<- BCP Index (101..200)
BCP 185
(also RFC 7115, RFC 9319)
[Note that this file is a concatenation of more than one RFC.]
Internet Engineering Task Force (IETF) R. Bush
Request for Comments: 7115 Internet Initiative Japan
BCP: 185 January 2014
Category: Best Current Practice
ISSN: 2070-1721
Origin Validation Operation
Based on the Resource Public Key Infrastructure (RPKI)
Abstract
Deployment of BGP origin validation that is based on the Resource
Public Key Infrastructure (RPKI) has many operational considerations.
This document attempts to collect and present those that are most
critical. It is expected to evolve as RPKI-based origin validation
continues to be deployed and the dynamics are better understood.
Status of This Memo
This memo documents an Internet Best Current Practice.
This document is a product of the Internet Engineering Task Force
(IETF). It has been approved for publication by the Internet
Engineering Steering Group (IESG). Further information on BCPs 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/rfc7115.
Copyright Notice
Copyright (c) 2014 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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Suggested Reading . . . . . . . . . . . . . . . . . . . . . . 3
3. RPKI Distribution and Maintenance . . . . . . . . . . . . . . 3
4. Within a Network . . . . . . . . . . . . . . . . . . . . . . 6
5. Routing Policy . . . . . . . . . . . . . . . . . . . . . . . 6
6. Notes and Recommendations . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . 10
1. Introduction
RPKI-based origin validation relies on widespread deployment of the
Resource Public Key Infrastructure (RPKI) [RFC6480]. How the RPKI is
distributed and maintained globally is a serious concern from many
aspects.
While the global RPKI is in the early stages of deployment, there is
no single root trust anchor, initial testing is being done by the
Regional Internet Registries (RIRs), and there are technical
testbeds. It is thought that origin validation based on the RPKI
will continue to be deployed incrementally over the next few years.
It is assumed that eventually there must be a single root trust
anchor for the public address space, see [IAB].
Origin validation needs to be done only by an AS's border routers and
is designed so that it can be used to protect announcements that are
originated by any network participating in Internet BGP routing:
large providers, upstream and downstream routers, and by edge
networks (e.g., small stub or enterprise networks).
Origin validation has been designed to be deployed on current routers
without significant hardware upgrades. It should be used in border
routers by operators from large backbones to small stub/enterprise/
edge networks.
RPKI-based origin validation has been designed so that, with prudent
local routing policies, there is little risk that what is seen as
today's normal Internet routing is threatened by imprudent deployment
of the global RPKI; see Section 5.
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1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to
be interpreted as described in RFC 2119 [RFC2119] only when they
appear in all upper case. They may also appear in lower or mixed
case as English words, without normative meaning.
2. Suggested Reading
It is assumed that the reader understands BGP [RFC4271], the RPKI
[RFC6480], the RPKI Repository Structure [RFC6481], Route Origin
Authorizations (ROAs) [RFC6482], the RPKI to Router Protocol
[RFC6810], RPKI-based Prefix Validation [RFC6811], and Ghostbusters
Records [RFC6493].
3. RPKI Distribution and Maintenance
The RPKI is a distributed database containing certificates,
Certificate Revocation Lists (CRLs), manifests, ROAs, and
Ghostbusters Records as described in [RFC6481]. Policies and
considerations for RPKI object generation and maintenance are
discussed elsewhere.
The RPKI repository design [RFC6481] anticipated a hierarchic
organization of repositories, as this seriously improves the
performance of relying parties that gather data over a non-hierarchic
organization. Publishing parties MUST implement hierarchic directory
structures.
A local relying party's valid cache containing all RPKI data may be
gathered from the global distributed database using the rsync
protocol [RFC5781] and a validation tool such as rcynic [rcynic].
A validated cache contains all RPKI objects that the RP has verified
to be valid according to the rules for validation RPKI certificates
and signed objects; see [RFC6487] and [RFC6488]. Entities that trust
the cache can use these RPKI objects without further validation.
Validated caches may also be created and maintained from other
validated caches. Network operators SHOULD take maximum advantage of
this feature to minimize load on the global distributed RPKI
database. Of course, the recipient relying parties should
re-validate the data.
As Trust Anchor Locators (TALs) [RFC6490] are critical to the RPKI
trust model, operators should be very careful in their initial
selection and vigilant in their maintenance.
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Timing of inter-cache synchronization, and synchronization between
caches and the global RPKI, is outside the scope of this document,
and depends on things such as how often routers feed from the caches,
how often the operator feels the global RPKI changes significantly,
etc.
As inter-cache synchronization within an operator's network does not
impact global RPKI resources, an operator may choose to synchronize
quite frequently.
To relieve routers of the load of performing certificate validation,
cryptographic operations, etc., the RPKI-Router protocol [RFC6810]
does not provide object-based security to the router. That is, the
router cannot validate the data cryptographically from a well-known
trust anchor. The router trusts the cache to provide correct data
and relies on transport-based security for the data received from the
cache. Therefore, the authenticity and integrity of the data from
the cache should be well protected; see Section 7 of [RFC6810].
As RPKI-based origin validation relies on the availability of RPKI
data, operators SHOULD locate RPKI caches close to routers that
require these data and services in order to minimize the impact of
likely failures in local routing, intermediate devices, long
circuits, etc. One should also consider trust boundaries, routing
bootstrap reachability, etc.
For example, a router should bootstrap from a cache that is reachable
with minimal reliance on other infrastructure such as DNS or routing
protocols. If a router needs its BGP and/or IGP to converge for the
router to reach a cache, once a cache is reachable, the router will
then have to reevaluate prefixes already learned via BGP. Such
configurations should be avoided if reasonably possible.
