Generation of IPv6 Atomic Fragments Considered Harmful
draft-ietf-6man-deprecate-atomfrag-generation-08

IPv6 maintenance Working Group (6man)                            F. Gont
Internet-Draft                                    SI6 Networks / UTN-FRH
Intended status: Informational                                    W. Liu
Expires: March 16, 2017                              Huawei Technologies
                                                             T. Anderson
                                                          Redpill Linpro
                                                      September 12, 2016

         Generation of IPv6 Atomic Fragments Considered Harmful
            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.

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
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   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.

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   to this document.  Code Components extracted from this document must
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   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 . . . . . . . . . . . . . . . . . . .   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

   [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).

   A legacy IPv4/IPv6 translator implementing the Stateless IP/ICMP
   Translation algorithm [RFC6145] may legitimately generate ICMPv6
   "Packet Too Big" messages [RFC4443] advertising a "Next-Hop MTU"
   smaller than 1280 (the minimum IPv6 MTU).  Section 5 of [RFC2460]
   states that, upon receiving such an ICMPv6 error message, hosts are
   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 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.

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   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 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 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 (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].  That is, employing fragmentation where not
   actually needed allows for fragmentation-based attack vectors to be
   employed, unnecessarily.

   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

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   packet has been sent, the aforementioned routers will themselves be
   the ones dropping their own traffic.

   The aforementioned attack vector is exacerbated by the following
   factors:

   o  The attacker does not need to forge the IPv6 Source Address of his
      attack packets.  Hence, deployment of simple BCP38 filters will
      not help as a counter-measure.

   o  Only the IPv6 addresses of the IPv6 packet embedded in the ICMPv6
      payload needs to be forged.  While one could envision filtering
      devices enforcing BCP38-style filters on the ICMPv6 payload, the
      use of extension headers (by the attacker) could make this
      difficult, if at all possible.

   o  Many implementations fail to perform validation checks on the
      received ICMPv6 error messages, as recommended in Section 5.2 of
      [RFC4443] and documented in [RFC5927].  It should be noted that in
      some cases, such as when an ICMPv6 error message has (supposedly)
      been elicited by a connection-less transport protocol (or some
      other connection-less protocol being encapsulated in IPv6), it may
      be virtually impossible to perform validation checks on the
      received ICMPv6 error message.  And, because of IPv6 extension
      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 3, SIIT (Stateless IP/ICMP Translation
      Algorithm) [RFC6145], including derivative protocols such as
      Stateful NAT64 [RFC6146], was the only technology making 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.

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3.  Additional Considerations

   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
       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
       reporting an MTU smaller than 1280 bytes (see Appendix A for a
       small survey).  Additionally, the results included in Section 6
       of [RFC6145] note that 57% of the tested web servers failed to
       produce IPv6 atomic fragments in response to ICMPv6 PTB messages
       reporting an MTU smaller than 1280 bytes.  Thus, any protocol
       relying on IPv6 atomic fragment generation for proper functioning
       will have interoperability problems with the aforementioned IPv6
       stacks.

   3.  IPv6 atomic fragment generation represents a case in which
       fragmented traffic is produced where otherwise it would not be
       needed.  Since there is widespread filtering of IPv6 fragments in
       the public Internet [RFC7872], this would mean that the
       (unnecessary) use of IPv6 fragmentation might result,

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       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:

   1.  An IPv6 node communicates with an IPv4 node (through SIIT).

   2.  The IPv4 node is located behind an IPv4 link with an MTU smaller
       than 1260 bytes.  An IPv4 Path MTU of 1260 corresponds to an IPv6
       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, as noted above, the value

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   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.

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
      IPv6 nodes will never have a legitimate need to start generating
      IPv6 atomic fragments.

   o  The recommendation in the previous bullet ensures there no longer
      are any valid reasons for ICMPv6 Packet Too Big errors containing
      a Path MTU value smaller than the minimum IPv6 MTU to exist.  IPv6
      nodes should therefore be updated to ignore them as invalid.

   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.

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6.  Security Considerations

   This document briefly discusses the security implications of the
   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, 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
   <http://go6lab.si/>, 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 <https://www.sixxs.net>
   for providing IPv6 connectivity.

8.  References

8.1.  Normative References

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <http://www.rfc-editor.org/info/rfc2460>.

   [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,
              <http://www.rfc-editor.org/info/rfc4443>.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
              <http://www.rfc-editor.org/info/rfc4821>.

   [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,
              <http://www.rfc-editor.org/info/rfc4861>.

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   [RFC6145]  Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
              Algorithm", RFC 6145, DOI 10.17487/RFC6145, April 2011,
              <http://www.rfc-editor.org/info/rfc6145>.

   [RFC7915]  Bao, C., Li, X., Baker, F., Anderson, T., and F. Gont,
              "IP/ICMP Translation Algorithm", RFC 7915,
              DOI 10.17487/RFC7915, June 2016,
              <http://www.rfc-editor.org/info/rfc7915>.

