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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) == Unused Reference: 'RFC2923' is defined on line 351, but no explicit reference was found in the text ** Obsolete normative reference: RFC 2460 (Obsoleted by RFC 8200) ** Obsolete normative reference: RFC 6145 (Obsoleted by RFC 7915) == Outdated reference: A later version (-10) exists of draft-ietf-6man-predictable-fragment-id-08 == Outdated reference: A later version (-02) exists of draft-ietf-v6ops-ipv6-ehs-in-real-world-00 Summary: 2 errors (**), 0 flaws (~~), 4 warnings (==), 3 comments (--).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, 6145 (if approved) W. Liu 5 Intended status: Standards Track Huawei Technologies 6 Expires: January 5, 2016 T. Anderson 7 Redpill Linpro 8 July 4, 2015 10 Deprecating the Generation of IPv6 Atomic Fragments 11 draft-ietf-6man-deprecate-atomfrag-generation-02 13 Abstract 15 The core IPv6 specification requires that when a host receives an 16 ICMPv6 "Packet Too Big" message reporting an MTU smaller than 1280 17 bytes, the host includes a Fragment Header in all subsequent packets 18 sent to that destination, without reducing the assumed Path-MTU. The 19 simplicity with which ICMPv6 "Packet Too Big" messages can be forged, 20 coupled with the widespread filtering of IPv6 fragments, results in 21 an attack vector that can be leveraged for Denial of Service 22 purposes. This document briefly discusses the aforementioned attack 23 vector, and formally updates RFC2460 such that generation of IPv6 24 atomic fragments is deprecated, thus eliminating the aforementioned 25 attack vector. 27 Status of This Memo 29 This Internet-Draft is submitted in full conformance with the 30 provisions of BCP 78 and BCP 79. 32 Internet-Drafts are working documents of the Internet Engineering 33 Task Force (IETF). Note that other groups may also distribute 34 working documents as Internet-Drafts. The list of current Internet- 35 Drafts is at http://datatracker.ietf.org/drafts/current/. 37 Internet-Drafts are draft documents valid for a maximum of six months 38 and may be updated, replaced, or obsoleted by other documents at any 39 time. It is inappropriate to use Internet-Drafts as reference 40 material or to cite them other than as "work in progress." 42 This Internet-Draft will expire on January 5, 2016. 44 Copyright Notice 46 Copyright (c) 2015 IETF Trust and the persons identified as the 47 document authors. All rights reserved. 49 This document is subject to BCP 78 and the IETF Trust's Legal 50 Provisions Relating to IETF Documents 51 (http://trustee.ietf.org/license-info) in effect on the date of 52 publication of this document. Please review these documents 53 carefully, as they describe your rights and restrictions with respect 54 to this document. Code Components extracted from this document must 55 include Simplified BSD License text as described in Section 4.e of 56 the Trust Legal Provisions and are provided without warranty as 57 described in the Simplified BSD License. 59 Table of Contents 61 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 62 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 63 3. Denial of Service (DoS) attack vector . . . . . . . . . . . . 3 64 4. Additional Considerations . . . . . . . . . . . . . . . . . . 5 65 5. Updating RFC2460 . . . . . . . . . . . . . . . . . . . . . . 6 66 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 67 7. Security Considerations . . . . . . . . . . . . . . . . . . . 7 68 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7 69 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 70 9.1. Normative References . . . . . . . . . . . . . . . . . . 7 71 9.2. Informative References . . . . . . . . . . . . . . . . . 8 72 Appendix A. Small Survey of OSes that Fail to Produce IPv6 73 Atomic Fragments . . . . . . . . . . . . . . . . . . 9 74 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 76 1. Introduction 78 [RFC2460] specifies the IPv6 fragmentation mechanism, which allows 79 IPv6 packets to be fragmented into smaller pieces such that they fit 80 in the Path-MTU to the intended destination(s). 82 Section 5 of [RFC2460] states that, when a host receives an ICMPv6 83 "Packet Too Big" message [RFC4443] advertising an MTU smaller than 84 1280 bytes (the minimum IPv6 MTU), the host is not required to reduce 85 the assumed Path-MTU, but must simply include a Fragment Header in 86 all subsequent packets sent to that destination. The resulting 87 packets will thus *not* be actually fragmented into several pieces, 88 but rather just include a Fragment Header with both the "Fragment 89 Offset" and the "M" flag set to 0 (we refer to these packets as 90 "atomic fragments"). As required by [RFC6946], these atomic 91 fragments are essentially processed by the destination host as non- 92 fragment traffic (since there are not really any fragments to be 93 reassembled). The goal of these atomic fragments has been to convey 94 an appropriate Fragment Identification value to be employed by IPv6/ 95 IPv4 translators for the resulting IPv4 fragments. 