DNSEXT Working Group                                            M. Stapp
Internet-Draft                                       Cisco Systems, Inc.
Expires: June 1, August 31, 2001                                        T. Lemon
                                                           A. Gustafsson
                                                           Nominum, Inc.
                                                           December 2000
                                                           March 2, 2001

                 A DNS RR for Encoding DHCP Information

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
   Task Force (IETF), its areas, and its working groups. Note that
   other groups may also distribute working documents as

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other documents
   at any time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on June 1, August 31, 2001.

Copyright Notice

   Copyright (C) The Internet Society (2000). (2001). All Rights Reserved.


   A situation can arise where multiple DHCP clients request the same
   DNS name from their (possibly distinct) DHCP servers.  To resolve
   such conflicts, 'Resolution of DNS Name Conflicts'[5] Conflicts'[6] proposes
   storing client identifiers in the DNS to unambiguously associate
   domain names with the DHCP clients "owning" them. This memo defines
   a distinct RR type for use by DHCP servers, the "DHCID" RR.

Table of Contents

   1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  The DHCID RR . . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  DHCID RDATA format . . . . . . . . . . . . . . . . . . . . . .  3
   4.1 Example  . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  4
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  4  5
   7.  Appendix A: Base 64 Encoding . . . . . . . . . . . . . . . . .  4  5
       References . . . . . . . . . . . . . . . . . . . . . . . . . .  6
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . .  6  7
       Full Copyright Statement . . . . . . . . . . . . . . . . . . .  8

1. Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119[1].

2. Introduction

   A set of procedures to allow DHCP[2] clients and servers to
   automatically update the DNS (RFC1034[3], RFC1035[4]) (RFC1034[4], RFC1035[5]) is proposed in
   "Resolution of DNS Name Conflicts"[5]. Conflicts"[6].

   A situation can arise where multiple DHCP clients wish to use the
   same DNS name. To resolve such conflicts, Resolution of DNS Name
   Conflicts[6] proposes storing client identifiers in the DNS to
   unambiguously associate domain names with the DHCP clients using
   them. In the interest of clarity, it would be preferable for this
   DHCP information to use a distinct RR type.

   This memo defines a distinct RR type for this purpose for use by
   DHCP clients or servers, the "DHCID" RR.


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

4. 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.  In DNS
   master files, the RDATA is represented in base 64 encoding (see
   Appendix A) A (Section 7)) and may be divided up into any number of
   white space separated substrings, down to single base 64 digits,
   which are concatenated to obtain the full signature. These
   substrings can span lines using the standard parenthesis. This
   format is identical to that used for representing binary data in
   DNSSEC (RFC2535[6]). (RFC2535[7]).

   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 safely perform dynamic DNS updates to the same zone.
   From the updater's perspective, the DHCID resource record consists
   of a 16-bit identifier type, followed by one or more bytes
   representing the actual identifier.  There are two possible forms
   for a DHCID RR -

   The type code can have one of three classes of values.  The first
   class contains just the value zero.  This type indicates that is used when the DHCP server is using
   remaining contents of the
   client's link-layer address to identify it, and one DHCID record encode an identifier that is used
   when the DHCP server is using some DHCP option that the DHCP client
   sent to identify it. When
   based on the client's link-layer address network address.

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

   The third class of types contains all the
   identifier, values not included in the
   first two bytes - that is, every value other than zero or 0xFFFF. Types in
   this class indicate that the remaining contents of the RRDATA are set to 0. When a DHCID record
   encode an identifier that is based on the DHCP option whose code is used
   the same as the identifier, specified type.  The most common value in this class
   at the first two bytes time of the
   RRDATA contain writing of this draft is 61, which is the DHCP
   option number, in network byte order. The two
   bytes 0xffff are reserved code[3] for future extensibility. In both cases, the remainder of Client Identifier option.

   The data following the RRDATA type code (for type codes other than 0xFFFF)
   is the result of performing derived by running a one-way hash across the identifier. identifying
   information.  The details of the method used to generate the data in the RR and
   the use to which a DHCP client or server may put this association are beyond the scope of this draft, and are discussed specified in the
   specification of the DNS update behavior, 'Resolution "Resolution of
   DNS Name
   Conflicts'[5]. Conflicts"[6].

