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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 2460 (Obsoleted by RFC 8200) == Outdated reference: A later version (-02) exists of draft-gont-v6ops-ipv6-ehs-in-real-world-00 Summary: 1 error (**), 0 flaws (~~), 2 warnings (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 IPv6 maintenance Working Group (6man) F. Gont 3 Internet-Draft SI6 Networks / UTN-FRH 4 Updates: 2460 (if approved) W. Liu 5 Intended status: Standards Track Huawei Technologies 6 Expires: February 20, 2015 August 19, 2014 8 Deprecating the Generation of IPv6 Atomic Fragments 9 draft-gont-6man-deprecate-atomfrag-generation-00 11 Abstract 13 The core IPv6 specification requires that when a host receives an 14 ICMPv6 "Packet Too Big" message reporting a "Next-Hop MTU" smaller 15 than 1280, the host includes a Fragment Header in all subsequent 16 packets sent to that destination, without reducing the assumed Path- 17 MTU. The simplicity with which ICMPv6 "Packet Too Big" messages can 18 be forged, coupled with the widespread filtering of IPv6 fragments, 19 results in an attack vector that can be leveraged for Denial of 20 Service purposes. This document briefly discusses the aforementioned 21 attack vector, and formally deprecates the generation of IPv6 atomic 22 fragments, such that the aforementioned attack vector is eliminated. 24 Status of This Memo 26 This Internet-Draft is submitted in full conformance with the 27 provisions of BCP 78 and BCP 79. 29 Internet-Drafts are working documents of the Internet Engineering 30 Task Force (IETF). Note that other groups may also distribute 31 working documents as Internet-Drafts. The list of current Internet- 32 Drafts is at http://datatracker.ietf.org/drafts/current/. 34 Internet-Drafts are draft documents valid for a maximum of six months 35 and may be updated, replaced, or obsoleted by other documents at any 36 time. It is inappropriate to use Internet-Drafts as reference 37 material or to cite them other than as "work in progress." 39 This Internet-Draft will expire on February 20, 2015. 41 Copyright Notice 43 Copyright (c) 2014 IETF Trust and the persons identified as the 44 document authors. All rights reserved. 46 This document is subject to BCP 78 and the IETF Trust's Legal 47 Provisions Relating to IETF Documents 48 (http://trustee.ietf.org/license-info) in effect on the date of 49 publication of this document. Please review these documents 50 carefully, as they describe your rights and restrictions with respect 51 to this document. Code Components extracted from this document must 52 include Simplified BSD License text as described in Section 4.e of 53 the Trust Legal Provisions and are provided without warranty as 54 described in the Simplified BSD License. 56 Table of Contents 58 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 59 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 60 3. Denial of Service (DoS) attack vector . . . . . . . . . . . . 3 61 4. Updating RFC2460 . . . . . . . . . . . . . . . . . . . . . . 4 62 5. Additional Considerations . . . . . . . . . . . . . . . . . . 5 63 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 64 7. Security Considerations . . . . . . . . . . . . . . . . . . . 5 65 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5 66 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 6 67 9.1. Normative References . . . . . . . . . . . . . . . . . . 6 68 9.2. Informative References . . . . . . . . . . . . . . . . . 6 69 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6 71 1. Introduction 73 [RFC2460] specifies the IPv6 fragmentation mechanism, which allows 74 IPv6 packets to be fragmented into smaller pieces such that they fit 75 in the Path-MTU to the intended destination(s). 77 Section 5 of [RFC2460] states that, when a host receives an ICMPv6 78 "Packet Too Big" message [RFC4443] advertising a "Next-Hop MTU" 79 smaller than 1280 (the minimum IPv6 MTU), the host is not required to 80 reduce the assumed Path-MTU, but must simply include a Fragment 81 Header in all subsequent packets sent to that destination. The 82 resulting packets will thus *not* be actually fragmented into several 83 pieces, but rather just include a Fragment Header with both the 84 "Fragment Offset" and the "M" flag set to 0 (we refer to these 85 packets as "atomic fragments"). As required by [RFC6946], these 86 atomic fragments are essentially processed by the destination host as 87 non-fragment traffic (since there are not really any fragments to be 88 reassembled). IPv6/IPv4 translators will typically employ the 89 Fragment Identification information found in the Fragment Header to 90 select an appropriate Fragment Identification value for the resulting 91 IPv4 fragments. 