the certificate has been found acceptable (either by path
validation or directly matching a fingerprint in this table),
this table is consulted to determine the appropriate
tmSecurityName to identify with the remote connection. This
is done by considering each active row from this table in
prioritized order according to its snmpTlstmCertToTSNID value.
Each row's snmpTlstmCertToTSNFingerprint value determines
whether the row is a match for the incoming connection:
1) If the row's snmpTlstmCertToTSNFingerprint value
identifies the presented certificate, then consider the
row as a successful match.
2) If the row's snmpTlstmCertToTSNFingerprint value
identifies a locally held copy of a trusted CA
certificate and that CA certificate was used to
validate the path to the presented certificate, then
consider the row as a successful match.
Once a matching row has been found, the
snmpTlstmCertToTSNMapType value can be used to determine how
the tmSecurityName to associate with the session should be
determined. See the snmpTlstmCertToTSNMapType column's
DESCRIPTION for details on determining the tmSecurityName
value. If it is impossible to determine a tmSecurityName from
the row's data combined with the data presented in the
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RFC 6353 TLS Transport Model for SNMP July 2011
certificate, then additional rows MUST be searched looking for
another potential match. If a resulting tmSecurityName mapped
from a given row is not compatible with the needed
requirements of a tmSecurityName (e.g., VACM imposes a
32-octet-maximum length and the certificate derived
securityName could be longer), then it must be considered an
invalid match and additional rows MUST be searched looking for
another potential match.
If no matching and valid row can be found, the connection MUST
be closed and SNMP messages MUST NOT be accepted over it.
Missing values of snmpTlstmCertToTSNID are acceptable and
implementations should continue to the next highest numbered
row. It is recommended that administrators skip index values
to leave room for the insertion of future rows (for example,
use values of 10 and 20 when creating initial rows).
Users are encouraged to make use of certificates with
subjectAltName fields that can be used as tmSecurityNames so
that a single root CA certificate can allow all child
certificate's subjectAltName to map directly to a
tmSecurityName via a 1:1 transformation. However, this table
is flexible to allow for situations where existing deployed
certificate infrastructures do not provide adequate
subjectAltName values for use as tmSecurityNames.
Certificates may also be mapped to tmSecurityNames using the
CommonName portion of the Subject field. However, the usage
of the CommonName field is deprecated and thus this usage is
NOT RECOMMENDED. Direct mapping from each individual
certificate fingerprint to a tmSecurityName is also possible
but requires one entry in the table per tmSecurityName and
requires more management operations to completely configure a
device."
::= { snmpTlstmCertificateMapping 3 }
snmpTlstmCertToTSNEntry OBJECT-TYPE
SYNTAX SnmpTlstmCertToTSNEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"A row in the snmpTlstmCertToTSNTable that specifies a mapping
for an incoming (D)TLS certificate to a tmSecurityName to use
for a connection."
INDEX { snmpTlstmCertToTSNID }
::= { snmpTlstmCertToTSNTable 1 }
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RFC 6353 TLS Transport Model for SNMP July 2011
SnmpTlstmCertToTSNEntry ::= SEQUENCE {
snmpTlstmCertToTSNID Unsigned32,
snmpTlstmCertToTSNFingerprint SnmpTLSFingerprint,
snmpTlstmCertToTSNMapType AutonomousType,
snmpTlstmCertToTSNData OCTET STRING,
snmpTlstmCertToTSNStorageType StorageType,
snmpTlstmCertToTSNRowStatus RowStatus
}
snmpTlstmCertToTSNID OBJECT-TYPE
SYNTAX Unsigned32 (1..4294967295)
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"A unique, prioritized index for the given entry. Lower
numbers indicate a higher priority."
::= { snmpTlstmCertToTSNEntry 1 }
snmpTlstmCertToTSNFingerprint OBJECT-TYPE
SYNTAX SnmpTLSFingerprint (SIZE(1..255))
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"A cryptographic hash of an X.509 certificate. The results of
a successful matching fingerprint to either the trusted CA in
the certificate validation path or to the certificate itself
is dictated by the snmpTlstmCertToTSNMapType column."
::= { snmpTlstmCertToTSNEntry 2 }
snmpTlstmCertToTSNMapType OBJECT-TYPE
SYNTAX AutonomousType
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"Specifies the mapping type for deriving a tmSecurityName from
a certificate. Details for mapping of a particular type SHALL
be specified in the DESCRIPTION clause of the OBJECT-IDENTITY
that describes the mapping. If a mapping succeeds it will
return a tmSecurityName for use by the TLSTM model and
processing stops.
If the resulting mapped value is not compatible with the
needed requirements of a tmSecurityName (e.g., VACM imposes a
32-octet-maximum length and the certificate derived
securityName could be longer), then future rows MUST be
searched for additional snmpTlstmCertToTSNFingerprint matches
to look for a mapping that succeeds.
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RFC 6353 TLS Transport Model for SNMP July 2011
Suitable values for assigning to this object that are defined
within the SNMP-TLS-TM-MIB can be found in the
snmpTlstmCertToTSNMIdentities portion of the MIB tree."