If insecure transports are used between an operator's cache and their
router(s), the Transport Security recommendations in [RFC6810] SHOULD
be followed. In particular, operators MUST NOT use insecure
transports between their routers and RPKI caches located in other
Autonomous Systems.
For redundancy, a router should peer with more than one cache at the
same time. Peering with two or more, at least one local and others
remote, is recommended.
If an operator trusts upstreams to carry their traffic, they may also
trust the RPKI data those upstreams cache and SHOULD peer with caches
made available to them by those upstreams. Note that this places an
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obligation on those upstreams to maintain fresh and reliable caches
and to make them available to their customers. And, as usual, the
recipient SHOULD re-validate the data.
A transit provider or a network with peers SHOULD validate origins in
announcements made by upstreams, downstreams, and peers. They still
should trust the caches provided by their upstreams.
Before issuing a ROA for a super-block, an operator MUST ensure that
all sub-allocations from that block that are announced by other ASes,
e.g., customers, have correct ROAs in the RPKI. Otherwise, issuing a
ROA for the super-block will cause the announcements of sub-
allocations with no ROAs to be viewed as Invalid; see [RFC6811].
While waiting for all recipients of sub-allocations to register ROAs,
the owner of the super-block may use live BGP data to populate ROAs
as a proxy, and then safely issue a ROA for the super-block.
Use of RPKI-based origin validation removes any need to inject more
specifics into BGP to protect against mis-origination of a less
specific prefix. Having a ROA for the covering prefix will protect
it.
To aid translation of ROAs into efficient search algorithms in
routers, ROAs should be as precise as possible, i.e., match prefixes
as announced in BGP. For example, software and operators SHOULD
avoid use of excessive max length values in ROAs unless they are
operationally necessary.
One advantage of minimal ROA length is that the forged origin attack
does not work for sub-prefixes that are not covered by overly long
max length. For example, if, instead of 10.0.0.0/16-24, one issues
10.0.0.0/16 and 10.0.42.0/24, a forged origin attack cannot succeed
against 10.0.666.0/24. They must attack the whole /16, which is more
likely to be noticed because of its size.
Therefore, ROA generation software MUST use the prefix length as the
max length if the user does not specify a max length.
Operators should be conservative in use of max length in ROAs. For
example, if a prefix will have only a few sub-prefixes announced,
multiple ROAs for the specific announcements should be used as
opposed to one ROA with a long max length.
Operators owning prefix P should issue ROAs for all ASes that may
announce P. If a prefix is legitimately announced by more than one
AS, ROAs for all of the ASes SHOULD be issued so that all are
considered Valid.
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In an environment where private address space is announced in
External BGP (eBGP), the operator may have private RPKI objects that
cover these private spaces. This will require a trust anchor created
and owned by that environment; see [LTA-USE].
Operators issuing ROAs may have customers that announce their own
prefixes and ASes into global eBGP, but who do not wish to go though
the work to manage the relevant certificates and ROAs. Operators
SHOULD offer to provision the RPKI data for these customers just as
they provision many other things for them.
An operator using RPKI data MAY choose any polling frequency they
wish for ensuring they have a fresh RPKI cache. However, if they use
RPKI data as an input to operational routing decisions, they SHOULD
ensure local caches inside their AS are synchronized with each other
at least every four to six hours.
Operators should use tools that warn them of any impending ROA or
certificate expiry that could affect the validity of their own data.
Ghostbusters Records [RFC6493] can be used to facilitate contact with
upstream Certification Authorities (CAs) to effect repair.
4. Within a Network
Origin validation need only be done by edge routers in a network,
those which border other networks or ASes.
A validating router will use the result of origin validation to
influence local policy within its network; see Section 5. In
deployment, this policy should fit into the AS's existing policy,
preferences, etc. This allows a network to incrementally deploy
validation-capable border routers.
The operator should be aware that RPKI-based origin validation, as
any other policy change, can cause traffic shifts in their network.
And, as with normal policy shift practice, a prudent operator has
tools and methods to predict, measure, modify, etc.
5. Routing Policy
Origin validation based on the RPKI marks a received announcement as
having an origin that is Valid, NotFound, or Invalid; see [RFC6811].
How this is used in routing should be specified by the operator's
local policy.
Local policy using relative preference is suggested to manage the
uncertainty associated with a system in early deployment; local
policy can be applied to eliminate the threat of unreachability of
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prefixes due to ill-advised certification policies and/or incorrect
certification data. For example, until the community feels
comfortable relying on RPKI data, routing on Invalid origin validity,
though at a low preference, MAY occur.
Operators should be aware that accepting Invalid announcements, no
matter how de-preferenced, will often be the equivalent of treating
them as fully Valid. Consider having a ROA for AS 42 for prefix
10.0.0.0/16-24. A BGP announcement for 10.0.666.0/24 from AS 666
would be Invalid. But if policy is not configured to discard it,
then longest-match forwarding will send packets toward AS 666, no
matter the value of local preference.
As origin validation will be rolled out incrementally, coverage will
be incomplete for a long time. Therefore, routing on NotFound
validity state SHOULD be done for a long time. As the transition
moves forward, the number of BGP announcements with validation state
NotFound should decrease. Hence, an operator's policy should not be
overly strict and should prefer Valid announcements; it should attach
a lower preference to, but still use, NotFound announcements, and
drop or give a very low preference to Invalid announcements. Merely
de-preferencing Invalid announcements is ill-advised; see previous
paragraph.
