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Network Working Group                                     F. Dupont, Ed.
Internet-Draft                                                    Point6
Expires: April 24, 2006                                 October 21, 2005

    Dynamic Home Agent Address Discovery (DHAAD) Considered Harmful

Status of this Memo

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   Dynamic Home Agent Address Discovery (DHAAD) mechanism of Mobile IPv6
   is nearly impossible to make secure.  As the service itself is
   useful, this document shows the security problems of the current
   mechanism and promotes an alternative solution.

1.  Introduction

   Mobile IPv6 specifications [RFC3775] contains a mechanism where a

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   home agent can help mobile nodes to discover the addresses of the
   home agents named the Dynamic Home Agent Address Discovery (DHAAD).
   This mechanism uses two ICMP message types:
      Home Agent Address Discovery Requests which are sent by mobile
      nodes to the Home Agents anycast addresses for their home subnet
      Home Agent Address Discovery Replies which are sent by home agents
      in response.
   A 16-bit identifier aids in matching requests and replies.

   The main security issue is in the anycast destination of requests,
   and as the mechanism is the first step of bootstrapping, there is no
   way to add reasonable security to it.

   So a solution is to change for a completely different mechanism
   providing the same service.  One is developed in the Mobile IPv6
   bootstrapping framework [ID-mip6-bootsplit]: it is based on the
   "Microsoft-style" for service discovery: DNS SRV Resource Records

   Security is provided by the DNSSEC [RFC4033] framework.  The needed
   pre-configured data on the mobile node for this mechanism is the
   domain name of the mobile service provider, which marginally better
   than the home subnet prefix.  For the security, a trust anchor which
   dominates the domain is needed.

   The mechanism can be extended to the Network Mobility (NEMO
   [RFC3963]) with "nemo" as the service name.

2.  ICMP Issue

   Specification of ICMP to carry DHAAD incurs a certain deployability
   risk.  Many ISPs are blocking ICMP on all links except the first hop,
   because ICMP is known to be a vehicle for DoS attacks and other sorts
   of threats.  It is theoretically possible to block ICMP types
   selectively, and therefore it would be possible to allow DHAAD
   messages through firewalls and still block those DHAAD messages that
   are a known threat.  However, because DHAAD is initiated from outside
   the firewall, the risk of a crude flooding DoS attack is unchanged
   since the firewall must allow any DHAAD message through.  The ISP
   could deploy some kind of authentication mechanism to validate that a
   DHAAD message comes from an authorized user before letting it
   through, but such sophisticated authentication is beyond current
   practice, and the advantages of deploying such a mechanism
   specifically for DHAAD are uncertain.  It is easier from a network
   management standpoint to simply uniformly block ICMP except on the
   last hop.

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3.  Security Considerations

   Securing the current DHAAD mechanism is a hopeless task.  The DNS SRV
   RR mechanism is already heavily used for service discovery and the
   standard way to make it secure is well known even not yet deployed in
   a large scale.

4.  Acknowledgments

   The authors of [ID-mip6-bootsplit] have done the hard work and should
   get all the credits.  Many early Mobile IPv6 operators pointed out
   that the current DHAAD mechanism does not provide a reasonable level
   of security.

   Nicolas Montavont proposed to extend the document to NEMO.

   James Kempf is the author of the Section 2 "ICMP issue".

5.  References

5.1.  Normative References

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

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

   [RFC3963]  Devarapalli, V., Wakikawa, R., Petrescu, A., and P.
              Thubert, "Network Mobility (NEMO) Basic Support Protocol",
              RFC 3963, January 2005.

5.2.  Informative References

              Giaretta, G., Ed., Kempf, J., and V. Devarapalli, "Mobile
              IPv6 bootstrapping in split scenario",
              draft-ietf-mip6-bootstrapping-split-00.txt (work in
              progress), June 2005.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

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Appendix A.  The new DHAAD mechanism

   The text of this annex is from [ID-mip6-bootsplit].

