--- 1/draft-ietf-6man-deprecate-atomfrag-generation-05.txt 2016-04-04 10:16:10.739145965 -0700 +++ 2/draft-ietf-6man-deprecate-atomfrag-generation-06.txt 2016-04-04 10:16:10.767146665 -0700 @@ -1,143 +1,124 @@ IPv6 maintenance Working Group (6man) F. Gont Internet-Draft SI6 Networks / UTN-FRH Intended status: Informational W. Liu -Expires: July 23, 2016 Huawei Technologies +Expires: October 6, 2016 Huawei Technologies T. Anderson Redpill Linpro - January 20, 2016 + April 4, 2016 Generation of IPv6 Atomic Fragments Considered Harmful - draft-ietf-6man-deprecate-atomfrag-generation-05 + draft-ietf-6man-deprecate-atomfrag-generation-06 Abstract - RFC2460 requires that when a host receives an ICMPv6 "Packet Too Big" - message reporting an MTU smaller than 1280 bytes, the host includes a - Fragment Header in all subsequent packets sent to that destination, - without reducing the assumed Path-MTU. The simplicity with which - ICMPv6 "Packet Too Big" messages can be forged means that an attacker - can leverage this functionality (the generation of IPv6 atomic - fragments) to trigger the use of fragmentation for any arbitrary IPv6 - flow, and subsequently perform any fragmentation-based attack. This - document discusses the security implications of the generation of - IPv6 atomic fragments and a number of interoperability issues + This document discusses the security implications of the generation + of IPv6 atomic fragments and a number of interoperability issues associated with IPv6 atomic fragments, and concludes that the aforementioned functionality is undesirable, thus documenting the motivation for removing this functionality in the revision of the - core IPv6 protocol specification [I-D.ietf-6man-rfc2460bis]. + core IPv6 protocol specification. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." - This Internet-Draft will expire on July 23, 2016. + This Internet-Draft will expire on October 6, 2016. Copyright Notice Copyright (c) 2016 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 - 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 - 3. Security Implications of the Generation of IPv6 Atomic + 2. Security Implications of the Generation of IPv6 Atomic Fragments . . . . . . . . . . . . . . . . . . . . . . . . . . 3 - 4. Additional Considerations . . . . . . . . . . . . . . . . . . 5 - 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 - 6. Security Considerations . . . . . . . . . . . . . . . . . . . 7 - 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7 - 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 - 8.1. Normative References . . . . . . . . . . . . . . . . . . 7 - 8.2. Informative References . . . . . . . . . . . . . . . . . 8 + 3. Additional Considerations . . . . . . . . . . . . . . . . . . 4 + 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 + 5. Security Considerations . . . . . . . . . . . . . . . . . . . 6 + 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7 + 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 + 7.1. Normative References . . . . . . . . . . . . . . . . . . 7 + 7.2. Informative References . . . . . . . . . . . . . . . . . 7 Appendix A. Small Survey of OSes that Fail to Produce IPv6 Atomic Fragments . . . . . . . . . . . . . . . . . . 9 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 1. Introduction [RFC2460] specifies the IPv6 fragmentation mechanism, which allows IPv6 packets to be fragmented into smaller pieces such that they can fit in the Path-MTU to the intended destination(s). Section 5 of [RFC2460] states that, when a host receives an ICMPv6 "Packet Too Big" message [RFC4443] advertising an MTU smaller than 1280 bytes (the minimum IPv6 MTU), the host is not required to reduce the assumed Path-MTU, but must simply include a Fragment Header in all subsequent packets sent to that destination. The resulting packets will thus *not* be actually fragmented into several pieces, - but rather just include a Fragment Header with both the "Fragment - Offset" and the "M" flag set to 0 (i.e., "atomic fragments" - [RFC6946]). [RFC6946] requires that these atomic fragments be - essentially processed by the destination host as non-fragmented - traffic (since there are not really any fragments to be reassembled). - The goal of these atomic fragments is simply to convey an appropriate + but rather be "atomic fragments" [RFC6946] (i.e., just include a + Fragment Header with both the "Fragment Offset" and the "M" flag set + to 0). [RFC6946] requires that these atomic fragments be essentially + processed by the destination host as non-fragmented traffic (since + there are not really any fragments to be reassembled). The goal of + these atomic fragments is simply to convey an appropriate Identification value to be employed by IPv6/IPv4 translators for the resulting IPv4 fragments. While atomic fragments might seem rather benign, there are scenarios in which the generation of IPv6 atomic fragments can be leveraged for performing