DEFVAL { snmpTlstmCertSpecified }
::= { snmpTlstmCertToTSNEntry 3 }
snmpTlstmCertToTSNData OBJECT-TYPE
SYNTAX OCTET STRING (SIZE(0..1024))
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"Auxiliary data used as optional configuration information for
a given mapping specified by the snmpTlstmCertToTSNMapType
column. Only some mapping systems will make use of this
column. The value in this column MUST be ignored for any
mapping type that does not require data present in this
column."
DEFVAL { "" }
::= { snmpTlstmCertToTSNEntry 4 }
snmpTlstmCertToTSNStorageType OBJECT-TYPE
SYNTAX StorageType
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The storage type for this conceptual row. Conceptual rows
having the value 'permanent' need not allow write-access to
any columnar objects in the row."
DEFVAL { nonVolatile }
::= { snmpTlstmCertToTSNEntry 5 }
snmpTlstmCertToTSNRowStatus OBJECT-TYPE
SYNTAX RowStatus
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The status of this conceptual row. This object may be used
to create or remove rows from this table.
To create a row in this table, an administrator must set this
object to either createAndGo(4) or createAndWait(5).
Until instances of all corresponding columns are appropriately
configured, the value of the corresponding instance of the
snmpTlstmParamsRowStatus column is notReady(3).
In particular, a newly created row cannot be made active until
the corresponding snmpTlstmCertToTSNFingerprint,
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RFC 6353 TLS Transport Model for SNMP July 2011
snmpTlstmCertToTSNMapType, and snmpTlstmCertToTSNData columns
have been set.
The following objects may not be modified while the
value of this object is active(1):
- snmpTlstmCertToTSNFingerprint
- snmpTlstmCertToTSNMapType
- snmpTlstmCertToTSNData
An attempt to set these objects while the value of
snmpTlstmParamsRowStatus is active(1) will result in
an inconsistentValue error."
::= { snmpTlstmCertToTSNEntry 6 }
-- Maps tmSecurityNames to certificates for use by the SNMP-TARGET-MIB
snmpTlstmParamsCount OBJECT-TYPE
SYNTAX Gauge32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"A count of the number of entries in the snmpTlstmParamsTable."
::= { snmpTlstmCertificateMapping 4 }
snmpTlstmParamsTableLastChanged OBJECT-TYPE
SYNTAX TimeStamp
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The value of sysUpTime.0 when the snmpTlstmParamsTable
was last modified through any means, or 0 if it has not been
modified since the command responder was started."
::= { snmpTlstmCertificateMapping 5 }
snmpTlstmParamsTable OBJECT-TYPE
SYNTAX SEQUENCE OF SnmpTlstmParamsEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"This table is used by a (D)TLS client when a (D)TLS
connection is being set up using an entry in the
SNMP-TARGET-MIB. It extends the SNMP-TARGET-MIB's
snmpTargetParamsTable with a fingerprint of a certificate to
use when establishing such a (D)TLS connection."
::= { snmpTlstmCertificateMapping 6 }
snmpTlstmParamsEntry OBJECT-TYPE
SYNTAX SnmpTlstmParamsEntry
MAX-ACCESS not-accessible
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RFC 6353 TLS Transport Model for SNMP July 2011
STATUS current
DESCRIPTION
"A conceptual row containing a fingerprint hash of a locally
held certificate for a given snmpTargetParamsEntry. The
values in this row should be ignored if the connection that
needs to be established, as indicated by the SNMP-TARGET-MIB
infrastructure, is not a certificate and (D)TLS based
connection. The connection SHOULD NOT be established if the
certificate fingerprint stored in this entry does not point to
a valid locally held certificate or if it points to an
unusable certificate (such as might happen when the
certificate's expiration date has been reached)."
INDEX { IMPLIED snmpTargetParamsName }
::= { snmpTlstmParamsTable 1 }
SnmpTlstmParamsEntry ::= SEQUENCE {
snmpTlstmParamsClientFingerprint SnmpTLSFingerprint,
snmpTlstmParamsStorageType StorageType,
snmpTlstmParamsRowStatus RowStatus
}
snmpTlstmParamsClientFingerprint OBJECT-TYPE
SYNTAX SnmpTLSFingerprint
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"This object stores the hash of the public portion of a
locally held X.509 certificate. The X.509 certificate, its
public key, and the corresponding private key will be used
when initiating a (D)TLS connection as a (D)TLS client."
::= { snmpTlstmParamsEntry 1 }
snmpTlstmParamsStorageType OBJECT-TYPE
SYNTAX StorageType
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The storage type for this conceptual row. Conceptual rows
having the value 'permanent' need not allow write-access to
any columnar objects in the row."
DEFVAL { nonVolatile }
::= { snmpTlstmParamsEntry 2 }
snmpTlstmParamsRowStatus OBJECT-TYPE
SYNTAX RowStatus
MAX-ACCESS read-create
STATUS current
DESCRIPTION
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RFC 6353 TLS Transport Model for SNMP July 2011
"The status of this conceptual row. This object may be used
to create or remove rows from this table.
To create a row in this table, an administrator must set this
object to either createAndGo(4) or createAndWait(5).
Until instances of all corresponding columns are appropriately
configured, the value of the corresponding instance of the
snmpTlstmParamsRowStatus column is notReady(3).
In particular, a newly created row cannot be made active until
the corresponding snmpTlstmParamsClientFingerprint column has
been set.
The snmpTlstmParamsClientFingerprint object may not be modified
while the value of this object is active(1).
