Network Working Group Bernard Aboba INTERNET-DRAFT Microsoft Category: Standards Track Stuart Cheshire <draft-aboba-dhc-domsearch-07.txt> Apple Computer 1 November 2001 DHCP Domain Search Option This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026. 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. The distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2001). All Rights Reserved. Abstract The Dynamic Host Configuration Protocol (DHCP) provides a mechanism for host configuration. RFC 2132 allows DHCP servers to specify configuration information for various kinds of name services to be passed to DHCP clients. In some circumstances, it is useful for the DHCP client to be configured with the domain search list. This document defines a new DHCP option which is passed from the DHCP Server to the DHCP Client to specify the domain search list when resolving hostnames. Aboba & Cheshire Standards Track [Page 1]
INTERNET-DRAFT DHCP Domain Search Option 1 November 2001 1. Introduction The Dynamic Host Configuration Protocol (DHCP) [2] provides a mechanism for host configuration. RFC 2132 [3] and RFC 2937 [6] allow DHCP servers to pass name service configuration information to DHCP clients. In some circumstances, it is useful for the DHCP client to be configured with the domain search list. This document defines a new DHCP option which is passed from the DHCP Server to the DHCP Client to specify the domain search list when resolving hostnames. 1.1. Terminology This document uses the following terms: DHCP client A DHCP client or "client" is an Internet host using DHCP to obtain configuration parameters such as a network address. DHCP server A DHCP server or "server" is an Internet host that returns configuration parameters to DHCP clients. 1.2. Requirements language 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 "Key words for use in RFCs to Indicate Requirement Levels" [1]. 2. Domain Search Option Format The code for this option is TBD. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TBD | Len | Searchstring... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Searchstring... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ In the above diagram, Searchstring is a string specifying the searchlist. If the length of the searchlist exceeds the maximum permissible within a single option (256 octets), then multiple options MAY be used, as described in "Encoding Long DHCP Options" [7]. To enable the searchlist to be encoded compactly, searchstrings in the searchlist are concatenated and encoded using the technique described in Aboba & Cheshire Standards Track [Page 2]
INTERNET-DRAFT DHCP Domain Search Option 1 November 2001 section 4.1.4 of "Domain Names - Implementation And Specification" [5]. In this scheme, an entire domain name or a list of labels at the end of a domain name is replaced with a pointer to a prior occurrence of the same name. Despite its complexity, this technique is valuable since the space available for encoding DHCP options is limited, and it is likely that a domain searchstring will contain repeated instances of the same domain name. Thus the DNS name compression is both useful and likely to be effective. For use in this specification, the pointer refers to the offset within the data portion of the DHCP option (not including the preceding DHCP option code byte or DHCP option length byte). If multiple Domain Search Options are present, then the data portions of all the Domain Search Options are concatenated together as specified in "Encoding Long DHCP Options" [7] and the pointer indicates an offset within the complete aggregate block of data. 3. Example Below is an example encoding of a search list consisting of "eng.apple.com." and "marketing.apple.com.": +---+---+---+---+---+---+---+---+---+---+---+ |TBD| 9 | 3 |'e'|'n'|'g'| 5 |'a'|'p'|'p'|'l'| +---+---+---+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+---+---+---+ |TBD| 9 |'e'| 3 |'c'|'o'|'m'| 0 | 9 |'m'|'a'| +---+---+---+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+---+---+---+ |TBD| 9 |'r'|'k'|'e'|'t'|'i'|'n'|'g'|xC0|x04| +---+---+---+---+---+---+---+---+---+---+---+ Note: i. The encoding of has been split (for this example) into three Domain Search Options. All Domain Search Options are logically concatenated into one block of data before being interpreted by the client. ii. The encoding of "eng.apple.com." ends with a zero, the null root label, to mark the end of the name, as required by RFC 1035. iii. The encoding of "marketing" (for "marketing.apple.com.") ends with the two-octet compression pointer C004 (hex), which points to Aboba & Cheshire Standards Track [Page 3]
INTERNET-DRAFT DHCP Domain Search Option 1 November 2001 offset 4 in the complete aggregated block of Domain Search Option data, where another validly encoded domain name can be found to complete the name ("apple.com."). Every search domain name must end either with a zero or with a two-octet compression pointer. If the receiver is part-way through decoding a search domain name when it reaches the end of the complete aggregated block of searchlist option data, without finding a zero or a valid two- octet compression pointer, then the partially read name must be discarded as invalid. 4. References [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [2] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997. [3] Alexander, S., Droms, R., "DHCP Options and BOOTP Vendor Extensions", RFC 2132, March 1997. [4] Droms, R., Arbaugh, W., "Authentication for DHCP Messages", RFC 3118, June 2001. [5] Mockapetris, P., "DOMAIN NAMES - IMPLEMENTATION AND SPECIFICATION", RFC 1035, November 1987. [6] Smith, C., "The Name Service Search Option for DHCP", RFC 2937, September 2000. [7] Lemon, T., Cheshire, S., "Encoding Long DHCP Options", Internet draft (work in progress), draft-ietf-dhc-concat-02.txt, October 2001. 5. Security Considerations Potential attacks on DHCP are discussed in section 7 of the DHCP protocol specification [2], as well as in the DHCP authentication specification [4]. In particular, using the domain search option, a rogue DHCP server might be able to redirect traffic to another site. To avert this attack, where DNS parameters such as the domain searchlist have been manually configured, these parameters SHOULD NOT be overridden by DHCP. Aboba & Cheshire Standards Track [Page 4]
INTERNET-DRAFT DHCP Domain Search Option 1 November 2001 6. IANA Considerations This draft requires assignment of a DHCP option code. It does not create any new number spaces for IANA administration. Acknowledgments The authors would like to thank Michael Patton, Erik Guttman, Olafur Gudmundsson, Thomas Narten, Mark Andrews, Erik Nordmark, Myron Hattig, Keith Moore and Bill Manning for comments on this draft. Authors' Addresses Bernard Aboba Microsoft Corporation One Microsoft Way Redmond, WA 98052 Phone: +1 425 706 6605 EMail: bernarda@microsoft.com Stuart Cheshire Apple Computer, Inc. 1 Infinite Loop Cupertino California 95014 USA Phone: +1 408 974 3207 EMail: rfc@stuartcheshire.org Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards- related documentation can be found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementors or users of this specification can be obtained from the IETF Secretariat. The IETF invites any interested party to bring to its attention any Aboba & Cheshire Standards Track [Page 5]
INTERNET-DRAFT DHCP Domain Search Option 1 November 2001 copyrights, patents or patent applications, or other proprietary rights which may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director. Full Copyright Statement Copyright (C) The Internet Society (2001). 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. 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." Expiration Date This memo is filed as <draft-aboba-dhc-domsearch-07.txt>, and expires May 27, 2002. Aboba & Cheshire Standards Track [Page 6]
