Network Working Group                                     R. Droms (ed.)Internet-Draft                                             Cisco SystemsExpires: August 29, 2003                               February 28, 2003                  DNS Configuration options for DHCPv6               draft-ietf-dhc-dhcpv6-opt-dnsconfig-03.txtStatus of this Memo   This document is an Internet-Draft and is in full conformance with   all provisions of Section 10 of RFC2026.   Internet-Drafts are working documents of the Internet Engineering   Task Force (IETF), its areas, and its working groups.  Note that   other groups may also distribute working documents as Internet-   Drafts.   Internet-Drafts are draft documents valid for a maximum of six months   and may be updated, replaced, or obsoleted by other documents at any   time.  It is inappropriate to use Internet-Drafts as reference   material or to cite them other than as "work in progress."   The list of current Internet-Drafts can be accessed at   http://www.ietf.org/ietf/1id-abstracts.txt.   The list of Internet-Draft Shadow Directories can be accessed at   http://www.ietf.org/shadow.html.   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,Droms (ed.)              Expires August 29, 2003                [Page 1]Internet-Draft    DNS Configuration Options for DHCPv6     February 2003   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 resolver4. 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 (ed.)              Expires August 29, 2003                [Page 2]Internet-Draft    DNS Configuration Options for DHCPv6     February 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            |      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+      |                         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 forDroms (ed.)              Expires August 29, 2003                [Page 3]Internet-Draft    DNS Configuration Options for DHCPv6     February 2003   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, writtenDroms (ed.)              Expires August 29, 2003                [Page 4]Internet-Draft    DNS Configuration Options for DHCPv6     February 2003   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 RFC3997Normative 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", RFCDroms (ed.)              Expires August 29, 2003                [Page 5]Internet-Draft    DNS Configuration Options for DHCPv6     February 2003        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.comDroms (ed.)              Expires August 29, 2003                [Page 6]Internet-Draft    DNS Configuration Options for DHCPv6     February 2003Full 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 assigns.   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