

Network Working Group                                     R. Droms (ed.)
Internet-Draft                                             Cisco Systems
Expires: August 29, 2003                               February 28, 2003


                  DNS Configuration options for DHCPv6
               draft-ietf-dhc-dhcpv6-opt-dnsconfig-03.txt

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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   This Internet-Draft will expire on August 29, 2003.

Copyright Notice

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

Abstract

   This document describes DHCPv6 options for passing a list of
   available DNS resolvers 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) [1, 6] in DHCPv6
   [2].

2. Terminology

   The key words MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,



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   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL in this document are to be
   interpreted as described in RFC2119 [3].

   This document uses terminology specific to IPv6 and DHCPv6 as defined
   in section "Terminology" of the DHCP specification [2].

3. DNS Resolver option

   The DNS Resolver option provides a list of one or more IPv6 addresses
   of DNS recursive resolvers 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 Resolver 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_RESOLVERS      |         option-len            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      |                 DNS-resolver (IPv6 address)                   |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      |                 DNS-resolver (IPv6 address)                   |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   option-code:   OPTION_DNS_RESOLVERS (tbd)

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

   DNS-server:    IPv6 address of DNS resolver


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            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         searchstring                          |
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   option-code:   OPTION_DOMAIN_LIST (tbd)

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

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

   The list of domain names in the 'searchstring' MUST be encoded as
   specified in section "Representation and use of domain names" of the
   DHCPv6 specification [2].

5. Appearance of these options

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

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

6. Security Considerations

   The DNS Resolver option may be used by an intruder DHCP server to
   cause DHCP clients to send DNS queries to an intruder DNS resolver.
   The results of these misdirected DNS queries may be used to spoof DNS
   names.

   To avoid attacks through the DNS Resolver option, the DHCP client
   SHOULD require DHCP authentication (see section "Authentication of
   DHCP messages" in the DHCPv6 specification) before installing a list
   of DNS resolvers 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



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   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 the DHCPv6 specification)
       prior to accepting a domain search option.


7. IANA Considerations

   IANA is requested to assign an option code to these options from the
   option-code space defined in section "DHCPv6 Options" of the DHCPv6
   specification [2].

8. Acknowledgments

   This option was originally part of the DHCPv6 specification, written



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   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. Changes from draft-ietf-dhc-dhcpv6-opt-dnsconfig-02.txt

   This document includes the following changes in response to comments
   made during the dhc/dnsext WG last call:

   o  Combined RFC2119 reference and reference to DHCPv6 specification
      into one "Terminology" section; added explicit normative reference
      to DHCPv6 specification.

   o  Changed name of "Domain Name Server" option to "DNS Resolver"
      option.

   o  Clarified and corrected filed names and descriptions of fields in
      the option format diagrams.

   o  Reworded "Appearance of these options" for clarity; removed
      Confirm from list of messages in which the options can appear.

   o  Clarified the type of attack that can be mounted through the
      Domain Search List option by copying text from RFC3997

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 XXXX, TBD 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



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        1536, October 1993.

Normative 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.


Author's Address

   Ralph Droms (ed.)
   Cisco Systems
   250 Apollo Drive
   Chelmsford, MA  01824
   USA

   Phone: +1 978 497 4733
   EMail: rdroms@cisco.com



























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Full Copyright Statement

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Acknowledgement

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



















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