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.








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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.











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  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



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  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.



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  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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Intellectual Property Statement

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  pertain to the implementation or use of the technology described in
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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: [email protected]



















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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
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Acknowledgement

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



















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