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Versions: (draft-gont-v6ops-ra-guard-implementation) 00 01 02 03 04 05 06 07 RFC 7113

IPv6 Operations Working Group (v6ops)                            F. Gont
Internet-Draft                                                   UK CPNI
Updates: 6105 (if approved)                                 May 22, 2012
Intended status: BCP
Expires: November 23, 2012


  Implementation Advice for IPv6 Router Advertisement Guard (RA-Guard)
              draft-ietf-v6ops-ra-guard-implementation-04

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 23, 2012.

Copyright Notice

   Copyright (c) 2012 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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   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 . . . . . . . . . . . . . . . . . . . . . . . . .  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
   Appendix B.  Advice and guidance to vendors  . . . . . . . . . . . 18
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19


























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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 those ports 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), pass the packet.

          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, pass the packet.

          Section 6.1.2 of [RFC4861] requires nodes to discard Router
          Advertisement messages if their Hop Limit is not 255.

   3.  Try to identify whether the packet is an ICMPv6 Router
       Advertisement message, by parsing the IPv6 header chain.  When
       doing so, enforce a limit on the maximum number of Extension
       Headers that is allowed for each packet, and if such limit is hit
       before the upper-layer protocol is identified, drop the packet.

          [RFC6564] specifies a uniform format for IPv6 Extension
          Header, thus meaning that an IPv6 node should be able to parse
          an IPv6 header chain even if it contains Extension Headers
          that are not currently supported by that node.

   4.  If the layer-2 device is unable to identify whether the packet is
       an ICMPv6 Router Advertisement message or not (i.e., the packet
       is a first-fragment, and the necessary information is missing),
       drop the packet.

          Note: This rule should only be applied to non-fragmented IPv6
          datagrams and 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.  If the packet is identified to be an ICMPv6 Router Advertisement
       message, drop the packet.

   6.  In all other cases, pass the packet as usual.





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      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.  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
   (possibly because the packet contains fragments that do not contain
   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.gont-6man-oversized-header-chain], IPv6 packets that fail to
   include the entire IPv6 header chain are anyway unlikely to survive
   in real networks.  Whilst currently legitimate from a specifications
   standpoint, they are virtually impossible to police with state-less
   filters and firewalls, and are hence likely to be blocked by such
   filters and firewalls.

   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



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   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.gont-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, 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.

   [RFC6564]  Krishnan, S., Woodyatt, J., Kline, E., Hoagland, J., and
              M. Bhatia, "A Uniform Format for IPv6 Extension Headers",
              RFC 6564, April 2012.

8.2.  Informative References

   [RFC6104]  Chown, T. and S. Venaas, "Rogue IPv6 Router Advertisement
              Problem Statement", RFC 6104, February 2011.

   [RFC6105]  Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
              Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,
              February 2011.

   [I-D.gont-6man-oversized-header-chain]
              Gont, F. and V. Manral, "Security and Interoperability
              Implications of Oversized IPv6 Header Chains",
              draft-gont-6man-oversized-header-chain-01 (work in
              progress), April 2012.

   [I-D.gont-6man-nd-extension-headers]
              Gont, F., "Security Implications of the Use of IPv6
              Extension Headers with IPv6 Neighbor Discovery",
              draft-gont-6man-nd-extension-headers-02 (work in
              progress), January 2012.

   [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>.

   [THC-IPV6]
              "The Hacker's Choice IPv6 Attack Toolkit",
              <http://www.thc.org/thc-ipv6/>.





























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Appendix A.  Assessment tools

   CPNI has produced assessment tools (which have not yet been made
   publicly available) to assess RA-Guard implementations with respect
   to the issues described in this document.  If you think that you
   would benefit from these tools, we might be able to provide a copy of
   the tools (please contact Fernando Gont at fernando@gont.com.ar).

   [THC-IPV6] is a publicly-available set of tools that implements some
   of the techniques described in this document.









































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Appendix B.  Advice and guidance to vendors

   Vendors are urged to contact CSIRTUK (csirt@cpni.gsi.gov.uk) if they
   think they may be affected by the issues described in this document.
   As the lead coordination centre for these issues, CPNI is well placed
   to give advice and guidance as required.

   CPNI works extensively with government departments and agencies,
   commercial organisations and the academic community to research
   vulnerabilities and potential threats to IT systems especially where
   they may have an impact on Critical National Infrastructure's (CNI).

   Other ways to contact CPNI, plus CPNI's PGP public key, are available
   at http://www.cpni.gov.uk.





































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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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