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Network Working Group                                           M. StappRequest for Comments: 4701                           Cisco Systems, Inc.Category: Standards Track                                       T. Lemon                                                           Nominum, Inc.                                                           A. Gustafsson                                          Araneus Information Systems Oy                                                            October 2006A DNS Resource Record (RR) for EncodingDynamic Host Configuration Protocol (DHCP) Information (DHCID RR)Status of This Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2006).Abstract   It is possible for Dynamic Host Configuration Protocol (DHCP) clients   to attempt to update the same DNS Fully Qualified Domain Name (FQDN)   or to update a DNS FQDN that has been added to the DNS for another   purpose as they obtain DHCP leases.  Whether the DHCP server or the   clients themselves perform the DNS updates, conflicts can arise.  To   resolve such conflicts,RFC 4703 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 Resource   Record (RR) type for this purpose for use by DHCP clients and   servers:  the "DHCID" RR.Stapp, et al.               Standards Track                     [Page 1]

RFC 4701                      The DHCID RR                  October 2006Table of Contents1. Introduction ....................................................32. Terminology .....................................................33. The DHCID RR ....................................................33.1. DHCID RDATA Format .........................................33.2. DHCID Presentation Format ..................................43.3. The DHCID RR Identifier Type Codes .........................43.4. The DHCID RR Digest Type Code ..............................43.5. Computation of the RDATA ...................................53.5.1. Using the Client's DUID .............................53.5.2. Using the Client Identifier Option ..................63.5.3. Using the Client's htype and chaddr .................63.6. Examples ...................................................63.6.1. Example 1 ...........................................63.6.2. Example 2 ...........................................73.6.3. Example 3 ...........................................74. Use of the DHCID RR .............................................85. Updater Behavior ................................................86. Security Considerations .........................................87. IANA Considerations .............................................98. Acknowledgements ................................................99. References ......................................................99.1. Normative References .......................................99.2. Informative References ....................................10Stapp, et al.               Standards Track                     [Page 2]

RFC 4701                      The DHCID RR                  October 20061.  Introduction   A set of procedures to allow DHCP [7] [11] clients and servers to   automatically update the DNS ([3], [4]) is proposed in [1].   Conflicts can arise if multiple DHCP clients wish to use the same DNS   name or a DHCP client attempts to use a name added for another   purpose.  To resolve such 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 obscure potentially sensitive client identifying   information, the data stored is the result of a one-way SHA-256 hash   computation.  The hash includes information from the DHCP client's   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.2.  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 [2].3.  The DHCID RR   The DHCID RR is defined with mnemonic DHCID and type code 49.  The   DHCID RR is only defined in the IN class.  DHCID RRs cause no   additional section processing.3.1.  DHCID RDATA Format   The RDATA section of a DHCID RR in transmission contains RDLENGTH   octets 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 2-octet identifier type, in network byte order,Stapp, et al.               Standards Track                     [Page 3]

RFC 4701                      The DHCID RR                  October 2006   followed by a 1-octet digest type, followed by one or more octets   representing the actual identifier:           < 2 octets >    Identifier type code           < 1 octet >     Digest type code           < n octets >    Digest (length depends on digest type)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 [8], Section 3.  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 Identifier Type Codes   The DHCID RR Identifier Type Code specifies what data from the DHCP   client's request was used as input into the hash function.  The   identifier type codes are defined in a registry maintained by IANA,   as specified inSection 7.  The initial list of assigned values for   the identifier type code and that type's identifier is:   +------------------+------------------------------------------------+   |  Identifier Type | Identifier                                     |   |       Code       |                                                |   +------------------+------------------------------------------------+   |      0x0000      | The 1-octet 'htype' followed by 'hlen' octets  |   |                  | of 'chaddr' from a DHCPv4 client's DHCPREQUEST |   |                  | [7].                                           |   |      0x0001      | The data octets (i.e., the Type and            |   |                  | Client-Identifier fields) from a DHCPv4        |   |                  | client's Client Identifier option [10].        |   |      0x0002      | The client's DUID (i.e., the data octets of a  |   |                  | DHCPv6 client's Client Identifier option [11]  |   |                  | or the DUID field from a DHCPv4 client's       |   |                  | Client Identifier option [6]).                 |   |  0x0003 - 0xfffe | Undefined; available to be assigned by IANA.   |   |      0xffff      | Undefined; RESERVED.                           |   +------------------+------------------------------------------------+3.4.  The DHCID RR Digest Type Code   The DHCID RR Digest Type Code is an identifier for the digest   algorithm used.  The digest is calculated over an identifier and the   canonical FQDN as described in the next section.Stapp, et al.               Standards Track                     [Page 4]

