--- 1/draft-ietf-6man-deprecate-atomfrag-generation-07.txt 2016-09-13 00:15:59.304871941 -0700 +++ 2/draft-ietf-6man-deprecate-atomfrag-generation-08.txt 2016-09-13 00:15:59.328872542 -0700 @@ -1,21 +1,21 @@ IPv6 maintenance Working Group (6man) F. Gont Internet-Draft SI6 Networks / UTN-FRH Intended status: Informational W. Liu -Expires: January 18, 2017 Huawei Technologies +Expires: March 16, 2017 Huawei Technologies T. Anderson Redpill Linpro - July 17, 2016 + September 12, 2016 Generation of IPv6 Atomic Fragments Considered Harmful - draft-ietf-6man-deprecate-atomfrag-generation-07 + draft-ietf-6man-deprecate-atomfrag-generation-08 Abstract 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. @@ -27,21 +27,21 @@ 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 January 18, 2017. + This Internet-Draft will expire on March 16, 2017. 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 @@ -52,21 +52,21 @@ described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Security Implications of the Generation of IPv6 Atomic Fragments . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Additional Considerations . . . . . . . . . . . . . . . . . . 5 4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 7 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 - 6. Security Considerations . . . . . . . . . . . . . . . . . . . 7 + 6. Security Considerations . . . . . . . . . . . . . . . . . . . 8 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 8.1. Normative References . . . . . . . . . . . . . . . . . . 8 8.2. Informative References . . . . . . . . . . . . . . . . . 9 Appendix A. Small Survey of OSes that Fail to Produce IPv6 Atomic Fragments . . . . . . . . . . . . . . . . . . 10 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11 1. Introduction @@ -106,38 +106,41 @@ of Service (DoS) attack vector that leverages the widespread filtering of IPv6 fragments in the public Internet. Section 3 provides additional considerations regarding the usefulness of generating 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 [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 in an arbitrary IPv6 flow (in scenarios in which actual + fragmentation of packets is not needed), and subsequently perform any fragmentation-based attack against legacy IPv6 nodes that do not - implement [RFC6946]. + implement [RFC6946]. That is, employing fragmentation where not + actually needed allows for fragmentation-based attack vectors to be + employed, unnecessarily. - 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) [RFC7872], 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 of IPv6 atomic fragments from that moment on - (as required by [RFC2460]). When server B starts sending IPv6 atomic - fragments (in response to the received ICMPv6 PTB), these packets - will be dropped, since we previously noted that IPv6 packets with - extension headers were being dropped between Host A and Server B. - Thus, this situation will result in a Denial of Service (DoS) + We note that, 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) + [RFC7872], some intermediate node filters fragments between Server B + and Host A. 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 of IPv6 atomic fragments from that + moment on (as required by [RFC2460]). When server B starts sending + IPv6 atomic fragments (in response to the received ICMPv6 PTB), these + packets will be dropped, since we previously noted that IPv6 packets + with extension headers were being dropped between Server B and Host + A. Thus, this situation will result in a Denial of Service (DoS) scenario. Another possible scenario is that in which two BGP peers are employing IPv6 transport, and they implement Access Control Lists (ACLs) to drop IPv6 fragments (to avoid control-plane attacks). If the aforementioned BGP peers drop IPv6 fragments but still honor received ICMPv6 Packet Too Big error messages, an attacker could easily attack the peering session by simply sending an ICMPv6 PTB message with a reported MTU smaller than 1280 bytes. Once the attack packet has been sent, the aforementioned routers will themselves be @@ -200,20 +203,24 @@ 2. The ability of the nodes receiving ICMPv6 PTB messages reporting an MTU smaller than 1280 bytes to actually produce atomic fragments 3. Support for IPv6 fragmentation on the IPv6 side of the translator 4. The ability of the translator implementation to access the information conveyed by the IPv6 Fragment Header + 5. The value extracted from the low-order 16-bits of the IPv6 + fragment Identification resulting in an appropriate IPv4 + Identification value + Unfortunately, 1. There exists a fair share of evidence of ICMPv6 Packet Too Big messages being dropped on the public Internet (for instance, that is one of the reasons for which PLPMTUD [RFC4821] was produced). Therefore, relying on such messages being successfully delivered will affect the robustness of the protocol that relies on them. 2. A number of IPv6 implementations have been known to fail to generate IPv6 atomic fragments in response to ICMPv6 PTB messages @@ -234,20 +241,33 @@ unnecessarily, in a Denial of Service situation even in legitimate cases. 