

DNSEXT Working Group                                            M. Stapp
Internet-Draft                                       Cisco Systems, Inc.
Expires: January 18, 2002                                       T. Lemon
                                                           A. Gustafsson
                                                           Nominum, Inc.
                                                           July 20, 2001


           A DNS RR for Encoding DHCP Information (DHCID RR)
                  <draft-ietf-dnsext-dhcid-rr-03.txt>

Status of this Memo

   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
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   This Internet-Draft will expire on January 18, 2002.

Copyright Notice

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

Abstract

   It is possible for multiple DHCP clients to attempt to update the
   same DNS FQDN as they obtain DHCP leases. Whether the DHCP server or
   the clients themselves perform the DNS updates, conflicts can arise.
   To resolve such conflicts, "Resolution of DNS Name Conflicts"[1]
   proposes storing client identifiers in the DNS to unambiguously
   associate domain names with the DHCP clients to which they refer.
   This memo defines a distinct RR type for this purpose for use by
   DHCP clients and servers, the "DHCID" RR.




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Table of Contents

   1.    Terminology  . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.    The DHCID RR . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.1   DHCID RDATA format . . . . . . . . . . . . . . . . . . . . .  3
   3.2   DHCID Presentation Format  . . . . . . . . . . . . . . . . .  4
   3.3   The DHCID RR Type Codes  . . . . . . . . . . . . . . . . . .  4
   3.4   Computation of the RDATA . . . . . . . . . . . . . . . . . .  4
   3.5   Use of the DHCID RR  . . . . . . . . . . . . . . . . . . . .  5
   3.6   Updater Behavior . . . . . . . . . . . . . . . . . . . . . .  5
   3.7   Examples . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.7.1 Example 1  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.7.2 Example 2  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.    Security Considerations  . . . . . . . . . . . . . . . . . .  6
   5.    IANA Considerations  . . . . . . . . . . . . . . . . . . . .  7
         References . . . . . . . . . . . . . . . . . . . . . . . . .  7
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  8
         Full Copyright Statement . . . . . . . . . . . . . . . . . .  9
































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1. Terminology

   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 RFC 2119[2].

2. Introduction

   A set of procedures to allow DHCP[3] clients and servers to
   automatically update the DNS (RFC1034[4], RFC1035[5]) is proposed in
   "Resolution of DNS Name Conflicts"[1].

   Conflicts can arise if multiple DHCP clients wish to use the same
   DNS name. To resolve such conflicts, "Resolution of DNS Name
   Conflicts"[1] proposes storing client identifiers in the DNS to
   unambiguously associate domain names with the DHCP clients using
   them. In the interest of clarity, it is preferable for this DHCP
   information to use a distinct RR type. This memo defines a distinct
   RR for this purpose for use by DHCP clients or servers, the "DHCID"
   RR.

   In order to avoid exposing potentially sensitive identifying
   information, the data stored is the result of a one-way MD5[6] hash
   computation. The hash includes information from the DHCP client's
   REQUEST message as well as the domain name itself, so that the data
   stored in the DHCID RR will be dependent on both the client
   identification used in the DHCP protocol interaction and the domain
   name. This means that the DHCID RDATA will vary if a single client
   is associated over time with more than one name. This makes it
   difficult to 'track' a client as it is associated with various
   domain names.

   The MD5 hash algorithm has been shown to be weaker than the SHA-1
   algorithm; it could therefore be argued that SHA-1 is a better
   choice. However, SHA-1 is significantly slower than MD5. A
   successful attack of MD5's weakness does not reveal the original
   data that was used to generate the signature, but rather provides a
   new set of input data that will produce the same signature. Because
   we are using the MD5 hash to conceal the original data, the fact
   that an attacker could produce a different plaintext resulting in
   the same MD5 output is not significant concern.

3. The DHCID RR

   The DHCID RR is defined with mnemonic DHCID and type code [TBD].

3.1 DHCID RDATA format

   The RDATA section of a DHCID RR in transmission contains RDLENGTH


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   bytes of binary data.  The format of this data and its
   interpretation by DHCP servers and clients are described below.