   [RFC6864]  Touch, J., "Updated Specification of the IPv4 ID Field",
              RFC 6864, DOI 10.17487/RFC6864, February 2013,
              <http://www.rfc-editor.org/info/rfc6864>.

8.2.  Informative References

   [RFC2992]  Hopps, C., "Analysis of an Equal-Cost Multi-Path
              Algorithm", RFC 2992, DOI 10.17487/RFC2992, November 2000,
              <http://www.rfc-editor.org/info/rfc2992>.

   [RFC5927]  Gont, F., "ICMP Attacks against TCP", RFC 5927,
              DOI 10.17487/RFC5927, July 2010,
              <http://www.rfc-editor.org/info/rfc5927>.

   [RFC4963]  Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
              Errors at High Data Rates", RFC 4963,
              DOI 10.17487/RFC4963, July 2007,
              <http://www.rfc-editor.org/info/rfc4963>.

   [RFC6052]  Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
              Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
              DOI 10.17487/RFC6052, October 2010,
              <http://www.rfc-editor.org/info/rfc6052>.

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
              April 2011, <http://www.rfc-editor.org/info/rfc6146>.

   [RFC6274]  Gont, F., "Security Assessment of the Internet Protocol
              Version 4", RFC 6274, DOI 10.17487/RFC6274, July 2011,
              <http://www.rfc-editor.org/info/rfc6274>.

   [RFC6946]  Gont, F., "Processing of IPv6 "Atomic" Fragments",
              RFC 6946, DOI 10.17487/RFC6946, May 2013,
              <http://www.rfc-editor.org/info/rfc6946>.

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   [RFC7739]  Gont, F., "Security Implications of Predictable Fragment
              Identification Values", RFC 7739, DOI 10.17487/RFC7739,
              February 2016, <http://www.rfc-editor.org/info/rfc7739>.

   [RFC7872]  Gont, F., Linkova, J., Chown, T., and W. Liu,
              "Observations on the Dropping of Packets with IPv6
              Extension Headers in the Real World", RFC 7872,
              DOI 10.17487/RFC7872, June 2016,
              <http://www.rfc-editor.org/info/rfc7872>.

   [I-D.ietf-6man-rfc2460bis]
              Deering, D. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", draft-ietf-6man-rfc2460bis-05 (work
              in progress), June 2016.

   [I-D.ietf-6man-rfc1981bis]
              <>, J., <>, S., <>, J., and R. Hinden, "Path MTU Discovery
              for IP version 6", draft-ietf-6man-rfc1981bis-02 (work in
              progress), April 2016.

   [Morbitzer]
              Morbitzer, M., "TCP Idle Scans in IPv6",  Master's Thesis.
              Thesis number: 670. Department of Computing Science,
              Radboud University Nijmegen. August 2013,
              <http://www.ru.nl/publish/pages/769526/
              m_morbitzer_masterthesis.pdf>.

   [JOOL]     Leiva Popper, A., "nf_defrag_ipv4 and nf_defrag_ipv6",
               April 2015, <https://github.com/NICMx/Jool/wiki/
              nf_defrag_ipv4-and-nf_defrag_ipv6#implementation-gotchas>.

Appendix A.  Small Survey of OSes that Fail to Produce IPv6 Atomic
             Fragments

   [This section will probably be removed from this document before it
   is published as an RFC].

   This section includes a non-exhaustive list of operating systems that
   *fail* to produce IPv6 atomic fragments.  It is based on the results
   published in [RFC6946] and [Morbitzer].  It is simply meant as a
   datapoint regarding the extent to which IPv6 implementations can be
   relied upon to generate IPv6 atomic fragments.

   The following Operating Systems fail to generate IPv6 atomic
   fragments in response to ICMPv6 PTB messages that report an MTU
   smaller than 1280 bytes:

   o  FreeBSD 8.0

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   o  Linux kernel 2.6.32

   o  Linux kernel 3.2

   o  Mac OS X 10.6.7

   o  NetBSD 5.1

Authors' Addresses

   Fernando Gont
   SI6 Networks / UTN-FRH
   Evaristo Carriego 2644
   Haedo, Provincia de Buenos Aires  1706
   Argentina

   Phone: +54 11 4650 8472
   Email: fgont@si6networks.com
   URI:   http://www.si6networks.com

   Will(Shucheng) Liu
   Huawei Technologies
   Bantian, Longgang District
   Shenzhen  518129
   P.R. China

   Email: liushucheng@huawei.com

   Tore Anderson
   Redpill Linpro
   Vitaminveien 1A
   Oslo  0485
   Norway

   Phone: +47 959 31 212
   Email: tore@redpill-linpro.com
   URI:   http://www.redpill-linpro.com

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