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2 IPv6 maintenance Working Group (6man) F. Gont
3 Internet-Draft SI6 Networks / UTN-FRH
4 Intended status: Informational W. Liu
5 Expires: March 16, 2017 Huawei Technologies
6 T. Anderson
7 Redpill Linpro
8 September 12, 2016
10 Generation of IPv6 Atomic Fragments Considered Harmful
11 draft-ietf-6man-deprecate-atomfrag-generation-08
13 Abstract
15 This document discusses the security implications of the generation
16 of IPv6 atomic fragments and a number of interoperability issues
17 associated with IPv6 atomic fragments, and concludes that the
18 aforementioned functionality is undesirable, thus documenting the
19 motivation for removing this functionality in the revision of the
20 core IPv6 protocol specification.
22 Status of This Memo
24 This Internet-Draft is submitted in full conformance with the
25 provisions of BCP 78 and BCP 79.
27 Internet-Drafts are working documents of the Internet Engineering
28 Task Force (IETF). Note that other groups may also distribute
29 working documents as Internet-Drafts. The list of current Internet-
30 Drafts is at http://datatracker.ietf.org/drafts/current/.
32 Internet-Drafts are draft documents valid for a maximum of six months
33 and may be updated, replaced, or obsoleted by other documents at any
34 time. It is inappropriate to use Internet-Drafts as reference
35 material or to cite them other than as "work in progress."
37 This Internet-Draft will expire on March 16, 2017.
39 Copyright Notice
41 Copyright (c) 2016 IETF Trust and the persons identified as the
42 document authors. All rights reserved.
44 This document is subject to BCP 78 and the IETF Trust's Legal
45 Provisions Relating to IETF Documents
46 (http://trustee.ietf.org/license-info) in effect on the date of
47 publication of this document. Please review these documents
48 carefully, as they describe your rights and restrictions with respect
49 to this document. Code Components extracted from this document must
50 include Simplified BSD License text as described in Section 4.e of
51 the Trust Legal Provisions and are provided without warranty as
52 described in the Simplified BSD License.
54 Table of Contents
56 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
57 2. Security Implications of the Generation of IPv6 Atomic
58 Fragments . . . . . . . . . . . . . . . . . . . . . . . . . . 3
59 3. Additional Considerations . . . . . . . . . . . . . . . . . . 5
60 4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 7
61 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
62 6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
63 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
64 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
65 8.1. Normative References . . . . . . . . . . . . . . . . . . 8
66 8.2. Informative References . . . . . . . . . . . . . . . . . 9
67 Appendix A. Small Survey of OSes that Fail to Produce IPv6
68 Atomic Fragments . . . . . . . . . . . . . . . . . . 10
69 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
71 1. Introduction
73 [RFC2460] specifies the IPv6 fragmentation mechanism, which allows
74 IPv6 packets to be fragmented into smaller pieces such that they can
75 fit in the Path-MTU to the intended destination(s).
77 A legacy IPv4/IPv6 translator implementing the Stateless IP/ICMP
78 Translation algorithm [RFC6145] may legitimately generate ICMPv6
79 "Packet Too Big" messages [RFC4443] advertising a "Next-Hop MTU"
80 smaller than 1280 (the minimum IPv6 MTU). Section 5 of [RFC2460]
81 states that, upon receiving such an ICMPv6 error message, hosts are
82 not required to reduce the assumed Path-MTU, but must simply include
83 a Fragment Header in all subsequent packets sent to that destination.
84 The resulting packets will thus *not* be actually fragmented into
85 several pieces, but rather be "atomic fragments" [RFC6946] (i.e.,
86 just include a Fragment Header with both the "Fragment Offset" and
87 the "M" flag set to 0). [RFC6946] requires that these atomic
88 fragments be essentially processed by the destination host as non-
89 fragmented traffic (since there are not really any fragments to be
90 reassembled). The goal of these atomic fragments is simply to convey
91 an appropriate Identification value to be employed by IPv6/IPv4
92 translators for the resulting IPv4 fragments.
94 While atomic fragments might seem rather benign, there are scenarios
95 in which the generation of IPv6 atomic fragments can be leveraged for
96 performing a number of attacks against the corresponding IPv6 flows.
