IPv6 maintenance Working Group (6man)                            F. Gont
Internet-Draft                                    SI6 Networks / UTN-FRH
Updates: 2460 (if approved)                                       W. Liu
Intended status: Standards Track                     Huawei Technologies Informational                                    W. Liu
Expires: January 5, May 29, 2016                                Huawei Technologies
                                                             T. Anderson
                                                          Redpill Linpro
                                                            July 4,
                                                       November 26, 2015


         Deprecation of the Generation of IPv6 Atomic Fragments


   The core IPv6 specification

   RFC2460 requires that when a host receives an ICMPv6 "Packet Too Big"
   message reporting an MTU smaller than 1280 bytes, the host includes a
   Fragment Header in all subsequent packets sent to that destination,
   without reducing the assumed Path-MTU.  The simplicity with which
   ICMPv6 "Packet Too Big" messages can be forged, coupled with the
   widespread filtering of IPv6 fragments, results in an attack vector
   that can be leveraged for Denial of Service purposes.  This document
   briefly discusses the aforementioned attack vector, and formally updates RFC2460 such that generation of IPv6
   atomic fragments is deprecated, thus eliminating why the
   attack vector. functionality is undesirable.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on January 5, May 29, 2016.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Denial of Service (DoS) attack vector . . . . . . . . . . . .   3
   4.  Additional Considerations . . . . . . . . . . . . . . . . . .   5   4
   5.  Updating RFC2460  . . . . . . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.   6
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   9.   6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8   7
   Appendix A.  Small Survey of OSes that Fail to Produce IPv6
                Atomic Fragments . . . . . . . . . . . . . . . . . .   9   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   [RFC2460] specifies the IPv6 fragmentation mechanism, which allows
   IPv6 packets to be fragmented into smaller pieces such that they fit
   in the Path-MTU to the intended destination(s).

   Section 5 of [RFC2460] states that, when a host receives an ICMPv6
   "Packet Too Big" message [RFC4443] advertising an MTU smaller than
   1280 bytes (the minimum IPv6 MTU), the host is 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 just include a Fragment Header with both the "Fragment
   Offset" and the "M" flag set to 0 (we refer to these packets as
   "atomic fragments").  As required by [RFC6946], these atomic
   fragments are 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 has been to convey
   an appropriate Fragment 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 introduce an
   attack vector that can be exploited for denial of service purposes.

   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),
   this document formally updates [RFC2460], forbidding the generation
   of IPv6 atomic fragments, such we
   conclude that the aforementioned attack vector this functionality is eliminated. undesirable.

   Section 3 describes some possible attack scenarios.  Section 4
   provides additional considerations regarding the usefulness of
   generating IPv6 atomic fragments.  Section 5 formally updates RFC2460
   such that this attack vector is eliminated.

2.  Terminology

   IPv6 atomic fragments
      IPv6 packets that contain a Fragment Header with the Fragment
      Offset set to 0 and the M flag set to 0 (as defined by [RFC6946]).

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Denial of Service (DoS) attack vector

   Let us assume that Host A is communicating with Server B, and that,
   as a result of the widespread filtering of IPv6 packets with
   extension headers (including fragmentation)
   [I-D.ietf-v6ops-ipv6-ehs-in-real-world], 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 packets with IPv6 EHs were being
   dropped between Host A and Server B.  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 ACLs 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, it will be the aforementioned routers
   themselves the ones dropping their own traffic.

   The aforementioned attack vector is exacerbated by the following

   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 messages.  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 4, SIIT [RFC6145] (including derivative
      protocols such as Stateful NAT64 [RFC6146]) is the only technology
      which currently makes 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 [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.

4.  Additional Considerations

   Besides the security assessment provided in Section 3, 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

   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

   3.  Support for IPv6 fragmentation on the IPv6 side of the translator


   o  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.

   o  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

   o  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 [I-D.ietf-v6ops-ipv6-ehs-in-real-world], this
      would mean that the (unnecessary) use of IPv6 fragmentation might
      result, unnecessarily, in a Denial of Service situation even in
      legitimate cases.

   Finally, 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 Fragment ID 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 < 1260
   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
   Fragment Identification on its own (rather than selecting the IPv4
   Fragment ID from the low-order 16-bits of the Fragment Identification
   of atomic fragments), this could possibly lead to IPv4 Fragment ID
   collisions.  However, since a number of implementations set IPv6
   Fragment ID according to the output of a Pseudo-Random Number
   Generator (PRNG) (see Appendix B of
   [I-D.ietf-6man-predictable-fragment-id]) and the translator only
   employs the low-order 16-bits of such value, it is very unlikely that
   relying on the Fragment ID of the IPv6 atomic fragment will result in
   a reduced Fragment ID collision rate (when compared to the case where
   the translator selects each IPv4 Fragment ID on its own).

