    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.

    Droms                       Standards Track                     [Page 1]

    RFC 3646          DNS Configuration Options for DHCPv6     December 2003

    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.

    Droms                       Standards Track                     [Page 2]

    RFC 3646          DNS Configuration Options for DHCPv6     December 2003

       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

    Droms                       Standards Track                     [Page 3]

    RFC 3646          DNS Configuration Options for DHCPv6     December 2003

       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.

    Droms                       Standards Track                     [Page 4]

    RFC 3646          DNS Configuration Options for DHCPv6     December 2003

       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.

    Droms                       Standards Track                     [Page 5]

    RFC 3646          DNS Configuration Options for DHCPv6     December 2003

    Intellectual Property Statement

       The IETF takes no position regarding the validity or scope of any
       intellectual property or other rights that might be claimed to
       pertain to the implementation or use of the technology described in
       this document or the extent to which any license under such rights
       might or might not be available; neither does it represent that it
       has made any effort to identify any such rights.  Information on the
       IETF's procedures with respect to rights in standards-track and
       standards-related documentation can be found in BCP-11.  Copies of
       claims of rights made available for publication and any assurances of
       licenses to be made available, or the result of an attempt made to
       obtain a general license or permission for the use of such
       proprietary rights by implementors or users of this specification can
       be obtained from the IETF Secretariat.

       The IETF invites any interested party to bring to its attention any
       copyrights, patents or patent applications, or other proprietary
       rights which may cover technology that may be required to practice
       this standard.  Please address the information to the IETF Executive
       Director.

    Author's Address

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

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

    Droms                       Standards Track                     [Page 6]

    RFC 3646          DNS Configuration Options for DHCPv6     December 2003

    Full Copyright Statement

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

       This document and translations of it may be copied and furnished to
       others, and derivative works that comment on or otherwise explain it
       or assist in its implementation may be prepared, copied, published
       and distributed, in whole or in part, without restriction of any
       kind, provided that the above copyright notice and this paragraph are
       included on all such copies and derivative works.  However, this
       document itself may not be modified in any way, such as by removing
       the copyright notice or references to the Internet Society or other
       Internet organizations, except as needed for the purpose of
       developing Internet standards in which case the procedures for
       copyrights defined in the Internet Standards process must be
       followed, or as required to translate it into languages other than
       English.

       The limited permissions granted above are perpetual and will not be
       revoked by the Internet Society or its successors or assignees.

       This document and the information contained herein is provided on an
       "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
       TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
       BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
       HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
       MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

    Acknowledgement

       Funding for the RFC Editor function is currently provided by the
       Internet Society.

    Droms                       Standards Track                     [Page 7]