An attempt to set these objects while the value of
snmpTlstmParamsRowStatus is active(1) will result in
an inconsistentValue error."
::= { snmpTlstmParamsEntry 3 }
snmpTlstmAddrCount OBJECT-TYPE
SYNTAX Gauge32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"A count of the number of entries in the snmpTlstmAddrTable."
::= { snmpTlstmCertificateMapping 7 }
snmpTlstmAddrTableLastChanged OBJECT-TYPE
SYNTAX TimeStamp
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The value of sysUpTime.0 when the snmpTlstmAddrTable
was last modified through any means, or 0 if it has not been
modified since the command responder was started."
::= { snmpTlstmCertificateMapping 8 }
snmpTlstmAddrTable OBJECT-TYPE
SYNTAX SEQUENCE OF SnmpTlstmAddrEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"This table is used by a (D)TLS client when a (D)TLS
connection is being set up using an entry in the
SNMP-TARGET-MIB. It extends the SNMP-TARGET-MIB's
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RFC 6353 TLS Transport Model for SNMP July 2011
snmpTargetAddrTable so that the client can verify that the
correct server has been reached. This verification can use
either a certificate fingerprint, or an identity
authenticated via certification path validation.
If there is an active row in this table corresponding to the
entry in the SNMP-TARGET-MIB that was used to establish the
connection, and the row's snmpTlstmAddrServerFingerprint
column has non-empty value, then the server's presented
certificate is compared with the
snmpTlstmAddrServerFingerprint value (and the
snmpTlstmAddrServerIdentity column is ignored). If the
fingerprint matches, the verification has succeeded. If the
fingerprint does not match, then the connection MUST be
closed.
If the server's presented certificate has passed
certification path validation [RFC5280] to a configured
trust anchor, and an active row exists with a zero-length
snmpTlstmAddrServerFingerprint value, then the
snmpTlstmAddrServerIdentity column contains the expected
host name. This expected host name is then compared against
the server's certificate as follows:
- Implementations MUST support matching the expected host
name against a dNSName in the subjectAltName extension
field and MAY support checking the name against the
CommonName portion of the subject distinguished name.
- The '*' (ASCII 0x2a) wildcard character is allowed in the
dNSName of the subjectAltName extension (and in common
name, if used to store the host name), but only as the
left-most (least significant) DNS label in that value.
This wildcard matches any left-most DNS label in the
server name. That is, the subject *.example.com matches
the server names a.example.com and b.example.com, but does
not match example.com or a.b.example.com. Implementations
MUST support wildcards in certificates as specified above,
but MAY provide a configuration option to disable them.
- If the locally configured name is an internationalized
domain name, conforming implementations MUST convert it to
the ASCII Compatible Encoding (ACE) format for performing
comparisons, as specified in Section 7 of [RFC5280].
If the expected host name fails these conditions then the
connection MUST be closed.
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RFC 6353 TLS Transport Model for SNMP July 2011
If there is no row in this table corresponding to the entry
in the SNMP-TARGET-MIB and the server can be authorized by
another, implementation-dependent means, then the connection
MAY still proceed."
::= { snmpTlstmCertificateMapping 9 }
snmpTlstmAddrEntry OBJECT-TYPE
SYNTAX SnmpTlstmAddrEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"A conceptual row containing a copy of a certificate's
fingerprint for a given snmpTargetAddrEntry. The values in
this row should be ignored if the connection that needs to be
established, as indicated by the SNMP-TARGET-MIB
infrastructure, is not a (D)TLS based connection. If an
snmpTlstmAddrEntry exists for a given snmpTargetAddrEntry, then
the presented server certificate MUST match or the connection
MUST NOT be established. If a row in this table does not
exist to match an snmpTargetAddrEntry row, then the connection
SHOULD still proceed if some other certificate validation path
algorithm (e.g., RFC 5280) can be used."
INDEX { IMPLIED snmpTargetAddrName }
::= { snmpTlstmAddrTable 1 }
SnmpTlstmAddrEntry ::= SEQUENCE {
snmpTlstmAddrServerFingerprint SnmpTLSFingerprint,
snmpTlstmAddrServerIdentity SnmpAdminString,
snmpTlstmAddrStorageType StorageType,
snmpTlstmAddrRowStatus RowStatus
}
snmpTlstmAddrServerFingerprint OBJECT-TYPE
SYNTAX SnmpTLSFingerprint
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"A cryptographic hash of a public X.509 certificate. This
object should store the hash of the public X.509 certificate
that the remote server should present during the (D)TLS
connection setup. The fingerprint of the presented
certificate and this hash value MUST match exactly or the
connection MUST NOT be established."
DEFVAL { "" }
::= { snmpTlstmAddrEntry 1 }
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RFC 6353 TLS Transport Model for SNMP July 2011
snmpTlstmAddrServerIdentity OBJECT-TYPE
SYNTAX SnmpAdminString
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The reference identity to check against the identity
presented by the remote system."
DEFVAL { "" }
::= { snmpTlstmAddrEntry 2 }
snmpTlstmAddrStorageType OBJECT-TYPE
SYNTAX StorageType
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The storage type for this conceptual row. Conceptual rows
having the value 'permanent' need not allow write-access to
any columnar objects in the row."
