Internet DRAFT - draft-ietf-6man-deprecate-atomfrag-generation
draft-ietf-6man-deprecate-atomfrag-generation
IPv6 maintenance Working Group (6man) F. Gont
Internet-Draft SI6 Networks / UTN-FRH
Intended status: Informational W. Liu
Expires: October 6, 2016 Huawei Technologies
T. Anderson
Redpill Linpro
April 4, 2016
Generation of IPv6 Atomic Fragments Considered Harmful
draft-ietf-6man-deprecate-atomfrag-generation-06
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, and concludes that the
aforementioned functionality is undesirable, thus documenting the
motivation for removing this functionality in the revision of the
core IPv6 protocol specification.
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 October 6, 2016.
Copyright Notice
Copyright (c) 2016 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
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to this document. Code Components extracted from this document must
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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 . . . . . . . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Small Survey of OSes that Fail to Produce IPv6
Atomic Fragments . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
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).
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 be "atomic fragments" [RFC6946] (i.e., 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 as non-fragmented traffic (since
there are not 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 e.g. having IPv6/IPv4
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translators generate a Fragment 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
filtering 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 and subsequently perform any
fragmentation-based attack against legacy IPv6 nodes that do not
implement [RFC6946].
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 Server B,
and that, as a result of the widespread dropping of IPv6 packets that
contain 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 IPv6 packets with extension headers
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 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
factors:
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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 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 "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 3, 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] 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:
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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
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 IPv6 Fragment Header
Unfortunately,
1. 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.
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 (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
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 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.
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 atomic fragments.
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
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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 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
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
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, since a number of
implementations set the IPv6 Fragment Identification according to the
output of a Pseudo-Random Number Generator (PRNG) (see Appendix B of
[RFC7739]) and the translator only employs the low-order 16-bits of
such 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).
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.
4. IANA Considerations
There are no IANA registries within this document.
5. Security Considerations
This document 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
filtering 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 [I-D.ietf-6man-rfc2460bis].
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6. Acknowledgements
The authors would like to thank (in alphabetical order) Congxiao Bao,
Bob Briscoe, Brian Carpenter, Tatuya Jinmei, Bob Hinden, Alberto
Leiva, Xing Li, Jeroen Massar, Erik Nordmark, Qiong Sun, Ole Troan,
and Tina Tsou, 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
tests that resulted in the publication of this document.
Additionally, he would like to thank SixXS <https://www.sixxs.net>
for providing IPv6 connectivity.
7. References
7.1. 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>.
[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>.
7.2. 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>.
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[RFC5927] Gont, F., "ICMP Attacks against TCP", RFC 5927,
DOI 10.17487/RFC5927, July 2010,
<http://www.rfc-editor.org/info/rfc5927>.
[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>.
[I-D.ietf-v6ops-ipv6-ehs-in-real-world]
Gont, F., Linkova, J., Chown, T., and S. LIU,
"Observations on the Dropping of Packets with IPv6
Extension Headers in the Real World", draft-ietf-v6ops-
ipv6-ehs-in-real-world-02 (work in progress), December
2015.
[I-D.ietf-6man-rfc2460bis]
Deering, S. and B. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", draft-ietf-6man-rfc2460bis-04 (work
in progress), March 2016.
[Morbitzer]
Morbitzer, M., "TCP Idle Scans in IPv6", Master's Thesis.
Thesis number: 670. Department of Computing Science,
Radboud University Nijmegen. August 2013,
<http://www.ru.nl/publish/pages/769526/
m_morbitzer_masterthesis.pdf>.
[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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Appendix A. Small Survey of OSes that Fail to Produce IPv6 Atomic
Fragments
[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]. It is simply meant as a
datapoint regarding the extent to which IPv6 implementations can be
relied upon to generate IPv6 atomic fragments.
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 Linux kernel current
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
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
P.R. China
Email: liushucheng@huawei.com
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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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