RFC 4701                      The DHCID RR                  October 2006   The digest type codes are defined in a registry maintained by IANA,   as specified inSection 7.  The initial list of assigned values for   the digest type codes is: value 0 is reserved, and value 1 is   SHA-256.  Reserving other types requires IETF standards action.   Defining new values will also require IETF standards action to   document how DNS updaters are to deal with multiple digest types.3.5.  Computation of the RDATA   The DHCID RDATA is formed by concatenating the 2-octet identifier   type code with variable-length data.   The RDATA for all type codes other than 0xffff, which is reserved for   future expansion, is formed by concatenating the 2-octet identifier   type code, the 1-octet digest type code, and the digest value (32   octets for SHA-256).       < identifier-type > < digest-type > < digest >   The input to the digest hash function is defined to be:       digest = SHA-256(< identifier > < FQDN >)   The FQDN is represented in the buffer in the canonical wire format as   described in [9], Section 6.2.  The identifier type code and the   identifier are related as specified inSection 3.3: the identifier   type code describes the source of the identifier.   A DHCPv4 updater uses the 0x0002 type code if a Client Identifier   option is present in the DHCPv4 messages and it is encoded as   specified in [6].  Otherwise, the updater uses 0x0001 if a Client   Identifier option is present, and 0x0000 if not.   A DHCPv6 updater always uses the 0x0002 type code.3.5.1.  Using the Client's DUID   When the updater is using the Client's DUID (either from a DHCPv6   Client Identifier option or from a portion of the DHCPv4 Client   Identifier option encoded as specified in [6]), the first two octets   of the DHCID RR MUST be 0x0002, in network byte order.  The third   octet is the digest type code (1 for SHA-256).  The rest of the DHCID   RR MUST contain the results of computing the SHA-256 hash across the   octets of the DUID followed by the FQDN.Stapp, et al.               Standards Track                     [Page 5]

RFC 4701                      The DHCID RR                  October 20063.5.2.  Using the Client Identifier Option   When the updater is using the DHCPv4 Client Identifier option sent by   the client in its DHCPREQUEST message, the first two octets of the   DHCID RR MUST be 0x0001, in network byte order.  The third octet is   the digest type code (1 for SHA-256).  The rest of the DHCID RR MUST   contain the results of computing the SHA-256 hash across the data   octets (i.e., the Type and Client-Identifier fields) of the option,   followed by the FQDN.3.5.3.  Using the Client's htype and chaddr   When the updater is using the client's link-layer address as the   identifier, the first two octets of the DHCID RDATA MUST be zero.   The third octet is the digest type code (1 for SHA-256).  To generate   the rest of the resource record, the updater computes a one-way hash   using the SHA-256 algorithm across a buffer containing the client's   network hardware type, link-layer address, and the FQDN data.   Specifically, the first octet 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 octets of the   'chaddr' field in the client's DHCPREQUEST message follow, in the   same order in which the octets appear in the DHCPREQUEST message.   The number of significant octets in the 'chaddr' field is specified   in the 'hlen' field of the DHCPREQUEST message.  The FQDN data, as   specified above, follows.3.6.  Examples3.6.1.  Example 1   A DHCP server allocates the IPv6 address 2001:DB8::1234:5678 to a   client that included the DHCPv6 client-identifier option data 00:01:   00:06:41:2d:f1:66:01:02:03:04:05:06 in its DHCPv6 request.  The   server updates the name "chi6.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 octets   to the value 0x0002, the 1-octet digest type to 1 for SHA-256, and   performing a SHA-256 hash computation across a buffer containing the   14 octets from the client-id option and the FQDN (represented as   specified inSection 3.5).     chi6.example.com.     AAAA    2001:DB8::1234:5678     chi6.example.com.     DHCID   ( AAIBY2/AuCccgoJbsaxcQc9TUapptP69l                                     OjxfNuVAA2kjEA= )   If the DHCID RR type is not supported, the RDATA would be encoded   [13] as:Stapp, et al.               Standards Track                     [Page 6]

RFC 4701                      The DHCID RR                  October 2006     \# 35 ( 000201636fc0b8271c82825bb1ac5c41cf5351aa69b4febd94e8f17cd             b95000da48c40 )3.6.2.  Example 2   A DHCP server allocates the IPv4 address 192.0.2.2 to a client that   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 octets to the value 0x0001, the 1-octet digest   type to 1 for SHA-256, and performing a SHA-256 hash computation   across a buffer containing the seven octets from the client-id option   and the FQDN (represented as specified inSection 3.5).     chi.example.com.      A       192.0.2.2     chi.example.com.      DHCID   ( AAEBOSD+XR3Os/0LozeXVqcNc7FwCfQdW                                     L3b/NaiUDlW2No= )   If the DHCID RR type is not supported, the RDATA would be encoded   [13] as:     \# 35 ( 0001013920fe5d1dceb3fd0ba3379756a70d73b17009f41d58bddbfcd             6a2503956d8da )3.6.3.  Example 3   A DHCP server allocating the IPv4 address 192.0.2.3 to a client with   the 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   octets to zero, the 1-octet digest type to 1 for SHA-256, and   performing an SHA-256 hash computation across a buffer containing the   1-octet 'htype' value for Ethernet, 0x01, followed by the six octets   of the Ethernet MAC address, and the domain name (represented as   specified inSection 3.5).     client.example.com.   A       192.0.2.3     client.example.com.   DHCID   ( AAABxLmlskllE0MVjd57zHcWmEH3pCQ6V                                     ytcKD//7es/deY= )   If the DHCID RR type is not supported, the RDATA would be encoded   [13] as:     \# 35 ( 000001c4b9a5b249651343158dde7bcc77169841f7a4243a572b5c283             fffedeb3f75e6 )Stapp, et al.               Standards Track                     [Page 7]

