IPv6 maintenance Working Group (6man)                            F. Gont
Internet-Draft                                    SI6 Networks / UTN-FRH
Updates: 3971, 4861 (if approved)                         March 22,                           June 3, 2013
Intended status: Standards Track
Expires: September 23, December 5, 2013

Security Implications of IPv6 Fragmentation with IPv6 Neighbor Discovery


   This document analyzes the security implications of employing IPv6
   fragmentation with Neighbor Discovery (ND) messages.  It updates RFC
   4861 such that use of the IPv6 Fragmentation Header is forbidden in
   all Neighbor Discovery messages, thus allowing for simple and
   effective counter-measures for Neighbor Discovery attacks.  Finally,
   it discusses the security implications of using IPv6 fragmentation
   with SEcure Neighbor Discovery (SEND), and formally updates RFC 3971
   to provide advice regarding how the aforementioned security
   implications can be prevented.

Status of this Memo

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Traditional Neighbor Discovery and IPv6 Fragmentation  . . . .  5
   3.  SEcure Neighbor Discovery (SEND) and IPv6 Fragmentation  . . .  6
   4.  Rationale for Forbidding IPv6 Fragmentation in Neighbor
       Discovery  . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   5.  Specification  . . . . . . . . . . . . . . . . . . . . . . . .  8
   6.  Operational Advice . . . . . . . . . . . . . . . . . . . . . .  9
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     10.1.  Normative References  . . . . . . . . . . . . . . . . . . 13
     10.2.  Informative References  . . . . . . . . . . . . . . . . . 13
   Appendix A.  Message Size When Carrying Certificates . . . . . . . 15
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15 16

1.  Introduction

   The Neighbor Discovery Protocol (NDP) is specified in RFC 4861
   [RFC4861].  It is used by both hosts and routers.  Its functions
   include Neighbor Discovery (ND), Router Discovery (RD), Address
   Autoconfiguration, Address Resolution, Neighbor Unreachability
   Detection (NUD), Duplicate Address Detection (DAD), and Redirection.

   Many of the possible attacks against the Neighbor Discovery Protocol
   are discussed in detail in [RFC3756].  In order to mitigate the
   aforementioned possible attacks, the SEcure Neighbor Discovery (SEND)
   was standardized.  SEND employs a number of mechanisms to certify the
   origin of Neighbor Discovery packets and the authority of routers,
   and to protect Neighbor Discovery packets from being the subject of
   modification or replay attacks.

   However, a number of factors, such as the use high administrative
   overhead of deploying trust anchors and the unavailability of SEND
   implementations for many widely-deployed operating systems, make SEND
   hard to deploy [Gont-DEEPSEC2011].  Thus, in many general scenarios
   it may be necessary and/or convenient to use other mitigation
   techniques for NDP-based attacks.  The following mitigations are
   currently available for NDP attacks:

   o  Static Access Control Lists (ACLs) in switches

   o  Layer-2 filtering of Neighbor Discovery packets (such as RA-Guard

   o  Neighbor Discovery monitoring tools (e.g., such as NDPMon
      [NDPMon], ramond [ramond])

   o  Intrusion Prevention Systems (IPS)

   IPv6 Router Advertisement Guard (RA-Guard) is a mitigation technique
   for attack vectors based on ICMPv6 Router Advertisement messages.  It
   is meant to block attack packets at a layer-2 device before the
   attack packets actually reach the target nodes.  [RFC6104] describes
   the problem statement of "Rogue IPv6 Router Advertisements", and
   [RFC6105] specifies the "IPv6 Router Advertisement Guard"

   Tools such as NDPMon [NDPMon] and ramond [ramond] aim at monitoring
   Neighbor Discovery traffic in the hopes of detecting possible attacks
   when there are discrepancies between the information advertised in
   Neighbor Discovery packets and the information stored on a local

   Some Intrusion Prevention Systems (IPS) can mitigate Neighbor
   Discovery attacks.  We recommend that Intrusion Prevention Systems
   (IPS) implement mitigations for NDP attacks.

