[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: (draft-gont-6man-ipv6-atomic-fragments) 00 01 02 03 04 RFC 6946

IPv6 maintenance Working Group (6man)                            F. Gont
Internet-Draft                                       Huawei Technologies
Updates: 2460, 5722 (if approved)                         March 21, 2013
Intended status: Standards Track
Expires: September 22, 2013


                 Processing of IPv6 "atomic" fragments
                draft-ietf-6man-ipv6-atomic-fragments-04

Abstract

   The IPv6 specification allows packets to contain a Fragment Header
   without the packet being actually fragmented into multiple pieces (we
   refer to these packets as "atomic fragments").  Such packets are
   typically sent by hosts that have received an ICMPv6 "Packet Too Big"
   error message that advertises a "Next-Hop MTU" smaller than 1280
   bytes, and are currently processed by some implementations as normal
   "fragmented traffic" (i.e., they are "reassembled" with any other
   queued fragments that supposedly correspond to the same original
   packet).  Thus, an attacker can cause hosts to employ "atomic
   fragments" by forging ICMPv6 "Packet Too Big" error messages, and
   then launch any fragmentation-based attacks against such traffic.
   This document discusses the generation of the aforementioned "atomic
   fragments" and the corresponding security implications.
   Additionally, this document formally updates RFC 2460 and RFC 5722
   such that IPv6 atomic fragments are processed independently of any
   other fragments, thus completely eliminating the aforementioned
   attack vector.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 22, 2013.

Copyright Notice



Gont                   Expires September 22, 2013               [Page 1]

Internet-Draft            IPv6 atomic fragments               March 2013


   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Generation of IPv6 'atomic fragments'  . . . . . . . . . . . .  6
   4.  Updating RFC 2460 and RFC 5722 . . . . . . . . . . . . . . . .  8
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Appendix A.  Survey of processing of IPv6 atomic fragments by
                different operating systems . . . . . . . . . . . . . 13
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14






















Gont                   Expires September 22, 2013               [Page 2]

Internet-Draft            IPv6 atomic fragments               March 2013


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).  [RFC2460] allowed
   fragments to overlap, thus leading to ambiguity in the result of the
   reassembly process, which could be leveraged by attackers to bypass
   firewall rules and/or evade Network Intrusion Detection Systems
   (NIDS) [RFC5722].

   [RFC5722] forbid overlapping fragments, specifying that when
   overlapping fragments are detected, all the fragments corresponding
   to that packet must be silently discarded.

   As specified in Section 5 of [RFC2460], when a host receives an
   ICMPv6 "Packet Too Big" message advertising a "Next-Hop MTU" smaller
   than 1280 (the minimum IPv6 MTU), it 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 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").
   IPv6/IPv4 translators employ the Fragment Identification information
   found in the Fragment Header to select an appropriate Fragment
   Identification value for the resulting IPv4 fragments.

   While these packets are really "atomic fragments" (they can be
   processed by the IPv6 module and handed to the upper-layer protocol
   without waiting for any other fragments), many IPv6 implementations
   process them as regular fragments.  Namely, they try to perform IPv6
   fragment reassembly with the "atomic fragment" and any other
   fragments already queued with the same set {IPv6 Source Address, IPv6
   Destination Address, Fragment Identification}.  For example, in the
   case of IPv6 implementations that have been updated to support
   [RFC5722], if a fragment with the same {IPv6 Source Address, IPv6
   Destination Address, Fragment Identification} is already queued for
   reassembly at a host when an "atomic fragment" is received with the
   same set {IPv6 Source Address, IPv6 Destination Address, Fragment
   Identification}, and both fragments overlap, all the fragments will
   be silently discarded.

   Processing of IPv6 "atomic fragments" as regular fragmented packets
   clearly provides an unnecessary vector to perform fragmentation-based
   attacks against non-fragmented traffic (i.e., IPv6 datagrams that are
   not really split into multiple pieces, but that just include a
   Fragment Header).

   IPv6 fragmentation attacks have been discussed in great detail in



Gont                   Expires September 22, 2013               [Page 3]

Internet-Draft            IPv6 atomic fragments               March 2013


   [I-D.gont-6man-predictable-fragment-id] and [CPNI-IPv6], and
   [RFC5722] describes a specific firewall-circumvention attack that
   could be performed by leveraging overlapping fragments.  The possible
   IPv6 fragmentation-based attacks are, in most cases, "ports" of the
   IPv4 fragmentation attacks discussed in [RFC6274].