97 While atomic fragments might seem rather benign, there are scenarios 98 in which the generation of IPv6 atomic fragments can introduce an 99 attack vector that can be exploited for denial of service purposes. 100 Since there are concrete security implications arising from the 101 generation of IPv6 atomic fragments, and there is no real gain in 102 generating IPv6 atomic fragments (as opposed to e.g. having IPv6/IPv4 103 translators generate a Fragment Identification value themselves), 104 this document formally updates [RFC2460], forbidding the generation 105 of IPv6 atomic fragments, such that the aforementioned attack vector 106 is eliminated. 108 Section 3 describes some possible attack scenarios. Section 4 109 provides additional considerations regarding the usefulness of 110 generating IPv6 atomic fragments. Section 5 formally updates RFC2460 111 such that this attack vector is eliminated. 113 2. Terminology 115 IPv6 atomic fragments 116 IPv6 packets that contain a Fragment Header with the Fragment 117 Offset set to 0 and the M flag set to 0 (as defined by [RFC6946]). 119 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 120 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 121 document are to be interpreted as described in RFC 2119 [RFC2119]. 123 3. Denial of Service (DoS) attack vector 125 Let us assume that Host A is communicating with Server B, and that, 126 as a result of the widespread filtering of IPv6 packets with 127 extension headers (including fragmentation) 128 [I-D.ietf-v6ops-ipv6-ehs-in-real-world], some intermediate node 129 filters fragments between Host A and Server B. If an attacker sends 130 a forged ICMPv6 "Packet Too Big" (PTB) error message to server B, 131 reporting an MTU smaller than 1280, this will trigger the generation 132 of IPv6 atomic fragments from that moment on (as required by 133 [RFC2460]). When server B starts sending IPv6 atomic fragments (in 134 response to the received ICMPv6 PTB), these packets will be dropped, 135 since we previously noted that packets with IPv6 EHs were being 136 dropped between Host A and Server B. Thus, this situation will 137 result in a Denial of Service (DoS) scenario. 139 Another possible scenario is that in which two BGP peers are 140 employing IPv6 transport, and they implement ACLs to drop IPv6 141 fragments (to avoid control-plane attacks). If the aforementioned 142 BGP peers drop IPv6 fragments but still honor received ICMPv6 Packet 143 Too Big error messages, an attacker could easily attack the peering 144 session by simply sending an ICMPv6 PTB message with a reported MTU 145 smaller than 1280 bytes. Once the attack packet has been sent, it 146 will be the aforementioned routers themselves the ones dropping their 147 own traffic. 149 The aforementioned attack vector is exacerbated by the following 150 factors: 152 o The attacker does not need to forge the IPv6 Source Address of his 153 attack packets. Hence, deployment of simple BCP38 filters will 154 not help as a counter-measure. 156 o Only the IPv6 addresses of the IPv6 packet embedded in the ICMPv6 157 payload needs to be forged. While one could envision filtering 158 devices enforcing BCP38-style filters on the ICMPv6 payload, the 159 use of extension headers (by the attacker) could make this 160 difficult, if at all possible. 162 o Many implementations fail to perform validation checks on the 163 received ICMPv6 error messages, as recommended in Section 5.2 of 164 [RFC4443] and documented in [RFC5927]. It should be noted that in 165 some cases, such as when an ICMPv6 error message has (supposedly) 166 been elicited by a connection-less transport protocol (or some 167 other connection-less protocol being encapsulated in IPv6), it may 168 be virtually impossible to perform validation checks on the 169 received ICMPv6 error messages. And, because of IPv6 extension 170 headers, the ICMPv6 payload might not even contain any useful 171 information on which to perform validation checks. 173 o Upon receipt of one of the aforementioned ICMPv6 "Packet Too Big" 174 error messages, the Destination Cache [RFC4861] is usually updated 175 to reflect that any subsequent packets to such destination should 176 include a Fragment Header. This means that a single ICMPv6 177 "Packet Too Big" error message might affect multiple communication 178 instances (e.g., TCP connections) with such destination. 180 o As noted in Section 4, SIIT [RFC6145] (including derivative 181 protocols such as Stateful NAT64 [RFC6146]) is the only technology 182 which currently makes use of atomic fragments. Unfortunately, an 183 IPv6 node cannot easily limit its exposure to the aforementioned 184 attack vector by only generating IPv6 atomic fragments towards 185 IPv4 destinations behind a stateless translator. This is due to 186 the fact that Section 3.3 of RFC6052 [RFC6052] encourages 187 operators to use a Network-Specific Prefix (NSP) that maps the 188 IPv4 address space into IPv6. When an NSP is being used, IPv6 189 addresses representing IPv4 nodes (reached through a stateless 190 translator) are indistinguishable from native IPv6 addresses. 