Some providers may choose to set Local-Preference based on the RPKI
validation result. Other providers may not want the RPKI validation
result to be more important than AS_PATH length -- these providers
would need to map the RPKI validation result to some BGP attribute
that is evaluated in BGP's path selection process after the AS_PATH
is evaluated. Routers implementing RPKI-based origin validation MUST
provide such options to operators.
Local-Preference may be used to carry both the validity state of a
prefix along with its traffic engineering (TE) characteristic(s). It
is likely that an operator already using Local-Preference will have
to change policy so they can encode these two separate
characteristics in the same BGP attribute without negative impact or
opening privilege escalation attacks. For example, do not encode
validation state in higher bits than used for TE.
When using a metric that is also influenced by other local policy, an
operator should be careful not to create privilege-upgrade
vulnerabilities. For example, if Local Pref is set depending on
validity state, peer community signaling SHOULD NOT upgrade an
Invalid announcement to Valid or better.
Announcements with Valid origins should be preferred over those with
NotFound or Invalid origins, if Invalid origins are accepted at all.
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Announcements with NotFound origins should be preferred over those
with Invalid origins.
Announcements with Invalid origins SHOULD NOT be used, but may be
used to meet special operational needs. In such circumstances, the
announcement should have a lower preference than that given to Valid
or NotFound.
When first deploying origin validation, it may be prudent not to drop
announcements with Invalid origins until inspection of logs, SNMP, or
other data indicates that the correct result would be obtained.
Validity state signaling SHOULD NOT be accepted from a neighbor AS.
The validity state of a received announcement has only local scope
due to issues such as scope of trust, RPKI synchrony, and management
of local trust anchors [LTA-USE].
6. Notes and Recommendations
Like the DNS, the global RPKI presents only a loosely consistent
view, depending on timing, updating, fetching, etc. Thus, one cache
or router may have different data about a particular prefix than
another cache or router. There is no 'fix' for this, it is the
nature of distributed data with distributed caches.
Operators should beware that RPKI caches are loosely synchronized,
even within a single AS. Thus, changes to the validity state of
prefixes could be different within an operator's network. In
addition, there is no guaranteed interval from when an RPKI cache is
updated to when that new information may be pushed or pulled into a
set of routers via this protocol. This may result in sudden shifts
of traffic in the operator's network, until all of the routers in the
AS have reached equilibrium with the validity state of prefixes
reflected in all of the RPKI caches.
It is hoped that testing and deployment will produce advice on cache
loading and timing for relying parties.
There is some uncertainty about the origin AS of aggregates and what,
if any, ROA can be used. The long-range solution to this is the
deprecation of AS_SETs; see [RFC6472].
As reliable access to the global RPKI and an operator's caches (and
possibly other hosts, e.g., DNS root servers) is important, an
operator should take advantage of relying-party tools that report
changes in BGP or RPKI data that would negatively affect validation
of such prefixes.
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Operators should be aware that there is a trade-off in placement of
an RPKI repository in address space for which the repository's
content is authoritative. On one hand, an operator will wish to
maximize control over the repository. On the other hand, if there
are reachability problems to the address space, changes in the
repository to correct them may not be easily accessed by others.
Operators who manage certificates should associate RPKI Ghostbusters
Records (see [RFC6493]) with each publication point they control.
These are publication points holding the CRL, ROAs, and other signed
objects issued by the operator, and made available to other ASes in
support of routing on the public Internet.
Routers that perform RPKI-based origin validation must support Four-
octet AS Numbers (see [RFC6793]), as, among other things, it is not
reasonable to generate ROAs for AS 23456.
Software that produces filter lists or other control forms for
routers where the target router does not support Four-octet AS
Numbers (see [RFC6793]) must be prepared to accept four-octet AS
Numbers and generate the appropriate two-octet output.
As a router must evaluate certificates and ROAs that are time
dependent, routers' clocks MUST be correct to a tolerance of
approximately an hour.
Servers should provide time service, such as NTPv4 [RFC5905], to
client routers.
7. Security Considerations
As the BGP origin AS of an update is not signed, origin validation is
open to malicious spoofing. Therefore, RPKI-based origin validation
is expected to deal only with inadvertent mis-advertisement.
Origin validation does not address the problem of AS_PATH validation.
Therefore, paths are open to manipulation, either malicious or
accidental.
As BGP does not ensure that traffic will flow via the paths it
advertises, the data plane may not follow the control plane.
Be aware of the class of privilege escalation issues discussed in
Section 5 above.
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8. Acknowledgments
The author wishes to thank Shane Amante, Rob Austein, Steve Bellovin,
Jay Borkenhagen, Wes George, Seiichi Kawamura, Steve Kent, Pradosh
Mohapatra, Chris Morrow, Sandy Murphy, Eric Osterweil, Keyur Patel,
Heather and Jason Schiller, John Scudder, Kotikalapudi Sriram,
Maureen Stillman, and Dave Ward.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC6481] Huston, G., Loomans, R., and G. Michaelson, "A Profile for
Resource Certificate Repository Structure", RFC 6481,
February 2012.
[RFC6482] Lepinski, M., Kent, S., and D. Kong, "A Profile for Route
Origin Authorizations (ROAs)", RFC 6482, February 2012.
[RFC6490] Huston, G., Weiler, S., Michaelson, G., and S. Kent,
"Resource Public Key Infrastructure (RPKI) Trust Anchor
Locator", RFC 6490, February 2012.
[RFC6493] Bush, R., "The Resource Public Key Infrastructure (RPKI)
Ghostbusters Record", RFC 6493, February 2012.
[RFC6793] Vohra, Q. and E. Chen, "BGP Support for Four-Octet
Autonomous System (AS) Number Space", RFC 6793, December
2012.
