Internet DRAFT - draft-ietf-v6ops-ra-guard-implementation
draft-ietf-v6ops-ra-guard-implementation
IPv6 Operations Working Group (v6ops) F. Gont
Internet-Draft UK CPNI
Updates: 6105 (if approved) November 14, 2012
Intended status: Informational
Expires: May 18, 2013
Implementation Advice for IPv6 Router Advertisement Guard (RA-Guard)
draft-ietf-v6ops-ra-guard-implementation-07
Abstract
The IPv6 Router Advertisement Guard (RA-Guard) mechanism is commonly
employed to mitigate attack vectors based on forged ICMPv6 Router
Advertisement messages. Many existing IPv6 deployments rely on RA-
Guard as the first line of defense against the aforementioned attack
vectors. However, some implementations of RA-Guard have been found
to be prone to circumvention by employing IPv6 Extension Headers.
This document describes the evasion techniques that affect the
aforementioned implementations, and formally updates RFC 6105, such
that the aforementioned RA-Guard evasion vectors are eliminated.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on May 18, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Evasion techniques for some Router Advertisement Guard (RA
Guard) implementations . . . . . . . . . . . . . . . . . . . . 4
2.1. Attack Vector based on IPv6 Extension Headers . . . . . . 4
2.2. Attack vector based on IPv6 fragmentation . . . . . . . . 4
3. RA-Guard implementation advice . . . . . . . . . . . . . . . . 8
4. Other Implications . . . . . . . . . . . . . . . . . . . . . . 11
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
8.1. Normative References . . . . . . . . . . . . . . . . . . . 15
8.2. Informative References . . . . . . . . . . . . . . . . . . 15
Appendix A. Assessment tools . . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18
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1. Introduction
IPv6 Router Advertisement Guard (RA-Guard) is a mitigation technique
for attack vectors based on ICMPv6 Router Advertisement messages.
[RFC6104] describes the problem statement of "Rogue IPv6 Router
Advertisements", and [RFC6105] specifies the "IPv6 Router
Advertisement Guard" functionality.
The concept behind RA-Guard is that a layer-2 device filters ICMPv6
Router Advertisement messages, according to a number of different
criteria. The most basic filtering criterion is that Router
Advertisement messages are discarded by the layer-2 device unless
they are received on a specified port of the layer-2 device.
Clearly, the effectiveness of the RA Guard mitigation relies on the
ability of the layer-2 device to identify ICMPv6 Router Advertisement
messages.
Some popular RA-Guard implementations have been found to be easy to
circumvent by employing IPv6 extension headers [CPNI-IPv6]. This
document describes such evasion techniques, and provides advice to
RA-Guard implementers such that the aforementioned evasion vectors
can be eliminated.
It should be noted that the aforementioned techniques could also be
exploited to evade network monitoring tools such as NDPMon [NDPMon],
ramond [ramond], and rafixd [rafixd], and could probably be exploited
to perform stealth DHCPv6 attacks.
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].
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2. Evasion techniques for some Router Advertisement Guard (RA Guard)
implementations
The following subsections describe two different vectors that have
been found to be effective for the evasion of popular implementations
of the RA-Guard protection. Section 2.1 describes an attack vector
based on the use of IPv6 Extension Headers with the ICMPv6 Router
Advertisement messages, which may be used to circumvent the RA-Guard
protection of those implementations that fail to process an entire
IPv6 header chain when trying to identify the ICMPv6 Router
Advertisement messages. Section 2.2 describes an attack method based
on the use of IPv6 fragmentation, possibly in conjunction with the
use of IPv6 Extension Headers. This later vector has been found to
be effective with all existing implementations of the RA-Guard
mechanism.
2.1. Attack Vector based on IPv6 Extension Headers
While there is currently no legitimate use for IPv6 Extension Headers
in ICMPv6 Router Advertisement messages, Neighbor Discovery
implementations allow the use of Extension Headers with these
messages, by simply ignoring the received options. Some RA-Guard
implementations try to identify ICMPv6 Router Advertisement messages
by simply looking at the "Next Header" field of the fixed IPv6
header, rather than following the entire header chain. As a result,
such implementations fail to identify any ICMPv6 Router Advertisement
messages that include any Extension Headers (for example, a Hop by
Hop Options header, a Destination Options Header, etc.), and can be
easily circumvented.