192 4. Additional Considerations 194 Besides the security assessment provided in Section 3, it is 195 interesting to evaluate the pros and cons of having an IPv6-to-IPv4 196 translating router rely on the generation of IPv6 atomic fragments. 198 Relying on the generation of IPv6 atomic fragments implies a reliance 199 on: 201 1. ICMPv6 packets arriving from the translator to the IPv6 node 203 2. The ability of the nodes receiving ICMPv6 PTB messages reporting 204 an MTU smaller than 1280 bytes to actually produce atomic 205 fragments 207 3. Support for IPv6 fragmentation on the IPv6 side of the translator 209 Unfortunately, 211 o There exists a fair share of evidence of ICMPv6 Packet Too Big 212 messages being dropped on the public Internet (for instance, that 213 is one of the reasons for which PLPMTUD [RFC4821] was produced). 214 Therefore, relying on such messages being successfully delivered 215 will affect the robustness of the protocol that relies on them. 217 o A number of IPv6 implementations have been known to fail to 218 generate IPv6 atomic fragments in response to ICMPv6 PTB messages 219 reporting an MTU smaller than 1280 bytes (see Appendix A for a 220 small survey). Additionally, the results included in Section 6 of 221 [RFC6145] note that 57% of the tested web servers failed to 222 produce IPv6 atomic fragments in response to ICMPv6 PTB messages 223 reporting an MTU smaller than 1280 bytes. Thus, any protocol 224 relying on IPv6 atomic fragment generation for proper functioning 225 will have interoperability problems with the aforementioned IPv6 226 stacks. 228 o IPv6 atomic fragment generation represents a case in which 229 fragmented traffic is produced where otherwise it would not be 230 needed. Since there is widespread filtering of IPv6 fragments in 231 the public Internet [I-D.ietf-v6ops-ipv6-ehs-in-real-world], this 232 would mean that the (unnecessary) use of IPv6 fragmentation might 233 result, unnecessarily, in a Denial of Service situation even in 234 legitimate cases. 236 Finally, we note that SIIT essentially employs the Fragment Header of 237 IPv6 atomic fragments to signal the translator how to set the DF bit 238 of IPv4 datagrams (the DF bit is cleared when the IPv6 packet 239 contains a Fragment Header, and is otherwise set to 1 when the IPv6 240 packet does not contain an IPv6 Fragment Header). Additionally, the 241 translator will employ the low-order 16-bits of the IPv6 Fragment 242 Identification for setting the IPv4 Fragment Identification. At 243 least in theory, this is expected to reduce the Fragment ID collision 244 rate in the following specific scenario: 246 1. An IPv6 node communicates with an IPv4 node (through SIIT) 248 2. The IPv4 node is located behind an IPv4 link with an MTU < 1260 250 3. ECMP routing [RFC2992] with more than one translator is employed 251 for e.g., redundancy purposes 253 In such a scenario, if each translator were to select the IPv4 254 Fragment Identification on its own (rather than selecting the IPv4 255 Fragment ID from the low-order 16-bits of the Fragment Identification 256 of atomic fragments), this could possibly lead to IPv4 Fragment ID 257 collisions. However, since a number of implementations set IPv6 258 Fragment ID according to the output of a Pseudo-Random Number 259 Generator (PRNG) (see Appendix B of 260 [I-D.ietf-6man-predictable-fragment-id]) and the translator only 261 employs the low-order 16-bits of such value, it is very unlikely that 262 relying on the Fragment ID of the IPv6 atomic fragment will result in 263 a reduced Fragment ID collision rate (when compared to the case where 264 the translator selects each IPv4 Fragment ID on its own). 266 Finally, we note that [RFC6145] is currently the only "consumer" of 267 IPv6 atomic fragments, and it correctly and diligently notes (in 268 Section 6) the possible interoperability problems of relying on IPv6 269 atomic fragments, proposing as a workaround that leads to more robust 270 behavior and simplified code. 272 5. Updating RFC2460 274 The following text from Section 5 of [RFC2460]: 276 "In response to an IPv6 packet that is sent to an IPv4 destination 277 (i.e., a packet that undergoes translation from IPv6 to IPv4), the 278 originating IPv6 node may receive an ICMP Packet Too Big message 279 reporting a Next-Hop MTU less than 1280. In that case, the IPv6 280 node is not required to reduce the size of subsequent packets to 281 less than 1280, but must include a Fragment header in those 282 packets so that the IPv6-to-IPv4 translating router can obtain a 283 suitable Identification value to use in resulting IPv4 fragments. 