DEFVAL { nonVolatile }
::= { snmpTlstmAddrEntry 3 }
snmpTlstmAddrRowStatus OBJECT-TYPE
SYNTAX RowStatus
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The status of this conceptual row. This object may be used
to create or remove rows from this table.
To create a row in this table, an administrator must set this
object to either createAndGo(4) or createAndWait(5).
Until instances of all corresponding columns are
appropriately configured, the value of the
corresponding instance of the snmpTlstmAddrRowStatus
column is notReady(3).
In particular, a newly created row cannot be made active until
the corresponding snmpTlstmAddrServerFingerprint column has been
set.
Rows MUST NOT be active if the snmpTlstmAddrServerFingerprint
column is blank and the snmpTlstmAddrServerIdentity is set to
'*' since this would insecurely accept any presented
certificate.
Hardaker Standards Track [Page 50]
RFC 6353 TLS Transport Model for SNMP July 2011
The snmpTlstmAddrServerFingerprint object may not be modified
while the value of this object is active(1).
An attempt to set these objects while the value of
snmpTlstmAddrRowStatus is active(1) will result in
an inconsistentValue error."
::= { snmpTlstmAddrEntry 4 }
-- ************************************************
-- snmpTlstmNotifications - Notifications Information
-- ************************************************
snmpTlstmServerCertificateUnknown NOTIFICATION-TYPE
OBJECTS { snmpTlstmSessionUnknownServerCertificate }
STATUS current
DESCRIPTION
"Notification that the server certificate presented by an SNMP
over (D)TLS server was invalid because no configured
fingerprint or CA was acceptable to validate it. This may be
because there was no entry in the snmpTlstmAddrTable or
because no path could be found to known Certification
Authority.
To avoid notification loops, this notification MUST NOT be
sent to servers that themselves have triggered the
notification."
::= { snmpTlstmNotifications 1 }
snmpTlstmServerInvalidCertificate NOTIFICATION-TYPE
OBJECTS { snmpTlstmAddrServerFingerprint,
snmpTlstmSessionInvalidServerCertificates}
STATUS current
DESCRIPTION
"Notification that the server certificate presented by an SNMP
over (D)TLS server could not be validated even if the
fingerprint or expected validation path was known. That is, a
cryptographic validation error occurred during certificate
validation processing.
To avoid notification loops, this notification MUST NOT be
sent to servers that themselves have triggered the
notification."
::= { snmpTlstmNotifications 2 }
-- ************************************************
-- snmpTlstmCompliances - Conformance Information
-- ************************************************
Hardaker Standards Track [Page 51]
RFC 6353 TLS Transport Model for SNMP July 2011
snmpTlstmCompliances OBJECT IDENTIFIER ::= { snmpTlstmConformance 1 }
snmpTlstmGroups OBJECT IDENTIFIER ::= { snmpTlstmConformance 2 }
-- ************************************************
-- Compliance statements
-- ************************************************
snmpTlstmCompliance MODULE-COMPLIANCE
STATUS current
DESCRIPTION
"The compliance statement for SNMP engines that support the
SNMP-TLS-TM-MIB"
MODULE
MANDATORY-GROUPS { snmpTlstmStatsGroup,
snmpTlstmIncomingGroup,
snmpTlstmOutgoingGroup,
snmpTlstmNotificationGroup }
::= { snmpTlstmCompliances 1 }
-- ************************************************
-- Units of conformance
-- ************************************************
snmpTlstmStatsGroup OBJECT-GROUP
OBJECTS {
snmpTlstmSessionOpens,
snmpTlstmSessionClientCloses,
snmpTlstmSessionOpenErrors,
snmpTlstmSessionAccepts,
snmpTlstmSessionServerCloses,
snmpTlstmSessionNoSessions,
snmpTlstmSessionInvalidClientCertificates,
snmpTlstmSessionUnknownServerCertificate,
snmpTlstmSessionInvalidServerCertificates,
snmpTlstmSessionInvalidCaches
}
STATUS current
DESCRIPTION
"A collection of objects for maintaining
statistical information of an SNMP engine that
implements the SNMP TLS Transport Model."
::= { snmpTlstmGroups 1 }
snmpTlstmIncomingGroup OBJECT-GROUP
OBJECTS {
snmpTlstmCertToTSNCount,
snmpTlstmCertToTSNTableLastChanged,
snmpTlstmCertToTSNFingerprint,
Hardaker Standards Track [Page 52]
RFC 6353 TLS Transport Model for SNMP July 2011
snmpTlstmCertToTSNMapType,
snmpTlstmCertToTSNData,
snmpTlstmCertToTSNStorageType,
snmpTlstmCertToTSNRowStatus
}
STATUS current
DESCRIPTION
"A collection of objects for maintaining
incoming connection certificate mappings to
tmSecurityNames of an SNMP engine that implements the
SNMP TLS Transport Model."
::= { snmpTlstmGroups 2 }
snmpTlstmOutgoingGroup OBJECT-GROUP
OBJECTS {
snmpTlstmParamsCount,
snmpTlstmParamsTableLastChanged,
snmpTlstmParamsClientFingerprint,
snmpTlstmParamsStorageType,
snmpTlstmParamsRowStatus,
snmpTlstmAddrCount,
snmpTlstmAddrTableLastChanged,
snmpTlstmAddrServerFingerprint,
snmpTlstmAddrServerIdentity,
snmpTlstmAddrStorageType,
snmpTlstmAddrRowStatus
}
STATUS current
DESCRIPTION
"A collection of objects for maintaining
outgoing connection certificates to use when opening
connections as a result of SNMP-TARGET-MIB settings."