a number of attacks against the corresponding IPv6 flows. Since there are concrete security implications arising from the generation of IPv6 atomic fragments, and there is no real gain in generating IPv6 atomic fragments (as opposed to e.g. having IPv6/IPv4 translators generate a Fragment Identification value themselves), we conclude that this functionality is undesirable. - Section 3 briefly discusses the security implications of the + Section 2 briefly discusses the security implications of the generation of IPv6 atomic fragments, and describes a specific Denial of Service (DoS) attack vector that leverages the widespread - filtering of IPv6 fragments in the public Internet. Section 4 + filtering of IPv6 fragments in the public Internet. Section 3 provides additional considerations regarding the usefulness of generating IPv6 atomic fragments. -2. Terminology - - IPv6 atomic fragments: - IPv6 packets that contain a Fragment Header with the Fragment - Offset set to 0 and the M flag set to 0 (as defined by [RFC6946]). - - The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", - "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this - document are to be interpreted as described in RFC 2119 [RFC2119]. - -3. Security Implications of the Generation of IPv6 Atomic Fragments +2. Security Implications of the Generation of IPv6 Atomic Fragments The security implications of IP fragmentation have been discussed at - length in [RFC6274] and [I-D.ietf-6man-predictable-fragment-id]. An - attacker can leverage the generation of IPv6 atomic fragments to - trigger the use of fragmentation in an arbitrary IPv6 flow and - subsequently perform any fragmentation-based attack against legacy - IPv6 nodes that do not implement [RFC6946]. + length in [RFC6274] and [RFC7739]. An attacker can leverage the + generation of IPv6 atomic fragments to trigger the use of + fragmentation in an arbitrary IPv6 flow and subsequently perform any + fragmentation-based attack against legacy IPv6 nodes that do not + implement [RFC6946]. Unfortunately, even nodes that already implement [RFC6946] can be subject to DoS attacks as a result of the generation of IPv6 atomic fragments. Let us assume that Host A is communicating with Server B, and that, as a result of the widespread dropping of IPv6 packets that contain extension headers (including fragmentation) [I-D.ietf-v6ops-ipv6-ehs-in-real-world], some intermediate node filters fragments between Host A and Server B. If an attacker sends a forged ICMPv6 "Packet Too Big" (PTB) error message to server B, reporting an MTU smaller than 1280, this will trigger the generation @@ -182,35 +163,35 @@ headers, the ICMPv6 payload might not even contain any useful information on which to perform validation checks. o Upon receipt of one of the aforementioned ICMPv6 "Packet Too Big" error messages, the Destination Cache [RFC4861] is usually updated to reflect that any subsequent packets to such destination should include a Fragment Header. This means that a single ICMPv6 "Packet Too Big" error message might affect multiple communication instances (e.g., TCP connections) with such destination. - o As noted in Section 4, SIIT [RFC6145] (including derivative + o As noted in Section 3, SIIT [RFC6145] (including derivative protocols such as Stateful NAT64 [RFC6146]) is the only technology which currently makes use of atomic fragments. Unfortunately, an IPv6 node cannot easily limit its exposure to the aforementioned attack vector by only generating IPv6 atomic fragments towards IPv4 destinations behind a stateless translator. This is due to the fact that Section 3.3 of [RFC6052] encourages operators to use a Network-Specific Prefix (NSP) that maps the IPv4 address space into IPv6. When an NSP is being used, IPv6 addresses representing IPv4 nodes (reached through a stateless translator) are indistinguishable from native IPv6 addresses. -4. Additional Considerations +3. Additional Considerations - Besides the security assessment provided in Section 3, it is + Besides the security assessment provided in Section 2, it is interesting to evaluate the pros and cons of having an IPv6-to-IPv4 translating router rely on the generation of IPv6 atomic fragments. Relying on the generation of IPv6 atomic fragments implies a reliance on: 1. ICMPv6 packets arriving from the translator to the IPv6 node 2. The ability of the nodes receiving ICMPv6 PTB messages reporting an MTU smaller than 1280 bytes to actually produce atomic @@ -273,101 +254,89 @@ 3. ECMP routing [RFC2992] with more than one translator is employed for e.g., redundancy purposes In such a scenario, if each translator were to select the IPv4 Identification on its own (rather than selecting the IPv4 Identification from the low-order 16-bits of the Fragment Identification of IPv6 atomic fragments), this could possibly lead to IPv4 Identification collisions. However, since a number of implementations set the IPv6 Fragment Identification