The following figure illustrates the structure of ICMPv6 Router
Advertisement messages that implement this evasion technique:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|NH=60| |NH=58| | |
+-+-+-+ +-+-+-+ + +
| IPv6 header | Dst Opt Hdr | ICMPv6 Router Advertisement |
+ + + +
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2.2. Attack vector based on IPv6 fragmentation
This section presents a different attack vector, which has been found
to be effective against all implementations of RA-Guard. The basic
idea behind this attack vector is that if the forged ICMPv6 Router
Advertisement is fragmented into at least two fragments, the layer-2
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device implementing "RA-Guard" would be unable to identify the attack
packet, and would thus fail to block it.
A first variant of this attack vector would be an original ICMPv6
Router Advertisement message preceded with a Destination Options
Header, that results in two fragments. The following figure
illustrates the "original" attack packet, prior to fragmentation, and
the two resulting fragments which are actually sent as part of the
attack.
Original packet:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|NH=60| |NH=58| | |
+-+-+-+ +-+-+-+ + +
| IPv6 header | Dst Opt Hdr | ICMPv6 RA |
+ + + +
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
First fragment:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|NH=44| |NH=60| |NH=58| |
+-+-+-+ +-+-+-+ +-+-+-+ +
| IPv6 Header | Frag Hdr | Dst Opt Hdr |
+ + + +
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Second fragment:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|NH=44| |NH=60| | | |
+-+-+-+ +-+-+-+ + + +
| IPv6 header | Frag Hdr | Dst Opt Hdr | ICMPv6 RA |
+ + + + +
| | | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
It should be noted that the "Hdr Ext Len" field of the Destination
Options Header is present in the first fragment (rather than the
second). Therefore, it is impossible for a device processing only
the second fragment to locate the ICMPv6 header contained in that
fragment, since it is unknown how many bytes should be "skipped" to
get to the next header following the Destination Options Header.
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Thus, by leveraging the use of the Fragment Header together with the
use of the Destination Options header, the attacker is able to
conceal the type and contents of the ICMPv6 message he is sending (an
ICMPv6 Router Advertisement in this example). Unless the layer-2
device were to implement IPv6 fragment reassembly, it would be
impossible for the device to identify the ICMPv6 type of the message.
A layer-2 device could, however, at least detect that that an
ICMPv6 message (or some type) is being sent, since the "Next
Header" field of the Destination Options header contained in the
first fragment is set to "58" (ICMPv6).
This idea can be taken further, such that it is also impossible for
the layer-2 device to detect that the attacker is sending an ICMPv6
message in the first place. This can be achieved with an original
ICMPv6 Router Advertisement message preceded with two Destination
Options Headers, that results in two fragments. The following figure
illustrates the "original" attack packet, prior to fragmentation, and
the two resulting packets which are actually sent as part of the
attack.
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Original packet:
+-+-+-+-+-+-+-+-+-+-+-+-//+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|NH=60| |NH=60| |NH=58| | |
+-+-+-+ +-+-+-+ +-+-+-+ + +
| IPv6 header | Dst Opt Hdr | Dst Opt Hdr | ICMPv6 RA |
+ + + + +
| | | | |
+-+-+-+-+-+-+-+-+-+-+-+-//+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
First fragment:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|NH=44| |NH=60| |NH=60| |
+-+-+-+ +-+-+-+ +-+-+-+ +
| IPv6 header | Frag Hdr | Dst Opt Hdr |
+ + + +
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Second fragment:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|NH=44| |NH=60| | |NH=58| | |
+-+-+-+ +-+-+-+ + +-+-+-+ + +
| IPv6 header | Frag Hdr | Dst O Hdr | Dst Opt Hdr | ICMPv6 RA |
+ + + + + +
| | | | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
In this variant, the "Next Header" field of the Destination Options
header contained in the first fragment is set "60" (Destination
Options header), and thus it is impossible for a device processing
only the first fragment to detect that an ICMPv6 message is being
sent in the first place.