::= { snmpTlstmGroups 3 }
snmpTlstmNotificationGroup NOTIFICATION-GROUP
NOTIFICATIONS {
snmpTlstmServerCertificateUnknown,
snmpTlstmServerInvalidCertificate
}
STATUS current
DESCRIPTION
"Notifications"
::= { snmpTlstmGroups 4 }
END
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RFC 6353 TLS Transport Model for SNMP July 2011 8. Operational Considerations This section discusses various operational aspects of deploying TLSTM. 8.1. Sessions A session is discussed throughout this document as meaning a security association between two TLSTM instances. State information for the sessions are maintained in each TLSTM implementation and this information is created and destroyed as sessions are opened and closed. A "broken" session (one side up and one side down) can result if one side of a session is brought down abruptly (i.e., reboot, power outage, etc.). Whenever possible, implementations SHOULD provide graceful session termination through the use of TLS disconnect messages. Implementations SHOULD also have a system in place for detecting "broken" sessions through the use of heartbeats [HEARTBEAT] or other detection mechanisms. Implementations SHOULD limit the lifetime of established sessions depending on the algorithms used for generation of the master session secret, the privacy and integrity algorithms used to protect messages, the environment of the session, the amount of data transferred, and the sensitivity of the data. 8.2. Notification Receiver Credential Selection When an SNMP engine needs to establish an outgoing session for notifications, the snmpTargetParamsTable includes an entry for the snmpTargetParamsSecurityName of the target. Servers that wish to support multiple principals at a particular port SHOULD make use of the Server Name Indication extension defined in Section 3.1 of [RFC4366]. Without the Server Name Indication the receiving SNMP engine (server) will not know which (D)TLS certificate to offer to the client so that the tmSecurityName identity-authentication will be successful. Another solution is to maintain a one-to-one mapping between certificates and incoming ports for notification receivers. This can be handled at the notification originator by configuring the snmpTargetAddrTable (snmpTargetAddrTDomain and snmpTargetAddrTAddress) and requiring the receiving SNMP engine to monitor multiple incoming static ports based on which principals are capable of receiving notifications. Implementations MAY also choose to designate a single Notification Receiver Principal to receive all incoming notifications or select an Hardaker Standards Track [Page 54]
RFC 6353 TLS Transport Model for SNMP July 2011 implementation specific method of selecting a server certificate to present to clients. 8.3. contextEngineID Discovery SNMPv3 requires that an application know the identifier (snmpEngineID) of the remote SNMP protocol engine in order to retrieve or manipulate objects maintained on the remote SNMP entity. [RFC5343] introduces a well-known localEngineID and a discovery mechanism that can be used to learn the snmpEngineID of a remote SNMP protocol engine. Implementations are RECOMMENDED to support and use the contextEngineID discovery mechanism defined in [RFC5343]. 8.4. Transport Considerations This document defines how SNMP messages can be transmitted over the TLS- and DTLS-based protocols. Each of these protocols is additionally based on other transports (TCP and UDP). These two base protocols also have operational considerations that must be taken into consideration when selecting a (D)TLS-based protocol to use such as its performance in degraded or limited networks. It is beyond the scope of this document to summarize the characteristics of these transport mechanisms. Please refer to the base protocol documents for details on messaging considerations with respect to MTU size, fragmentation, performance in lossy networks, etc. 9. Security Considerations This document describes a transport model that permits SNMP to utilize (D)TLS security services. The security threats and how the (D)TLS transport model mitigates these threats are covered in detail throughout this document. Security considerations for DTLS are covered in [RFC4347] and security considerations for TLS are described in Section 11 and Appendices D, E, and F of TLS 1.2 [RFC5246]. When run over a connectionless transport such as UDP, DTLS is more vulnerable to denial-of-service attacks from spoofed IP addresses; see Section 4.2 for details how the cookie exchange is used to address this issue. 9.1. Certificates, Authentication, and Authorization Implementations are responsible for providing a security certificate installation and configuration mechanism. Implementations SHOULD support certificate revocation lists. (D)TLS provides for authentication of the identity of both the (D)TLS server and the (D)TLS client. Access to MIB objects for the Hardaker Standards Track [Page 55]
RFC 6353 TLS Transport Model for SNMP July 2011 authenticated principal MUST be enforced by an access control subsystem (e.g., the VACM). Authentication of the command generator principal's identity is important for use with the SNMP access control subsystem to ensure that only authorized principals have access to potentially sensitive data. The authenticated identity of the command generator principal's certificate is mapped to an SNMP model-independent securityName for use with SNMP access control. The (D)TLS handshake only provides assurance that the certificate of the authenticated identity has been signed by a configured accepted Certification Authority. (D)TLS has no way to further authorize or reject access based on the authenticated identity. An Access Control Model (such as the VACM) provides access control and authorization of a command generator's requests to a command responder and a notification receiver's authorization to receive Notifications from a notification originator. However, to avoid man-in-the-middle attacks, both ends of the (D)TLS-based connection MUST check the certificate presented by the other side against what was expected. For example, command generators must check that the command responder presented and authenticated itself with an X.509 certificate that was expected. Not doing so would allow an impostor, at a minimum, to present false data, receive sensitive information, and/or provide a false belief that configuration was actually received and acted upon. Authenticating and verifying the identity of the (D)TLS server and the (D)TLS client for all operations ensures the authenticity of the SNMP engine that provides MIB data. The instructions found in the DESCRIPTION clause of the snmpTlstmCertToTSNTable object must be followed exactly. It is also important that the rows of the table be searched in prioritized order starting with the row containing the lowest numbered snmpTlstmCertToTSNID value. 9.2. (D)TLS Security Considerations This section discusses security considerations specific to the usage of (D)TLS. 9.2.1. TLS Version Requirements Implementations of TLS typically support multiple versions of the Transport Layer Security protocol as well as the older Secure Sockets Layer (SSL) protocol. Because of known security vulnerabilities, TLSTM clients and servers MUST NOT request, offer, or use SSL 2.0. See Appendix E.2 of [RFC5246] for further details. Hardaker Standards Track [Page 56]
RFC 6353 TLS Transport Model for SNMP July 2011
9.2.2. Perfect Forward Secrecy
The use of Perfect Forward Secrecy is RECOMMENDED and can be provided
by (D)TLS with appropriately selected cipher_suites, as discussed in
Appendix F of [RFC5246].