   DNS software should consider the RDATA section to be opaque. DHCP
   clients or servers use the DHCID RR to associate a DHCP client's
   identity with a DNS name, so that multiple DHCP clients and servers
   may deterministically perform dynamic DNS updates to the same zone.
   From the updater's perspective, the DHCID resource record RDATA
   consists of a 16-bit identifier type, in network byte order,
   followed by one or more bytes representing the actual identifier:

     	< 16 bits >	DHCP identifier used
     	< n bytes >	MD5 digest

3.2 DHCID Presentation Format

   In DNS master files, the RDATA is represented as a single block in
   base 64 encoding and may be divided up into any number of white
   space separated substrings, down to single base 64 digits, which are
   concatenated to form the complete RDATA.  These substrings can span
   lines using the standard parentheses. This format is identical to
   that used for representing binary data in RFC2535[7].

3.3 The DHCID RR Type Codes

   The type code can have one of three classes of values.  The first
   class contains just the value zero.  This type indicates that the
   remaining contents of the DHCID record encode an identifier that is
   based on the client's link-layer network address.

   The second class of types contains just the value 0xFFFF.  This type
   code is reserved for future extensibility.

   The third class of types contains all the values not included in the
   first two - that is, every value other than zero or 0xFFFF. Types in
   this class indicate that the remaining contents of the DHCID record
   encode an identifier that is based on the DHCP option whose code is
   the same as the specified type.  The most common value in this class
   at the time of the writing of this specification is 0x3d (61
   decimal), which is the DHCP option code for the Client Identifier
   option [8].

3.4 Computation of the RDATA

   The data following the type code (for type codes other than 0xFFFF)
   is derived by running the MD5 hash algorithm across a buffer
   containing the identifying information. The identifying information
   includes some data from the DHCP client's DHCPREQUEST message, and
   the FQDN which is the target of the update.


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   The domain name is represented in the buffer in dns wire-format as
   described in RFC1035[5], section 3.1. The domain name MUST NOT be
   compressed as described in RFC1035[5], section 4.1.4. Any uppercase
   alphabetic ASCII character in a label MUST be converted to lowercase
   before being used to compute the hash.

   When the updater is using the client's link-layer address as the
   identifier, the first two bytes of the DHCID RDATA MUST be zero. To
   generate the rest of the resource record, the updater computes a
   one-way hash using the MD5 algorithm across a buffer containing the
   client's network hardware type, link-layer address, and the FQDN
   data.  Specifically, the first byte of the buffer contains the
   network hardware type as it appeared in the DHCP 'htype' field of
   the client's DHCPREQUEST message. All of the significant bytes of
   the chaddr field in the client's DHCPREQUEST message follow, in the
   same order in which the bytes appear in the DHCPREQUEST message. The
   number of significant bytes in the 'chaddr' field is specified in
   the 'hlen' field of the DHCPREQUEST message. The FQDN data, as
   specified above, follows.

   When the updater is using a DHCP option sent by the client in its
   DHCPREQUEST message, the first two bytes of the DHCID RR MUST be the
   option code of that option, in network byte order. For example, if
   the DHCP client identifier option is being used, the first byte of
   the DHCID RR should be zero, and the second byte should be 61
   decimal. The rest of the DHCID RR MUST contain the results of
   computing an MD5 hash across the payload of the option being used,
   followed by the FQDN. The payload of a DHCP option consists of the
   bytes of the option following the option code and length.

   The "Resolution of DNS Name Conflicts"[1] specification describes
   the selection process that updaters follow to choose an identifier
   from the information presented in a client's DHCPREQUEST message.

3.5 Use of the DHCID RR

   This RR MUST NOT be used for any purpose other than that detailed in
   "Resolution of DNS Name Conflicts"[1]. Although this RR contains
   data that is opaque to DNS servers, the data must be consistent
   across all entities that update and interpret this record.
   Therefore, new data formats may only be defined through actions of
   the DHC Working Group, as a result of revising [1].

3.6 Updater Behavior

   The data in the DHCID RR allows updaters to determine whether more
   than one DHCP client desires to use a particular FQDN.  This allows
   site administrators to establish policy about DNS updates. The DHCID
   RR does not establish any policy itself.