98 Since there are concrete security implications arising from the
99 generation of IPv6 atomic fragments, and there is no real gain in
100 generating IPv6 atomic fragments (as opposed to e.g. having IPv6/IPv4
101 translators generate a Fragment Identification value themselves), we
102 conclude that this functionality is undesirable.
104 Section 2 briefly discusses the security implications of the
105 generation of IPv6 atomic fragments, and describes a specific Denial
106 of Service (DoS) attack vector that leverages the widespread
107 filtering of IPv6 fragments in the public Internet. Section 3
108 provides additional considerations regarding the usefulness of
109 generating IPv6 atomic fragments.
111 2. Security Implications of the Generation of IPv6 Atomic Fragments
113 The security implications of IP fragmentation have been discussed at
114 length in [RFC6274] and [RFC7739]. An attacker can leverage the
115 generation of IPv6 atomic fragments to trigger the use of
116 fragmentation in an arbitrary IPv6 flow (in scenarios in which actual
117 fragmentation of packets is not needed), and subsequently perform any
118 fragmentation-based attack against legacy IPv6 nodes that do not
119 implement [RFC6946]. That is, employing fragmentation where not
120 actually needed allows for fragmentation-based attack vectors to be
121 employed, unnecessarily.
123 We note that, Unfortunately, even nodes that already implement
124 [RFC6946] can be subject to DoS attacks as a result of the generation
125 of IPv6 atomic fragments. Let us assume that Host A is communicating
126 with Server B, and that, as a result of the widespread dropping of
127 IPv6 packets that contain extension headers (including fragmentation)
128 [RFC7872], some intermediate node filters fragments between Server B
129 and Host A. If an attacker sends a forged ICMPv6 "Packet Too Big"
130 (PTB) error message to server B, reporting an MTU smaller than 1280,
131 this will trigger the generation of IPv6 atomic fragments from that
132 moment on (as required by [RFC2460]). When server B starts sending
133 IPv6 atomic fragments (in response to the received ICMPv6 PTB), these
134 packets will be dropped, since we previously noted that IPv6 packets
135 with extension headers were being dropped between Server B and Host
136 A. Thus, this situation will result in a Denial of Service (DoS)
137 scenario.
139 Another possible scenario is that in which two BGP peers are
140 employing IPv6 transport, and they implement Access Control Lists
141 (ACLs) to drop IPv6 fragments (to avoid control-plane attacks). If
142 the aforementioned BGP peers drop IPv6 fragments but still honor
143 received ICMPv6 Packet Too Big error messages, an attacker could
144 easily attack the peering session by simply sending an ICMPv6 PTB
145 message with a reported MTU smaller than 1280 bytes. Once the attack
146 packet has been sent, the aforementioned routers will themselves be
147 the ones dropping their 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 message. 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 3, SIIT (Stateless IP/ICMP Translation
181 Algorithm) [RFC6145], including derivative protocols such as
182 Stateful NAT64 [RFC6146], was the only technology making use of
183 atomic fragments. Unfortunately, an IPv6 node cannot easily limit
184 its exposure to the aforementioned attack vector by only
185 generating IPv6 atomic fragments towards IPv4 destinations behind
186 a stateless translator. This is due to the fact that Section 3.3
187 of [RFC6052] encourages operators to use a Network-Specific Prefix
188 (NSP) that maps the IPv4 address space into IPv6. When an NSP is
189 being used, IPv6 addresses representing IPv4 nodes (reached
190 through a stateless translator) are indistinguishable from native
191 IPv6 addresses.
193 3. Additional Considerations
195 Besides the security assessment provided in Section 2, it is
196 interesting to evaluate the pros and cons of having an IPv6-to-IPv4
197 translating router rely on the generation of IPv6 atomic fragments.