   A key challenge that these mitigation or monitoring techniques face
   is that introduced by IPv6 fragmentation, since it is trivial for an
   attacker to conceal his attack by fragmenting his packets into
   multiple fragments.  This may limit or even eliminate the
   effectiveness of the aforementioned mitigation or monitoring
   techniques.  Recent work [CPNI-IPv6] indicates that current
   implementations of the aforementioned mitigations for NDP attacks can
   be trivially evaded.  For example, as noted in
   [I-D.ietf-v6ops-ra-guard-implementation], current RA-Guard
   implementations can be trivially evaded by fragmenting the attack
   packets into multiple fragments, such that the layer-2 device cannot
   find all the necessary information to perform packet filtering in the
   same packet.  While Neighbor Discovery monitoring tools could (in
   theory implement IPv6 fragment reassembly, this is usually an arms-
   race with the attacker (an attacker generate lots of forged fragments
   to "confuse" the monitoring tools), and therefore the aforementioned
   tools are unreliable for the detection of such attacks.

   Section 2 analyzes the use of IPv6 fragmentation in traditional
   Neighbor discovery.  Section 3 analyzes the use of IPv6 fragmentation
   in SEcure Neighbor Discovery (SEND).  Section 4 provides the
   rationale for forbidding the use of IPv6 fragmentation with Neighbor
   Discovery.  Section 5 formally updates RFC 4861 such that use of the
   IPv6 Fragment Header with traditional Neighbor Discovery is
   forbidden, and also formally updates RFC 3971 providing advice on the
   use of IPv6 fragmentation with SEND.  Section 6 provides operational
   advice about interoperability problems arising from the use of IPv6
   fragmentation with Neighbor Discovery.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Traditional Neighbor Discovery and IPv6 Fragmentation

   The only potential use case for IPv6 fragmentation with traditional
   (i.e., non-SEND) IPv6 Neighbor Discovery would be that in which a
   Router Advertisement must include a large number of options (Prefix
   Information Options, Route Information Options, etc.).  However, this
   could still be achieved without employing fragmentation, by splitting
   the aforementioned information into multiple Router Advertisement

      Some Neighbor Discovery implementations are known to silently
      ignore Router Advertisement messages that employ fragmentation.
      Therefore, splitting the necessary information into multiple RA
      messages (rather than sending a large RA message that is
      fragmented into multiple IPv6 fragments) is probably desirable
      even from an interoperability point of view.

   Thus, avoiding the use of IPv6 fragmentation in traditional Neighbor
   Discovery would greatly simplify and improve the effectiveness of
   monitoring and filtering Neighbor Discovery traffic, and would also
   prevent interoperability problems with those implementations that do
   not support fragmentation in Neighbor Discovery messages.

3.  SEcure Neighbor Discovery (SEND) and IPv6 Fragmentation

   SEND packets typically carry more information than traditional
   Neighbor Discovery packets: for example, they include additional
   options such as the CGA option and the RSA signature option.

   When SEND nodes employ any of the Neighbor Discovery messages
   specified in [RFC4861], the situation is roughly the same: if more
   information than would fit in a non-fragmented Neighbor Discovery
   packet needs to be sent, it should be split into multiple Neighbor
   Discovery messages (such that IPv6 fragmentation is avoided).