284 Note that this means the payload may have to be reduced to 1232 285 octets (1280 minus 40 for the IPv6 header and 8 for the Fragment 286 header), and smaller still if additional extension headers are 287 used." 289 is formally replaced with: 291 "An IPv6 node that receives an ICMPv6 Packet Too Big error message 292 that reports a Next-Hop MTU smaller than 1280 bytes (the minimum 293 IPv6 MTU) MUST NOT include a Fragment header in subsequent packets 294 sent to the corresponding destination. That is, IPv6 nodes MUST 295 NOT generate IPv6 atomic fragments." 297 6. IANA Considerations 299 There are no IANA registries within this document. The RFC-Editor 300 can remove this section before publication of this document as an 301 RFC. 303 7. Security Considerations 305 This document describes a Denial of Service (DoS) attack vector that 306 leverages the widespread filtering of IPv6 fragments in the public 307 Internet by means of ICMPv6 PTB error messages. Additionally, it 308 formally updates [RFC2460] such that this attack vector is 309 eliminated. 311 8. Acknowledgements 313 The authors would like to thank (in alphabetical order) Alberto 314 Leiva, Bob Briscoe, Brian Carpenter, Tatuya Jinmei, Jeroen Massar, 315 and Erik Nordmark, for providing valuable comments on earlier 316 versions of this document. 318 Fernando Gont would like to thank Fernando Gont would like to thank 319 Jan Zorz / Go6 Lab , and Jared Mauch / NTT 320 America, for providing access to systems and networks that were 321 employed to produce some of tests that resulted in the publication of 322 this document. Additionally, he would like to thank SixXS 323 for providing IPv6 connectivity. 325 9. References 327 9.1. Normative References 329 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 330 (IPv6) Specification", RFC 2460, December 1998. 332 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 333 Requirement Levels", BCP 14, RFC 2119, March 1997. 335 [RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control 336 Message Protocol (ICMPv6) for the Internet Protocol 337 Version 6 (IPv6) Specification", RFC 4443, March 2006. 339 [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU 340 Discovery", RFC 4821, March 2007. 342 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 343 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 344 September 2007. 346 [RFC6145] Li, X., Bao, C., and F. Baker, "IP/ICMP Translation 347 Algorithm", RFC 6145, April 2011. 349 9.2. Informative References 351 [RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery", RFC 352 2923, September 2000. 354 [RFC2992] Hopps, C., "Analysis of an Equal-Cost Multi-Path 355 Algorithm", RFC 2992, November 2000. 357 [RFC5927] Gont, F., "ICMP Attacks against TCP", RFC 5927, July 2010. 359 [RFC6052] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X. 360 Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052, 361 October 2010. 363 [RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful 364 NAT64: Network Address and Protocol Translation from IPv6 365 Clients to IPv4 Servers", RFC 6146, April 2011. 367 [RFC6946] Gont, F., "Processing of IPv6 "Atomic" Fragments", RFC 368 6946, May 2013. 370 [I-D.ietf-6man-predictable-fragment-id] 371 Gont, F., "Security Implications of Predictable Fragment 372 Identification Values", draft-ietf-6man-predictable- 373 fragment-id-08 (work in progress), June 2015. 375 [I-D.ietf-v6ops-ipv6-ehs-in-real-world] 376 Gont, F., Linkova, J., Chown, T., and S. LIU, 377 "Observations on IPv6 EH Filtering in the Real World", 378 draft-ietf-v6ops-ipv6-ehs-in-real-world-00 (work in 379 progress), April 2015. 381 [Morbitzer] 382 Morbitzer, M., "TCP Idle Scans in IPv6", Master's Thesis. 383 Thesis number: 670. Department of Computing Science, 384 Radboud University Nijmegen. August 2013, 385 . 388 Appendix A. Small Survey of OSes that Fail to Produce IPv6 Atomic 389 Fragments 391 [This section will probably be removed from this document before it 392 is published as an RFC]. 394 This section includes a non-exhaustive list of operating systems that 395 *fail* to produce IPv6 atomic fragments. It is based on the results 396 published in [RFC6946] and [Morbitzer]. 398 The following Operating Systems fail to generate IPv6 atomic 399 fragments in response to ICMPv6 PTB messages that report an MTU 400 smaller than 1280 bytes: 402 o FreeBSD 8.0 404 o Linux kernel 2.6.32 406 o Linux kernel 3.2 408 o Mac OS X 10.6.7 410 o NetBSD 5.1 412 Authors' Addresses 414 Fernando Gont 415 SI6 Networks / UTN-FRH 416 Evaristo Carriego 2644 417 Haedo, Provincia de Buenos Aires 1706 418 Argentina 420 Phone: +54 11 4650 8472 421 Email: fgont@si6networks.com 422 URI: http://www.si6networks.com 423 Will(Shucheng) Liu 424 Huawei Technologies 425 Bantian, Longgang District 426 Shenzhen 518129 427 P.R. China 429 Email: liushucheng@huawei.com 431 Tore Anderson 432 Redpill Linpro 433 Vitaminveien 1A 434 Oslo 0485 435 Norway 437 Phone: +47 959 31 212 438 Email: tore@redpill-linpro.com 439 URI: http://www.redpill-linpro.com