[RFC6810] Bush, R. and R. Austein, "The Resource Public Key
Infrastructure (RPKI) to Router Protocol", RFC 6810,
January 2013.
[RFC6811] Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R.
Austein, "BGP Prefix Origin Validation", RFC 6811, January
2013.
9.2. Informative References
[LTA-USE] Bush, R., "RPKI Local Trust Anchor Use Cases", Work in
Progress, September 2013.
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
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[RFC5781] Weiler, S., Ward, D., and R. Housley, "The rsync URI
Scheme", RFC 5781, February 2010.
[RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010.
[RFC6472] Kumari, W. and K. Sriram, "Recommendation for Not Using
AS_SET and AS_CONFED_SET in BGP", BCP 172, RFC 6472,
December 2011.
[RFC6480] Lepinski, M. and S. Kent, "An Infrastructure to Support
Secure Internet Routing", RFC 6480, February 2012.
[RFC6487] Huston, G., Michaelson, G., and R. Loomans, "A Profile for
X.509 PKIX Resource Certificates", RFC 6487, February
2012.
[RFC6488] Lepinski, M., Chi, A., and S. Kent, "Signed Object
Template for the Resource Public Key Infrastructure
(RPKI)", RFC 6488, February 2012.
[IAB] IAB, "IAB statement on the RPKI", January 2010,
<http://www.iab.org/documents/
correspondence-reports-documents/docs2010/
iab-statement-on-the-rpki/>.
[rcynic] "rcynic RPKI validator", November 2013,
<http://rpki.net/rcynic>.
Author's Address
Randy Bush
Internet Initiative Japan
5147 Crystal Springs
Bainbridge Island, Washington 98110
US
EMail: randy@psg.com
Bush Best Current Practice [Page 11]
=========================================================================
Internet Engineering Task Force (IETF) Y. Gilad
Request for Comments: 9319 Hebrew University of Jerusalem
BCP: 185 S. Goldberg
Category: Best Current Practice Boston University
ISSN: 2070-1721 K. Sriram
USA NIST
J. Snijders
Fastly
B. Maddison
Workonline Communications
October 2022
The Use of maxLength in the Resource Public Key Infrastructure (RPKI)
Abstract
This document recommends ways to reduce the forged-origin hijack
attack surface by prudently limiting the set of IP prefixes that are
included in a Route Origin Authorization (ROA). One recommendation
is to avoid using the maxLength attribute in ROAs except in some
specific cases. The recommendations complement and extend those in
RFC 7115. This document also discusses the creation of ROAs for
facilitating the use of Distributed Denial of Service (DDoS)
mitigation services. Considerations related to ROAs and RPKI-based
Route Origin Validation (RPKI-ROV) in the context of destination-
based Remotely Triggered Discard Route (RTDR) (elsewhere referred to
as "Remotely Triggered Black Hole") filtering are also highlighted.
Status of This Memo
This memo documents an Internet Best Current Practice.
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
BCPs is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9319.
Copyright Notice
Copyright (c) 2022 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
(https://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 Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
1.1. Requirements
1.2. Documentation Prefixes
2. Suggested Reading
3. Forged-Origin Sub-Prefix Hijack
4. Measurements of the RPKI
5. Recommendations about Minimal ROAs and maxLength
5.1. Facilitating Ad Hoc Routing Changes and DDoS Mitigation
5.2. Defensive De-aggregation in Response to Prefix Hijacks
6. Considerations for RTDR Filtering Scenarios
7. User Interface Design Recommendations
8. Operational Considerations
9. Security Considerations
10. IANA Considerations
11. References
11.1. Normative References
11.2. Informative References
Acknowledgments
Authors' Addresses
1. Introduction
The Resource Public Key Infrastructure (RPKI) [RFC6480] uses Route
Origin Authorizations (ROAs) to create a cryptographically verifiable
mapping from an IP prefix to a set of Autonomous Systems (ASes) that
are authorized to originate that prefix. Each ROA contains a set of
IP prefixes and the AS number of one of the ASes authorized to
originate all the IP prefixes in the set [RFC6482]. The ROA is
cryptographically signed by the party that holds a certificate for
the set of IP prefixes.
The ROA format also supports a maxLength attribute. According to
[RFC6482], "When present, the maxLength specifies the maximum length
of the IP address prefix that the AS is authorized to advertise."
Thus, rather than requiring the ROA to list each prefix that the AS
is authorized to originate, the maxLength attribute provides a
shorthand that authorizes an AS to originate a set of IP prefixes.
However, measurements of RPKI deployments have found that the use of
the maxLength attribute in ROAs tends to lead to security problems.
In particular, measurements taken in June 2017 showed that of the
prefixes specified in ROAs that use the maxLength attribute, 84% were
vulnerable to a forged-origin sub-prefix hijack [GSG17]. The forged-
origin prefix or sub-prefix hijack involves inserting the legitimate
AS as specified in the ROA as the origin AS in the AS_PATH; the
hijack can be launched against any IP prefix/sub-prefix that has a
ROA. Consider a prefix/sub-prefix that has a ROA that is unused
(i.e., not announced in BGP by a legitimate AS). A forged-origin
hijack involving such a prefix/sub-prefix can propagate widely
throughout the Internet. On the other hand, if the prefix/sub-prefix
were announced by the legitimate AS, then the propagation of the
forged-origin hijack is somewhat limited because of its increased
AS_PATH length relative to the legitimate announcement. Of course,
forged-origin hijacks are harmful in both cases, but the extent of
harm is greater for unannounced prefixes. See Section 3 for detailed
discussion.