9.3. Use with SNMPv1/SNMPv2c Messages
The SNMPv1 and SNMPv2c message processing described in [RFC3584] (BCP
74) always selects the SNMPv1 or SNMPv2c Security Models,
respectively. Both of these and the User-based Security Model
typically used with SNMPv3 derive the securityName and securityLevel
from the SNMP message received, even when the message was received
over a secure transport. Access control decisions are therefore made
based on the contents of the SNMP message, rather than using the
authenticated identity and securityLevel provided by the TLS
Transport Model. It is RECOMMENDED that only SNMPv3 messages using
the Transport Security Model (TSM) or another secure-transport aware
security model be sent over the TLSTM transport.
Using a non-transport-aware Security Model with a secure Transport
Model is NOT RECOMMENDED. See [RFC5590], Section 7.1 for additional
details on the coexistence of security-aware transports and non-
transport-aware security models.
9.4. MIB Module Security
There are a number of management objects defined in this MIB module
with a MAX-ACCESS clause of read-write and/or read-create. Such
objects may be considered sensitive or vulnerable in some network
environments. The support for SET operations in a non-secure
environment without proper protection can have a negative effect on
network operations. These are the tables and objects and their
sensitivity/vulnerability:
o The snmpTlstmParamsTable can be used to change the outgoing X.509
certificate used to establish a (D)TLS connection. Modifications
to objects in this table need to be adequately authenticated since
modifying the values in this table will have profound impacts to
the security of outbound connections from the device. Since
knowledge of authorization rules and certificate usage mechanisms
may be considered sensitive, protection from disclosure of the
SNMP traffic via encryption is also highly recommended.
o The snmpTlstmAddrTable can be used to change the expectations of
the certificates presented by a remote (D)TLS server.
Modifications to objects in this table need to be adequately
authenticated since modifying the values in this table will have
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RFC 6353 TLS Transport Model for SNMP July 2011
profound impacts to the security of outbound connections from the
device. Since knowledge of authorization rules and certificate
usage mechanisms may be considered sensitive, protection from
disclosure of the SNMP traffic via encryption is also highly
recommended.
o The snmpTlstmCertToTSNTable is used to specify the mapping of
incoming X.509 certificates to tmSecurityNames, which eventually
get mapped to an SNMPv3 securityName. Modifications to objects in
this table need to be adequately authenticated since modifying the
values in this table will have profound impacts to the security of
incoming connections to the device. Since knowledge of
authorization rules and certificate usage mechanisms may be
considered sensitive, protection from disclosure of the SNMP
traffic via encryption is also highly recommended. When this
table contains a significant number of rows it may affect the
system performance when accepting new (D)TLS connections.
Some of the readable objects in this MIB module (i.e., objects with a
MAX-ACCESS other than not-accessible) may be considered sensitive or
vulnerable in some network environments. It is thus important to
control even GET and/or NOTIFY access to these objects and possibly
to even encrypt the values of these objects when sending them over
the network via SNMP. These are the tables and objects and their
sensitivity/vulnerability:
o This MIB contains a collection of counters that monitor the (D)TLS
connections being established with a device. Since knowledge of
connection and certificate usage mechanisms may be considered
sensitive, protection from disclosure of the SNMP traffic via
encryption is highly recommended.
SNMP versions prior to SNMPv3 did not include adequate security.
Even if the network itself is secure (for example, by using IPsec),
even then, there is no control as to who on the secure network is
allowed to access and GET/SET (read/change/create/delete) the objects
in this MIB module.
It is RECOMMENDED that implementers consider the security features as
provided by the SNMPv3 framework (see [RFC3410], Section 8),
including full support for the SNMPv3 cryptographic mechanisms (for
authentication and privacy).
