Network Working Group                                     R. Droms (ed.)
Internet-Draft                                             Cisco Systems
Expires: September 30, 2002                                     Apr 2002 August 29, 2003                               February 28, 2003

                  DNS Configuration options for DHCPv6
               draft-ietf-dhc-dhcpv6-opt-dnsconfig-02.txt
               draft-ietf-dhc-dhcpv6-opt-dnsconfig-03.txt

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Copyright Notice

   Copyright (C) The Internet Society (2002). (2003).  All Rights Reserved.

Abstract

   This document describes DHCPv6 options for passing a list of
   available DNS Servers 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, 2] 6] in DHCPv6
   [4].
   [2].

2. Requirements 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 RFC2119 [3].

3. Terminology

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

4. Domain Name Server specification [2].

3. DNS Resolver option

   The Domain Name Server DNS Resolver option provides a list of one or more IP IPv6 addresses
   of DNS servers recursive resolvers to which a client's DNS resolver MAY send
   DNS queries [2]. [1].  The DNS servers are listed in the order of
   preference for use by the client resolver.

   The format of the Domain Name Server 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_SERVERS     OPTION_DNS_RESOLVERS      |         option-len            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      |                   DNS-server (IP                 DNS-resolver (IPv6 address)                   |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      |                   DNS-server (IP                 DNS-resolver (IPv6 address)                   |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   option-code:   OPTION_DNS_SERVERS   OPTION_DNS_RESOLVERS (tbd)

   option-length:

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

   DNS-server:    IP    IPv6 address of DNS server

5. resolver

4. Domain Search List option

   A server sends a

   The Domain Search List option to the DHCP client to
   specify 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            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         searchstring                          |
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   option-code:   OPTION_DOMAIN_LIST (tbd)

   option-length:

   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 [4].

6. [2].

5. Appearance of these options

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

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

7.

6. Security Considerations

   The Domain Name Server DNS Resolver option may be used by an intruder DHCP server to
   cause DHCP clients to send DNS queries to an intruder DNS server. 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
   incompletely specified domain names.  The results of these
   misdirected searches may be used to spoof DNS names.

   To avoid attacks through the Domain Name Server option,  Note that
   support for DNSSEC [4] will not avert this attack, because the DHCP
   client SHOULD use authenticated DHCP (see section "Authentication of
   DHCP messages"
   resource records in the DHCPv6 specification).

   Because the Domain Search List option invalid domains may be used legitimately signed.

   The degree to spoof DNS name
   resolution 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 way 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 cannot resolvers:

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

   2.  Resolve a name that contains any dots by DNS security
   mechanisms like DNSSEC [5], DHCP clients 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 MUST
       have been manually configured, these parameters SHOULD NOT be
       overridden by DHCP.

   3.  A host SHOULD require the use
   authenticated of DHCP when a Domain Search List option is included authentication (see section
       "Authentication of DHCP messages" in the DHCPv6 specification)
       prior to accepting a
   DHCP message.

8. 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 [4].

References

   [1]  Mockapetris, P., [2].

8. Acknowledgments

   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. 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 - concepts and facilities", STD
        13, RFC 1034, November 1987.

   [2] 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.

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

   [4]

   [2]  Bound, J., Carney, M., Perkins, C., Lemon, T., Volz, B. and R.
        Droms (ed.), "Dynamic Host Configuration Protocol for IPv6
        (DHCPv6)", draft-ietf-dhc-dhcpv6-23 (work RFC XXXX, TBD 2003.

   [3]  Bradner, S., "Key words for use in progress), February
        2002.

   [5] 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.

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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