93 While atomic fragments might seem rather benign, there are scenarios 94 in which the generation of IPv6 atomic fragments can introduce an 95 attack vector that can be exploited for denial of service purposes. 96 Since there are concrete security implications arising from the 97 generation of IPv6 atomic fragments, and there is no real gain in 98 generating IPv6 atomic fragments (as opposed to e.g. having IPv6/IPv4 99 translators generate a Fragment Identification value themselves), 100 this document formally updates [RFC2460], forbidding the generation 101 of IPv6 atomic fragments, such that the aforementioned attack vector 102 is eliminated. 104 Section 3 describes some possible attack scenarios. Section 5 105 provides additional considerations regarding the usefulness of 106 generating IPv6 atomic fragments. Section 4 formally updates RFC2460 107 such that this attack vector is eliminated. 109 2. Terminology 111 IPv6 atomic fragments 112 IPv6 packets that contain a Fragment Header with the Fragment 113 Offset set to 0 and the M flag set to 0 (as defined by [RFC6946]). 115 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 116 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 117 document are to be interpreted as described in RFC 2119 [RFC2119]. 119 3. Denial of Service (DoS) attack vector 121 Let us assume that Host A is communicating with Server B, and that, 122 as a result of the widespread filtering of IPv6 packets with 123 extension headers (including fragmentation) 124 [I-D.gont-v6ops-ipv6-ehs-in-real-world], some intermediate node 125 filters fragments between Host A and Server B. If an attacker sends 126 a forged ICMPv6 "Packet Too Big" (PTB) error message to server B, 127 reporting a Next-Hop MTU smaller than 1280, this will trigger the 128 generation of IPv6 atomic fragments from that moment on (as required 129 by [RFC2460]). When server server B starts sending IPv6 atomic 130 fragments (in response to the received ICMPv6 PTB), these packets 131 will be dropped, since we previously noted that packets with IPv6 EHs 132 were being dropped between Host A and Server B. Thus, this situation 133 will result in a Denial of Service (DoS) scenario. 135 Another possible scenario is that in which two BGP peers are 136 employing IPv6 transport, and they implement ACLs to drop IPv6 137 fragments (to avoid control-plane attacks). If the aforementioned 138 BGP peers drop IPv6 fragments but still honor received ICMPv6 Packet 139 Too Big error messages, an attacker could easily attack the peering 140 session by simply sending an ICMPv6 PTB message with a reported MTU 141 smaller than 1280 bytes. Once the attack packet has been fired, it 142 will be the aforementioned routers themselves the ones dropping their 143 own traffic. 145 The aforementioned attack vector is exacerbated by the following 146 factors: 148 o The attacker does not need to forge the IPv6 Source Address of his 149 attack packets. Hence, deployment of simple BCP38 filters will 150 not help as a counter-measure. 152 o Only the IPv6 addresses of the IPv6 packet embedded in the ICMPv6 153 payload need to be forged. While one could envision filtering 154 devices enforcing BCP38-style filters on the ICMPv6 payload, the 155 use of extension (by the attacker) could make this difficult, if 156 at all possible. 158 o Many implementations fail to perform validation checks on the 159 received ICMPv6 error messages, as recommended in Section 5.2 of 160 [RFC4443] and documented in [RFC5927]. It should be noted that in 161 some cases, such as when an ICMPv6 error message has (supposedly) 162 been elicited by a connection-less transport protocol (or some 163 other connection-less protocol being encapsulated in IPv6), it may 164 be virtually impossible to perform validation checks on the 165 received ICMPv6 error messages. And, because of IPv6 extension 166 headers, the ICMPv6 payload might not even contain any useful 167 information on which to perform validation checks. 