199 Relying on the generation of IPv6 atomic fragments implies a reliance
200 on:
202 1. ICMPv6 packets arriving from the translator to the IPv6 node
204 2. The ability of the nodes receiving ICMPv6 PTB messages reporting
205 an MTU smaller than 1280 bytes to actually produce atomic
206 fragments
208 3. Support for IPv6 fragmentation on the IPv6 side of the translator
210 4. The ability of the translator implementation to access the
211 information conveyed by the IPv6 Fragment Header
213 5. The value extracted from the low-order 16-bits of the IPv6
214 fragment Identification resulting in an appropriate IPv4
215 Identification value
217 Unfortunately,
219 1. There exists a fair share of evidence of ICMPv6 Packet Too Big
220 messages being dropped on the public Internet (for instance, that
221 is one of the reasons for which PLPMTUD [RFC4821] was produced).
222 Therefore, relying on such messages being successfully delivered
223 will affect the robustness of the protocol that relies on them.
225 2. A number of IPv6 implementations have been known to fail to
226 generate IPv6 atomic fragments in response to ICMPv6 PTB messages
227 reporting an MTU smaller than 1280 bytes (see Appendix A for a
228 small survey). Additionally, the results included in Section 6
229 of [RFC6145] note that 57% of the tested web servers failed to
230 produce IPv6 atomic fragments in response to ICMPv6 PTB messages
231 reporting an MTU smaller than 1280 bytes. Thus, any protocol
232 relying on IPv6 atomic fragment generation for proper functioning
233 will have interoperability problems with the aforementioned IPv6
234 stacks.
236 3. IPv6 atomic fragment generation represents a case in which
237 fragmented traffic is produced where otherwise it would not be
238 needed. Since there is widespread filtering of IPv6 fragments in
239 the public Internet [RFC7872], this would mean that the
240 (unnecessary) use of IPv6 fragmentation might result,
241 unnecessarily, in a Denial of Service situation even in
242 legitimate cases.
244 4. The packet-handling API at the node where the translator is
245 running may obscure fragmentation-related information. In such
246 scenarios, the information conveyed by the Fragment Header may be
247 unavailable to the translator. [JOOL] discusses a sample
248 framework (Linux Netfilter) that hinders access to the
249 information conveyed in IPv6 atomic fragments.
251 5. While [RFC2460] requires that the IPv6 fragment Identification of
252 a fragmented packet be different that of any other fragmented
253 packet sent recently with the same Source Address and Destination
254 Address, there is no requirement on the low-order 16-bits of such
255 value. Thus, there is no guarantee that, by employing the low-
256 order 16-bits of the IPv6 fragment Identification of a packet
257 sent by a source host, IPv4 fragment identification collisions
258 will be avoided or reduced. Besides, collisions might occur
259 where two distinct IPv6 Destination Addresses are translated into
260 the same IPv4 address, such that Identification values that might
261 have been generated to be unique in the IPv6 context end up
262 colliding when used in the translated IPv4 context.
264 We note that SIIT essentially employs the Fragment Header of IPv6
265 atomic fragments to signal the translator how to set the DF bit of
266 IPv4 datagrams (the DF bit is cleared when the IPv6 packet contains a
267 Fragment Header, and is otherwise set to 1 when the IPv6 packet does
268 not contain an IPv6 Fragment Header). Additionally, the translator
269 will employ the low-order 16-bits of the IPv6 Fragment Identification
270 for setting the IPv4 Fragment Identification. At least in theory,
271 this is expected to reduce the IPv4 Identification collision rate in
272 the following specific scenario:
274 1. An IPv6 node communicates with an IPv4 node (through SIIT).
276 2. The IPv4 node is located behind an IPv4 link with an MTU smaller
277 than 1260 bytes. An IPv4 Path MTU of 1260 corresponds to an IPv6
278 Path MTU of 1280, due to an option-less IPv4 header being 20
279 bytes shorter than the IPv6 header.
281 3. ECMP routing [RFC2992] with more than one translator is employed
282 for e.g., redundancy purposes.