   Section 3 describes the generation of IPv6 "atomic fragments", and
   how they can be remotely "triggered" by a remote attacker.  Section 4
   formally updates [RFC2460] and [RFC5722] such that the aforementioned
   attack vector is eliminated.  Appendix A contains a survey of the
   generation and processing of IPv6 atomic fragments in different
   versions of a number of popular IPv6 implementations.







































Gont                   Expires September 22, 2013               [Page 4]

Internet-Draft            IPv6 atomic fragments               March 2013


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.

   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].










































Gont                   Expires September 22, 2013               [Page 5]

Internet-Draft            IPv6 atomic fragments               March 2013


3.  Generation of IPv6 'atomic fragments'

   Section 5 of [RFC2460] states:

      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.

   This means that any ICMPv6 "Packet Too Big" message advertising a
   "Next-Hop MTU" smaller than 1280 could trigger the generation of the
   so-called "atomic fragments" (i.e., IPv6 datagrams that include a
   Fragment Header, but that are composed of a single fragment, with
   both the "Fragment Offset" and the "M" fields of the Fragment Header
   set to 0).  This can be leveraged to perform a variety of
   fragmentation-based attacks [I-D.gont-6man-predictable-fragment-id]
   [CPNI-IPv6].

      For example, an attacker could forge IPv6 fragments with an
      appropriate {IPv6 Source Address, IPv6 Destination Address,
      Fragment Identification} tuple, such that these malicious
      fragments are incorrectly "reassembled" by the destination host
      together with some of the legitimate fragments of the original
      packet, thus leading to packet drops (and a potential Denial of
      Service).

   From a security standpoint, this situation is exacerbated by the
   following factors:

   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.

   o  Upon receipt of one of the aforementioned ICMPv6 "Packet Too Big"
      error messages, the Destination Cache [RFC4861] is usually updated



Gont                   Expires September 22, 2013               [Page 6]

Internet-Draft            IPv6 atomic fragments               March 2013


      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  Some implementations employ predictable Fragment Identification
      values, thus greatly improving the chances of an attacker of
      successfully performing fragmentation-based attacks
      [I-D.gont-6man-predictable-fragment-id].










































Gont                   Expires September 22, 2013               [Page 7]

Internet-Draft            IPv6 atomic fragments               March 2013


4.  Updating RFC 2460 and RFC 5722

   Section 4.5 of [RFC2460] and Section 4 of [RFC5722] are updated as
   follows:

      A host that receives an IPv6 packet which includes a Fragment
      Header with the "Fragment Offset" equal to 0 and the "M" flag
      equal to 0 MUST process such packet in isolation from any other
      packets/fragments, even if such packets/fragments contain the same
      set {IPv6 Source Address, IPv6 Destination Address, Fragment
      Identification}.  A received "atomic fragments" should be
      "reassembled" from the contents of that sole fragment.

         The Unfragmentable Part of the reassembled packet consists of
         all headers up to, but not including, the Fragment header of
         the received atomic fragment.

         The Next Header field of the last header of the Unfragmentable
         Part of the reassembled packet is obtained from the Next Header
         field of the Fragment header of the received atomic fragment.

         The Payload Length of the reassembled packet is obtained by
         subtracting the length of the Fragment Header (that is, 8) from
         the Payload Length of the received atomic fragment.

      Additionally, if any fragments with the same set {IPv6 Source
      Address, IPv6 Destination Address, Fragment Identification} are
      present in the fragment reassembly queue when the atomic fragment
      is received, such fragments MUST NOT be discarded upon receipt of
      the "colliding" IPv6 atomic fragment, since IPv6 atomic fragments
      MUST NOT interfere with "normal" fragmented traffic.




















Gont                   Expires September 22, 2013               [Page 8]

Internet-Draft            IPv6 atomic fragments               March 2013


5.  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.














































Gont                   Expires September 22, 2013               [Page 9]

Internet-Draft            IPv6 atomic fragments               March 2013


6.  Security Considerations

   This document describes how the traditional processing of IPv6 atomic
   fragments enables the exploitation of fragmentation-based attacks
   (such as those described in [I-D.gont-6man-predictable-fragment-id]
   and [CPNI-IPv6]).  This document formally updates [RFC2460] and
   [RFC5722], such that IPv6 atomic fragments are processed
   independently of any other fragments, thus completely eliminating the
   aforementioned attack vector.










































Gont                   Expires September 22, 2013              [Page 10]

Internet-Draft            IPv6 atomic fragments               March 2013


7.  Acknowledgements

   The author would like to thank (in alphabetical order) Tore Anderson,
   Ran Atkinson, Remi Despres, Stephen Farrell, Brian Haberman, Timothy
   Hartrick, Steinar Haug, Philip Homburg, Simon Josefsson, Simon
   Perreault, Sean Turner, Florian Weimer, and Bjoern A. Zeeb, for
   providing valuable comments on earlier versions of this document.
   Additionally, the author would like to thank Alexander Bluhm, who
   implemented this specification for OpenBSD.

