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.
AbstractThis 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. RFC 1034  and RFC 1035 ) in DHCPv6 (RFC 3315 ). RFC 2119 . 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.
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
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. 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  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  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  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. section "IANA Considerations" of RFC 3315.
The analysis of the potential attack through the domain search list is taken from the specification of the DHCPv4 Domain Search option, RFC3397 . Thanks to Rob Austein, Alain Durand, Peter Koch, Tony Lindstrom and Pekka Savola for their contributions to this document.  Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987.  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.  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.  Eastlake, D., "Domain Name System Security Extensions", RFC 2535, March 1999.  Kumar, A., Postel, J., Neuman, C., Danzig, P. and S. Miller, "Common DNS Implementation Errors and Suggested Fixes", RFC 1536, October 1993.  Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, November 1987.  Gavron, E., "A Security Problem and Proposed Correction With Widely Deployed DNS Software", RFC 1535, October 1993.  Aboba, B. and S. Cheshire, "Dynamic Host Configuration Protocol (DHCP) Domain Search Option", RFC 3397, November 2002.
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.