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 Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This document is a product of an individual. Comments are solicited and should be addressed to the author(s). Copyright Notice Copyright (C) The Internet Society (2002). 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. Gill, Heasly, Meyer [Page 1]
INTERNET-DRAFT Expires: April 2003 October 2002 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]. 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. Gill, Heasly, Meyer Section 2. [Page 2]
INTERNET-DRAFT Expires: April 2003 October 2002 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 Gill, Heasly, Meyer Section 3. [Page 3]
INTERNET-DRAFT Expires: April 2003 October 2002 <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. Gill, Heasly, Meyer Section 3.1.1. [Page 4]
INTERNET-DRAFT Expires: April 2003 October 2002 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. Gill, Heasly, Meyer Section 4. [Page 5]
INTERNET-DRAFT Expires: April 2003 October 2002 5. Non-normative References [BALDWIN2001] http://www.hacker.pl/gminick/pliki/notmine/detecting_spoof.txt [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, 1997. [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, http://www.ir.bbn.com/projects/s-bgp/draft-clynn-s- bgp-protocol-00a.txt 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 Gill, Heasly, Meyer Section 7. [Page 6]
INTERNET-DRAFT Expires: April 2003 October 2002 Vijay Gill AOL Email: vijay@umbc.edu John Heasley Verio Email: heas@shrubbery.net David Meyer Sprint Email: dmm@sprint.net 8. Full Copyright Statement Copyright (C) The Internet Society (2002). 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. Gill, Heasly, Meyer Section 8. [Page 7]