IPv6 maintenance Working Group (6man) F. Gont
Internet-Draft SI6 Networks / UTN-FRH
Updates: 2460 (if approved) W. Liu
Intended status: Standards Track Huawei Technologies
Expires: February 20, 2015 August 19, 2014
Deprecating the Generation of IPv6 Atomic Fragments
draft-gont-6man-deprecate-atomfrag-generation-00
Abstract
The core IPv6 specification requires that when a host receives an
ICMPv6 "Packet Too Big" message reporting a "Next-Hop MTU" smaller
than 1280, 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 formally deprecates the generation of IPv6 atomic
fragments, such that the aforementioned attack vector is eliminated.
Status of This Memo
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This Internet-Draft will expire on February 20, 2015.
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Denial of Service (DoS) attack vector . . . . . . . . . . . . 3
4. Updating RFC2460 . . . . . . . . . . . . . . . . . . . . . . 4
5. Additional Considerations . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 5
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
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 a "Next-Hop MTU"
smaller than 1280 (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-fragment traffic (since there are not really any fragments to be
reassembled). IPv6/IPv4 translators will typically employ the
Fragment Identification information found in the Fragment Header to
select an appropriate Fragment Identification value 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.
Since there are concrete security implications arising from the
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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),
this document formally updates [RFC2460], forbidding the generation
of IPv6 atomic fragments, such that the aforementioned attack vector
is eliminated.
Section 3 describes some possible attack scenarios. Section 5
provides additional considerations regarding the usefulness of
generating IPv6 atomic fragments. Section 4 formally updates RFC2460
such that this attack vector is eliminated.
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.gont-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 a Next-Hop MTU smaller than 1280, this will trigger the
generation of IPv6 atomic fragments from that moment on (as required
by [RFC2460]). When server 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 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 fired, it
will be the aforementioned routers themselves the ones dropping their
own traffic.
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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 BCP38 filters will
not help as a counter-measure.
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 BCP38-style filters on the ICMPv6 payload, the
use of extension (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.
4. Updating RFC2460
The following text from Section 5 of [RFC2460]:
"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."
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is formally replaced with:
"IPv6 nodes MUST discard ICMPv6 Packet Too Big error messages that
report a Next-Hop MTU smaller than 1280 bytes (the minimum IPv6
MTU)."
5. Additional Considerations
Besides the security assessment provided in Section 3, it is
interesting to evaluate if there is any gain in generating IPv6
atomic fragments (to provide for Fragment Identification value) as
opposed to just let IPv6/IPv4 translators select an appropriate IPv4
Fragment Identification value.
After some analysis, one can conclude that, if anything, an IPv6/IPv4
translator is in a much better position to select an appropriate
Fragment Identification value for the packet that are to be
translated from the IPv6 to the IPv4 world. For instance, an IPv6
node will generate Fragment Identification values without any
knowledge of the Fragment ID values being generated by other IPv6
nodes employing the translator. Thus, an IPv6/IPv4 translator is in
a much better position to generate Fragment IDs that will not result
in collisions (i.e., that will not be reused for the same tuple
{Source Address, Destination Address}.
6. 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.
7. 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. Additionally, it
formally updates [RFC2460] such that this attack vector is
eliminated.
8. Acknowledgements
Fernando Gont would like to thank Jan Zorz and Go6 Lab
for providing access to systems and networks that
were employed to produce some of the measurement results presented in
this document. Additionally, he would like to thank SixXS
for providing IPv6 connectivity.
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9. References
9.1. 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.
9.2. Informative References
[RFC5927] Gont, F., "ICMP Attacks against TCP", RFC 5927, July 2010.
[RFC6946] Gont, F., "Processing of IPv6 "Atomic" Fragments", RFC
6946, May 2013.
[I-D.gont-v6ops-ipv6-ehs-in-real-world]
Gont, F., Linkova, J., Chown, T., and W. Will, "IPv6
Extension Headers in the Real World", draft-gont-v6ops-
ipv6-ehs-in-real-world-00 (work in progress), August 2014.
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
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Will(Shucheng) Liu
Huawei Technologies
Bantian, Longgang District
Shenzhen 518129
P.R. China
Email: liushucheng@huawei.com
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