IPv6 maintenance Working Group (6man) F. Gont
Internet-Draft SI6 Networks / UTN-FRH
Intended status: Informational W. Liu
Expires: May 29, 2016 Huawei Technologies
T. Anderson
Redpill Linpro
November 26, 2015
Deprecation of the Generation of IPv6 Atomic Fragments
draft-ietf-6man-deprecate-atomfrag-generation-04
Abstract
RFC2460 requires that when a host receives an ICMPv6 "Packet Too Big"
message reporting an MTU smaller than 1280 bytes, 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 why the
aforementioned functionality is undesirable.
Status of This Memo
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This Internet-Draft will expire on May 29, 2016.
Copyright Notice
Copyright (c) 2015 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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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Denial of Service (DoS) attack vector . . . . . . . . . . . . 3
4. Additional Considerations . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Small Survey of OSes that Fail to Produce IPv6
Atomic Fragments . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
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 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 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-
fragmented traffic (since there are not really any fragments to be
reassembled). The goal of these atomic fragments has been to convey
an appropriate Fragment 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 introduce an
attack vector that can be exploited for denial of service purposes.
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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
translators generate a Fragment Identification value themselves), we
conclude that this functionality is undesirable.
Section 3 describes some possible attack scenarios. Section 4
provides additional considerations regarding the usefulness of
generating IPv6 atomic fragments.
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.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 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 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 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.
o As noted in Section 4, 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 [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.
4. Additional Considerations
Besides the security assessment provided in Section 3, 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
Unfortunately,
o 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.
o 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.
o 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.
Finally, 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 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 Fragment ID 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 < 1260
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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
Fragment Identification on its own (rather than selecting the IPv4
Fragment ID from the low-order 16-bits of the Fragment Identification
of atomic fragments), this could possibly lead to IPv4 Fragment ID
collisions. However, since a number of implementations set IPv6
Fragment ID according to the output of a Pseudo-Random Number
Generator (PRNG) (see Appendix B of
[I-D.ietf-6man-predictable-fragment-id]) and the translator only
employs the low-order 16-bits of such value, it is very unlikely that
relying on the Fragment ID of the IPv6 atomic fragment will result in
a reduced Fragment ID collision rate (when compared to the case where
the translator selects each IPv4 Fragment ID 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.
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.
6. 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. It concludes that
the generation of IPv6 atomic fragments is an undesirable feature.
7. Acknowledgements
The authors would like to thank (in alphabetical order) Alberto
Leiva, Bob Briscoe, Brian Carpenter, Tatuya Jinmei, Jeroen Massar,
Erik Nordmark, and Ole Troan, for providing valuable comments on
earlier versions of this document.
Fernando Gont would like to thank 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.
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8. References
8.1. 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>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[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>.
8.2. Informative References
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery",
RFC 2923, DOI 10.17487/RFC2923, September 2000,
<http://www.rfc-editor.org/info/rfc2923>.
[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>.
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[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>.
[RFC6946] Gont, F., "Processing of IPv6 "Atomic" Fragments",
RFC 6946, DOI 10.17487/RFC6946, May 2013,
<http://www.rfc-editor.org/info/rfc6946>.
[I-D.ietf-6man-predictable-fragment-id]
Gont, F., "Security Implications of Predictable Fragment
Identification Values", draft-ietf-6man-predictable-
fragment-id-10 (work in progress), October 2015.
[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-01 (work in progress), October
2015.
[Morbitzer]
Morbitzer, M., "TCP Idle Scans in IPv6", Master's Thesis.
Thesis number: 670. Department of Computing Science,
Radboud University Nijmegen. August 2013,
<https://www.ru.nl/publish/pages/578936/
m_morbitzer_masterthesis.pdf>.
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].
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
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o Linux kernel 2.6.32
o Linux kernel 3.2
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
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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