RFC3397
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.
Contents
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.
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.
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.
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.
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.
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.
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.
Informative References
RFC1535 Gavron, E., "A Security Problem and Proposed Correction
With Widely Deployed DNS Software", RFC 1535, October 1993.
RFC2132 Alexander, S. and R. Droms, "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.
IANA Considerations
The IANA has assigned DHCP option code 119 to the Domain Search Option.
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.
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10. Authors' Addresses
Bernard Aboba Microsoft Corporation One Microsoft Way Redmond, WA 98052
Phone: +1 425 706 6605 EMail: [email protected]
Stuart Cheshire Apple Computer, Inc. 1 Infinite Loop Cupertino California 95014 USA
Phone: +1 408 974 3207 EMail: [email protected]
11. Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
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