   However, Certification Path Advertisement messages (specified in
   [RFC3971]) pose a different situation, since the Certificate Option
   they include typically contains much more information than any other
   Neighbor Discovery option.  For example, Appendix C of [RFC3971]
   reports Certification Path Advertisement messages from 1050 to 1066
   bytes on an Ethernet link layer.  Since the size of CPA messages
   could potentially exceed the MTU of the local link, Section 5
   recommends that fragmented CPA messages be normally processed, but
   discourages the use of keys that would result in fragmented CPA

   It should be noted that relying on fragmentation opens the door to a
   variety of IPv6 fragmentation-based attacks. attacks against SEND.  In
   particular, if an attacker is located on the same broadcast domain as
   the victim host, and Certification Path Advertisement messages employ
   IPv6 fragmentation, it would be trivial for the attacker to forge
   IPv6 fragments such that they result in "Fragment ID collisions",
   causing both the attack fragments and the legitimate fragments to be
   discarded by the victim node.  This would eventually cause the
   Authorization Delegation Discovery (Section 6 of [RFC3971]) to fail,
   thus leading the host to fall back (depending on local configuration)
   either to unsecured mode, or to reject the corresponding Router
   Advertisement messages (possibly resulting in a Denial of Service).

4.  Rationale for Forbidding IPv6 Fragmentation in Neighbor Discovery

   A number of considerations should be made regarding the use of IPv6
   fragmentation with Neighbor Discovery:

   o  A significant number of existing implementations already silently
      drop fragmented ND messages, so the use of IPv6 fragmentation may
      hamper interoperability among IPv6 implementations.

   o  Although it is possible to build an ND message that needs to be
      fragmented, such packets are unlikely to exist in the real world
      because of the large number of options that would be required for
      the resulting packet to exceed the minimum IPv6 MTU of 1280

   o  If an ND message was so large as to need fragmentation, there is
      an option to distribute the same information amongst more than one
      message, each of which is small enough to not need fragmentation.

   Thus, forbidding the use of IPv6 fragmentation with Neighbor
   Discovery normalizes existing behavior and sets the expectations of
   all implementations to the existing lowest common denominator.

5.  Specification

   Nodes MUST NOT employ IPv6 fragmentation for sending any of the
   following Neighbor Discovery and SEcure Neighbor Discovery messages:

   o  Neighbor Solicitation

   o  Neighbor Advertisement

   o  Router Solicitation

   o  Router Advertisement

   o  Redirect

   o  Certification Path Solicitation

   Nodes SHOULD NOT employ IPv6 fragmentation for sending the following
   messages (see Section 6.4.2 of [RFC3971]):

   o  Certification Path Advertisement messages

   Nodes MUST silently ignore the following Neighbor Discovery and
   SEcure Neighbor Discovery messages if the packets carrying them
   include an IPv6 Fragmentation Header:

   o  Neighbor Solicitation

   o  Neighbor Advertisement

   o  Router Solicitation

   o  Router Advertisement

   o  Redirect

   o  Certification Path Solicitation

   Nodes SHOULD normally process the following messages when the packets
   carrying them include an IPv6 Fragmentation Header:

   o  Certification Path Advertisement

   SEND nodes SHOULD NOT employ keys that would result in fragmented CPA

6.  Operational Advice

   An operator detecting that Neighbor Discovery traffic is being
   silently dropped should find whether the corresponding Neighbor
   Discovery are employing IPv6 fragmentation.  If they are, it is
   likely that the devices receiving such packets are silently dropping
   them merely because they employ IPv6 fragmentation.  In such case, an
   operator should check whether the sending device has an option to
   prevent fragmentation of ND messages, and/or see whether it is
   possible to reduce the options carried on such messages.  We note
   that solving this (unlikely) problem might need a software upgrade to
   a version that does not employ IPv6 fragmentation with Neighbor

7.  IANA Considerations

   There are no IANA registries within this document.  The RFC-Editor
   can remove this section before publication of this document as an

8.  Security Considerations

   The IPv6 Fragmentation Header can be leveraged to circumvent network
   monitoring tools and current implementations of mechanisms such as
   RA-Guard [I-D.ietf-v6ops-ra-guard-implementation].  By updating the
   relevant specifications such that the IPv6 Fragment Header is not
   allowed in any Neighbor Discovery messages except "Certification Path
   Advertisement", protection of local nodes against Neighbor Discovery
   attacks, and monitoring of Neighbor Discovery traffic is greatly

   [I-D.ietf-v6ops-ra-guard-implementation] discusses an improvement to
   the RA-Guard mechanism that can mitigate Neighbor Discovery attacks
   that employ IPv6 Fragmentation.  However, it should be noted that
   unless [RFC4861] is updated (as proposed in this document), Neighbor
   Discovery monitoring tools (such as NDPMon [NDPMon], and ramond
   [ramond]) would remain unreliable and trivial to circumvent by a
   skilled attacker.

