Internet DRAFT - draft-aboba-dhc-domsearch


Network Working Group                                      Bernard Aboba
INTERNET-DRAFT                                                 Microsoft
Category: Standards Track                                Stuart Cheshire
<draft-aboba-dhc-domsearch-09.txt>                        Apple Computer
11 January 2002

                       DHCP Domain Search Option

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC 2026.

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Copyright Notice

Copyright (C) The Internet Society (2002).  All Rights Reserved.


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 using DNS.

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Table of Contents

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

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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

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 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" [CONCAT].

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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" [CONCAT] 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 "":

|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|


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

ii.  The encoding of "" ends with a zero, the null root
     label, to mark the end of the name, as required by RFC 1035.

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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 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

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.

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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], section 6.

[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

          SPECIFICATION", RFC 1035, November 1987.

[RFC1536] Kumar, A., Postel, J., Neuman, C., Danzig, P., Miller, S.,
          "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., Arbaugh, W., "Authentication for DHCP Messages",
          RFC 3118, June 2001.

[CONCAT]  Lemon, T., Cheshire, S., "Encoding Long DHCP Options",
          Internet draft (work in progress), draft-ietf-dhc-
          concat-02.txt, October 2001.

6.  Informative references

[RFC1535] Gavron, E., "A Security Problem and Proposed Correction With
          Widely Deployed DNS Software", RFC 1535, October 1993.

[RFC2132] Alexander, S., Droms, R., "DHCP Options and BOOTP Vendor
          Extensions", RFC 2132, March 1997.

[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.

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7.  IANA Considerations

This draft requires assignment of a DHCP option code. It does not create
any new number spaces for IANA administration.


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

Stuart Cheshire
Apple Computer, Inc.
1 Infinite Loop
California 95014

Phone: +1 408 974 3207

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Expiration Date

This memo is filed as <draft-aboba-dhc-domsearch-09.txt>,  and  expires
June 27, 2002.

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