   This RR MUST NOT be used for any purpose other than that detailed in the DHC document.
   "Resolution of DNS Name Conflicts"[6]. Althought this RR contains
   data that is opaque to DNS servers, the data is meaningful to DHCP
   updaters. 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. Group, as a result of revising [6].

4.1 Example

   A DHCP server allocating the IPv4 address to a client
   "" might use the client's link-layer address to
   identify the client:	A 	DHCID 	AAAY KREX Igqt wYgQ o93/ yNlJ 	AAAYKREXIgqtwYgQo93/yNlJ

   A DHCP server allocating the IPv4 address to a client
   "" might use the DHCP client identifier option to
   identify the client:	A	DHCID 	AGGS cSLa AYjd OhGM HKD/ lJ2B 	AGGScSLaAYjdOhGMHKD/lJ2B

5. 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.

6. IANA Considerations

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

7. Appendix A: Base 64 Encoding

   The following encoding technique is taken from RFC 2045[7] 2045[8] by N.
   Borenstein and N. Freed.  It is reproduced here in an edited form
   for convenience.

   A 65-character subset of US-ASCII is used, enabling 6 bits to be
   represented per printable character. (The extra 65th character, "=",
   is used to signify a special processing function.)

   The encoding process represents 24-bit groups of input bits as
   output strings of 4 encoded characters. Proceeding from left to
   right, a 24-bit input group is formed by concatenating 3 8-bit input
   groups.  These 24 bits are then treated as 4 concatenated 6-bit
   groups, each of which is translated into a single digit in the base
   64 alphabet.

   Each 6-bit group is used as an index into an array of 64 printable
   characters. The character referenced by the index is placed in the
   output string.

                            The Base 64 Alphabet

         Value Encoding  Value Encoding  Value Encoding  Value Encoding
             0 A            17 R            34 i            51 z
             1 B            18 S            35 j            52 0
             2 C            19 T            36 k            53 1
             3 D            20 U            37 l            54 2
             4 E            21 V            38 m            55 3
             5 F            22 W            39 n            56 4
             6 G            23 X            40 o            57 5
             7 H            24 Y            41 p            58 6
             8 I            25 Z            42 q            59 7
             9 J            26 a            43 r            60 8
            10 K            27 b            44 s            61 9
            11 L            28 c            45 t            62 +
            12 M            29 d            46 u            63 /
            13 N            30 e            47 v
            14 O            31 f            48 w         (pad) =
            15 P            32 g            49 x
            16 Q            33 h            50 y

   Special processing is performed if fewer than 24 bits are available
   at the end of the data being encoded.  A full encoding quantum is
   always completed at the end of a quantity.  When fewer than 24 input
   bits are available in an input group, zero bits are added (on the
   right) to form an integral number of 6-bit groups. Padding at the
   end of the data is performed using the '=' character.  Since all
   base 64 input is an integral number of octets, only the following
   cases can arise: (1) the final quantum of encoding input is an
   integral multiple of 24 bits; here, the final unit of encoded output
   will be an integral multiple of 4 characters with no "=" padding,
   (2) the final quantum of encoding input is exactly 8 bits; here, the
   final unit of encoded output will be two characters followed by two
   "=" padding characters, or (3) the final quantum of encoding input
   is exactly 16 bits; here, the final unit of encoded output will be
   three characters followed by one "=" padding character.


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

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

   [3]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
        Extensions", RFC 2132, 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]  Stapp, M., "Resolution of DNS Name Conflicts Among DHCP Clients
        (draft-ietf-dhc-dns-resolution-*)", July 2000.


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


   [8]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
        Extensions (MIME) Part One: Format of Internet Message Bodies",
        RFC 2045, November 1996.

Authors' Addresses

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

   Phone: 978.244.8498

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


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


Full Copyright Statement

   Copyright (C) The Internet Society (2000). (2001). All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph
   are included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an


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