according to the output of a Pseudo-Random Number Generator (PRNG) (see Appendix B of - [I-D.ietf-6man-predictable-fragment-id]) and the translator only - employs the low-order 16-bits of such value, it is very unlikely that - relying on the Fragment Identification of the IPv6 atomic fragment - will result in a reduced IPv4 Identification collision rate (when - compared to the case where the translator selects each IPv4 - Identification on its own). + [RFC7739]) and the translator only employs the low-order 16-bits of + such value, it is very unlikely that relying on the Fragment + Identification of the IPv6 atomic fragment will result in a reduced + IPv4 Identification collision rate (when compared to the case where + the translator selects each IPv4 Identification on its own). Finally, we note that [RFC6145] is currently the only "consumer" of IPv6 atomic fragments, and it correctly and diligently notes (in Section 6) the possible interoperability problems of relying on IPv6 atomic fragments, proposing as a workaround that leads to more robust behavior and simplified code. -5. IANA Considerations +4. IANA Considerations - There are no IANA registries within this document. The RFC-Editor - can remove this section before publication of this document as an - RFC. + There are no IANA registries within this document. -6. Security Considerations +5. Security Considerations This document briefly discusses the security implications of the generation of IPv6 atomic fragments, and describes a specific Denial of Service (DoS) attack vector that leverages the widespread filtering of IPv6 fragments in the public Internet. It concludes that the generation of IPv6 atomic fragments is an undesirable feature, and documents the motivation for removing this functionality from [I-D.ietf-6man-rfc2460bis]. -7. Acknowledgements +6. Acknowledgements The authors would like to thank (in alphabetical order) Congxiao Bao, Bob Briscoe, Brian Carpenter, Tatuya Jinmei, Bob Hinden, Alberto Leiva, Xing Li, Jeroen Massar, Erik Nordmark, Qiong Sun, Ole Troan, and Tina Tsou, for providing valuable comments on earlier versions of this document. Fernando Gont would like to thank Jan Zorz / Go6 Lab , and Jared Mauch / NTT America, for providing access to systems and networks that were employed to produce some of tests that resulted in the publication of this document. Additionally, he would like to thank SixXS for providing IPv6 connectivity. -8. References +7. References -8.1. Normative References +7.1. Normative References [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460, December 1998, . - [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate - Requirement Levels", BCP 14, RFC 2119, - DOI 10.17487/RFC2119, March 1997, - . - [RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", RFC 4443, DOI 10.17487/RFC4443, March 2006, . [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007, . [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, DOI 10.17487/RFC4861, September 2007, . [RFC6145] Li, X., Bao, C., and F. Baker, "IP/ICMP Translation Algorithm", RFC 6145, DOI 10.17487/RFC6145, April 2011, . -8.2. Informative References - - [RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery", - RFC 2923, DOI 10.17487/RFC2923, September 2000, - . +7.2. Informative References [RFC2992] Hopps, C., "Analysis of an Equal-Cost Multi-Path Algorithm", RFC 2992, DOI 10.17487/RFC2992, November 2000, . [RFC5927] Gont, F., "ICMP Attacks against TCP", RFC 5927, DOI 10.17487/RFC5927, July 2010, . [RFC6052] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X. @@ -381,36 +350,35 @@ April 2011, . [RFC6274] Gont, F., "Security Assessment of the Internet Protocol Version 4", RFC 6274, DOI 10.17487/RFC6274, July 2011, . [RFC6946] Gont, F., "Processing of IPv6 "Atomic" Fragments", RFC 6946, DOI 10.17487/RFC6946, May 2013, . - [I-D.ietf-6man-predictable-fragment-id] - Gont, F., "Security Implications of Predictable Fragment - Identification Values", draft-ietf-6man-predictable- - fragment-id-10 (work in progress), October 2015. + [RFC7739] Gont, F., "Security Implications of Predictable Fragment + Identification Values", RFC 7739, DOI 10.17487/RFC7739, + February 2016, . [I-D.ietf-v6ops-ipv6-ehs-in-real-world] Gont, F., Linkova, J., Chown, T., and S. LIU, "Observations on the Dropping of Packets with IPv6 Extension Headers in the Real World", draft-ietf-v6ops- ipv6-ehs-in-real-world-02 (work in progress), December 2015. [I-D.ietf-6man-rfc2460bis] Deering, S. and B. Hinden, "Internet Protocol, Version 6 - (IPv6) Specification", draft-ietf-6man-rfc2460bis-02 (work - in progress), December 2015. + (IPv6) Specification", draft-ietf-6man-rfc2460bis-04 (work + in progress), March 2016. [Morbitzer] Morbitzer, M., "TCP Idle Scans in IPv6", Master's Thesis. Thesis number: 670. Department of Computing Science, Radboud University Nijmegen. August 2013, . [JOOL] Leiva Popper, A., "nf_defrag_ipv4 and nf_defrag_ipv6", April 2015,