   Finally, we note that [RFC6145] is currently the only "consumer" of
   IPv6 atomic fragments, and it correctly and diligently notes (in
   Section 6) the possible interoperability problems of relying on IPv6
   atomic fragments, proposing as a workaround that leads to more robust
   behavior and simplified code.

5.  Updating RFC2460

   The following text from Section 5 of [RFC2460]:

      "In response to an IPv6 packet that is sent to an IPv4 destination
      (i.e., a packet that undergoes translation from IPv6 to IPv4), the
      originating IPv6 node may receive an ICMP Packet Too Big message
      reporting a Next-Hop MTU less than 1280.  In that case, the IPv6
      node is not required to reduce the size of subsequent packets to
      less than 1280, but must include a Fragment header in those
      packets so that the IPv6-to-IPv4 translating router can obtain a
      suitable Identification value to use in resulting IPv4 fragments.
      Note that this means the payload may have to be reduced to 1232
      octets (1280 minus 40 for the IPv6 header and 8 for the Fragment
      header), and smaller still if additional extension headers are

   is formally replaced with:

      "An IPv6 node that receives an ICMPv6 Packet Too Big error message
      that reports a Next-Hop MTU smaller than 1280 bytes (the minimum
      IPv6 MTU) MUST NOT include a Fragment header in subsequent packets
      sent to the corresponding destination.  That is, IPv6 nodes MUST
      NOT generate IPv6 atomic fragments."

6.  IANA Considerations

   There are no IANA registries within this document.  The RFC-Editor
   can remove this section before publication of this document as an


6.  Security Considerations

   This document describes a Denial of Service (DoS) attack vector that
   leverages the widespread filtering of IPv6 fragments in the public
   Internet by means of ICMPv6 PTB error messages.  Additionally, it
   formally updates [RFC2460] such  It concludes that this attack vector
   the generation of IPv6 atomic fragments is

8. an undesirable feature.

7.  Acknowledgements

   The authors would like to thank (in alphabetical order) Alberto
   Leiva, Bob Briscoe, Brian Carpenter, Tatuya Jinmei, Jeroen Massar,
   Erik Nordmark, and Ole Troan, for providing valuable comments on
   earlier versions of this document.

   Fernando Gont would like to thank 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 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. 1998, <http://www.rfc-editor.org/info/rfc2460>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997. 1997,

   [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. 2006,

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007. 2007,

   [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. 2007,

   [RFC6145]  Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
              Algorithm", RFC 6145, DOI 10.17487/RFC6145, April 2011.

9.2. 2011,

8.2.  Informative References

   [RFC2923]  Lahey, K., "TCP Problems with Path MTU Discovery",
              RFC 2923, DOI 10.17487/RFC2923, September 2000. 2000,

   [RFC2992]  Hopps, C., "Analysis of an Equal-Cost Multi-Path
              Algorithm", RFC 2992, DOI 10.17487/RFC2992, November 2000. 2000,

   [RFC5927]  Gont, F., "ICMP Attacks against TCP", RFC 5927,
              DOI 10.17487/RFC5927, July 2010. 2010,

   [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. 2010,

   [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. 2011, <http://www.rfc-editor.org/info/rfc6146>.

   [RFC6946]  Gont, F., "Processing of IPv6 "Atomic" Fragments",
              RFC 6946, DOI 10.17487/RFC6946, May 2013. 2013,

              Gont, F., "Security Implications of Predictable Fragment
              Identification Values", draft-ietf-6man-predictable-
              fragment-id-10 (work in progress), June October 2015.

              Gont, F., Linkova, J., Chown, T., and S. LIU,
              "Observations on the Dropping of Packets with IPv6 EH Filtering
              Extension Headers in the Real World",
              draft-ietf-v6ops-ipv6-ehs-in-real-world-00 draft-ietf-v6ops-
              ipv6-ehs-in-real-world-01 (work in progress), April October

              Morbitzer, M., "TCP Idle Scans in IPv6",  Master's Thesis.
              Thesis number: 670. Department of Computing Science,
              Radboud University Nijmegen. August 2013,

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

   [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].

   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
   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

   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

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