Further, deployment of SNMP versions prior to SNMPv3 is NOT
RECOMMENDED. Instead, it is RECOMMENDED to deploy SNMPv3 and to
enable cryptographic security. It is then a customer/operator
responsibility to ensure that the SNMP entity giving access to an
instance of this MIB module is properly configured to give access to
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the objects only to those principals (users) that have legitimate
rights to indeed GET or SET (change/create/delete) them.
10. IANA Considerations
IANA has assigned:
1. Two TCP/UDP port numbers from the "Registered Ports" range of the
Port Numbers registry, with the following keywords:
Keyword Decimal Description References
------- ------- ----------- ----------
snmptls 10161/tcp SNMP-TLS [RFC6353]
snmpdtls 10161/udp SNMP-DTLS [RFC6353]
snmptls-trap 10162/tcp SNMP-Trap-TLS [RFC6353]
snmpdtls-trap 10162/udp SNMP-Trap-DTLS [RFC6353]
These are the default ports for receipt of SNMP command messages
(snmptls and snmpdtls) and SNMP notification messages (snmptls-trap
and snmpdtls-trap) over a TLS Transport Model as defined in this
document.
2. An SMI number (8) under snmpDomains for the snmpTLSTCPDomain
object identifier
3. An SMI number (9) under snmpDomains for the snmpDTLSUDPDomain
object identifier
4. An SMI number (198) under mib-2, for the MIB module in this
document
5. "tls" as the corresponding prefix for the snmpTLSTCPDomain in the
SNMP Transport Domains registry
6. "dtls" as the corresponding prefix for the snmpDTLSUDPDomain in
the SNMP Transport Domains registry
11. Acknowledgements
This document closely follows and copies the Secure Shell Transport
Model for SNMP documented by David Harrington and Joseph Salowey in
[RFC5592].
This document was reviewed by the following people who helped provide
useful comments (in alphabetical order): Andy Donati, Pasi Eronen,
David Harrington, Jeffrey Hutzelman, Alan Luchuk, Michael Peck, Tom
Petch, Randy Presuhn, Ray Purvis, Peter Saint-Andre, Joseph Salowey,
Juergen Schoenwaelder, Dave Shield, and Robert Story.
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RFC 6353 TLS Transport Model for SNMP July 2011
This work was supported in part by the United States Department of
Defense. Large portions of this document are based on work by
General Dynamics C4 Systems and the following individuals: Brian
Baril, Kim Bryant, Dana Deluca, Dan Hanson, Tim Huemiller, John
Holzhauer, Colin Hoogeboom, Dave Kornbau, Chris Knaian, Dan Knaul,
Charles Limoges, Steve Moccaldi, Gerardo Orlando, and Brandon Yip.
12. References
12.1. Normative References
[RFC1123] Braden, R., "Requirements for Internet Hosts -
Application and Support", STD 3, RFC 1123, October 1989.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2578] McCloghrie, K., Ed., Perkins, D., Ed., and J.
Schoenwaelder, Ed., "Structure of Management Information
Version 2 (SMIv2)", STD 58, RFC 2578, April 1999.
[RFC2579] McCloghrie, K., Ed., Perkins, D., Ed., and J.
Schoenwaelder, Ed., "Textual Conventions for SMIv2",
STD 58, RFC 2579, April 1999.
[RFC2580] McCloghrie, K., Perkins, D., and J. Schoenwaelder,
"Conformance Statements for SMIv2", STD 58, RFC 2580,
April 1999.
[RFC3411] Harrington, D., Presuhn, R., and B. Wijnen, "An
Architecture for Describing Simple Network Management
Protocol (SNMP) Management Frameworks", STD 62,
RFC 3411, December 2002.
[RFC3413] Levi, D., Meyer, P., and B. Stewart, "Simple Network
Management Protocol (SNMP) Applications", STD 62,
RFC 3413, December 2002.
[RFC3414] Blumenthal, U. and B. Wijnen, "User-based Security Model
(USM) for version 3 of the Simple Network Management
Protocol (SNMPv3)", STD 62, RFC 3414, December 2002.
[RFC3415] Wijnen, B., Presuhn, R., and K. McCloghrie, "View-based
Access Control Model (VACM) for the Simple Network
Management Protocol (SNMP)", STD 62, RFC 3415,
December 2002.
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RFC 6353 TLS Transport Model for SNMP July 2011
[RFC3418] Presuhn, R., "Management Information Base (MIB) for the
Simple Network Management Protocol (SNMP)", STD 62,
RFC 3418, December 2002.
[RFC3584] Frye, R., Levi, D., Routhier, S., and B. Wijnen,
"Coexistence between Version 1, Version 2, and Version 3
of the Internet-standard Network Management Framework",
BCP 74, RFC 3584, August 2003.
[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006.
[RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen,
J., and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 4366, April 2006.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer
Security (TLS) Protocol Version 1.2", RFC 5246,
August 2008.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation
List (CRL) Profile", RFC 5280, May 2008.
[RFC5590] Harrington, D. and J. Schoenwaelder, "Transport
Subsystem for the Simple Network Management Protocol
(SNMP)", RFC 5590, June 2009.
[RFC5591] Harrington, D. and W. Hardaker, "Transport Security
Model for the Simple Network Management Protocol
(SNMP)", RFC 5591, June 2009.
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for
IPv6 Address Text Representation", RFC 5952,
August 2010.