For this reason, this document recommends that, whenever possible,
operators SHOULD use "minimal ROAs" that authorize only those IP
prefixes that are actually originated in BGP, and no other prefixes.
Further, it recommends ways to reduce the forged-origin attack
surface by prudently limiting the address space that is included in
ROAs. One recommendation is to avoid using the maxLength attribute
in ROAs except in some specific cases. The recommendations
complement and extend those in [RFC7115]. The document also
discusses the creation of ROAs for facilitating the use of DDoS
mitigation services. Considerations related to ROAs and RPKI-ROV in
the context of destination-based Remotely Triggered Discard Route
(RTDR) (elsewhere referred to as "Remotely Triggered Black Hole")
filtering are also highlighted.
Please note that the term "RPKI-based Route Origin Validation" and
the corresponding acronym "RPKI-ROV" that are used in this document
mean the same as the term "Prefix Origin Validation" used in
[RFC6811].
One ideal place to implement the ROA-related recommendations is in
the user interfaces for configuring ROAs. Recommendations for
implementors of such user interfaces are provided in Section 7.
The practices described in this document require no changes to the
RPKI specifications and will not increase the number of signed ROAs
in the RPKI because ROAs already support lists of IP prefixes
[RFC6482].
1.1. Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
1.2. Documentation Prefixes
The documentation prefixes recommended in [RFC5737] are insufficient
for use as example prefixes in this document. Therefore, this
document uses the address space defined in [RFC1918] for constructing
example prefixes.
Note that although the examples in this document are presented using
IPv4 prefixes, all the analysis thereof and the recommendations made
are equally valid for the equivalent IPv6 cases.
2. Suggested Reading
It is assumed that the reader understands BGP [RFC4271], RPKI
[RFC6480], ROAs [RFC6482], RPKI-ROV [RFC6811], and BGPsec [RFC8205].
3. Forged-Origin Sub-Prefix Hijack
A detailed description and discussion of forged-origin sub-prefix
hijacks are presented here, especially considering the case when the
sub-prefix is not announced in BGP. The forged-origin sub-prefix
hijack is relevant to a scenario in which:
(1) the RPKI [RFC6480] is deployed, and
(2) routers use RPKI-ROV to drop invalid routes [RFC6811], but
(3) BGPsec [RFC8205] (or any similar method to validate the
truthfulness of the BGP AS_PATH attribute) is not deployed.
Note that this set of assumptions accurately describes a substantial
and growing number of large Internet networks at the time of writing.
The forged-origin sub-prefix hijack [RFC7115] [GCHSS] is described
here using a running example.
Consider the IP prefix 192.168.0.0/16, which is allocated to an
organization that also operates AS 64496. In BGP, AS 64496
originates the IP prefix 192.168.0.0/16 as well as its sub-prefix
192.168.225.0/24. Therefore, the RPKI should contain a ROA
authorizing AS 64496 to originate these two IP prefixes.
Suppose, however, the organization issues and publishes a ROA
including a maxLength value of 24:
ROA:(192.168.0.0/16-24, AS 64496)
We refer to the above as a "loose ROA" since it authorizes AS 64496
to originate any sub-prefix of 192.168.0.0/16 up to and including
length /24, rather than only those prefixes that are intended to be
announced in BGP.
Because AS 64496 only originates two prefixes in BGP (192.168.0.0/16
and 192.168.225.0/24), all other prefixes authorized by the loose ROA
(for instance, 192.168.0.0/24) are vulnerable to the following
forged-origin sub-prefix hijack [RFC7115] [GCHSS]:
The hijacker AS 64511 sends a BGP announcement "192.168.0.0/24: AS
64511, AS 64496", falsely claiming that AS 64511 is a neighbor of
AS 64496 and that AS 64496 originates the IP prefix
192.168.0.0/24. In fact, the IP prefix 192.168.0.0/24 is not
originated by AS 64496.
The hijacker's BGP announcement is valid according to the RPKI
since the ROA (192.168.0.0/16-24, AS 64496) authorizes AS 64496 to
originate BGP routes for 192.168.0.0/24.
Because AS 64496 does not actually originate a route for
192.168.0.0/24, the hijacker's route is the only route for
192.168.0.0/24. Longest-prefix-match routing ensures that the
hijacker's route to the sub-prefix 192.168.0.0/24 is always
preferred over the legitimate route to 192.168.0.0/16 originated
by AS 64496.
Thus, the hijacker's route propagates through the Internet, and
traffic destined for IP addresses in 192.168.0.0/24 will be delivered
to the hijacker.
The forged-origin sub-prefix hijack would have failed if a minimal
ROA as described in Section 5 was used instead of the loose ROA. In
this example, a minimal ROA would be:
ROA:(192.168.0.0/16, 192.168.225.0/24, AS 64496)
This ROA is "minimal" because it includes only those IP prefixes that
AS 64496 originates in BGP, but no other IP prefixes [RFC6907].
The minimal ROA renders AS 64511's BGP announcement invalid because:
(1) this ROA "covers" the attacker's announcement (since
192.168.0.0/24 is a sub-prefix of 192.168.0.0/16), and
(2) there is no ROA "matching" the attacker's announcement (there is
no ROA for AS 64511 and IP prefix 192.168.0.0/24) [RFC6811].
If routers ignore invalid BGP announcements, the minimal ROA above
ensures that the sub-prefix hijack will fail.
Thus, if a minimal ROA had been used, the attacker would be forced to
launch a forged-origin prefix hijack in order to attract traffic as
follows:
The hijacker AS 64511 sends a BGP announcement "192.168.0.0/16: AS
64511, AS 64496", falsely claiming that AS 64511 is a neighbor of
AS 64496.