The second fragment presents the same challenges as the second
fragment of the previous variant. That is, it would be impossible
for a device processing only the second fragment to locate the second
Destination Options header (and hence the ICMPv6 header), since the
"Hdr Ext Len" field of the first Destination Options header is
present in the first fragment (rather than the second).
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3. RA-Guard implementation advice
The following filtering rules must be implemented as part of an "RA-
Guard" implementation on ports that face interfaces that are not
allowed to send ICMPv6 Router Advertisement messages, such that the
vulnerabilities discussed in this document are eliminated:
1. If the IPv6 Source Address of the packet is not a link-local
address (fe80::/10), RA-Guard must pass the packet.
RATIONALE: This prevents "RA-Guard" from dedicating compute
cycles to filtering packets that originate off-net and, if
they are RA's, would not be accepted by the host. Section
6.1.2 of [RFC4861] requires nodes to discard Router
Advertisement messages if their IPv6 Source Address is not a
link-local address.
2. If the Hop Limit is not 255, RA-Guard must pass the packet.
RATIONALE: This prevents "RA-Guard" from dedicating compute
cycles to filtering packets that originate off-net and, if
they are RA's, would not be accepted by the host. Section
6.1.2 of [RFC4861] requires nodes to discard Router
Advertisement messages if their Hop Limit is not 255.
3. RA-Guard must parse the IPv6 entire header chain present in the
packet, to identify whether the packet is a Router Advertisement
message.
RATIONALE: [RFC6564] specifies a uniform format for IPv6
Extension Header, thus meaning that an IPv6 node can parse an
IPv6 header chain even if it contains Extension Headers that
are not currently supported by that node. Additionally,
[I-D.ietf-6man-oversized-header-chain] requires that if a
packet is fragmented, the first fragment contains the entire
IPv6 header chain.
RA-Guard implementations must not enforce a limit on the
number of bytes they can inspect (starting from the beginning
of the IPv6 packet), since this could introduce false-
positives: legitimate packets could be dropped simply because
the RA-Guard device does not parse the entire IPv6 header
chain present in the packet. An implementation that has such
an implementation-specific limit must not claim compliance
with this specification, and must pass the packet when such
implementation-specific limit is reached.
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4. When parsing the IPv6 header chain, if the packet is a first-
fragment (i.e., a packet containing a Fragment Header with the
Fragment Offset set to 0) and it fails to contain the entire IPv6
header chain (i.e., all the headers starting from the IPv6 header
up to, and including, the upper-layer header), RA-Guard must drop
the packet, and should log the packet drop event in an
implementation-specific manner as a security fault.
RATIONALE: [I-D.ietf-6man-oversized-header-chain] specifies
that the first-fragment (i.e., the fragment with the Fragment
Offset set to 0) MUST contain the entire IPv6 header chain,
and allows intermmediate systems such as routers to drop those
packets that fail to comply with this requirement.
NOTE: This rule should only be applied to IPv6 fragments with
a Fragment Offset of 0 (non-first fragments can be safely
passed, since they will never reassemble into a complete
datagram if they are part of a Router Advertisement received
on a port where such packets are not allowed).
5. When parsing the IPv6 header chain, if the packet is identified
to be an ICMPv6 Router Advertisement message, RA-Guard must drop
the packet, and should log the packet drop event in an
implementation-specific manner as a security fault.
RATIONALE: By definition, Router Advertisement messages MUST
originate on-link, MUST have a link-local IPv6 Source Address,
and MUST have a Hop Limit value of 255. [RFC4861].
6. In all other cases, RA-Guard must pass the packet as usual.
NOTE: For the purpose of enforcing the RA-Guard filtering policy,
an ESP header [RFC4303] should be considered to be an "upper-layer
protocol" (that is, it should be considered the last header in the
IPv6 header chain). This means that packets employing ESP would
be passed by the RA-Guard device to the intended destination. If
the destination host does not have a security association with the
sender of the aforementioned IPv6 packet, the packet would be
dropped. Otherwise, if the packet is considered valid by the
IPsec implementation at the receiving host and encapsulates a
Router Advertisement message, it is up to the receiving host what
to do with such packet.