   As noted in Section 3, use of SEND could potentially result in
   fragmented "Certification Path Advertisement" messages, thus allowing
   an attacker to employ IPv6 fragmentation-based attacks against such
   messages.  Therefore, to the extent that is possible, such use of
   fragmentation should be avoided.

9.  Acknowledgements

   The author would like to thank (in alphabetical order) Mikael
   Abrahamsson, Ran Atkinson, Ron Bonica, Jean-Michel Combes, David
   Farmer, Adrian Farrel, Stephen Farrell, Roque Gagliano, Brian
   Haberman, Bob Hinden, Philip Homburg, Ray Hunter, Arturo Servin, Mark
   Smith, and Martin Stiemerling, for providing valuable comments on
   earlier versions of this document.

   The author would like to thank Roque Gagliano, who contributed the
   information regarding messages sizes in Appendix A.

   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.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3971]  Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
              Neighbor Discovery (SEND)", RFC 3971, March 2005.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

   [RFC6494]  Gagliano, R., Krishnan, S., and A. Kukec, "Certificate
              Profile and Certificate Management for SEcure Neighbor
              Discovery (SEND)", RFC 6494, February 2012.

10.2.  Informative References

   [RFC3756]  Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
              Discovery (ND) Trust Models and Threats", RFC 3756,
              May 2004.

   [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.

   [NDPMon]   "NDPMon - IPv6 Neighbor Discovery Protocol Monitor",

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

              Gont, F., "Implementation Advice for IPv6 Router
              Advertisement Guard (RA-Guard)",
              draft-ietf-v6ops-ra-guard-implementation-07 (work in
              progress), November 2012.

              Gont, F., "Security Assessment of the Internet Protocol
              version 6 (IPv6)",  UK Centre for the Protection of
              National Infrastructure, (available on request).

              Gont, "Results of a Security Assessment of the Internet
              Protocol version 6 (IPv6)",  DEEPSEC 2011 Conference,
              Vienna, Austria, November 2011, <http://

Appendix A.  Message Size When Carrying Certificates

   This section aims at estimating the size of normal Certification Path
   Advertisement messages.

   Considering a Certification Path Advertisement (CPA) such as that of
   Appendix C of [RFC3971] (certification path length of 4, between 1
   and 4 address prefix extensions, and a key length of 1024 bits), the
   certificate lengths range between 864 to 888 bytes (and the
   corresponding Ethernet packets from 1050 to 1066 bytes) [RFC3971].

   Updating the aforementioned packet size to account for the larger
   (2048 bits) keys required by [RFC6494] results in packet sizes
   ranging from 1127 to 1238 bytes, which are smaller than the minimum
   IPv6 MTU (1280 bytes), and much smaller than the ubiquitous Ethernet
   MTU (1500 bytes).

   However, we note that packet sizes may vary depending on a number of
   factors, including:

   o  the number of prefixes included in the certificate

   o  the length of Fully-Qualified Domain Names (FQDNs) in Trust Anchor
      (TA) options [RFC3971] (if present)

   If larger key sizes (i.e. 4096 bits) were required in the future, a
   larger MTU size might be required to to convey such information in
   Neighbor Discovery packets without the need to employ fragmentation.

Author's Address

   Fernando Gont
   SI6 Networks / UTN-FRH
   Evaristo Carriego 2644
   Haedo, Provincia de Buenos Aires  1706

   Phone: +54 11 4650 8472
   Email: fgont@si6networks.com
   URI:   http://www.si6networks.com