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INTERNET-DRAFT Vijay Gill
draft-gill-btsh-01.txt John Heasley
David Meyer
Category Informational
Expires: November 2003 May 2003
The BGP TTL Security Hack (BTSH)
<draft-gill-btsh-02.txt>
Status of this Document
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
The BGP TTL Security Hack (BTSH) is designed to protect the BGP
[RFC1771] infrastructure from CPU-utilization based attacks. While
BTSH is most effective in protecting directly connected BGP peers, it
can also provide a lower level of protection to multi-hop sessions.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Assumptions Underlying BTSH. . . . . . . . . . . . . . . . . . 3
2.1. Assumptions on Attack Sophistication. . . . . . . . . . . . 3
3. BTSH Procedure . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Multi-hop Scenarios . . . . . . . . . . . . . . . . . . . . 5
3.1.1. iBGP Handling. . . . . . . . . . . . . . . . . . . . . . 5
4. Intellectual Property. . . . . . . . . . . . . . . . . . . . . 5
5. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations. . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References. . . . . . . . . . . . . . . . . . . . 7
8.2. Informative References. . . . . . . . . . . . . . . . . . . 7
9. Author's Addresses . . . . . . . . . . . . . . . . . . . . . . 8
10. Full Copyright Statement. . . . . . . . . . . . . . . . . . . 8
1. Introduction
The BGP TTL Security Hack (BTSH) is designed to protect the BGP
[RFC1771] infrastructure from CPU-utilization based attacks. In
particular, while cryptographic techniques can protect the routed
infrastructure from a wide variety of attacks, many attacks based on
CPU-overload can be prevented by the simple mechanism described in
this document.
BTSH is based on the fact that the vast majority of ISP eBGP peerings
are established between routers that are adjacent [PEERING]. Thus
most eBGP peerings are either directly between connected interfaces
or at the worst case, are between loopback and loopback, with static
routes to loopbacks. Since TTL spoofing [BALDWIN2001] is considered
nearly impossible, a mechanism based on an expected TTL value can
provide a simple and reasonably robust defense from infrastructure
attacks based on forged BGP packets.
The keywords MUST, MUST NOT, MAY, OPTIONAL, REQUIRED, RECOMMENDED,
SHALL, SHALL NOT, SHOULD, SHOULD NOT are to be interpreted as defined
in RFC 2119 [RFC2119].
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2. Assumptions Underlying BTSH
BTSH is predicated upon the following assumptions:
(i). The vast majority of eBGP peerings are between adjacent
routers [PEERING].
(ii). It is common practice for many service providers to
ingress filter (deny) packets that have the provider's
loopback addresses as the source IP address.
(iii). Use of BTSH is OPTIONAL, and can be configured on a
per-peer (group) basis.
(iv). The router supports a method of classifying traffic
destined for the route processor into interesting/control
and not-control queues.
(iv). The peer routers both implement BTSH.
2.1. Assumptions on Attack Sophistication
Throughout this document, we assume that attackers have evolved in
both sophistication and access to the point that they can send
control traffic to a BGP session, and that this traffic appears to be
valid control traffic (i.e., has the source/destination of configured
peer routers).
We also assume that each router in the path between the attacker and
the victim BGP speaker decrements TTL properly (clearly, if the
either the path or the adjacent peer is compromised, then there are
worse problems we have to worry about).
Since the vast majority of our peerings are between adjacent routers,
we can set the TTL on the BGP packets to 255 (the maximum possible
for IP) and then reject any BGP packets that come in from configured
peers which do NOT have a TTL in the range 255-254. That is, the
receive TTL is expected to be within a small range of 1 or 2
(254-255). The actual value depends upon the architecture, but is it
is expected that the receiver will verify the range.
BTSH can be disabled for applications such as route-servers and other
large diameter multi-hop peerings. In the event that an the attack
comes in from a compromised multi-hop peering, that peering can be
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shut down (a method to reduce exposure to multi-hop attacks is
outlined below).
3. BTSH Procedure
BTSH SHOULD not be enabled by default. The following process
described the per-peer behavior:
(i). If BTSH is enabled, do the following:
(a). For directly connected routers,
o Set the TCP TTL for the BGP connection a value
in the range 255-254.
o For each configured eBGP peer:
Update the receive path ACL/firewall to only
allow BGP packets to pass onto the Route
Processor (RP) that have the correct
<source,destination,TTL> tuple. The TTL must
either be in the range 255-254 (directly
connected peer), or 255-(configured-range-of-hops)
for a multi-hop peer. We specify a range here
to achieve some robustness to changes in
topology. The connected check should be
disabled for such non-direct peerings.