If a packet is dropped due to this filtering policy, then the packet
drop event should be logged in an implementation-specific manner as a
security fault. The logging mechanism should include a drop counter
dedicated to RA-Guard packet drops.
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In order to protect current end-node IPv6 implementations, Rule #4
has been defined as a default rule to drop packets that cannot be
positively identified as not being Router Advertisement (RA) messages
(because the packet is a fragment that fails to include the entire
IPv6 header chain). This means that, at least in theory, RA-Guard
could result in false-positive blocking of some legitimate non-RA
packets that could not be positively identified as being non-RA. In
order to reduce the likelihood of false positives, Rule #1 and Rule
#2 require that packets that would not pass the required validation
checks for RA messages (Section 6.1.2 of [RFC4861]) be passed without
further inspection. In any case, as noted in
[I-D.ietf-6man-oversized-header-chain], IPv6 packets that fail to
include the entire IPv6 header chain are virtually impossible to
police with state-less filters and firewalls, and hence are unlikely
to survive in real networks. [I-D.ietf-6man-oversized-header-chain]
requires that hosts employing fragmentation include the entire IPv6
header chain in the first fragment (the fragment with the Fragment
Offset set to 0), thus eliminating the aforementioned false
positives.
This filtering policy assumes that host implementations require that
the IPv6 Source Address of ICMPv6 Router Advertisement messages be a
link-local address, and that they discard the packet if this check
fails, as required by the current IETF specifications [RFC4861].
Additionally, it assumes that hosts require the Hop Limit of Neighbor
Discovery messages to be 255, and discard those packets otherwise.
The aforementioned filtering rules implicitly handle the case of
fragmented packets: if the RA-Guard device fails to identify the
upper-layer protocol as a result of the use of fragmentation, the
corresponding packets would be dropped.
Finally, we note that IPv6 implementations that allow overlapping
fragments (i.e. that do not comply with [RFC5722]) might still be
subject of RA-based attacks. However, a recent assessment of IPv6
implementations [SI6-FRAG] with respect to their fragment reassembly
policy seems to indicate that most current implementations comply
with [RFC5722].
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4. Other Implications
A similar concept to that of "RA-Guard" has been implemented for
protecting against forged DHCPv6 messages. Such protection can be
circumvented with the same techniques discussed in this document, and
the counter-measures for such evasion attack are analogous to those
described in Section 3 of this document.
[DHCPv6-Shield] specifies a mechanism to protect against rogue
DHCPv6 servers, while taking into consideration the evasion
techniques discussed in this document.
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5. IANA Considerations
This document has no actions for IANA.
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6. Security Considerations
This document describes a number of techniques that have been found
to be effective to circumvent popular RA-Guard implementations, and
provides advice to RA-Guard implementations such that those evasion
vulnerabilities are eliminated.
As noted in Section 3, IPv6 implementations that allow overlapping
fragments (i.e. that do not comply with [RFC5722]) might still be
subject of RA-based attacks. However, most current
implementations seem to comply with [RFC5722].
We note that if an attacker sends a fragmented Router Advertisement
message on a port not allowed to send such packets, the first-
fragment would be dropped, and the rest of the fragments would be
passed. This means that the victim node would tie memory buffers for
the aforementioned fragments, which would never reassemble into a
complete datagram. If a large number of such packets were sent by an
attacker, and the victim node failed to implement proper resource
management for the fragment reassembly buffer, this could lead to a
Denial of Service (DoS). However, this does not really introduce a
new attack vector, since an attacker could always perform the same
attack by sending forged fragmented datagram in which at least one of
the fragments is missing. [CPNI-IPv6] discusses some resource
management strategies that could be implemented for the fragment
reassembly buffer.
We note that most effective and efficient mitigation for these
attacks would be to prohibit the use of IPv6 fragmentation with
Router Advertisement messages (as proposed by
[I-D.ietf-6man-nd-extension-headers]), such that the RA-Guard
functionality is easier to implement. However, since such mitigation
would require an update to existing implementations, it cannot be
relied upon in the short or near term.
