DNSEXT Working Group                                            M. Stapp
Internet-Draft                                       Cisco Systems, Inc.
Expires: December 20, 2002 May 2, 2003                                            T. Lemon
                                                           A. Gustafsson
                                                           Nominum, Inc.
                                                           June 21,
                                                        November 1, 2002

           A DNS RR for Encoding DHCP Information (DHCID RR)

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 December 20, 2002. May 2, 2003.

Copyright Notice

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


   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.

Table of Contents

   1.    Terminology  . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.    The DHCID RR . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.1   DHCID RDATA format . . . . . . . . . . . . . . . . . . . . .  4
   3.2   DHCID Presentation Format  . . . . . . . . . . . . . . . . .  4
   3.3   The DHCID RR Type Codes  . . . . . . . . . . . . . . . . . .  4
   3.4   Computation of the RDATA . . . . . . . . . . . . . . . . . .  4  5
   3.5   Examples . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.5.1 Example 1  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.5.2 Example 2  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.    Use of the DHCID RR  . . . . . . . . . . . . . . . . . . . .  6
   5.    Updater Behavior . . . . . . . . . . . . . . . . . . . . . .  6
   6.    Security Considerations  . . . . . . . . . . . . . . . . . .  7
   7.    IANA Considerations  . . . . . . . . . . . . . . . . . . . .  7
   8.    Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  7
         References . . . . . . . . . . . . . . . . . . . . . . . . .  7
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  8
         Full Copyright Statement . . . . . . . . . . . . . . . . . .  9 10

1. Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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"

   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.


   The DHCID RR is defined with mnemonic DHCID and type code [TBD]. The
   DHCID RR is only defined in the IN class. DHCID RRs cause no
   additional section processing. The DHCID RR is not a singleton type.

3.1 DHCID RDATA format

   The RDATA section of a DHCID RR in transmission contains RDLENGTH
   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 identical to that used for representing binary data
   in RFC2535[7]. The data 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.

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 RR Type Code specifies what data from the DHCP client's link-layer network address.

   The second class of types contains just
   request was used as input into the value 0xFFFF.  This type
   code is reserved for future extensibility. hash function. 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 type codes are
   defined 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 a registry maintained by IANA, as the specified type.  The most common value in this class
   at the time of the writing Section 7.
   The initial list of this specification is 0x3d (61
   decimal), which is assigned values for the DHCP option type code for is:

     0x0000 = htype, chaddr from a DHCPv4 client's
              DHCPREQUEST (RFC 2131)
     0x0001 = The data portion from a DHCPv4 client's Client
              Identifier option (RFC 2132)
     0x0002 = The data portion (i.e., the DUID) from a DHCPv6
              client's Client Identifier option [8].

     0x0003 - 0xfffe = Available to be assigned by IANA

     0xffff = RESERVED

3.4 Computation of the RDATA

   The DHCID RDATA is formed by concatenating the two type bytes with
   some variable-length identifying data.

       < type > < data >

   The RDATA for all type codes other than 0xffff, which is reserved
   for future expansion, is formed by concatenating the two type bytes
   and a 16-byte MD5 hash value. The input to the hash function is
   defined to be:

       data = MD5(< identifier > < FQDN >)

   The FQDN is represented in the buffer in unambiguous canonical form
   as described in RFC2535[7], section 8.1. The type code and the
   identifier are related as specified in Section 3.3: the type code
   describes the source of the identifier.

       type code           identifier

       0x0000              htype,hlen,chaddr from the client's DHCPREQUEST

       0x0001-             'data' portion of a DHCP option from the
       0xfffe              client's DHCPREQUEST

       0xffff              RESERVED

   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.

   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 the DHCPv4 Client Identifier 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, 0x0001, in network byte order. For example, if The rest of the
   DHCID RR MUST contain the results of computing an MD5 hash across
   the payload of the option, followed by the FQDN. The payload of the
   option consists of the bytes of the option following the DHCP client identifier option code
   and length.

   When the updater is being used, using the DHCPv6 DUID sent by the client in its
   REQUEST message, the first byte two bytes of the DHCID RR should MUST be zero, and the second 0x0002,
   in network byte should be 61
   decimal. order. The rest of the DHCID RR MUST contain the
   results of computing an MD5 hash across the payload of the option being used, option,
   followed by the FQDN. The payload of a DHCP the option consists of the
   bytes of the option following the option code and length.

3.5 Examples

3.5.1 Example 1

   A DHCP server allocating the IPv4 address to a client with
   Ethernet MAC address 01:02:03:04:05:06 using domain name
   "client.example.com" 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.example.com.	A
     client.example.com. 	DHCID 	AAAUMru0ZM5OK/PdVAJgZ/HU

3.5.2 Example 2

   A DHCP server allocates the IPv4 address 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.example.com" 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, value
   0x0001, 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.example.com.	A
     chi.example.com.	DHCID 	AD3dquu0xNqYn/4zw2FXy8X3 	AAHdd5jiQ3kEjANDm82cbObk\012

4. 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].

5. 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.

   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.

6. 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
   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[10]) when performing DNS updates.

7. IANA Considerations

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

   This specification defines a new number-space for the 16-bit type
   codes associated with the DHCID RR. IANA is requested to establish a
   registry of the values for this number-space.

   Three initial values are assigned in Section 3.3, and the value
   0xFFFF is reserved for future use. New DHCID RR type codes are
   tentatively assigned after the specification for the associated type
   code, published as an Internet Draft, has received expert review by
   a designated expert. The final assignment of DHCID RR type codes is
   through Standards Action, as defined in RFC2434[11].

8. Acknowledgements

   Many thanks to Josh Littlefield, Olafur Gudmundsson, Bernie Volz,
   and Ralph Droms for their review and suggestions.


   [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]   Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and M.
         Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)
         (draft-ietf-dhc-dhcpv6-*.txt)", November 2002.

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

   [11]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
         Considerations Section in RFCs", RFC 2434, October 1998.

Authors' Addresses

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

   Phone: 978.244.8498
   EMail: mjs@cisco.com
   Ted Lemon
   Nominum, Inc.
   950 Charter St.
   Redwood City, CA  94063

   EMail: mellon@nominum.com

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

   EMail: gson@nominum.com

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