It is assumed that a receive path ACL is an ACL
that is designed to control which packets are
allowed to go to the RP. This procedure will
only allow BGP packets from adjacent router to
pass onto the RP.
(c). If the TTL is not in the range 255-254 (or
255-(configured-range-of-hops) for multi-hop
peers), punt into low priority queue, log, or
silently discard.
(ii). If BTSH is not enabled for a particular peering, normal
RFC 1772 [RFC1772] protocol behavior is followed.
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3.1. Multi-hop Scenarios
When a multi-hop BGP session is required, we set the expected TTL
value to be 255-(configured-range-of-acceptable-of-hops). While this
approach provides a qualitatively lower degree of security for BGP
(i.e., an DoS attack could be theoretically be launched by
compromising some box in the path). However, BTSH will still catch
the vast majority of observed DDoS attacks against eBGP.
3.1.1. iBGP Handling
BTSH is not used for iBGP peer groups. Current best practice is to
protect peers (both eBGP and iBGP) with an MD5 signature [RFC2385].
Such sessions can be further protected by filtering (deny) at the
network edge for any packet that has a source address of one of the
loopbacks addresses used for iBGP peering.
4. Intellectual Property
The IETF takes no position regarding the validity or scope of any
intellectual property 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; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication 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 implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
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5. Acknowledgments
The BTSH concept originated with many different people, including
Paul Traina and Jon Stewart. Ryan McDowell also suggested a similar
idea. Steve Bellovin, Jay Borkenhagen and Randy Bush also provided
useful feedback on early versions of this document.
6. Security Considerations
BTSH is a simple procedure that protects single hop BGP sessions,
except in those cases where the directly connected peer has been
compromised. While the method is less effective for multi-hop BGP
sessions, it still closes the window on several forms of attack.
Protection of the BGP infrastructure beyond this method will likely
require cryptographic machinery such as is envisioned by Secure BGP
(S-BGP) [SBGP1,SBGP2], and/or other extensions. For example, consider
the class of attacks based on forged SYN packets directed to port
179/tcp on a large core infrastructure routers. In this case, the
routers respond with SYN/ACKs (or ICMP messages) towards the victim,
resulting in flooding of the victim's link being flooded with SYN/ACK
or ICMP traffic. Preventing such attacks will likely require that BGP
speakers send SYN/ACKs only to configured neighbors, and they never
send ICMP messages related to these events.
Finally, note that in the multi-hop case described above, we specify
a range of acceptable TTLs in order to achieve some robustness to
topology changes. This robustness to topological change comes at the
cost of the loss some robustness to different forms of attack.
7. IANA Considerations
This document creates a no new requirements on IANA namespaces
[RFC2434].
8. References
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8.1. Normative References
[RFC1771] "A Border Gateway Protocol (BGP-4)", Y. Rekhter,
T. Li, Editors, RFC 1771, March, 1995
[RFC1772] "Application of the Border Gateway Protocol in
the Internet", Y. Rekhter, P. Gross, RFC 1772,
March, 1995
[RFC2385] "Protection of BGP Sessions via the TCP MD5
Signature Option", A. Heffernan, RFC 2385,
August, 1998.
[SBGP1] "Secure Border Gateway Protocol (Secure-BGP)",
Stephen Kent and Charles Lynn and Karen Seo,
IEEE Journal on Selected Areas in Communications,
volume 18, number 4, April, 2000.
[SBGP2] "Secure Border Gateway Protocol (S-BGP) -- Real
World Performance and Deployment Issues", Stephen
Kent and Charles Lynn and Joanne Mikkelson and
Karen Seo, Proceedings of the IEEE Network and
Distributed System Security Symposium, February,
2000.
8.2. Informative References
[BALDWIN2001] http://www.sekure.net/docs/detecting_spoof.txt
[PEERING] Empirical data gathered from the Sprint and AOL
backbones, October, 2002.
[RFC2119] "Key words for use in RFCs to Indicate
Requirement Levels", S. Bradner, RFC 2119, March,
1997.
[RFC2434] Narten, T., and H. Alvestrand, "Guidelines for
Writing an IANA Considerations Section in
RFCs", RFC 2434/BCP 0026, October, 1998.
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9. Author's Addresses
Vijay Gill
AOL
Email: vijay@umbc.edu
John Heasley
Verio
Email: heas@shrubbery.net
David Meyer
Sprint
Email: dmm@1-4-5.net
10. Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS 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.
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