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Network Working Group                                          F. Dupont
Internet-Draft                                                    Point6
Expires: June 24, 2006                                 December 21, 2005

             A note about 3rd party bombing in Mobile IPv6

Status of this Memo

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   Mobile IPv6 introduces some new kinds of reflection attacks, as known
   as 3rd party bombing.  This memo analyses these attacks and makes
   some recommendations: the goal is to avoid anything, including in new
   optimized mechanisms, which can make the life of bad guys easier.

1.  Introduction

   The standard reflection attack is based on the "bombing" of a 3rd

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   party, the victim, by a reflector:
   -  the attacker A sends requests to the reflector R using the address
      of the victim V as the source address of requests
   -  the reflector R sends (large) answers to V.
   In the mobile IPv6 [RFC3775] context, the attacker is a mobile node,
   the reflector a correspondent node (a common one or the home agent).

   There are two basic defenses against the reflection attack:
   -  ingress filtering [BCP38] (improved in [BCP84]), i.e., checking
      that source addresses are topologically correct
   -  all protocols which needs some kind of positive feedback from
      peers, as TCP or RTP/RTCP.
   As a correspondent node sees two addresses (a transient care-of
   address and the home address) per mobile node, it is clear that
   mobile IPv6 can add new opportunities to reflection attacks...

2.  Analysis

   There are three different kinds of reflection attacks using mobile
   IPv6 mechanisms, one for each mode of operation: triangular routing,
   bidirectional tunneling and routing optimization.

   Note that the attack is always from the mobile node itself, i.e., no
   intermediate node can successfully modify packets in transit to
   launch an attack as in the transient pseudo-nat attack [ID.transient-

   At the opposite, a local / visited network access control with an AAA
   architecture [ID.aaa-mipv6-requirements] or an equivalent can give
   some real guarantees about a care-of address, i.e., something better
   than just trusting mobile nodes.

   The basic argument of this document is the ingress filtering is the
   right defense against reflection attacks and similar threats like
   address stealing and network bombing.

2.1.  Triangular routing

   In this case the mobile node sends packets carrying in the home
   address option a fake home address.  The correspondent node sends
   back traffic to this home address.  In order to avoid a very
   dangerous attack (bombing traffic with no trace of the attacker) the
   mobile IPv6 specifications [RFC3775] mandates the verification of
   home address options:
   -  when a packet with a home address option matches a binding cache
      entry it is accepted, but routing optimization is active: this
      case will be analyzed further.

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   -  when a proper IPsec security association exists, but in this case
      the home address is proven.
   So there is no reflection attack issue with triangular routing.

2.2.  Bidirectional tunneling

   This mode is the basic one: all traffic from or to the mobile node
   goes through a bidirectional tunnel between the mobile node and its
   home agent.

   In order to launch a reflection attack the mobile node has to put a
   fake care-of address in binding updates sent to its home agent (home
   registrations).  The home agent does not perform a routing
   routability check, i.e., it does not check if the mobile node is
   really reachable at its current care-of address.  This point was
   discussed (issue 34 [ISSUE34]) by the mobileip working group which

   "The above mechanisms do not show that the care-of address given in
   the Binding Update is correct.  This opens the possibility for
   Denial-of-Service attacks against third parties.  However, since the
   mobile node and home agent have a security association, the home
   agent can always identify an ill-behaving mobile node.  This allows
   the home agent operator to discontinue the mobile node's service, and
   possibly take further actions based on the business relationship with
   the mobile node's owner."

   Note that the registration of a fake care-of address diverts all the
   traffic to the mobile node via the home agent and at least 40 octets
   is added to each packet, so for an attacker this attack is more
   effective than the next one.

2.3.  Routing optimization

   The last mode is the routing optimization: binding updates sent by
   the mobile node create or update binding cache entries on the
   correspondent node.  A routing header of type 2 is added to each
   packet to the mobile node with its home address, so the extra
   information uses 24 octets and reveals the home address.

   The reflection attack in the routing optimization case uses fake
   care-of addresses in binding updates sent from the mobile node to the
   correspondent node.  Usually the care-of address is in the source
   address field of the IPv6 header but it can be, with a different
   value, in an alternate care-of address option too.  So the only
   security issue can come from this option because a fake care-of
   address in the IPv6 header is not a different case than a fake source
   address, i.e., the routing optimization does not modify the basic

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   reflection attack.

   When the return routability procedure is used with an alternate
   care-of address, it is applied to the right address (issue 5
   [ISSUE5]).  When some other mechanisms is used, usually a longer term
   one, either a return routability check must be performed, using a
   third packet (aka a hand-shake) repeating a cookie from the binding
   acknowledgment [ID.mipv6-rrcookie] or binding updates with different
   care-of addresses in the IPv6 header and in the alternate care-of
   address option must be refused.

