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Network Working Group                                      Bernard Aboba
INTERNET-DRAFT                                                 Microsoft
Category: Standards Track
<draft-aboba-dhc-domsearch-01.txt>
26 December 2000

                       DHCP Domain Search Option

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
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Internet-Drafts are draft documents valid for a maximum of six months
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or to cite them other than as "work in progress."

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The distribution of this memo is unlimited.

11..  Copyright Notice

Copyright (C) The Internet Society (2001).  All Rights Reserved.

22..  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 is
particularly useful for cases where the client may be configured to use
multicast DNS (mDNS). This document defines a new DHCP option which is
passed from the DHCP Server to the DHCP Client to specify the domain
seach list when resolving hostnames.








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33..  Introduction

The Dynamic Host Configuration Protocol (DHCP) [2] provides a mechanism
for host configuration. RFC 2132 [3] 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 is
particularly useful for cases where the client may be configured to use
multicast DNS (mDNS) [5]. This document defines a new DHCP option which
is passed from the DHCP Server to the DHCP Client to specify the domain
seach list when resolving hostnames.

33..11..  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.

33..22..  Requirements language

In this document, the key words "MAY", "MUST,  "MUST  NOT",  "optional",
"recommended",  "SHOULD",  and  "SHOULD  NOT",  are to be interpreted as
described in [1].

44..  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 [6].





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To enable the searchlist to be encoded compactly, searchstrings in the
searchlist are concatenated and encoded using the technique described in
section 4.1.4 of [7].  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 occurance of the same name.  Despite its complexity, this
technique is valuable since the space available for encoding if DHCP
options is limited, and it is likely that a domain searchstring will
contain repeated instances of the same domain name. Thus compression
method described in [7] is both useful and likely to be effective.

For use in this specification, the pointer refers to the offset within
the SearchString portion of the option. If multiple searchlist options
are present, then the SearchString portion of all the options are
concatenated and the pointer refers to the offset within the
concatenated SearchString.

55..  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",
     Internet draft (work in progress), draft-ietf-dhc-
     authentication-14.txt, July 2000.

[5]  Esibov, L., Aboba, B., Thaler, D., "Multicast DNS", Internet draft
     (work in progress), draft-aboba-dnsext-mdns-03.txt, October 2000.

[6]  Arbaugh, W. A., Keromytis, A. D.,"DHCP Continuation Option Code",
     Internet draft (work in progress), draft-ietf-dhc-options-
     cont-01.txt, January 2000.

[7]  Mockapetris, P., "DOMAIN NAMES - IMPLEMENTATION AND SPECIFICATION",
     RFC 1035, November 1987.

66..  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]. Since the domain search option may be used to control
the use of mDNS as described in [5], it is possible for a host



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masquerading as a DNS server to return a domain search option enabling
use of mDNS. Since mDNS uses only linklocal multicast, such an attack
would not have repercussions beyond the local segment. However, on a
heavily populated segment, it might result in increased multicast
traffic levels and some degradation in host performance.

77..  IANA Considerations

This draft does not create any new number spaces for IANA
administration.

88..  Acknowledgments

The authors would like to thank Stuart Cheshire, Michael Patton, Erik
Guttman, Olafur Gudmundsson, Thomas Narten, Mark Andrews, Erik Nordmark,
Myron Hattig and Bill Manning for comments on this draft, provided at
the mDNS lunch in Adelaide, Australia on 3/29/00.

99..  Authors' Addresses

Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052

Phone: +1 (425) 936-6605
EMail: bernarda@microsoft.com

1100..  Intellectual Property Statement

The IETF takes no position regarding the validity or scope of any
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to the implementation or use of the technology described in this
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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



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Director.

1111..  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
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perpetual and will not be revoked by the Internet Society or its
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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."

1122..  Expiration Date

This memo is filed as <draft-aboba-dhc-domsearch-01.txt>,  and  expires
September 1, 2001.






















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