One document matched: draft-aboba-dhc-domsearch-09.txt-14918.txt
Differences from 09.txt-08.txt
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.
Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups. Note that other groups
may also distribute working documents as Internet-Drafts.
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and may be updated, replaced, or obsoleted by other documents at any
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or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
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
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 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
"eng.apple.com." and "marketing.apple.com.":
+---+---+---+---+---+---+---+---+---+---+---+
|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|
+---+---+---+---+---+---+---+---+---+---+---+
Note:
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
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.
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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 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.
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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
[RFC1035] Mockapetris, P., "DOMAIN NAMES - IMPLEMENTATION AND
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.
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 draft.
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
Intellectual Property Statement
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The IETF invites any interested party to bring to its attention any
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INTERNET-DRAFT DHCP Domain Search Option 11 January 2002
copyrights, patents or patent applications, or other proprietary rights
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Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
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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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