A.1.  Component of the solution

   HA address discovery.  The Mobile Node needs to discover the address
   of its Home Agent.  The main objective of a bootstrapping solution is
   to minimize the data pre-configured on the Mobile Node.  For this
   reason, the DHAAD defined in [RFC3775] may not be applicable in real
   deployments since it is required that the Mobile Node is pre-
   configured with the home network prefix and it does not allow an
   operator to load balance by having Mobile Nodes dynamically assigned
   to Home Agents located in different subnets.  This document defines a
   solution for Home Agent address discovery that is based on Domain
   Name Service (DNS), introducing a new DNS SRV record [RFC2782].  The
   unique information that needs to be pre- configured on the Mobile
   Node is the domain name of the MSP.

A.2.  Home Agent Address Discovery

   Once a Mobile Node has obtained network access, it can perform Mobile
   IPv6 bootstrapping.  For this purpose, the Mobile Node queries the
   DNS server to request information on Home Agent service.  As
   mentioned before in the document, the only information that needs to
   be auto- configured on the Mobile Node is the domain name of the
   Mobility Service Provider.

   The Mobile Node needs to obtain the IP address of the DNS server
   before it can send a DNS request.  This can be pre-configured on the
   Mobile Node or obtained through DHCPv6 from the visited link.  In any
   case, it is assumed that there is some kind of mechanism by which the
   Mobile Node is configured with a DNS server since a DNS server is
   needed for many other reasons.

   Two options for DNS lookup for a Home Agent address are identified in
   this document: DNS lookup by Home Agent Name and DNS lookup by
   service name.

   While this document specifies a Home Agent Address Discovery solution
   based on DNS, when the ASP and the MSP are the same entity DHCP may
   be used.  See "DCHPv6 option for home agent discovery in MIPv6" for

A.2.1.  DNS lookup by Home Agent Name

   In this case, the Mobile Node is configured with the Fully Qualified
   Domain Name of the Home Agent.  As an example, the Mobile Node could

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   be configured with the name "ha1.example.com", where "example.com" is
   the domain name of the MSP granting the mobility service.

   The Mobile Node constructs a DNS request, by setting the QNAME to the
   name of the Home Agent.  The request has QTYPE set to 'AAAA', so that
   the DNS server sends the IPv6 address of the Home Agent.  Once the
   DNS server replies to this query, the Mobile Node knows its Home
   Agent address and can run IKEv2 in order to set up an IPsec SA and
   get a Home Address.

   Additionally, it could be useful to provide an ability for the Mobile
   Node to discover a Home Agent placed in a particular location (e.g.
   on the visited link).  In order to achieve this, the Mobile Node must
   be able to construct a DNS request such that the DNS server will be
   able to reply with a Home Agent from the requested location.  This
   can be accomplished by using a specific naming convention for the
   FQDNs of the Home Agents.  As an example, an operator might assign
   the FQDN "ha.locationA.operator.com" to the Home Agent located in
   "location A" and the FQDN "ha.locationB.operator.com" to the Home
   Agent located in "location B".  If the Mobile Node wants to use a
   Home Agent located in "location A", it will set the QNAME to
   "ha.locationA.operator.com" in the DNS request.

A.2.2.  DNS lookup by service name

   [RFC2782] defines the service resource record (SRV RR), that allows
   an operator to use several servers for a single domain, to move
   services from host to host, and to designate some hosts as primary
   servers for a service and others as backups.  Clients ask for a
   specific service/protocol for a specific domain and get back the
   names of any available servers.

   [RFC2782] describes also the policies to choose a service agent based
   on the preference and weight values.  The DNS SRV record may contain
   the preference and weight values for multiple Home Agents available
   to the Mobile Node in addition to the Home Agent FQDNs.  If multiple
   Home Agents are available in the DNS SRV record then Mobile Node is
   responsible for processing the information as per policy and for
   picking one Home Agent.  If the Home Agent of choice does not respond
   for some reason or the IKEv2 authentication fails, the Mobile Node
   SHOULD try other Home Agents on the list.

   The service name for Mobile IPv6 Home Agent service as required by
   [RFC2782] is "mip6" and the protocol name is "ipv6".  Note that a
   transport name cannot be used here because Mobile IPv6 does not run
   over a transport protocol.