12.2. Informative References
[HEARTBEAT] Seggelmann, R., Tuexen, M., and M. Williams, "Transport
Layer Security (TLS) and Datagram Transport Layer
Security (DTLS) Heartbeat Extension", Work in Progress,
July 2011.
[RFC3410] Case, J., Mundy, R., Partain, D., and B. Stewart,
"Introduction and Applicability Statements for Internet-
Standard Management Framework", RFC 3410, December 2002.
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RFC 6353 TLS Transport Model for SNMP July 2011
[RFC5343] Schoenwaelder, J., "Simple Network Management Protocol
(SNMP) Context EngineID Discovery", RFC 5343,
September 2008.
[RFC5592] Harrington, D., Salowey, J., and W. Hardaker, "Secure
Shell Transport Model for the Simple Network Management
Protocol (SNMP)", RFC 5592, June 2009.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document
Framework", RFC 5890, August 2010.
[RFC5953] Hardaker, W., "Transport Layer Security (TLS) Transport
Model for the Simple Network Management Protocol
(SNMP)", RFC 5953, August 2010.
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Appendix A. Target and Notification Configuration Example
The following sections describe example configuration for the SNMP-
TLS-TM-MIB, the SNMP-TARGET-MIB, the NOTIFICATION-MIB, and the SNMP-
VIEW-BASED-ACM-MIB.
A.1. Configuring a Notification Originator
The following row adds the "Joe Cool" user to the "administrators"
group:
vacmSecurityModel = 4 (TSM)
vacmSecurityName = "Joe Cool"
vacmGroupName = "administrators"
vacmSecurityToGroupStorageType = 3 (nonVolatile)
vacmSecurityToGroupStatus = 4 (createAndGo)
The following row configures the snmpTlstmAddrTable to use
certificate path validation and to require the remote notification
receiver to present a certificate for the "server.example.org"
identity.
snmpTargetAddrName = "toNRAddr"
snmpTlstmAddrServerFingerprint = ""
snmpTlstmAddrServerIdentity = "server.example.org"
snmpTlstmAddrStorageType = 3 (nonVolatile)
snmpTlstmAddrRowStatus = 4 (createAndGo)
The following row configures the snmpTargetAddrTable to send
notifications using TLS/TCP to the snmptls-trap port at 192.0.2.1:
snmpTargetAddrName = "toNRAddr"
snmpTargetAddrTDomain = snmpTLSTCPDomain
snmpTargetAddrTAddress = "192.0.2.1:10162"
snmpTargetAddrTimeout = 1500
snmpTargetAddrRetryCount = 3
snmpTargetAddrTagList = "toNRTag"
snmpTargetAddrParams = "toNR" (MUST match below)
snmpTargetAddrStorageType = 3 (nonVolatile)
snmpTargetAddrRowStatus = 4 (createAndGo)
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The following row configures the snmpTargetParamsTable to send the
notifications to "Joe Cool", using authPriv SNMPv3 notifications
through the TransportSecurityModel [RFC5591]:
snmpTargetParamsName = "toNR" (must match above)
snmpTargetParamsMPModel = 3 (SNMPv3)
snmpTargetParamsSecurityModel = 4 (TransportSecurityModel)
snmpTargetParamsSecurityName = "Joe Cool"
snmpTargetParamsSecurityLevel = 3 (authPriv)
snmpTargetParamsStorageType = 3 (nonVolatile)
snmpTargetParamsRowStatus = 4 (createAndGo)
A.2. Configuring TLSTM to Utilize a Simple Derivation of tmSecurityName
The following row configures the snmpTlstmCertToTSNTable to map a
validated client certificate, referenced by the client's public X.509
hash fingerprint, to a tmSecurityName using the subjectAltName
component of the certificate.
snmpTlstmCertToTSNID = 1
(chosen by ordering preference)
snmpTlstmCertToTSNFingerprint = HASH (appropriate fingerprint)
snmpTlstmCertToTSNMapType = snmpTlstmCertSANAny
snmpTlstmCertToTSNData = "" (not used)
snmpTlstmCertToTSNStorageType = 3 (nonVolatile)
snmpTlstmCertToTSNRowStatus = 4 (createAndGo)
This type of configuration should only be used when the naming
conventions of the (possibly multiple) Certification Authorities are
well understood, so two different principals cannot inadvertently be
identified by the same derived tmSecurityName.
A.3. Configuring TLSTM to Utilize Table-Driven Certificate Mapping
The following row configures the snmpTlstmCertToTSNTable to map a
validated client certificate, referenced by the client's public X.509
hash fingerprint, to the directly specified tmSecurityName of "Joe
Cool".
snmpTlstmCertToTSNID = 2
(chosen by ordering preference)
snmpTlstmCertToTSNFingerprint = HASH (appropriate fingerprint)
snmpTlstmCertToTSNMapType = snmpTlstmCertSpecified
snmpTlstmCertToTSNSecurityName = "Joe Cool"
snmpTlstmCertToTSNStorageType = 3 (nonVolatile)
snmpTlstmCertToTSNRowStatus = 4 (createAndGo)
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RFC 6353 TLS Transport Model for SNMP July 2011 Author's Address Wes Hardaker SPARTA, Inc. P.O. Box 382 Davis, CA 95617 USA Phone: +1 530 792 1913 EMail: ietf@hardakers.net Hardaker Standards Track [Page 65]