Network Working Group                                      R. Droms, Ed.
Request for Comments: 3646                                 Cisco Systems
Category: Standards Track                                  December 2003


              DNS Configuration options for Dynamic Host
                Configuration Protocol for IPv6 (DHCPv6)

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   This document describes Dynamic Host Configuration Protocol for IPv6
   (DHCPv6) options for passing a list of available DNS recursive name
   servers and a domain search list to a client.

1.  Introduction

   This document describes two options for passing configuration
   information related to Domain Name Service (DNS) (RFC 1034 [6] and
   RFC 1035 [1]) in DHCPv6 (RFC 3315 [2]).

2.  Terminology

   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 BCP 14, RFC 2119 [3].

   Throughout this document, unless otherwise specified, the acronym
   DHCP refers to DHCP for IPv6 (DHCPv6) as specified in RFC 3315.

   This document uses terminology specific to IPv6 and DHCP as defined
   in section "Terminology" of RFC 3315.

3.  DNS Recursive Name Server option

   The DNS Recursive Name Server option provides a list of one or more
   IPv6 addresses of DNS recursive name servers to which a client's DNS
   resolver MAY send DNS queries [1].  The DNS servers are listed in the
   order of preference for use by the client resolver.

   The format of the DNS Recursive Name Server option is:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      OPTION_DNS_SERVERS       |         option-len            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |            DNS-recursive-name-server (IPv6 address)           |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |            DNS-recursive-name-server (IPv6 address)           |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   option-code:               OPTION_DNS_SERVERS (23)

   option-len:                Length of the list of DNS recursive name
                              servers in octets; must be a multiple of
                              16

   DNS-recursive-name-server: IPv6 address of DNS recursive name server

4.  Domain Search List option

   The Domain Search List option specifies the domain search list the
   client is to use when resolving hostnames with DNS.  This option does
   not apply to other name resolution mechanisms.

   The format of the Domain Search List option is:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      OPTION_DOMAIN_LIST       |         option-len            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          searchlist                           |
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   option-code:  OPTION_DOMAIN_LIST (24)

   option-len:   Length of the 'searchlist' field in octets

   searchlist:   The specification of the list of domain names in the
                 Domain Search List

   The list of domain names in the 'searchlist' MUST be encoded as
   specified in section "Representation and use of domain names" of RFC
   3315.

5.  Appearance of these options

   The DNS Recursive Name Server option MUST NOT appear in any other
   than the following messages: Solicit, Advertise, Request, Renew,
   Rebind, Information-Request, and Reply.

   The Domain Search List option MUST NOT appear in any other than the
   following messages: Solicit, Advertise, Request, Renew, Rebind,
   Information-Request, and Reply.

6.  Security Considerations

   The DNS Recursive Name Server option may be used by an intruder DHCP
   server to cause DHCP clients to send DNS queries to an intruder DNS
   recursive name server.  The results of these misdirected DNS queries
   may be used to spoof DNS names.

   To avoid attacks through the DNS Recursive Name Server option, the
   DHCP client SHOULD require DHCP authentication (see section
   "Authentication of DHCP messages" in RFC 3315) before installing a
   list of DNS recursive name servers obtained through authenticated
   DHCP.

   The Domain Search List option may be used by an intruder DHCP server
   to cause DHCP clients to search through invalid domains for
   incompletely specified domain names.  The results of these

   misdirected searches may be used to spoof DNS names.  Note that
   support for DNSSEC [4] will not avert this attack, because the
   resource records in the invalid domains may be legitimately signed.

   The degree to which a host is vulnerable to attack via an invalid
   domain search option is determined in part by DNS resolver behavior.
   RFC1535 [7] contains a discussion of security weaknesses related to
   implicit as well as explicit domain searchlists, and provides
   recommendations relating to resolver searchlist processing.  Section
   6 of RFC1536 [5] also addresses this vulnerability, and recommends
   that resolvers:

   1. Use searchlists only when explicitly specified; no implicit
      searchlists should be used.

   2. Resolve a name that contains any dots by first trying it as an
      FQDN and if that fails, with the names in the searchlist appended.

   3. Resolve a name containing no dots by appending with the searchlist
      right away, but once again, no implicit searchlists should be
      used.

   In order to minimize potential vulnerabilities it is recommended
   that:

   1. Hosts implementing the domain search option SHOULD also implement
      the searchlist recommendations of RFC1536, section 6.

   2. Where DNS parameters such as the domain searchlist or DNS servers
      have been manually configured, these parameters SHOULD NOT be
      overridden by DHCP.

   3. A host SHOULD require the use of DHCP authentication (see section
      "Authentication of DHCP messages" in RFC 3315) prior to accepting
      a domain search option.

7.  IANA Considerations

   IANA has assigned an option code to the DNS Recursive Name Server
   option (23) and to the Domain Search List option (24) from the DHCP
   option code space defined in section "IANA Considerations" of RFC
   3315.

8.  Acknowledgements

   This option was originally part of the DHCPv6 specification, written
   by Jim Bound, Mike Carney, Charlie Perkins, Ted Lemon, Bernie Volz
   and Ralph Droms.

   The analysis of the potential attack through the domain search list
   is taken from the specification of the DHCPv4 Domain Search option,
   RFC3397 [8].

   Thanks to Rob Austein, Alain Durand, Peter Koch, Tony Lindstrom and
   Pekka Savola for their contributions to this document.

9.  References

9.1.  Normative References

   [1]  Mockapetris, P., "Domain names - implementation and
        specification", STD 13, RFC 1035, November 1987.

   [2]  Bound, J., Carney, M., Perkins, C., Lemon, T., Volz, B. and R.
        Droms (ed.), "Dynamic Host Configuration Protocol for IPv6
        (DHCPv6)", RFC 3315, May 2003.

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

   [4]  Eastlake, D., "Domain Name System Security Extensions", RFC
        2535, March 1999.

   [5]  Kumar, A., Postel, J., Neuman, C., Danzig, P. and S. Miller,
        "Common DNS Implementation Errors and Suggested Fixes", RFC
        1536, October 1993.

9.2.  Informative References

   [6]  Mockapetris, P., "Domain names - concepts and facilities", STD
        13, RFC 1034, November 1987.

   [7]  Gavron, E., "A Security Problem and Proposed Correction With
        Widely Deployed DNS Software", RFC 1535, October 1993.

   [8]  Aboba, B. and S. Cheshire, "Dynamic Host Configuration Protocol
        (DHCP) Domain Search Option", RFC 3397, November 2002.

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Author's Address

   Ralph Droms, Editor
   Cisco Systems
   1414 Massachusetts Ave.
   Boxboro, MA  01719
   USA

   Phone: +1 978 936 1674
   EMail: rdroms@cisco.com

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