INTERNET-DRAFT                                    Vijay Gill
draft-gill-btsh-00.txt                          John Heasley
                                                 David Meyer
Category                                       Informational
Expires: April 2003                             October 2002

                    The BGP TTL Security Hack (BTSH)

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 (2002). All Rights Reserved.


   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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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.

   SHALL, SHALL NOT, SHOULD, SHOULD NOT are to be interpreted as defined
   in RFC 2119 [RFC2119].

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.

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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
   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

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                     <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.

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.

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4.  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) [SBGP], 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.

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5.  Non-normative References


   [PEERING]       Empirical data gathered from the Sprint and AOL
                   backbones, October, 2002.

   [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

   [RFC2119]       "Key words for use in RFCs to Indicate
                   Requirement Levels", S. Bradner, RFC 2119, March,

   [RFC2385]       "Protection of BGP Sessions via the TCP MD5
                   Signature Option", A. Heffernan, RFC 2385,
                   August, 1998.

   [SBGP]          "Secure BGP (S-BGP)", Charles Lynn, Joanne
                   Mikkelson, and Karen Seo, December 2001,

6.  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.

7.  Author's Address

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   Vijay Gill

   John Heasley

   David Meyer

8.  Full Copyright Statement

   Copyright (C) The Internet Society (2002). All Rights Reserved.

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   This document and the information contained herein is provided on an

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