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Network Working Group                                           B. AbobaRequest for Comments: 3397                                     MicrosoftCategory: Standards Track                                    S. Cheshire                                                    Apple Computer, Inc.                                                           November 2002Dynamic Host Configuration Protocol (DHCP) Domain Search OptionStatus 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 (2002).  All Rights Reserved.Abstract   This document defines a new Dynamic Host Configuration Protocol   (DHCP) option which is passed from the DHCP Server to the DHCP Client   to specify the domain search list used when resolving hostnames using   DNS.Table of Contents1.  Introduction ................................................21.1 Terminology ............................................21.2 Requirements Language ..................................22.  Domain Search Option Format .................................23.  Example .....................................................34.  Security Considerations .....................................45.  Normative References ........................................56.  Informative References ......................................57.  IANA Considerations .........................................68.  Acknowledgments .............................................69.  Intellectual Property Statement .............................610. Authors' Addresses ..........................................711. Full Copyright Statement ....................................8Aboba & Cheshire            Standards Track                     [Page 1]

RFC 3397               DHCP Domain Search Option           November 20021.  Introduction   The Dynamic Host Configuration Protocol (DHCP) [RFC2131] provides a   mechanism for host configuration.  [RFC2132] and [RFC2937] 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 used when resolving   hostnames with DNS.  This option applies only to DNS and does not   apply to other name resolution mechanisms.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" [RFC2119].2.  Domain Search Option Format   The code for this option is 119.    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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |     119       |     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 (255 octets), then multiple   options MAY be used, as described in "Encoding Long Options in the   Dynamic Host Configuration Protocol (DHCPv4)" [RFC3396].Aboba & Cheshire            Standards Track                     [Page 2]

RFC 3397               DHCP Domain Search Option           November 2002   To enable the searchlist to be encoded compactly, searchstrings in   the searchlist MUST be concatenated and encoded using the technique   described insection 4.1.4 of "Domain Names - Implementation And   Specification" [RFC1035].  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 in the Dynamic Host   Configuration Protocol (DHCPv4)" [RFC3396] 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.":   +---+---+---+---+---+---+---+---+---+---+---+   |119| 9 | 3 |'e'|'n'|'g'| 5 |'a'|'p'|'p'|'l'|   +---+---+---+---+---+---+---+---+---+---+---+   +---+---+---+---+---+---+---+---+---+---+---+   |119| 9 |'e'| 3 |'c'|'o'|'m'| 0 | 9 |'m'|'a'|   +---+---+---+---+---+---+---+---+---+---+---+   +---+---+---+---+---+---+---+---+---+---+---+   |119| 9 |'r'|'k'|'e'|'t'|'i'|'n'|'g'|xC0|x04|   +---+---+---+---+---+---+---+---+---+---+---+   Note:   i.    The encoding 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 byRFC1035.Aboba & Cheshire            Standards Track                     [Page 3]

RFC 3397               DHCP Domain Search Option           November 2002   iii.  The encoding of "marketing" (for "marketing.apple.com.") ends         with the two-octet compression pointer C004 (hex), which points         to 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 the 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.  Security Considerations   Potential attacks on DHCP are discussed insection 7 of the DHCP   protocol specification [RFC2131], as well as in the DHCP   authentication specification [RFC3118].  In particular, using the   domain search option, a rogue DHCP server might be able to redirect   traffic to another site.   For example, a user requesting a connection to "myhost", expecting to   reach "myhost.bigco.com" might instead be directed to   "myhost.roguedomain.com".  Note that support for DNSSEC [RFC2535]   will not avert this attack, since the resource records for   "myhost.roguedomain.com" might be legitimately signed.  This makes   the domain search option a more fruitful avenue of attack for a rogue   DHCP server than providing an illegitimate DNS server option   (described in [RFC2132]).   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] discusses security weaknesses related to implicit as well   as explicit domain searchlists, and provides recommendations relating   to resolver searchlist processing.[RFC1536] section 6 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 local domain name (or         searchlist if specified) appended.   [3]   Resolve a name containing no dots by appending with the         searchlist right away, but once again, no implicit searchlists         should be used.Aboba & Cheshire            Standards Track                     [Page 4]

RFC 3397               DHCP Domain Search Option           November 2002   In order to minimize potential vulnerabilities it is recommended   that:   [a]   Hosts implementing the domain search option SHOULD also         implement the searchlist recommendations of [RFC1536],section6.   [b]   Where DNS parameters such as the domain searchlist or DNS         servers have been manually configured, these parameters SHOULD         NOT be overridden by DHCP.   [c]   Domain search option implementations MAY require DHCP         authentication [RFC3118] prior to accepting a domain search         option.5.  Normative References   [RFC1035]   Mockapetris, P., "Domain Names - Implementation and               Specification", STD 13,RFC 1035, November 1987.   [RFC1536]   Kumar, A., Postel, J., Neuman, C., Danzig, P. and S.               Miller, "Common DNS Implementation Errors and Suggested               Fixes",RFC 1536, October 1993.   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate               Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC2131]   Droms, R., "Dynamic Host Configuration Protocol",RFC2131, March 1997.   [RFC3118]   Droms, R. and W. Arbaugh, "Authentication for DHCP               Messages",RFC 3118, June 2001.   [RFC3396]   Lemon, T. and S. Cheshire, "Encoding Long Options in the               Dynamic Host Configuration Protocol (DHCPv4)",RFC 3396,               November 2002.6.  Informative References   [RFC1535]   Gavron, E., "A Security Problem and Proposed Correction               With Widely Deployed DNS Software",RFC 1535, October               1993.   [RFC2132]   Alexander, S. and R. Droms, "DHCP Options and BOOTP               Vendor Extensions",RFC 2132, March 1997.Aboba & Cheshire            Standards Track                     [Page 5]

RFC 3397               DHCP Domain Search Option           November 2002   [RFC2535]   Eastlake, D., "Domain Name System Security Extensions",RFC 2535, March 1999.   [RFC2937]   Smith, C., "The Name Service Search Option for DHCP",RFC2937, September 2000.7.  IANA Considerations   The IANA has assigned DHCP option code 119 to the Domain Search   Option.8. 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 memo.9. 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 inBCP-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   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.Aboba & Cheshire            Standards Track                     [Page 6]

RFC 3397               DHCP Domain Search Option           November 200210. 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.orgAboba & Cheshire            Standards Track                     [Page 7]

RFC 3397               DHCP Domain Search Option           November 200211. Full Copyright Statement   Copyright (C) The Internet Society (2002).  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.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Aboba & Cheshire            Standards Track                     [Page 8]