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   Updaters use data from a DHCP client's request and the domain name
   that the client desires to use to compute a client identity hash,
   and then compare that hash to the data in any DHCID RRs on the name
   that they wish to associate with the client's IP address. If an
   updater discovers DHCID RRs whose RDATA does not match the client
   identity that they have computed, the updater SHOULD conclude that a
   different client is currently associated with the name in question.
   The updater SHOULD then proceed according to the site's
   administrative policy. That policy might dictate that a different
   name be selected, or it might permit the updater to continue.

3.7 Examples

3.7.1 Example 1

   A DHCP server allocating the IPv4 address 10.0.0.1 to a client with
   Ethernet MAC address 01:02:03:04:05:06 using domain name
   "client.org.nil" uses the client's link-layer address to identify
   the client. The DHCID RDATA is composed by setting the two type
   bytes to zero, and performing an MD5 hash computation across a
   buffer containing the Ethernet MAC type byte, 0x01, the six bytes of
   MAC address, and the domain name (represented as specified in
   Section 3.4).

     client.org.nil.	A   	10.0.0.1
     client.org.nil. 	DHCID 	AAAUMru0ZM5OK/PdVAJgZ/HU

3.7.2 Example 2

   A DHCP server allocates the IPv4 address 10.0.12.99 to a client
   which included the DHCP client-identifier option data
   01:07:08:09:0a:0b:0c in its DHCP request. The server updates the
   name "chi.org.nil" on the client's behalf, and uses the DHCP client
   identifier option data as input in forming a DHCID RR. The DHCID
   RDATA is formed by setting the two type bytes to the option code,
   0x003d, and performing an MD5 hash computation across a buffer
   containing the seven bytes from the client-id option and the FQDN
   (represented as specified in Section 3.4).

     chi.org.nil.	A    	10.0.12.99
     chi.org.nil.	DHCID 	AD3dquu0xNqYn/4zw2FXy8X3

4. Security Considerations

   The DHCID record as such does not introduce any new security
   problems into the DNS.  In order to avoid exposing private
   information about DHCP clients to public scrutiny, a one-way hash is
   used to obscure all client information. In order to make it
   difficult to 'track' a client by examining the names associated with


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   a particular hash value, the FQDN is included in the hash
   computation. Thus, the RDATA is dependent on both the DHCP client
   identification data and on each FQDN associated with the client.

   Administrators should be wary of permitting unsecured DNS updates to
   zones which are exposed to the global Internet. Both DHCP clients
   and servers SHOULD use some form of update authentication (e.g.,
   TSIG[9]) when performing DNS updates.

5. IANA Considerations

   IANA is requested to allocate an RR type number for the DHCID record
   type.

References

   [1]  Stapp, M., "Resolution of DNS Name Conflicts Among DHCP Clients
        (draft-ietf-dhc-dns-resolution-*)", March 2001.

   [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", RFC 2119, March 1997.

   [3]  Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, Mar
        1997.

   [4]  Mockapetris, P., "Domain names - Concepts and Facilities", RFC
        1034, Nov 1987.

   [5]  Mockapetris, P., "Domain names - Implementation and
        Specification", RFC 1035, Nov 1987.

   [6]  Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321, April
        1992.

   [7]  Eastlake, D., "Domain Name System Security Extensions", RFC
        2535, March 1999.

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

   [9]  Vixie, P., Gudmundsson, O., Eastlake, D. and B. Wellington,
        "Secret Key Transaction Authentication for DNS (TSIG)", RFC
        2845, May 2000.








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Authors' Addresses

   Mark Stapp
   Cisco Systems, Inc.
   250 Apollo Dr.
   Chelmsford, MA  01824
   USA

   Phone: 978.244.8498
   EMail: mjs@cisco.com


   Ted Lemon
   Nominum, Inc.
   950 Charter St.
   Redwood City, CA  94063
   USA

   EMail: mellon@nominum.com


   Andreas Gustafsson
   Nominum, Inc.
   950 Charter St.
   Redwood City, CA  94063
   USA

   EMail: gson@nominum.com























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Full Copyright Statement

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

   This document and translations of it may be copied and furnished to
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Acknowledgement

   Funding for the RFC editor function is currently provided by the
   Internet Society.



















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