Deprecating the Generation of IPv6 Atomic Fragments
draft-gont-6man-deprecate-atomfrag-generation-00

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
Expires: February 20, 2015                               August 19, 2014

          Deprecating the Generation of IPv6 Atomic Fragments
            draft-gont-6man-deprecate-atomfrag-generation-00

Abstract

   The core IPv6 specification requires that when a host receives an
   ICMPv6 "Packet Too Big" message reporting a "Next-Hop MTU" smaller
   than 1280, 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 deprecates the generation of IPv6 atomic
   fragments, such that the aforementioned attack vector is eliminated.

Status of This Memo

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   Copyright (c) 2014 IETF Trust and the persons identified as the
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   This document is subject to BCP 78 and the IETF Trust's Legal
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   (http://trustee.ietf.org/license-info) in effect on the date of

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   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Denial of Service (DoS) attack vector . . . . . . . . . . . .   3
   4.  Updating RFC2460  . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Additional Considerations . . . . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

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 a "Next-Hop MTU"
   smaller than 1280 (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-fragment traffic (since there are not really any fragments to be
   reassembled).  IPv6/IPv4 translators will typically employ the
   Fragment Identification information found in the Fragment Header to
   select an appropriate Fragment Identification value 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

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   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 that the aforementioned attack vector
   is eliminated.

   Section 3 describes some possible attack scenarios.  Section 5
   provides additional considerations regarding the usefulness of
   generating IPv6 atomic fragments.  Section 4 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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   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.gont-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 a Next-Hop MTU smaller than 1280, this will trigger the
   generation of IPv6 atomic fragments from that moment on (as required
   by [RFC2460]).  When server 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 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 fired, it
   will be the aforementioned routers themselves the ones dropping their
   own traffic.

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   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 need to be forged.  While one could envision filtering
      devices enforcing BCP38-style filters on the ICMPv6 payload, the
      use of extension (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.

4.  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
      used."

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   is formally replaced with:

      "IPv6 nodes MUST discard ICMPv6 Packet Too Big error messages that
      report a Next-Hop MTU smaller than 1280 bytes (the minimum IPv6
      MTU)."

5.  Additional Considerations

   Besides the security assessment provided in Section 3, it is
   interesting to evaluate if there is any gain in generating IPv6
   atomic fragments (to provide for Fragment Identification value) as
   opposed to just let IPv6/IPv4 translators select an appropriate IPv4
   Fragment Identification value.

   After some analysis, one can conclude that, if anything, an IPv6/IPv4
   translator is in a much better position to select an appropriate
   Fragment Identification value for the packet that are to be
   translated from the IPv6 to the IPv4 world.  For instance, an IPv6
   node will generate Fragment Identification values without any
   knowledge of the Fragment ID values being generated by other IPv6
   nodes employing the translator.  Thus, an IPv6/IPv4 translator is in
   a much better position to generate Fragment IDs that will not result
   in collisions (i.e., that will not be reused for the same tuple
   {Source Address, Destination Address}.

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
   RFC.

7.  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 that this attack vector is
   eliminated.

8.  Acknowledgements

   Fernando Gont would like to thank Jan Zorz and Go6 Lab
   <http://go6lab.si/> for providing access to systems and networks that
   were employed to produce some of the measurement results presented in
   this document.  Additionally, he would like to thank SixXS
   <https://www.sixxs.net> for providing IPv6 connectivity.

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9.  References

9.1.  Normative References

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

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

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

9.2.  Informative References

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

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

   [I-D.gont-v6ops-ipv6-ehs-in-real-world]
              Gont, F., Linkova, J., Chown, T., and W. Will, "IPv6
              Extension Headers in the Real World", draft-gont-v6ops-
              ipv6-ehs-in-real-world-00 (work in progress), August 2014.

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

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

   Email: liushucheng@huawei.com

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