RFC3397 - Dynamic Host Configuration Protocol (DHCP) Domain Search Option

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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.

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 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

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",
"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].

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
"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 by RFC
1035.

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 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 "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.

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

[RFC1035] Mockapetris, P., "Domain Names - Implementation and
Specification", STD 13, RFC1035, November 1987.

[RFC1536] Kumar, A., Postel, J., Neuman, C., Danzig, P. and S.
Miller, "Common DNS Implementation Errors and Suggested
Fixes", RFC1536, October 1993.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC2119, March 1997.

[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, March 1997.

[RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP
Messages", RFC3118, June 2001.

[RFC3396] Lemon, T. and S. Cheshire, "Encoding Long Options in the
Dynamic Host Configuration Protocol (DHCPv4)", RFC3396,
November 2002.

6. Informative References

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

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

[RFC2535] Eastlake, D., "Domain Name System Security Extensions",
RFC2535, 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
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
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standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
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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.

10. 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

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
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Acknowledgement

Funding for the RFCEditor function is currently provided by the
Internet Society.

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