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Keywords: Routing, BGP, Security, AS_PATH, and RPKI
Internet Engineering Task Force (IETF) S. Bellovin
Request for Comments: 7353 Columbia University
Category: Informational R. Bush
ISSN: 2070-1721 Internet Initiative Japan
D. Ward
Cisco Systems
August 2014
Security Requirements for BGP Path Validation
Abstract
This document describes requirements for a BGP security protocol
design to provide cryptographic assurance that the origin Autonomous
System (AS) has the right to announce the prefix and to provide
assurance of the AS Path of the announcement.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
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). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see 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/rfc7353.
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 . . . . . . . . . . . . . . . . . . 2
2. Recommended Reading . . . . . . . . . . . . . . . . . . . . . 2
3. General Requirements . . . . . . . . . . . . . . . . . . . . 3
4. BGP UPDATE Security Requirements . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 7
1. Introduction
Origin validation based on Resource Public Key Infrastructure (RPKI)
[RFC6811] provides a measure of resilience to accidental
mis-origination of prefixes; however, it provides neither
cryptographic assurance (announcements are not signed) nor assurance
of the AS Path of the announcement.
This document describes requirements to be placed on a BGP security
protocol, herein termed "BGPsec", intended to rectify these gaps.
The threat model assumed here is documented in [RFC4593] and
[RFC7132].
As noted in the threat model [RFC7132], this work is limited to
threats to the BGP protocol. Issues of business relationship
conformance, while quite important to operators, are not security
issues per se and are outside the scope of this document. It is
hoped that these issues will be better understood in the future.
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, without normative meaning.
2. Recommended Reading
This document assumes knowledge of the RPKI [RFC6480] and the RPKI
Repository Structure [RFC6481].
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This document assumes ongoing incremental deployment of Route Origin
Authorizations (ROAs) [RFC6482], the RPKI to the Router Protocol
[RFC6810], and RPKI-based Prefix Validation [RFC6811].
And, of course, a knowledge of BGP [RFC4271] is required.
3. General Requirements
The following are general requirements for a BGPsec protocol:
3.1 A BGPsec design MUST allow the receiver of a BGP announcement
to determine, to a strong level of certainty, that the
originating AS in the received PATH attribute possessed the
authority to announce the prefix.
3.2 A BGPsec design MUST allow the receiver of a BGP announcement
to determine, to a strong level of certainty, that the received
PATH attribute accurately represents the sequence of External
BGP (eBGP) exchanges that propagated the prefix from the origin
AS to the receiver, particularly if an AS has added or deleted
any AS number other than its own in the PATH attribute. This
includes modification to the number of AS prepends.
3.3 BGP attributes other than the AS_PATH are used only locally, or
have meaning only between immediate neighbors, may be modified
by intermediate systems and figure less prominently in the
decision process. Consequently, it is not appropriate to try
to protect such attributes in a BGPsec design.
3.4 A BGPsec design MUST be amenable to incremental deployment.
This implies that incompatible protocol capabilities MUST be
negotiated.
3.5 A BGPsec design MUST provide analysis of the operational
considerations for deployment and particularly of incremental
deployment, e.g., contiguous islands, non-contiguous islands,
universal deployment, etc.
3.6 As proofs of possession and authentication may require
cryptographic payloads and/or storage and computation, likely
increasing processing and memory requirements on routers, a
BGPsec design MAY require use of new hardware. That is,
compatibility with current hardware abilities is not a
requirement that this document imposes on a solution.
3.7 A BGPsec design need not prevent attacks on data-plane traffic.
It need not provide assurance that the data plane even follows
the control plane.
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3.8 A BGPsec design MUST resist attacks by an enemy who has access
to the inter-router link layer, per Section 3.1.1.2 of
[RFC4593]. In particular, such a design MUST provide
mechanisms for authentication of all data, including protecting
against message insertion, deletion, modification, or replay.
Mechanisms that suffice include TCP sessions authenticated with
the TCP Authentication Option (TCP-AO) [RFC5925], IPsec
[RFC4301], or Transport Layer Security (TLS) [RFC5246].
3.9 It is assumed that a BGPsec design will require information
about holdings of address space and Autonomous System Numbers
(ASNs), and assertions about binding of address space to ASNs.
A BGPsec design MAY make use of a security infrastructure
(e.g., a PKI) to distribute such authenticated data.
3.10 It is entirely OPTIONAL to secure AS SETs and prefix
aggregation. The long-range solution to this is the
deprecation of AS_SETs; see [RFC6472].
3.11 If a BGPsec design uses signed prefixes, given the difficulty
of splitting a signed message while preserving the signature,
it need not handle multiple prefixes in a single UPDATE PDU.
3.12 A BGPsec design MUST enable each BGPsec speaker to configure
use of the security mechanism on a per-peer basis.
3.13 A BGPsec design MUST provide backward compatibility in the
message formatting, transmission, and processing of routing
information carried through a mixed security environment.
Message formatting in a fully secured environment MAY be
handled in a non-backward compatible manner.
3.14 While the formal validity of a routing announcement should be
determined by the BGPsec protocol, local routing policy MUST be
the final arbiter of the best path and other routing decisions.
3.15 A BGPsec design MUST support 'transparent' route servers,
meaning that the AS of the route server is not counted in
downstream BGP AS-path-length tie-breaking decisions.