   The SRV RR has a DNS type code of 33.  As an example, the Mobile

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   constructs a request with QNAME set to "mip6.example.com" and QTYPE
   to SRV.  The reply contains the FQDNs of one or more servers, that
   can then be resolved in a separate DNS transaction to the IP
   addresses.  However, it is RECOMMENDED that the DNS server also
   return the IP addresses of the Home Agents in AAAA records as part of
   the additional data section in order to avoid requiring an additional
   DNS round trip to resolve the FQDNs, if there is room in the SRV

A.3.  HA Address Discovery Security

   Use of DNS for address discovery carries certain security risks.  DNS
   transactions in the Internet are typically done without any
   authentication of the DNS server by the client or of the client by
   the server.  There are two risks involved:

   1) A legitimate client obtains a bogus Home Agent address from a
   bogus DNS server.  This is sometimes called a "pharming" attack,

   2) An attacking client obtains a legitimate Home Agent address from a
   legitimate server.

   The risk in Case 1 is mitigated because the Mobile Node is required
   to conduct an IKE transaction with the Home Agent prior to performing
   a Binding Update to establish Mobile IPv6 service.  According to the
   IKEv2 specification, the responder must present the initiator with a
   valid certificate containing the responder's public key, and the
   responder to initiator IKE_AUTH message must be protected with an
   authenticator calculated using the public key in the certificate.
   Thus, an attacker would have to set up both a bogus DNS server and a
   bogus Home Agent, and provision the Home Agent with a certificate
   that a victim Mobile Node could verify.  If the Mobile Node can
   detect that the certificate is not trustworthy, the attack will be
   foiled when the Mobile Node attempts to set up the IKE SA.

   The risk in Case 2 is limited for a single Mobile Node to Home Agent
   transaction if the attacker does not possess proper credentials to
   authenticate with the Home Agent.  The IKE SA establishment will fail
   when the attacking Mobile Node attempts to authenticate itself with
   the Home Agent.  Regardless of whether the Home Agent utilizes EAP or
   host-side certificates to authenticate the Mobile Node, the
   authentication will fail unless the Mobile Node has valid

   Another risk exists in Case 2 because the attacker may be attempting
   to propagate a DoS attack on the Home Agent.  In that case, the
   attacker obtains the Home Agent address from the DNS, then propagates
   the address to a network of attacking hosts that bombard the Home

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   Agent with traffic.  This attack is not unique to the bootstrapping
   solution, however, it is actually a risk that any Mobile IPv6 Home
   Agent installation faces.  In fact, the risk is faced by any service
   in the Internet that distributes a unicast globally routable address
   to clients.  Since Mobile IPv6 requires that the Mobile Node
   communicate through a globally routable unicast address of a Home
   Agent, it is possible that the Home Agent address could be propagated
   to an attacker by various means (theft of the Mobile Node, malware
   installed on the Mobile Node, evil intent of the Mobile Node owner
   him/herself, etc.) even if the home address is manually configured on
   the Mobile Node.  Consequently, every Mobile IPv6 Home Agent
   installation will likely be required to mount anti-DoS measures.
   Such measures include overprovisioning of links to and from Home
   Agents and of Home Agent processing capacity, vigilant monitoring of
   traffic on the Home Agent networks to detect when traffic volume
   increases abnormally indicating a possible DoS attack, and hot spares
   that can quickly be switched on in the event an attack is mounted on
   an operating collection of Home Agents.  DoS attacks of moderate
   intensity should be foiled during the IKEv2 transaction.  IKEv2
   implementations are expected to generate their cookies without any
   saved state, and to time out cookie generation parameters frequently,
   with the timeout value increasing if a DoS attack is suspected.  This
   should prevent state depletion attacks, and should assure continued
   service to legitimate clients until the practical limits on the
   network bandwith and processing capacity of the Home Agent network
   are reached.

   Explicit security measures between the DNS server and host, such
   DNSSEC or TSIG/TKEY can mitigate the risk of 1) and 2), but these
   security measures are not widely deployed on end nodes.  These
   security measures are not sufficient to protect the Home Agent
   address against DoS attack, however, because a node having a
   legitimate security association with the DNS server could
   nevertheless intentionally or inadvertently cause the Home Agent
   address to become the target of DoS.

   Security considerations for discovering HA using DHCP are covered in

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Author's Address

   Francis Dupont (editor)
   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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