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RFC 7113

Implementation Advice for IPv6 Router Advertisement Guard (RA-Guard)

Pages: 13
Informational
Updates:  6105

Top   ToC   RFC7113 - Page 1
Internet Engineering Task Force (IETF)                           F. Gont
Request for Comments: 7113                           Huawei Technologies
Updates: 6105                                              February 2014
Category: Informational
ISSN: 2070-1721


  Implementation Advice for IPv6 Router Advertisement Guard (RA-Guard)

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 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 5741. 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/rfc7113.
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Copyright Notice

   Copyright (c) 2014 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Evasion Techniques for Some RA-Guard Implementations . . . . . 3 2.1. Attack Vector Based on IPv6 Extension Headers . . . . . . 3 2.2. Attack Vector Based on IPv6 Fragmentation . . . . . . . . 4 3. RA-Guard Implementation Advice . . . . . . . . . . . . . . . . 6 4. Other Implications . . . . . . . . . . . . . . . . . . . . . . 9 5. Security Considerations . . . . . . . . . . . . . . . . . . . 9 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 7.1. Normative References . . . . . . . . . . . . . . . . . . . 10 7.2. Informative References . . . . . . . . . . . . . . . . . . 11 Appendix A. Assessment Tools . . . . . . . . . . . . . . . . . . 12

1. Introduction

IPv6 Router Advertisement Guard (RA-Guard) is a mitigation technique for attack vectors based on ICMPv6 Router Advertisement [RFC4861] 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 (L2) 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 L2 device unless they are received on a specified port of the L2 device. Clearly, the effectiveness of RA-Guard relies on the ability of the L2 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
Top   ToC   RFC7113 - Page 3
   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 previously mentioned 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 [RFC3315] attacks.

2. Evasion Techniques for Some RA-Guard Implementations

The following subsections describe two different vectors that have been found to be effective for the evasion of popular implementations of RA-Guard. Section 2.1 describes an attack vector based on the use of IPv6 Extension Headers with 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 against all existing implementations of RA-Guard.

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 [RFC4861] 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 | + + + + | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Top   ToC   RFC7113 - Page 4

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 L2 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, which results in two fragments. The following figure illustrates the "original" attack packet, prior to fragmentation, and the two resulting fragments that 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 | + + + + + | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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   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.

   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 L2 device
   were to implement IPv6 fragment reassembly, it would be impossible
   for the device to identify the ICMPv6 type of the message.

      An L2 device could, however, at least detect that an ICMPv6
      message (of 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 L2 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 that are actually sent as part of the
   attack.

    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       |
    +             +             +                         +
    |             |             |                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Top   ToC   RFC7113 - Page 6
    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 to "60" (Destination
   Options header); 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).

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 processing cycles to filtering packets that originate off-net and that, 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 processing cycles to filtering packets that originate off-net and that, if they are RA's, would not be accepted by the destination host. Section 6.1.2 of [RFC4861] requires nodes to discard Router Advertisement messages if their Hop Limit is not 255.
Top   ToC   RFC7113 - Page 7
   3.  RA-Guard must parse the entire IPv6 header chain present in the
       packet, to identify whether the packet is a Router Advertisement
       message.

          NOTE: 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.

   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: [RFC7112] 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 intermediate 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 or the packet
       contains an unrecognized Next Header value [IANA-IP-PROTO],
       RA-Guard must drop the packet, and should log the packet drop
       event in an implementation-specific manner as a security fault.
       RA-Guard must provide a configuration knob that controls whether
       packets with unrecognized Next Header values are dropped; this
       configuration knob must default to "drop".

          RATIONALE: By definition, Router Advertisement messages are
          required to originate on-link, have a link-local IPv6 Source
          Address, and have a Hop Limit value of 255 [RFC4861].
          [RFC7045] requires that nodes be configurable with respect to
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          whether packets with unrecognized headers are forwarded, and
          allows the default behavior to be that such packets be
          dropped.

   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 Encapsulating Security Payload (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 a 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.

   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 [RFC7112], 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.  [RFC7112] 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 that they discard those packets
   otherwise.
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   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].

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 countermeasures 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.

5. 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 implementers 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 ICMPv6 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 IPv6 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 datagrams in which at
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   least one of the fragments is missing.  [CPNI-IPv6] discusses some
   resource management strategies that could be implemented for the IPv6
   fragment reassembly buffer.

   We note that the most effective and efficient mitigation for these
   attacks would rely on the prohibiting the use of IPv6 fragmentation
   with Router Advertisement messages (as specified by [RFC6980]), 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 is left for other documents (e.g.,
   [DHCPv6-Shield]).

6. 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, Mike Heard, 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).

7. References

7.1. Normative References

[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003. [RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, December 2005.
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   [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.

   [RFC6980]        Gont, F., "Security Implications of IPv6
                    Fragmentation with IPv6 Neighbor Discovery",
                    RFC 6980, August 2013.

   [RFC7045]        Carpenter, B. and S. Jiang, "Transmission and
                    Processing of IPv6 Extension Headers", RFC 7045,
                    December 2013.

   [RFC7112]        Gont, F., Manral, V., and R. Bonica, "Implications
                    of Oversized IPv6 Header Chains", RFC 7112,
                    January 2014.

7.2. Informative References

[CPNI-IPv6] Gont, F., "Security Assessment of the Internet Protocol version 6 (IPv6)", UK Centre for the Protection of National Infrastructure, (available on request). [DHCPv6-Shield] Gont, F., Liu, W., and G. Van de Velde, "DHCPv6- Shield: Protecting Against Rogue DHCPv6 Servers", Work in Progress, October 2013. [IANA-IP-PROTO] IANA, "Assigned Internet Protocol Numbers", <http://www.iana.org/assignments/protocol-numbers/>. [NDPMon] "NDPMon - IPv6 Neighbor Discovery Protocol Monitor", <http://ndpmon.sourceforge.net/>. [RFC6104] Chown, T. and S. Venaas, "Rogue IPv6 Router Advertisement Problem Statement", RFC 6104, February 2011. [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>.
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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.

Author's Address

Fernando Gont Huawei Technologies Evaristo Carriego 2644 Haedo, Provincia de Buenos Aires 1706 Argentina Phone: +54 11 4650 8472 EMail: fgont@si6networks.com