3.16 A BGPsec design MUST support AS aliasing. This technique is
not well defined or universally implemented but is being
documented in [AS-MIGRATION]. A BGPsec design SHOULD
accommodate AS 'migration' techniques such as common
proprietary and non-standard methods that allow a router to
have two AS identities, without lengthening the effective AS
Path.
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3.17 If a BGPsec design makes use of a security infrastructure, that
infrastructure SHOULD enable each network operator to select
the entities it will trust when authenticating data in the
security infrastructure. See, for example, [LTA-USE-CASES].
3.18 A BGPsec design MUST NOT require operators to reveal more than
is currently revealed in the operational inter-domain routing
environment, other than the inclusion of necessary security
credentials to allow others to ascertain for themselves the
necessary degree of assurance regarding the validity of Network
Layer Reachability Information (NLRI) received via BGPsec.
This includes peering, customer/provider relationships, an
ISP's internal infrastructure, etc. It is understood that some
data are revealed to the savvy seeker by BGP, traceroute, etc.,
today.
3.19 A BGPsec design MUST signal (e.g., via logging or SNMP)
security exceptions that are significant to the operator. The
specific data to be signaled are an implementation matter.
3.20 Any routing information database MUST be re-authenticated
periodically or in an event-driven manner, especially in
response to events such as, for example, PKI updates.
3.21 Any inter-AS use of cryptographic hashes or signatures MUST
provide mechanisms for algorithm agility. For a discussion,
see [ALG-AGILITY].
3.22 A BGPsec design SHOULD NOT presume to know the intent of the
originator of a NLRI, nor that of any AS on the AS Path, other
than that they intend to pass it to the next AS in the path.
3.23 A BGPsec listener SHOULD NOT trust non-BGPsec markings, such as
communities, across trust boundaries.
4. BGP UPDATE Security Requirements
The following requirements MUST be met in the processing of BGP
UPDATE messages:
4.1 A BGPsec design MUST enable each recipient of an UPDATE to
formally validate that the origin AS in the message is
authorized to originate a route to the prefix(es) in the
message.
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4.2 A BGPsec design MUST enable the recipient of an UPDATE to
formally determine that the NLRI has traversed the AS Path
indicated in the UPDATE. Note that this is more stringent than
showing that the path is merely not impossible.
4.3 Replay of BGP UPDATE messages need not be completely prevented,
but a BGPsec design SHOULD provide a mechanism to control the
window of exposure to replay attacks.
4.4 A BGPsec design SHOULD provide some level of assurance that the
origin of a prefix is still 'alive', i.e., that a monkey in the
middle has not withheld a WITHDRAW message or the effects
thereof.
4.5 The AS Path of an UPDATE message SHOULD be able to be
authenticated as the message is processed.
4.6 Normal sanity checks of received announcements MUST be done,
e.g., verification that the first element of the AS_PATH list
corresponds to the locally configured AS of the peer from which
the UPDATE was received.
4.7 The output of a router applying BGPsec validation to a received
UPDATE MUST be unequivocal and conform to a fully specified
state in the design.
5. Security Considerations
If an external "security infrastructure" is used, as mentioned in
Section 3, paragraphs 9 and 17 above, the authenticity and integrity
of the data of such an infrastructure MUST be assured. In addition,
the integrity of those data MUST be assured when they are used by
BGPsec, e.g., in transport.
The requirement of backward compatibility to BGP4 may open an avenue
to downgrade attacks.
The data plane might not follow the path signaled by the control
plane.
Security for subscriber traffic is outside the scope of this document
and of BGP security in general. IETF standards for payload data
security should be employed. While adoption of BGP security measures
may ameliorate some classes of attacks on traffic, these measures are
not a substitute for use of subscriber-based security.
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6. Acknowledgments
The authors wish to thank the authors of [BGP-SECURITY] from whom we
liberally stole, Roque Gagliano, Russ Housley, Geoff Huston, Steve
Kent, Sandy Murphy, Eric Osterweil, John Scudder, Kotikalapudi
Sriram, Sam Weiler, and a number of others.
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.
[RFC4593] Barbir, A., Murphy, S., and Y. Yang, "Generic Threats to
Routing Protocols", RFC 4593, October 2006.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, June 2010.
[RFC7132] Kent, S. and A. Chi, "Threat Model for BGP Path Security",
RFC 7132, February 2014.
7.2. Informative References
[ALG-AGILITY]
Housley, R., "Guidelines for Cryptographic Algorithm
Agility", Work in Progress, June 2014.
[AS-MIGRATION]
George, W. and S. Amante, "Autonomous System (AS)
Migration Features and Their Effects on the BGP AS_PATH
Attribute", Work in Progress, January 2014.
[BGP-SECURITY]
Christian, B. and T. Tauber, "BGP Security Requirements",
Work in Progress, November 2008.
[LTA-USE-CASES]
Bush, R., "RPKI Local Trust Anchor Use Cases", Work in
Progress, June 2014.
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
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[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[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.
[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.
[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.
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Authors' Addresses
Steven M. Bellovin
Columbia University
1214 Amsterdam Avenue, MC 0401
New York, New York 10027
USA
Phone: +1 212 939 7149
EMail: bellovin@acm.org
Randy Bush
Internet Initiative Japan
5147 Crystal Springs
Bainbridge Island, Washington 98110
USA
EMail: randy@psg.com
David Ward
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
USA
EMail: dward@cisco.com
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