169 o Upon receipt of one of the aforementioned ICMPv6 "Packet Too Big" 170 error messages, the Destination Cache [RFC4861] is usually updated 171 to reflect that any subsequent packets to such destination should 172 include a Fragment Header. This means that a single ICMPv6 173 "Packet Too Big" error message might affect multiple communication 174 instances (e.g., TCP connections) with such destination. 176 4. Updating RFC2460 178 The following text from Section 5 of [RFC2460]: 180 "In response to an IPv6 packet that is sent to an IPv4 destination 181 (i.e., a packet that undergoes translation from IPv6 to IPv4), the 182 originating IPv6 node may receive an ICMP Packet Too Big message 183 reporting a Next-Hop MTU less than 1280. In that case, the IPv6 184 node is not required to reduce the size of subsequent packets to 185 less than 1280, but must include a Fragment header in those 186 packets so that the IPv6-to-IPv4 translating router can obtain a 187 suitable Identification value to use in resulting IPv4 fragments. 188 Note that this means the payload may have to be reduced to 1232 189 octets (1280 minus 40 for the IPv6 header and 8 for the Fragment 190 header), and smaller still if additional extension headers are 191 used." 193 is formally replaced with: 195 "IPv6 nodes MUST discard ICMPv6 Packet Too Big error messages that 196 report a Next-Hop MTU smaller than 1280 bytes (the minimum IPv6 197 MTU)." 199 5. Additional Considerations 201 Besides the security assessment provided in Section 3, it is 202 interesting to evaluate if there is any gain in generating IPv6 203 atomic fragments (to provide for Fragment Identification value) as 204 opposed to just let IPv6/IPv4 translators select an appropriate IPv4 205 Fragment Identification value. 207 After some analysis, one can conclude that, if anything, an IPv6/IPv4 208 translator is in a much better position to select an appropriate 209 Fragment Identification value for the packet that are to be 210 translated from the IPv6 to the IPv4 world. For instance, an IPv6 211 node will generate Fragment Identification values without any 212 knowledge of the Fragment ID values being generated by other IPv6 213 nodes employing the translator. Thus, an IPv6/IPv4 translator is in 214 a much better position to generate Fragment IDs that will not result 215 in collisions (i.e., that will not be reused for the same tuple 216 {Source Address, Destination Address}. 218 6. IANA Considerations 220 There are no IANA registries within this document. The RFC-Editor 221 can remove this section before publication of this document as an 222 RFC. 224 7. Security Considerations 226 This document describes a Denial of Service (DoS) attack vector that 227 leverages the widespread filtering of IPv6 fragments in the public 228 Internet by means of ICMPv6 PTB error messages. Additionally, it 229 formally updates [RFC2460] such that this attack vector is 230 eliminated. 232 8. Acknowledgements 234 Fernando Gont would like to thank Jan Zorz and Go6 Lab 235 for providing access to systems and networks that 236 were employed to produce some of the measurement results presented in 237 this document. Additionally, he would like to thank SixXS 238 for providing IPv6 connectivity. 240 9. References 242 9.1. Normative References 244 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 245 (IPv6) Specification", RFC 2460, December 1998. 247 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 248 Requirement Levels", BCP 14, RFC 2119, March 1997. 250 [RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control 251 Message Protocol (ICMPv6) for the Internet Protocol 252 Version 6 (IPv6) Specification", RFC 4443, March 2006. 254 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 255 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 256 September 2007. 258 9.2. Informative References 260 [RFC5927] Gont, F., "ICMP Attacks against TCP", RFC 5927, July 2010. 262 [RFC6946] Gont, F., "Processing of IPv6 "Atomic" Fragments", RFC 263 6946, May 2013. 265 [I-D.gont-v6ops-ipv6-ehs-in-real-world] 266 Gont, F., Linkova, J., Chown, T., and W. Will, "IPv6 267 Extension Headers in the Real World", draft-gont-v6ops- 268 ipv6-ehs-in-real-world-00 (work in progress), August 2014. 270 Authors' Addresses 272 Fernando Gont 273 SI6 Networks / UTN-FRH 274 Evaristo Carriego 2644 275 Haedo, Provincia de Buenos Aires 1706 276 Argentina 278 Phone: +54 11 4650 8472 279 Email: fgont@si6networks.com 280 URI: http://www.si6networks.com 281 Will(Shucheng) Liu 282 Huawei Technologies 283 Bantian, Longgang District 284 Shenzhen 518129 285 P.R. China 287 Email: liushucheng@huawei.com