4. The packet-handling API at the node where the translator is running may obscure fragmentation-related information. In such scenarios, the information conveyed by the Fragment Header may be unavailable to the translator. [JOOL] discusses a sample framework (Linux Netfilter) that hinders access to the information conveyed in IPv6 atomic fragments. + 5. While [RFC2460] requires that the IPv6 fragment Identification of + a fragmented packet be different that of any other fragmented + packet sent recently with the same Source Address and Destination + Address, there is no requirement on the low-order 16-bits of such + value. Thus, there is no guarantee that, by employing the low- + order 16-bits of the IPv6 fragment Identification of a packet + sent by a source host, IPv4 fragment identification collisions + will be avoided or reduced. Besides, collisions might occur + where two distinct IPv6 Destination Addresses are translated into + the same IPv4 address, such that Identification values that might + have been generated to be unique in the IPv6 context end up + colliding when used in the translated IPv4 context. + We note that SIIT essentially employs the Fragment Header of IPv6 atomic fragments to signal the translator how to set the DF bit of IPv4 datagrams (the DF bit is cleared when the IPv6 packet contains a Fragment Header, and is otherwise set to 1 when the IPv6 packet does not contain an IPv6 Fragment Header). Additionally, the translator will employ the low-order 16-bits of the IPv6 Fragment Identification for setting the IPv4 Fragment Identification. At least in theory, this is expected to reduce the IPv4 Identification collision rate in the following specific scenario: @@ -258,47 +278,43 @@ Path MTU of 1280, due to an option-less IPv4 header being 20 bytes shorter than the IPv6 header. 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 - [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). - Besides, because of the limited sized of the IPv4 identification - field, it is nevertheless virtually impossible to guarantee - uniqueness of the IPv4 identification values without artificially - limiting the data rate of fragmented traffic [RFC6864] [RFC4963]. + IPv4 Identification collisions. However, as noted above, the value + extracted from the low-order 16-bits of the IPv6 fragment + Identification might not result in an appropriate IPv4 + identification: for example, a number of implementations set the IPv6 + Fragment Identification according to the output of a Pseudo-Random + Number Generator (PRNG) (see Appendix B of [RFC7739]); hence,if 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). Besides, because of + the limited sized of the IPv4 identification field, it is + nevertheless virtually impossible to guarantee uniqueness of the IPv4 + identification values without artificially limiting the data rate of + fragmented traffic [RFC6864] [RFC4963]. [RFC6145] was the only "consumer" of IPv6 atomic fragments, and it correctly and diligently noted (in Section 6) the possible interoperability problems of relying on IPv6 atomic fragments, proposing a workaround that led to more robust behavior and simplified code. [RFC6145] has been obsoleted by [RFC7915], such that SIIT does not rely on IPv6 atomic fragments. - Finally, we believe that IPv4 links with an MTU smaller than 1260 - bytes are very uncommonly found in the modern Internet. At the same - time, we note that the sole purpose of IPv6 atomic fragments is to - make such links compatible with IPv4/IPv6 translation. We surmise, - therefore, that IPv6 atomic fragments are useful in only a minuscule - number of "real world" situations. - 4. Conclusions Taking all of the above considerations into account, we recommend that IPv6 atomic fragments be deprecated. In particular: o IPv4/IPv6 translators should be updated to not generate ICMPv6 Packet Too Big errors containing a Path MTU value smaller than the minimum IPv6 MTU of 1280 bytes. This will ensure that current @@ -313,34 +329,35 @@ We note that these recommendations have been incorporated in [I-D.ietf-6man-rfc1981bis], [I-D.ietf-6man-rfc2460bis] and [RFC7915]. 5. IANA Considerations There are no IANA registries within this document. 6. 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 + generation of IPv6 atomic fragments, and describes one 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 The authors would like to thank (in alphabetical order) Congxiao Bao, Carlos Jesus Bernardos Cano, Bob Briscoe, Brian Carpenter, Tatuya Jinmei, Bob Hinden, Alberto Leiva, Ted Lemon, Xing Li, Jeroen Massar, - Erik Nordmark, Qiong Sun, Ole Troan, Tina Tsou, and Bernie Volz, for - providing valuable comments on earlier versions of this document. + Erik Nordmark, Joe Touch, Qiong Sun, Ole Troan, Tina Tsou, and Bernie + Volz, 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 the tests that resulted in the publication of this document. Additionally, he would like to thank SixXS for providing IPv6 connectivity. 8. References