   This document is based on the technical report "Security Assessment
   of the Internet Protocol version 6 (IPv6)" [CPNI-IPv6] authored by
   Fernando Gont on behalf of the UK Centre for the Protection of
   National Infrastructure (CPNI).

   Fernando Gont would like to thank CPNI (http://www.cpni.gov.uk) for
   their continued support.


































Gont                   Expires September 22, 2013              [Page 11]

Internet-Draft            IPv6 atomic fragments               March 2013


8.  References

8.1.  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.

   [RFC5722]  Krishnan, S., "Handling of Overlapping IPv6 Fragments",
              RFC 5722, December 2009.

8.2.  Informative References

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

   [RFC6274]  Gont, F., "Security Assessment of the Internet Protocol
              Version 4", RFC 6274, July 2011.

   [CPNI-IPv6]
              Gont, F., "Security Assessment of the Internet Protocol
              version 6 (IPv6)",  UK Centre for the Protection of
              National Infrastructure, (available on request).

   [I-D.gont-6man-predictable-fragment-id]
              Gont, F., "Security Implications of Predictable Fragment
              Identification Values",
              draft-gont-6man-predictable-fragment-id-03 (work in
              progress), January 2013.













Gont                   Expires September 22, 2013              [Page 12]

Internet-Draft            IPv6 atomic fragments               March 2013


Appendix A.  Survey of processing of IPv6 atomic fragments by different
             operating systems

   This section includes a survey of the support of IPv6 atomic
   fragments in popular operating systems, as tested in October 30,
   2012.

   +---------------------+---------------------+-----------------------+
   |   Operating System  |   Generates atomic  |    Implements this    |
   |                     |      fragments      |     specification     |
   +---------------------+---------------------+-----------------------+
   |     FreeBSD 8.0     |          No         |           No          |
   +---------------------+---------------------+-----------------------+
   |     FreeBSD 8.2     |         Yes         |           No          |
   +---------------------+---------------------+-----------------------+
   |     FreeBSD 9.0     |         Yes         |           No          |
   +---------------------+---------------------+-----------------------+
   |    Linux 3.0.0-15   |         Yes         |          Yes          |
   +---------------------+---------------------+-----------------------+
   |      NetBSD 5.1     |          No         |           No          |
   +---------------------+---------------------+-----------------------+
   |    NetBSD-current   |          No         |          Yes          |
   +---------------------+---------------------+-----------------------+
   |   OpenBSD-current   |         Yes         |          Yes          |
   +---------------------+---------------------+-----------------------+
   |      Solaris 11     |         Yes         |          Yes          |
   +---------------------+---------------------+-----------------------+
   |    Windows XP SP2   |         Yes         |           No          |
   +---------------------+---------------------+-----------------------+
   |    Windows Vista    |         Yes         |           No          |
   |     (Build 6000)    |                     |                       |
   +---------------------+---------------------+-----------------------+
   |    Windows 7 Home   |         Yes         |           No          |
   |       Premium       |                     |                       |
   +---------------------+---------------------+-----------------------+

      Table 1: Processing of IPv6 atomic fragments by different OSes

      In the table above, "generates atomic fragments" notes whether an
      implementation generates atomic fragments in response to received
      ICMPv6 Packet Too Big error messages that advertise a MTU smaller
      than 1280 bytes.









Gont                   Expires September 22, 2013              [Page 13]

Internet-Draft            IPv6 atomic fragments               March 2013


Author's Address

   Fernando Gont
   Huawei Technologies
   Evaristo Carriego 2644
   Haedo, Provincia de Buenos Aires  1706
   Argentina

   Phone: +54 11 4650 8472
   Email: fgont@si6networks.com









































Gont                   Expires September 22, 2013              [Page 14]


Html markup produced by rfcmarkup 1.109, available from https://tools.ietf.org/tools/rfcmarkup/