284 In such a scenario, if each translator were to select the IPv4
285 Identification on its own (rather than selecting the IPv4
286 Identification from the low-order 16-bits of the Fragment
287 Identification of IPv6 atomic fragments), this could possibly lead to
288 IPv4 Identification collisions. However, as noted above, the value
289 extracted from the low-order 16-bits of the IPv6 fragment
290 Identification might not result in an appropriate IPv4
291 identification: for example, a number of implementations set the IPv6
292 Fragment Identification according to the output of a Pseudo-Random
293 Number Generator (PRNG) (see Appendix B of [RFC7739]); hence,if the
294 translator only employs the low-order 16-bits of such value, it is
295 very unlikely that relying on the Fragment Identification of the IPv6
296 atomic fragment will result in a reduced IPv4 Identification
297 collision rate (when compared to the case where the translator
298 selects each IPv4 Identification on its own). Besides, because of
299 the limited sized of the IPv4 identification field, it is
300 nevertheless virtually impossible to guarantee uniqueness of the IPv4
301 identification values without artificially limiting the data rate of
302 fragmented traffic [RFC6864] [RFC4963].
304 [RFC6145] was the only "consumer" of IPv6 atomic fragments, and it
305 correctly and diligently noted (in Section 6) the possible
306 interoperability problems of relying on IPv6 atomic fragments,
307 proposing a workaround that led to more robust behavior and
308 simplified code. [RFC6145] has been obsoleted by [RFC7915], such
309 that SIIT does not rely on IPv6 atomic fragments.
311 4. Conclusions
313 Taking all of the above considerations into account, we recommend
314 that IPv6 atomic fragments be deprecated.
316 In particular:
318 o IPv4/IPv6 translators should be updated to not generate ICMPv6
319 Packet Too Big errors containing a Path MTU value smaller than the
320 minimum IPv6 MTU of 1280 bytes. This will ensure that current
321 IPv6 nodes will never have a legitimate need to start generating
322 IPv6 atomic fragments.
324 o The recommendation in the previous bullet ensures there no longer
325 are any valid reasons for ICMPv6 Packet Too Big errors containing
326 a Path MTU value smaller than the minimum IPv6 MTU to exist. IPv6
327 nodes should therefore be updated to ignore them as invalid.
329 We note that these recommendations have been incorporated in
330 [I-D.ietf-6man-rfc1981bis], [I-D.ietf-6man-rfc2460bis] and [RFC7915].
332 5. IANA Considerations
334 There are no IANA registries within this document.
336 6. Security Considerations
338 This document briefly discusses the security implications of the
339 generation of IPv6 atomic fragments, and describes one specific
340 Denial of Service (DoS) attack vector that leverages the widespread
341 filtering of IPv6 fragments in the public Internet. It concludes
342 that the generation of IPv6 atomic fragments is an undesirable
343 feature, and documents the motivation for removing this functionality
344 from [I-D.ietf-6man-rfc2460bis].
346 7. Acknowledgements
348 The authors would like to thank (in alphabetical order) Congxiao Bao,
349 Carlos Jesus Bernardos Cano, Bob Briscoe, Brian Carpenter, Tatuya
350 Jinmei, Bob Hinden, Alberto Leiva, Ted Lemon, Xing Li, Jeroen Massar,
351 Erik Nordmark, Joe Touch, Qiong Sun, Ole Troan, Tina Tsou, and Bernie
352 Volz, for providing valuable comments on earlier versions of this
353 document.
355 Fernando Gont would like to thank Jan Zorz / Go6 Lab
356 , and Jared Mauch / NTT America, for providing
357 access to systems and networks that were employed to produce some of
358 the tests that resulted in the publication of this document.
359 Additionally, he would like to thank SixXS
360 for providing IPv6 connectivity.
362 8. References
364 8.1. Normative References
366 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
367 (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
368 December 1998, .
370 [RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
371 Control Message Protocol (ICMPv6) for the Internet
372 Protocol Version 6 (IPv6) Specification", RFC 4443,
373 DOI 10.17487/RFC4443, March 2006,
374 .
376 [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
377 Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
378 .
380 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
381 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
382 DOI 10.17487/RFC4861, September 2007,
383 .
385 [RFC6145] Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
386 Algorithm", RFC 6145, DOI 10.17487/RFC6145, April 2011,
387 .
389 [RFC7915] Bao, C., Li, X., Baker, F., Anderson, T., and F. Gont,
390 "IP/ICMP Translation Algorithm", RFC 7915,
391 DOI 10.17487/RFC7915, June 2016,
392 .