This forged-origin prefix hijack is significantly less damaging than
the forged-origin sub-prefix hijack:
AS 64496 legitimately originates 192.168.0.0/16 in BGP, so the
hijacker AS 64511 is not presenting the only route to
192.168.0.0/16.
Moreover, the path originated by AS 64511 is one hop longer than
the path originated by the legitimate origin AS 64496.
As discussed in [LSG16], this means that the hijacker will attract
less traffic than it would have in the forged-origin sub-prefix
hijack where the hijacker presents the only route to the hijacked
sub-prefix.
In summary, a forged-origin sub-prefix hijack has the same impact as
a regular sub-prefix hijack, despite the increased AS_PATH length of
the illegitimate route. A forged-origin sub-prefix hijack is also
more damaging than the forged-origin prefix hijack.
4. Measurements of the RPKI
Network measurements taken in June 2017 showed that 12% of the IP
prefixes authorized in ROAs have a maxLength value longer than their
prefix length. Of these, the vast majority (84%) were non-minimal,
as they included sub-prefixes that are not announced in BGP by the
legitimate AS and were thus vulnerable to forged-origin sub-prefix
hijacks. See [GSG17] for details.
These measurements suggest that operators commonly misconfigure the
maxLength attribute and unwittingly open themselves up to forged-
origin sub-prefix hijacks. That is, they are exposing a much larger
attack surface for forged-origin hijacks than necessary.
5. Recommendations about Minimal ROAs and maxLength
Operators SHOULD use minimal ROAs whenever possible. A minimal ROA
contains only those IP prefixes that are actually originated by an AS
in BGP and no other IP prefixes. See Section 3 for an example.
In general, operators SHOULD avoid using the maxLength attribute in
their ROAs, since its inclusion will usually make the ROA non-
minimal.
One such exception may be when all more specific prefixes permitted
by the maxLength value are actually announced by the AS in the ROA.
Another exception is where: (a) the maxLength value is substantially
larger compared to the specified prefix length in the ROA, and (b) a
large number of more specific prefixes in that range are announced by
the AS in the ROA. In practice, this case should occur rarely (if at
all). Operator discretion is necessary in this case.
This practice requires no changes to the RPKI specifications and need
not increase the number of signed ROAs in the RPKI because ROAs
already support lists of IP prefixes [RFC6482]. See [GSG17] for
further discussion of why this practice will have minimal impact on
the performance of the RPKI ecosystem.
Operators that implement these recommendations and have existing ROAs
published in the RPKI system MUST perform a review of such objects,
especially where they make use of the maxLength attribute, to ensure
that the set of included prefixes is "minimal" with respect to the
current BGP origination and routing policies. Published ROAs MUST be
replaced as necessary. Such an exercise MUST be repeated whenever
the operator makes changes to either policy.
5.1. Facilitating Ad Hoc Routing Changes and DDoS Mitigation
Operational requirements may require that a route for an IP prefix be
originated on an ad hoc basis, with little or no prior warning. An
example of such a situation arises when an operator wishes to make
use of DDoS mitigation services that use BGP to redirect traffic via
a "scrubbing center".
In order to ensure that such ad hoc routing changes are effective, a
ROA validating the new route should exist. However, a difficulty
arises due to the fact that newly created objects in the RPKI are
made visible to relying parties considerably more slowly than routing
updates in BGP.
Ideally, it would not be necessary to pre-create the ROA, which
validates the ad hoc route, and instead create it "on the fly" as
required. However, this is practical only if the latency imposed by
the propagation of RPKI data is guaranteed to be within acceptable
limits in the circumstances. For time-critical interventions such as
responding to a DDoS attack, this is unlikely to be the case.
Thus, the ROA in question will usually need to be created well in
advance of the routing intervention, but such a ROA will be non-
minimal, since it includes an IP prefix that is sometimes (but not
always) originated in BGP.
In this case, the ROA SHOULD only include:
(1) the set of IP prefixes that are always originated in BGP, and
(2) the set of IP prefixes that are sometimes, but not always,
originated in BGP.
The ROA SHOULD NOT include any IP prefixes that the operator knows
will not be originated in BGP. In general, the ROA SHOULD NOT make
use of the maxLength attribute unless doing so has no impact on the
set of included prefixes.
The running example is now extended to illustrate one situation where
it is not possible to issue a minimal ROA.
Consider the following scenario prior to the deployment of RPKI.
Suppose AS 64496 announced 192.168.0.0/16 and has a contract with a
DDoS mitigation service provider that holds AS 64500. Further,
assume that the DDoS mitigation service contract applies to all IP
addresses covered by 192.168.0.0/22. When a DDoS attack is detected
and reported by AS 64496, AS 64500 immediately originates
192.168.0.0/22, thus attracting all the DDoS traffic to itself. The
traffic is scrubbed at AS 64500 and then sent back to AS 64496 over a
backhaul link. Notice that, during a DDoS attack, the DDoS
mitigation service provider AS 64500 originates a /22 prefix that is
longer than AS 64496's /16 prefix, so all the traffic (destined to
addresses in 192.168.0.0/22) that normally goes to AS 64496 goes to
AS 64500 instead. In some deployments, the origination of the /22
route is performed by AS 64496 and announced only to AS 64500, which
then announces transit for that prefix. This variation does not
change the properties considered here.
First, suppose the RPKI only had the minimal ROA for AS 64496, as
described in Section 3. However, if there is no ROA authorizing AS
64500 to announce the /22 prefix, then the DDoS mitigation (and
traffic scrubbing) scheme would not work. That is, if AS 64500
originates the /22 prefix in BGP during DDoS attacks, the
announcement would be invalid [RFC6811].
