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INFORMATIONAL
Internet Engineering Task Force (IETF) F. Gont
Request for Comments: 8021 SI6 Networks / UTN-FRH
Category: Informational W. Liu
ISSN: 2070-1721 Huawei Technologies
T. Anderson
Redpill Linpro
January 2017
Generation of IPv6 Atomic Fragments Considered Harmful
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. It concludes that the
aforementioned functionality is undesirable and thus documents the
motivation for removing this functionality from an upcoming revision
of the core IPv6 protocol specification (RFC 2460).
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
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). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 7841.
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/rfc8021.
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Copyright Notice
Copyright (c) 2017 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
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publication of this document. Please review these documents
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to this document. Code Components extracted from this document must
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the Trust Legal Provisions and are provided without warranty as
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 .....................................................8
5. Security Considerations .........................................8
6. References ......................................................9
6.1. Normative References .......................................9
6.2. Informative References ....................................10
Acknowledgements ..................................................12
Authors' Addresses ................................................12
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" (PTB) error messages [RFC4443] advertising an 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; rather, they will be "atomic" fragments [RFC6946]
(i.e., they will 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(s) as non-fragmented traffic (since there are not
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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.
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, for example, having
IPv6/IPv4 translators generate an IPv4 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
dropping 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 can subsequently perform
any type of 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 Host 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 Host B
and Host A. If an attacker sends a forged ICMPv6 PTB error message
to Host 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 Host B starts sending IPv6 atomic fragments (in
response to the received ICMPv6 PTB error message), these packets
will be dropped, since we previously noted that IPv6 packets with
extension headers were being dropped between Host B and Host A.
Thus, this situation will result in a DoS scenario.
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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 PTB error messages, an attacker could easily attack
the corresponding peering session by simply sending an ICMPv6 PTB
message with a reported MTU smaller than 1280 bytes. Once the attack
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 filters as per BCP 38
[BCP38] does not help as a countermeasure.
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 filters in the style of BCP 38 on the ICMPv6
payload, the use of extension headers (by the attacker) could make
this difficult, if not impossible.
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 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 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 PTB error
messages, the Destination Cache [RFC4861] is usually updated to
reflect that any subsequent packets to such a destination should
include a Fragment Header. This means that a single ICMPv6 PTB
error message might affect multiple communication instances (e.g.,
TCP connections) with such a destination.
o As noted in Section 3, SIIT (the Stateless IP/ICMP Translation
Algorithm) [RFC6145], including derivative protocols such as
Stateful NAT64 (Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers) [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
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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.
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 destination
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 Fragment Header
5. The value extracted from the low-order 16 bits of the IPv6
fragment header Identification field resulting in an appropriate
IPv4 Identification value
Unfortunately,
1. There exists a fair share of evidence of ICMPv6 PTB error
messages being dropped on the public Internet (for instance, that
is one of the reasons for which Packetization Layer Path MTU
Discovery (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. 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
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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 dropping of IPv6 fragments in
the public Internet [RFC7872], this would mean that the
(unnecessary) use of IPv6 fragmentation might result,
unnecessarily, in a DoS 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 fragments.
5. While [RFC2460] requires that the IPv6 fragment header
Identification field of a fragmented packet be different than
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 a value. Thus, there is no
guarantee that IPv4 fragment Identification collisions will be
avoided or reduced by employing the low-order 16 bits of the IPv6
fragment header Identification field of a packet sent by a source
host. 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 context of IPv6 end up colliding when used in
the context of translated IPv4.
We note that SIIT essentially employs the Fragment Header of IPv6
atomic fragments to signal the translator how to set the Don't
Fragment (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 a Fragment Header). Additionally,
the translator will employ the low-order 16 bits of the IPv6 fragment
header Identification field for setting the IPv4 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 optionless IPv4 header being 20 bytes
shorter than the IPv6 header.
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3. ECMP routing [RFC2992] with more than one translator is employed,
for example, for 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
extracted from the low-order 16 bits of the IPv6 fragment header
Identification field might not result in an appropriate IPv4
Identification: for example, a number of implementations set the IPv6
fragment header Identification field according to the output of a
Pseudorandom Number Generator (PRNG) (see Appendix B of [RFC7739]);
hence, if the translator only employs the low-order 16 bits of such a
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 size 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 its 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.
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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 PTB
error messages containing an 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 that there are
no longer any valid reasons for ICMPv6 PTB error messages
reporting an MTU value smaller than the minimum IPv6 MTU
(1280 bytes). IPv6 nodes should therefore be updated to ignore
ICMPv6 PTB error messages reporting an MTU smaller than 1280 bytes
as invalid.
We note that these recommendations have been incorporated in
[IPv6-PMTUD], [IPv6-Spec], and [RFC7915].
5. Security Considerations
This document briefly discusses the security implications of the
generation of IPv6 atomic fragments and describes one specific DoS
attack vector that leverages the widespread dropping 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 [IPv6-Spec].
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6. References
6.1. 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>.
[BCP38] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000,
<http://www.rfc-editor.org/info/rfc2827>.
[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>.
[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>.
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6.2. 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>.
[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>.
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[IPv6-Spec]
Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", Work in Progress,
draft-ietf-6man-rfc2460bis-08, November 2016.
[IPv6-PMTUD]
McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
"Path MTU Discovery for IP version 6", Work in Progress,
draft-ietf-6man-rfc1981bis-03, October 2016.
[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>.
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Acknowledgements
The authors would like to thank (in alphabetical order) Congxiao Bao,
Bob Briscoe, Carlos Jesus Bernardos Cano, Brian Carpenter, Bob
Hinden, Tatuya Jinmei, Alberto Leiva Popper, Ted Lemon, Xing Li,
Jeroen Massar, Erik Nordmark, Qiong Sun, Joe Touch, 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 Ivan Arce, Guillermo Gont, and
Diego Armando Maradona for their inspiration.
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
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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