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IETF RFC 6946
Last modified on Tuesday, May 14th, 2013
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Internet Engineering Task Force (IETF) F. Gont
Request for Comments: 6946 Huawei Technologies
Updates: 2460, 5722 May 2013
Category: Standards Track
ISSN: 2070-1721
Processing of IPv6 "Atomic" Fragments
 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 is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 6946.
Gont Standards Track PAGE 1 
RFC 6946 IPv6 Atomic Fragments May 2013
Copyright Notice
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 ....................................................2
2. Terminology .....................................................4
3. Generation of IPv6 Atomic Fragments .............................4
4. Updating RFC 2460 and RFC 5722 ..................................5
5. Security Considerations .........................................6
6. Acknowledgements ................................................6
7. References ......................................................7
7.1. Normative References .......................................7
7.2. Informative References .....................................7
Appendix A. Survey of Processing of IPv6 Atomic Fragments by
Different Operating Systems ............................8
1. Introduction
[RFC 2460] 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). [RFC 2460] allows
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) [RFC 5722].
[RFC 5722] forbids 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 [RFC 2460], 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
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RFC 6946 IPv6 Atomic Fragments May 2013
will thus not actually be fragmented into several pieces but will
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
[RFC 5722], 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
[PREDICTABLE-ID] and [CPNI-IPv6], and [RFC 5722] 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 [RFC 6274].
Section 3 describes the generation of IPv6 atomic fragments and how
they can be remotely "triggered" by a remote attacker. Section 4
formally updates [RFC 2460] and [RFC 5722] 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.
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RFC 6946 IPv6 Atomic Fragments May 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 [RFC 2119].
3. Generation of IPv6 Atomic Fragments
Section 5 of [RFC 2460] 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 [PREDICTABLE-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).
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RFC 6946 IPv6 Atomic Fragments May 2013
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
[RFC 4443] and documented in [RFC 5927]. It should be noted that in
some cases, such as when an ICMPv6 error message has (supposedly)
been elicited by a connectionless transport protocol (or some
other connectionless 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 [RFC 4861] is usually updated
to reflect that any subsequent packets to that 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 that IPv6 destination
address.
o Some implementations employ predictable Fragment Identification
values, thus greatly improving the chances of an attacker
successfully performing fragmentation-based attacks
[PREDICTABLE-ID].
4. Updating RFC 2460 and RFC 5722
Section 4.5 of [RFC 2460] and Section 4 of [RFC 5722] are updated as
follows:
A host that receives an IPv6 packet that includes a Fragment
Header with the "Fragment Offset" equal to 0 and the "M" flag
equal to 0 MUST process that 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 fragment 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.
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RFC 6946 IPv6 Atomic Fragments May 2013
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.
5. 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 [PREDICTABLE-ID] and [CPNI-IPv6]). 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.
6. 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).
Finally, the author wishes to thank Nelida Garcia and Guillermo Gont
for their love and support.
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RFC 6946 IPv6 Atomic Fragments May 2013
7. References
7.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC 4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
Message Protocol (ICMPv6) for the Internet Protocol
Version 6 (IPv6) Specification", RFC 4443, March 2006.
[RFC 4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC 5722] Krishnan, S., "Handling of Overlapping IPv6 Fragments",
RFC 5722, December 2009.
7.2. Informative References
[CPNI-IPv6]
Gont, F., "Security Assessment of the Internet Protocol
version 6 (IPv6)", UK Centre for the Protection of
National Infrastructure, (available on request).
[PREDICTABLE-ID]
Gont, F., "Security Implications of Predictable Fragment
Identification Values", Work in Progress, March 2013.
[RFC 5927] Gont, F., "ICMP Attacks against TCP", RFC 5927, July 2010.
[RFC 6274] Gont, F., "Security Assessment of the Internet Protocol
Version 4", RFC 6274, July 2011.
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RFC 6946 IPv6 Atomic Fragments May 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 on 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 an MTU
smaller than 1280 bytes.
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RFC 6946 IPv6 Atomic Fragments May 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 Standards Track PAGE 9
RFC TOTAL SIZE: 18843 bytes
PUBLICATION DATE: Tuesday, May 14th, 2013
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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