Therefore, the RPKI should have two ROAs: one for AS 64496 and one
for AS 64500.
ROA:(192.168.0.0/16, 192.168.225.0/24, AS 64496)
ROA:(192.168.0.0/22, AS 64500)
Neither ROA uses the maxLength attribute, but the second ROA is not
"minimal" because it contains a /22 prefix that is not originated by
anyone in BGP during normal operations. The /22 prefix is only
originated by AS 64500 as part of its DDoS mitigation service during
a DDoS attack.
Notice, however, that this scheme does not come without risks.
Namely, all IP addresses in 192.168.0.0/22 are vulnerable to a
forged-origin sub-prefix hijack during normal operations when the /22
prefix is not originated. (The hijacker AS 64511 would send the BGP
announcement "192.168.0.0/22: AS 64511, AS 64500", falsely claiming
that AS 64511 is a neighbor of AS 64500 and falsely claiming that AS
64500 originates 192.168.0.0/22.)
In some situations, the DDoS mitigation service at AS 64500 might
want to limit the amount of DDoS traffic that it attracts and scrubs.
Suppose that a DDoS attack only targets IP addresses in
192.168.0.0/24. Then, the DDoS mitigation service at AS 64500 only
wants to attract the traffic designated for the /24 prefix that is
under attack, but not the entire /22 prefix. To allow for this, the
RPKI should have two ROAs: one for AS 64496 and one for AS 64500.
ROA:(192.168.0.0/16, 192.168.225.0/24, AS 64496)
ROA:(192.168.0.0/22-24, AS 64500)
The second ROA uses the maxLength attribute because it is designed to
explicitly enable AS 64500 to originate any /24 sub-prefix of
192.168.0.0/22.
As before, the second ROA is not "minimal" because it contains
prefixes that are not originated by anyone in BGP during normal
operations. Also, all IP addresses in 192.168.0.0/22 are vulnerable
to a forged-origin sub-prefix hijack during normal operations when
the /22 prefix is not originated.
The use of the maxLength attribute in this second ROA also comes with
additional risk. While it permits the DDoS mitigation service at AS
64500 to originate prefix 192.168.0.0/24 during a DDoS attack in that
space, it also makes the other /24 prefixes covered by the /22 prefix
(i.e., 192.168.1.0/24, 192.168.2.0/24, and 192.168.3.0/24) vulnerable
to forged-origin sub-prefix attacks.
5.2. Defensive De-aggregation in Response to Prefix Hijacks
When responding to certain classes of prefix hijack (in particular,
the forged-origin sub-prefix hijack described above), it may be
desirable for the victim to perform "defensive de-aggregation", i.e.,
to begin originating more-specific prefixes in order to compete with
the hijack routes for selection as the best path in networks that are
not performing RPKI-ROV [RFC6811].
In topologies where at least one AS on every path between the victim
and hijacker filters RPKI-ROV invalid prefixes, it may be the case
that the existence of a minimal ROA issued by the victim prevents the
defensive more-specific prefixes from being propagated to the
networks topologically close to the attacker, thus hampering the
effectiveness of this response.
Nevertheless, this document recommends that, where possible, network
operators publish minimal ROAs even in the face of this risk. This
is because:
* Minimal ROAs offer the best possible protection against the
immediate impact of such an attack, rendering the need for such a
response less likely;
* Increasing RPKI-ROV adoption by network operators will, over time,
decrease the size of the neighborhoods in which this risk exists;
and
* Other methods for reducing the size of such neighborhoods are
available to potential victims, such as establishing direct
External BGP (EBGP) adjacencies with networks from whom the
defensive routes would otherwise be hidden.
6. Considerations for RTDR Filtering Scenarios
Considerations related to ROAs and RPKI-ROV [RFC6811] for the case of
destination-based RTDR (elsewhere referred to as "Remotely Triggered
Black Hole") filtering are addressed here. In RTDR filtering, highly
specific prefixes (greater than /24 in IPv4 and greater than /48 in
IPv6, or possibly even /32 in IPv4 and /128 in IPv6) are announced in
BGP. These announcements are tagged with the well-known BGP
community defined by [RFC7999]. For the reasons set out above, it is
obviously not desirable to use a large maxLength value or include any
such highly specific prefixes in the ROAs to accommodate destination-
based RTDR filtering.
As a result, RPKI-ROV [RFC6811] is a poor fit for the validation of
RTDR routes. Specification of new procedures to address this use
case through the use of the RPKI is outside the scope of this
document.
Therefore:
* Operators SHOULD NOT create non-minimal ROAs (by either creating
additional ROAs or using the maxLength attribute) for the purpose
of advertising RTDR routes; and
* Operators providing a means for operators of neighboring
autonomous systems to advertise RTDR routes via BGP MUST NOT make
the creation of non-minimal ROAs a pre-requisite for its use.
7. User Interface Design Recommendations
Most operator interaction with the RPKI system when creating or
modifying ROAs will occur via a user interface that abstracts the
underlying encoding, signing, and publishing operations.
This document recommends that designers and/or providers of such user
interfaces SHOULD provide warnings to draw the user's attention to
the risks of creating non-minimal ROAs in general and using the
maxLength attribute in particular.
Warnings provided by such a system may vary in nature from generic
warnings based purely on the inclusion of the maxLength attribute to
customised guidance based on the observable BGP routing policy of the
operator in question. The choices made in this respect are expected
to be dependent on the target user audience of the implementation.