2.4.  Home network bombing

   The last attack uses a fake home address and simulates a "return to
   the home".  Pending traffic is redirected to the the home address and
   can flood the home network.  Further traffic has to be sent from the
   fake home address so this is the standard reflection attack.

3.  Current attacks using fake source addresses

   Reflection attacks are only one kind of attacks based on the use of
   fake source addresses.  Current distributed denials of service (DDoS)
   use a large number of compromised nodes (BOTs [Botnets]) and do not
   take advantage of possible amplification by reflection, and this fact
   should not change in the future.

4.  Conclusion

   We recommend that alternative ways to build security associations to
   protect signaling between mobile and correspondent nodes do not
   blindly accept "hidden" care-of addresses.  As the goal of these ways
   is to be more efficient than the return routability procedure, IMHO
   their specifications must either forbid alternate care-of address
   options which carry different addresses than source addresses of IPv6
   headers or must provide an explicit counter-measure (like [ID.mipv6-

   We recommend to avoid unnecessary return routability checks because:
   -  return routability does not give better assurance than ingress
      filtering: both check if the source is on the returning path.
   -  return routability is an active device which has a cost in extra
      messages and delays, ingress filtering is a passive device which
      can be done at any line speed with hardware support.
   -  if ingress filtering is not applied, the network is open to any
      attacks based on fake source addresses, including current DDoS
      which are (as we get the proof everyday) far more then enough to

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      kill a target attached as any speed to the network.

5.  Security Considerations

   This goal of this document is to verify that mobile IPv6 mechanisms
   cannot be misused to make reflection attacks easier.  As the return
   routability procedure [ID.ro-sec] is not considered by many people as
   very safe or efficient, some new ways to build security associations
   to protect mobile IPv6 signaling are likely to appear [ID.ro-
   enhance], so they should be carefully designed...

6.  Acknowledgments

   Some persons stressed Wassim Haddad to add some defense against
   reflection attacks to his optimized mobile IPv6 [ID.mipv6-omipv6]
   when this is both unnecessary and of course against the idea of

   Pekka Savola raised our attention about [BCP84] which improved
   ingress filtering in multi-homed environments.

7.  References

7.1.  Normative References

   [RFC3775]  Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
              in IPv6", RFC 3775, June 2004.

7.2.  Informative References

   [BCP38]    Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", RFC 2827, BCP 38, May 2000.

   [BCP84]    Baker, F. and P. Savola, "Ingress Filtering for Multihomed
              Networks", RFC 3704, BCP 84, March 2004.

   [Botnets]  Kristoff, J., "Presentation at NANOG 32 on botnets",
              October 2004,

              Faccin, S., Le, F., Patil, B., Perkins, C., Dupont, F.,
              Laurent-Maknavicius, M., and J. Bournelle, "Mobile IPv6
              Authentication, Authorization, and Accounting

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              Requirements", draft-le-aaa-mipv6-requirements-03.txt
              (work in progress), February 2004.

              Haddad, W., Dupont, F., Madour, L., Krishnan, S., and S.
              Park, "Optimizing Mobile IPv6 (OMIPv6)",
              draft-haddad-mipv6-omipv6-01.txt (work in progress),
              February 2004.

              Dupont, F. and J-M. Combes, "Care-of Address Test for
              MIPv6 using a State Cookie",
              draft-dupont-mipv6-rrcookie-02.txt (work in progress),
              December 2005.

              Arkko, J. and C. Vogt, "A Taxonomy and Analysis of
              Enhancements to Mobile IPv6 Route Optimization",
              draft-irtf-mobopts-ro-enhancements-04.txt (work in
              progress), October 2005.

              Nikander, P., Arkko, J., Aura, T., Montenegro, G., and E.
              Nordmark, "Mobile IP version 6 Route Optimization Security
              Design Background", RFC 4225, December 2005.

              Dupont, F. and J-J. Bernard, "Transient pseudo-NAT attacks
              or how NATs are even more evil than you believed",
              draft-dupont-transient-pseudonat-04.txt (work in
              progress), June 2004.

   [ISSUE34]  Yegin, A., Arkko, J., and F. Dupont, "MIPv6 issue 34: CoA
              test also for home registrations? (Rejected, included in
              draft 18)", May 2004, <http://users.piuha.net/jarkko/

   [ISSUE5]   Arkko, J., Ed., "MIPv6 issue 5: Alternative-CoA usage
              rules (Adopted, included in draft 17)", May 2004, <http://

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Author's Address

   Francis Dupont
   c/o GET/ENST Bretagne
   2 rue de la Chataigneraie
   CS 17607
   35576 Cesson-Sevigne Cedex

   Fax:   +33 2 99 12 70 30
   Email: Francis.Dupont@enst-bretagne.fr

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