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Network Working Group B. Aboba Request for Comments: 3397 Microsoft Category: Standards Track S. Cheshire

                                                  Apple Computer, Inc.
                                                         November 2002
  Dynamic Host Configuration Protocol (DHCP) Domain Search Option

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 (2002).  All Rights Reserved.


 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

Table of Contents

 1.  Introduction ................................................  2
      1.1 Terminology ............................................  2
      1.2 Requirements Language ..................................  2
 2.  Domain Search Option Format .................................  2
 3.  Example .....................................................  3
 4.  Security Considerations .....................................  4
 5.  Normative References ........................................  5
 6.  Informative References ......................................  5
 7.  IANA Considerations .........................................  6
 8.  Acknowledgments .............................................  6
 9.  Intellectual Property Statement .............................  6
 10. Authors' Addresses ..........................................  7
 11. Full Copyright Statement ....................................  8

Aboba & Cheshire Standards Track [Page 1] RFC 3397 DHCP Domain Search Option November 2002

1. 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",
 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 in section 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
 "" and "":
 |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|
 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 "" ends with a zero, the null
       root label, to mark the end of the name, as required by RFC

Aboba & Cheshire Standards Track [Page 3] RFC 3397 DHCP Domain Search Option November 2002

 iii.  The encoding of "marketing" (for "") 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 ("").
 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 in section 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 "" might instead be directed to
 "".  Note that support for DNSSEC [RFC2535]
 will not avert this attack, since the resource records for
 "" 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
 [a]   Hosts implementing the domain search option SHOULD also
       implement the searchlist recommendations of [RFC1536], section
 [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

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", RFC
             2131, 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
 [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", RFC
             2937, September 2000.

7. IANA Considerations

 The IANA has assigned DHCP option code 119 to the Domain Search

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 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
 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

Aboba & Cheshire Standards Track [Page 6] RFC 3397 DHCP Domain Search Option November 2002

10. Authors' Addresses

 Bernard Aboba
 Microsoft Corporation
 One Microsoft Way
 Redmond, WA 98052
 Phone: +1 425 706 6605
 Stuart Cheshire
 Apple Computer, Inc.
 1 Infinite Loop
 California 95014
 Phone: +1 408 974 3207

Aboba & Cheshire Standards Track [Page 7] RFC 3397 DHCP Domain Search Option November 2002

11. 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
 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


 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Aboba & Cheshire Standards Track [Page 8]

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