8. Operational Considerations
The recommendations specified in this document (in particular, those
in Section 5) involve trade-offs between operational agility and
security.
Operators adopting the recommended practice of issuing minimal ROAs
will, by definition, need to make changes to their existing set of
issued ROAs in order to effect changes to the set of prefixes that
are originated in BGP.
Even in the case of routing changes that are planned in advance,
existing procedures may need to be updated to incorporate changes to
issued ROAs and may require additional time allowed for those changes
to propagate.
Operators are encouraged to carefully review the issues highlighted
(especially those in Sections 5.1 and 5.2) in light of their specific
operational requirements. Failure to do so could, in the worst case,
result in a self-inflicted denial of service.
The recommendations made in Section 5 are likely to be more onerous
for operators utilising large IP address space allocations from which
many more-specific advertisements are made in BGP. Operators of such
networks are encouraged to seek opportunities to automate the
required procedures in order to minimise manual operational burden.
9. Security Considerations
This document makes recommendations regarding the use of RPKI-ROV as
defined in [RFC6811] and, as such, introduces no additional security
considerations beyond those specified therein.
10. IANA Considerations
This document has no IANA actions.
11. References
11.1. Normative References
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
J., and E. Lear, "Address Allocation for Private
Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,
February 1996, <https://www.rfc-editor.org/info/rfc1918>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
[RFC6480] Lepinski, M. and S. Kent, "An Infrastructure to Support
Secure Internet Routing", RFC 6480, DOI 10.17487/RFC6480,
February 2012, <https://www.rfc-editor.org/info/rfc6480>.
[RFC6482] Lepinski, M., Kent, S., and D. Kong, "A Profile for Route
Origin Authorizations (ROAs)", RFC 6482,
DOI 10.17487/RFC6482, February 2012,
<https://www.rfc-editor.org/info/rfc6482>.
[RFC6811] Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R.
Austein, "BGP Prefix Origin Validation", RFC 6811,
DOI 10.17487/RFC6811, January 2013,
<https://www.rfc-editor.org/info/rfc6811>.
[RFC7115] Bush, R., "Origin Validation Operation Based on the
Resource Public Key Infrastructure (RPKI)", BCP 185,
RFC 7115, DOI 10.17487/RFC7115, January 2014,
<https://www.rfc-editor.org/info/rfc7115>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
11.2. Informative References
[GCHSS] Gilad, Y., Cohen, A., Herzberg, A., Schapira, M., and H.
Shulman, "Are We There Yet? On RPKI's Deployment and
Security", NDSS 2017, February 2017,
<https://eprint.iacr.org/2016/1010.pdf>.
[GSG17] Gilad, Y., Sagga, O., and S. Goldberg, "MaxLength
Considered Harmful to the RPKI", CoNEXT '17,
DOI 10.1145/3143361.3143363, December 2017,
<https://eprint.iacr.org/2016/1015.pdf>.
[LSG16] Lychev, R., Shapira, M., and S. Goldberg, "Rethinking
security for internet routing", Communications of the ACM,
DOI 10.1145/2896817, October 2016, <http://cacm.acm.org/
magazines/2016/10/207763-rethinking-security-for-internet-
routing/>.
[RFC5737] Arkko, J., Cotton, M., and L. Vegoda, "IPv4 Address Blocks
Reserved for Documentation", RFC 5737,
DOI 10.17487/RFC5737, January 2010,
<https://www.rfc-editor.org/info/rfc5737>.
[RFC6907] Manderson, T., Sriram, K., and R. White, "Use Cases and
Interpretations of Resource Public Key Infrastructure
(RPKI) Objects for Issuers and Relying Parties", RFC 6907,
DOI 10.17487/RFC6907, March 2013,
<https://www.rfc-editor.org/info/rfc6907>.
[RFC7999] King, T., Dietzel, C., Snijders, J., Doering, G., and G.
Hankins, "BLACKHOLE Community", RFC 7999,
DOI 10.17487/RFC7999, October 2016,
<https://www.rfc-editor.org/info/rfc7999>.
[RFC8205] Lepinski, M., Ed. and K. Sriram, Ed., "BGPsec Protocol
Specification", RFC 8205, DOI 10.17487/RFC8205, September
2017, <https://www.rfc-editor.org/info/rfc8205>.
Acknowledgments
The authors would like to thank the following people for their review
and contributions to this document: Omar Sagga and Aris Lambrianidis.
Thanks are also due to Matthias Waehlisch, Ties de Kock, Amreesh
Phokeer, Éric Vyncke, Alvaro Retana, John Scudder, Roman Danyliw,
Andrew Alston, and Murray Kucherawy for comments and suggestions, to
Roni Even for the Gen-ART review, to Jean Mahoney for the ART-ART
review, to Acee Lindem for the Routing Area Directorate review, and
to Sean Turner for the Security Area Directorate review.
Authors' Addresses
Yossi Gilad
Hebrew University of Jerusalem
Rothburg Family Buildings
Edmond J. Safra Campus
Jerusalem 9190416
Israel
Email: yossigi@cs.huji.ac.il
Sharon Goldberg
Boston University
111 Cummington St, MCS135
Boston, MA 02215
United States of America
Email: goldbe@cs.bu.edu
Kotikalapudi Sriram
USA National Institute of Standards and Technology
100 Bureau Drive
Gaithersburg, MD 20899
United States of America
Email: kotikalapudi.sriram@nist.gov
Job Snijders
Fastly
Amsterdam
Netherlands
Email: job@fastly.com
Ben Maddison
Workonline Communications
114 West St
Johannesburg
2196
South Africa
Email: benm@workonline.africa