DNSEXT Working Group                                            M. Stapp
Internet-Draft                                       Cisco Systems, Inc.
Expires: August 31, 2001 January 18, 2002                                       T. Lemon
                                                           A. Gustafsson
                                                           Nominum, Inc.
                                                           March 2,
                                                           July 20, 2001

           A DNS RR for Encoding DHCP Information
                  <draft-ietf-dnsext-dhcid-rr-02.txt> (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 August 31, 2001. January 18, 2002.

Copyright Notice

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

Abstract

   A situation can arise where

   It is possible for multiple DHCP clients request to attempt to update the
   same DNS name from their (possibly distinct) FQDN as they obtain DHCP leases. Whether the DHCP servers. server or
   the clients themselves perform the DNS updates, conflicts can arise.
   To resolve such conflicts, 'Resolution "Resolution of DNS Name Conflicts'[6] Conflicts"[1]
   proposes storing client identifiers in the DNS to unambiguously
   associate domain names with the DHCP clients "owning" them. 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
   4.
   3.1   DHCID RDATA format . . . . . . . . . . . . . . . . . . . . . .  3
   4.1 Example
   3.2   DHCID Presentation Format  . . . . . . . . . . . . . . . . .  4
   3.3   The DHCID RR Type Codes  . . . . . . . . . . . . .  4
   5.  Security Considerations . . . . .  4
   3.4   Computation of the RDATA . . . . . . . . . . . . . . .  4
   6.  IANA Considerations . . .  4
   3.5   Use of the DHCID RR  . . . . . . . . . . . . . . . . . . . .  5
   7.  Appendix A: Base 64 Encoding
   3.6   Updater Behavior . . . . . . . . . . . . . . . . . . . . . .  5
       References
   3.7   Examples . . . . . . . . . . . . . . . . . . . . . . . . . .  6
       Authors' Addresses
   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 . . . . . . . . . . . . . . . . . . .  8  9

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[1]. 2119[2].

2. Introduction

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

   A situation Conflicts"[1].

   Conflicts can arise where if multiple DHCP clients wish to use the same
   DNS name. To resolve such conflicts, Resolution "Resolution of DNS Name
   Conflicts[6]
   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 would be is 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.

3. The DHCID RR

   The DHCID RR

   In order to avoid exposing potentially sensitive identifying
   information, the data stored is defined with mnemonic DHCID and type code [TBD].

4. DHCID RDATA format

   The RDATA section the result of a DHCID RR in 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
   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 (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[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 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.

   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:

     	< 16 bits >	DHCP identifier that is
   based on used
     	< n bytes >	MD5 digest

3.2 DHCID Presentation Format

   In DNS master files, the client's link-layer network address.

   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 draft specification is 61, 0x3d (61
   decimal), which is the DHCP option code[3] code for the Client Identifier option.
   option [8].

3.4 Computation of the RDATA

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

   The domain name is represented in "Resolution of
   DNS Name Conflicts"[6].

   This RR the buffer in dns wire-format as
   described in RFC1035[5], section 3.1. The domain name MUST NOT be used for any purpose other than that detailed
   compressed as described in
   "Resolution of DNS Name Conflicts"[6]. Althought this RR contains
   data that is opaque RFC1035[5], section 4.1.4. Any uppercase
   alphabetic ASCII character in a label MUST be converted to DNS servers, lowercase
   before being used to compute the data must be consistent
   across all entities that update and interpret this record.
   Therefore, new data formats may only 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 defined through actions zero. To
   generate the rest of the DHC Working Group, as resource record, the updater computes a result of revising [6].

4.1 Example

   A DHCP server allocating
   one-way hash using the IPv4 address 10.0.0.1 to MD5 algorithm across a client
   "client.org.nil" might use buffer containing the
   client's network hardware type, link-layer address to
   identify 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 client:

     client.org.nil.	A   	10.0.0.1
     client.org.nil. DHCID 	AAAYKREXIgqtwYgQo93/yNlJ 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.

   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 10.0.12.99 10.0.0.1 to a client
   "chi.org.nil" might use with
   Ethernet MAC address 01:02:03:04:05:06 using domain name
   "client.org.nil" uses the DHCP client identifier option client's link-layer address to identify
   the client:

     chi.org.nil.	A    	10.0.12.99
     chi.org.nil.	DHCID 	AGGScSLaAYjdOhGMHKD/lJ2B

5. Security Considerations client. 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 RDATA is requested composed by setting the two type
   bytes to allocate zero, and performing an RR MD5 hash computation across a
   buffer containing the Ethernet MAC type number for byte, 0x01, the DHCID record
   type.

7. Appendix A: Base 64 Encoding

   The following encoding technique is taken from RFC 2045[8] by N.
   Borenstein six bytes of
   MAC address, and N. Freed.  It is reproduced here the domain name (represented as specified in an edited form
   for convenience.
   Section 3.4).

     client.org.nil.	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   	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
   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 client
   which is translated into a single digit included the DHCP client-identifier option data
   01:07:08:09:0a:0b:0c in its DHCP request. The server updates the base
   64 alphabet.

   Each 6-bit group is used
   name "chi.org.nil" on the client's behalf, and uses the DHCP client
   identifier option data as an index into an array of 64 printable
   characters. input in forming a DHCID RR. The character referenced DHCID
   RDATA is formed by setting the index is placed in two type bytes to 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 option code,
   0x003d, and performing an MD5 hash computation across 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 buffer
   containing the seven bytes from the end of client-id option and the data being encoded. FQDN
   (represented as specified in Section 3.4).

     chi.org.nil.	A full encoding quantum is
   always completed at    	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 end of DNS.  In order to avoid exposing private
   information about DHCP clients to public scrutiny, a quantity.  When fewer than 24 input
   bits are available in an input group, zero bits are added (on the
   right) one-way hash is
   used to form an integral number of 6-bit groups. Padding at the
   end of obscure all client information. In order to make it
   difficult to 'track' a client by examining the data is performed using names associated with
   a particular hash value, the '=' character.  Since all
   base 64 input FQDN is an integral number of octets, only included in the following
   cases can arise: (1) hash
   computation. Thus, the final quantum of encoding input RDATA is an
   integral multiple of 24 bits; here, dependent on both the final unit of encoded output
   will be an integral multiple of 4 characters DHCP client
   identification data and on each FQDN associated with no "=" padding,
   (2) the final quantum of encoding input is exactly 8 bits; here, the
   final unit of encoded output will client.

   Administrators should be two characters followed by two
   "=" padding characters, or (3) wary of permitting unsecured DNS updates to
   zones which are exposed to the final quantum global Internet. Both DHCP clients
   and servers SHOULD use some form of encoding input update authentication (e.g.,
   TSIG[9]) when performing DNS updates.

5. IANA Considerations

   IANA is exactly 16 bits; here, requested to allocate an RR type number for the final unit of encoded output will be
   three characters followed by one "=" padding character. 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.

   [2]

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

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

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

   [8]  Freed, N.  Alexander, S. and N. Borenstein, "Multipurpose Internet Mail
        Extensions (MIME) Part One: Format of Internet Message Bodies", 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 2045, November 1996.
        2845, May 2000.

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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