394 [RFC6864] Touch, J., "Updated Specification of the IPv4 ID Field",
395 RFC 6864, DOI 10.17487/RFC6864, February 2013,
396 .
398 8.2. Informative References
400 [RFC2992] Hopps, C., "Analysis of an Equal-Cost Multi-Path
401 Algorithm", RFC 2992, DOI 10.17487/RFC2992, November 2000,
402 .
404 [RFC5927] Gont, F., "ICMP Attacks against TCP", RFC 5927,
405 DOI 10.17487/RFC5927, July 2010,
406 .
408 [RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
409 Errors at High Data Rates", RFC 4963,
410 DOI 10.17487/RFC4963, July 2007,
411 .
413 [RFC6052] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
414 Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
415 DOI 10.17487/RFC6052, October 2010,
416 .
418 [RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
419 NAT64: Network Address and Protocol Translation from IPv6
420 Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
421 April 2011, .
423 [RFC6274] Gont, F., "Security Assessment of the Internet Protocol
424 Version 4", RFC 6274, DOI 10.17487/RFC6274, July 2011,
425 .
427 [RFC6946] Gont, F., "Processing of IPv6 "Atomic" Fragments",
428 RFC 6946, DOI 10.17487/RFC6946, May 2013,
429 .
431 [RFC7739] Gont, F., "Security Implications of Predictable Fragment
432 Identification Values", RFC 7739, DOI 10.17487/RFC7739,
433 February 2016, .
435 [RFC7872] Gont, F., Linkova, J., Chown, T., and W. Liu,
436 "Observations on the Dropping of Packets with IPv6
437 Extension Headers in the Real World", RFC 7872,
438 DOI 10.17487/RFC7872, June 2016,
439 .
441 [I-D.ietf-6man-rfc2460bis]
442 Deering, D. and R. Hinden, "Internet Protocol, Version 6
443 (IPv6) Specification", draft-ietf-6man-rfc2460bis-05 (work
444 in progress), June 2016.
446 [I-D.ietf-6man-rfc1981bis]
447 <>, J., <>, S., <>, J., and R. Hinden, "Path MTU Discovery
448 for IP version 6", draft-ietf-6man-rfc1981bis-02 (work in
449 progress), April 2016.
451 [Morbitzer]
452 Morbitzer, M., "TCP Idle Scans in IPv6", Master's Thesis.
453 Thesis number: 670. Department of Computing Science,
454 Radboud University Nijmegen. August 2013,
455 .
458 [JOOL] Leiva Popper, A., "nf_defrag_ipv4 and nf_defrag_ipv6",
459 April 2015, .
462 Appendix A. Small Survey of OSes that Fail to Produce IPv6 Atomic
463 Fragments
465 [This section will probably be removed from this document before it
466 is published as an RFC].
468 This section includes a non-exhaustive list of operating systems that
469 *fail* to produce IPv6 atomic fragments. It is based on the results
470 published in [RFC6946] and [Morbitzer]. It is simply meant as a
471 datapoint regarding the extent to which IPv6 implementations can be
472 relied upon to generate IPv6 atomic fragments.
474 The following Operating Systems fail to generate IPv6 atomic
475 fragments in response to ICMPv6 PTB messages that report an MTU
476 smaller than 1280 bytes:
478 o FreeBSD 8.0
479 o Linux kernel 2.6.32
481 o Linux kernel 3.2
483 o Mac OS X 10.6.7
485 o NetBSD 5.1
487 Authors' Addresses
489 Fernando Gont
490 SI6 Networks / UTN-FRH
491 Evaristo Carriego 2644
492 Haedo, Provincia de Buenos Aires 1706
493 Argentina
495 Phone: +54 11 4650 8472
496 Email: fgont@si6networks.com
497 URI: http://www.si6networks.com
499 Will(Shucheng) Liu
500 Huawei Technologies
501 Bantian, Longgang District
502 Shenzhen 518129
503 P.R. China
505 Email: liushucheng@huawei.com
507 Tore Anderson
508 Redpill Linpro
509 Vitaminveien 1A
510 Oslo 0485
511 Norway
513 Phone: +47 959 31 212
514 Email: tore@redpill-linpro.com
515 URI: http://www.redpill-linpro.com