Finally, we note that RA-Guard only mitigates attack vectors based on
ICMPv6 Router advertisement messages. Protection against similar
attacks based on other messages (such as DCHPv6) is considered out of
the scope of this document, and left for other documents(e.g.
[DHCPv6-Shield]).
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7. Acknowledgements
The author would like to thank Ran Atkinson, who provided very
detailed comments and suggested text that was incorporated into this
document.
The author would like to thank Ran Atkinson, Karl Auer, Robert
Downie, Washam Fan, David Farmer, Marc Heuse, Nick Hilliard, Ray
Hunter, Joel Jaeggli, Simon Perreault, Arturo Servin, Gunter van de
Velde, James Woodyatt, and Bjoern A. Zeeb, for providing valuable
comments on earlier versions of this document.
The author would like to thank Arturo Servin, who presented this
document at IETF 81.
This document resulted from the project "Security Assessment of the
Internet Protocol version 6 (IPv6)" [CPNI-IPv6], carried out by
Fernando Gont on behalf of the UK Centre for the Protection of
National Infrastructure (CPNI). The author would like to thank the
UK CPNI, for their continued support.
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8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC5722] Krishnan, S., "Handling of Overlapping IPv6 Fragments",
RFC 5722, December 2009.
[RFC6105] Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,
February 2011.
[RFC6564] Krishnan, S., Woodyatt, J., Kline, E., Hoagland, J., and
M. Bhatia, "A Uniform Format for IPv6 Extension Headers",
RFC 6564, April 2012.
[I-D.ietf-6man-oversized-header-chain]
Gont, F. and V. Manral, "Security and Interoperability
Implications of Oversized IPv6 Header Chains",
draft-ietf-6man-oversized-header-chain-02 (work in
progress), November 2012.
[I-D.ietf-6man-nd-extension-headers]
Gont, F., "Security Implications of IPv6 Fragmentation
with IPv6 Neighbor Discovery",
draft-ietf-6man-nd-extension-headers-01 (work in
progress), November 2012.
8.2. Informative References
[RFC6104] Chown, T. and S. Venaas, "Rogue IPv6 Router Advertisement
Problem Statement", RFC 6104, February 2011.
[DHCPv6-Shield]
Gont, F., "DHCPv6-Shield: Protecting Against Rogue DHCPv6
Servers", IETF Internet Draft,
draft-gont-opsec-dhcpv6-shield, work in progress,
May 2012.
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[CPNI-IPv6]
Gont, F., "Security Assessment of the Internet Protocol
version 6 (IPv6)", UK Centre for the Protection of
National Infrastructure, (available on request).
[NDPMon] "NDPMon - IPv6 Neighbor Discovery Protocol Monitor",
<http://ndpmon.sourceforge.net/>.
[rafixd] "rafixd", <http://www.kame.net/dev/cvsweb2.cgi/kame/kame/
kame/rafixd/>.
[ramond] "ramond", <http://ramond.sourceforge.net/>.
[SI6-FRAG]
SI6 Networks, "IPv6 NIDS evasion and improvements in IPv6
fragmentation/reassembly", 2012, <http://
blog.si6networks.com/2012/02/
ipv6-nids-evasion-and-improvements-in.html>.
[SI6-IPv6]
"SI6 Networks' IPv6 toolkit",
<http://www.si6networks.com/tools/ipv6toolkit>.
[THC-IPV6]
"The Hacker's Choice IPv6 Attack Toolkit",
<http://www.thc.org/thc-ipv6/>.
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Appendix A. Assessment tools
[SI6-IPv6] is a publicly-available set of tools (for Linux, *BSD, and
Mac OS) that implements the techniques described in this document.
[THC-IPV6] is a publicly-available set of tools (for Linux) that
implements some of the techniques described in this document.
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Author's Address
Fernando Gont
Centre for the Protection of National Infrastructure
Email: fgont@si6networks.com
URI: http://www.cpni.gov.uk
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