draft-ietf-dnsext-dhcid-rr-06.txt

bind-3.2.

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


           A DNS RR for Encoding DHCP Information (DHCID RR)
                  <draft-ietf-dnsext-dhcid-rr-06.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
   Task Force (IETF), its areas, and its working groups. Note that
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   Internet-Drafts.

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   This Internet-Draft will expire on May 2, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2002). 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 . . . . . . . . . . . . . . . . . . . . .  4
   3.2   DHCID Presentation Format  . . . . . . . . . . . . . . . . .  4
   3.3   The DHCID RR Type Codes  . . . . . . . . . . . . . . . . . .  4
   3.4   Computation of the RDATA . . . . . . . . . . . . . . . . . .  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 . . . . . . . . . . . . . . . . . . 10































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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]. 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.




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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 DHCID RR Type Code specifies what data from the DHCP client's
   request was used as input into the hash function. The type codes are
   defined in a registry maintained by IANA, as specified in Section 7.
   The initial list of assigned values for the type code 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
   	   (draft-ietf-dhc-dhcpv6-*.txt)

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

     0xffff = RESERVED







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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.

   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 the DHCPv4 Client Identifier option sent
   by the client in its DHCPREQUEST message, the first two bytes of the
   DHCID RR MUST be 0x0001, in network byte order. 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 option code
   and length.

   When the updater is using the DHCPv6 DUID sent by the client in its
   REQUEST message, the first two bytes of the DHCID RR MUST be 0x0002,
   in network byte order. 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 option code and length.




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