Implementation Advice for IPv6 Router Advertisement Guard (RA-Guard)
RFC 7113

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
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   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

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   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   |
      +             +              +                                +
      |             |              |                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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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       |
    +             +             +                         +
    |             |             |                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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    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.

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   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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   [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/>.

   [rafixd]         "rafixd", <http://www.kame.net/dev/cvsweb2.cgi/kame/
                    kame/kame/rafixd/>.

   [ramond]         "ramond", <http://ramond.sourceforge.net/>.

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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

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