RFC 4701                      The DHCID RR                  October 20064.  Use of the DHCID RR   This RR MUST NOT be used for any purpose other than that detailed in   [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 obscure the client's identity information,   a one-way hash is used.  Further, 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.   However, it should be noted that an attacker that has some knowledge,   such as of MAC addresses commonly used in DHCP client identification   data, may be able to discover the client's DHCP identify by using a   brute-force attack.  Even without any additional knowledge, the   number of unknown bits used in computing the hash is typically only   48 to 80.   Administrators should be wary of permitting unsecured DNS updates to   zones, whether or not they are exposed to the global Internet.  Both   DHCP clients and servers SHOULD use some form of update   authentication (e.g., [12]) when performing DNS updates.Stapp, et al.               Standards Track                     [Page 8]

RFC 4701                      The DHCID RR                  October 20067.  IANA Considerations   IANA has allocated a DNS RR type number for the DHCID record type.   This specification defines a new number-space for the 2-octet   identifier type codes associated with the DHCID RR.  IANA has   established a registry of the values for this number-space.  Three   initial values are assigned inSection 3.3, and the value 0xFFFF is   reserved for future use.  New DHCID RR identifier type codes are   assigned through Standards Action, as defined in [5].   This specification defines a new number-space for the 1-octet digest   type codes associated with the DHCID RR.  IANA has established a   registry of the values for this number-space.  Two initial values are   assigned inSection 3.4.  New DHCID RR digest type codes are assigned   through Standards Action, as defined in [5].8.  Acknowledgements   Many thanks to Harald Alvestrand, Ralph Droms, Olafur Gudmundsson,   Sam Hartman, Josh Littlefield, Pekka Savola, and especially Bernie   Volz for their review and suggestions.9.  References9.1.  Normative References   [1]  Stapp, M. and B. Volz, "Resolution of Fully Qualified Domain        Name (FQDN) Conflicts among Dynamic Host Configuration Protocol        (DHCP) Clients",RFC 4703, October 2006.   [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement        Levels",BCP 14,RFC 2119, March 1997.   [3]  Mockapetris, P., "Domain names - concepts and facilities",        STD 13,RFC 1034, November 1987.   [4]  Mockapetris, P., "Domain names - implementation and        specification", STD 13,RFC 1035, November 1987.   [5]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA        Considerations Section in RFCs",BCP 26,RFC 2434, October 1998.   [6]  Lemon, T. and B. Sommerfeld, "Node-specific Client Identifiers        for Dynamic Host Configuration Protocol Version Four (DHCPv4)",RFC 4361, February 2006.Stapp, et al.               Standards Track                     [Page 9]

RFC 4701                      The DHCID RR                  October 20069.2.  Informative References   [7]   Droms, R., "Dynamic Host Configuration Protocol",RFC 2131,         March 1997.   [8]   Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",RFC 3548, July 2003.   [9]   Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,         "Resource Records for the DNS Security Extensions",RFC 4034,         March 2005.   [10]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor         Extensions",RFC 2132, March 1997.   [11]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M.         Carney, "Dynamic Host Configuration Protocol for IPv6         (DHCPv6)",RFC 3315, July 2003.   [12]  Vixie, P., Gudmundsson, O., Eastlake, D., and B. Wellington,         "Secret Key Transaction Authentication for DNS (TSIG)",RFC 2845, May 2000.   [13]  Gustafsson, A., "Handling of Unknown DNS Resource Record (RR)         Types",RFC 3597, September 2003.Stapp, et al.               Standards Track                    [Page 10]

RFC 4701                      The DHCID RR                  October 2006Authors' Addresses   Mark Stapp   Cisco Systems, Inc.   1414 Massachusetts Ave.   Boxborough, MA  01719   USA   Phone: 978.936.1535   EMail: mjs@cisco.com   Ted Lemon   Nominum, Inc.   950 Charter St.   Redwood City, CA  94063   USA   EMail: mellon@nominum.com   Andreas Gustafsson   Araneus Information Systems Oy   Ulappakatu 1   02320 Espoo   Finland   EMail: gson@araneus.fiStapp, et al.               Standards Track                    [Page 11]

RFC 4701                      The DHCID RR                  October 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is provided by the IETF   Administrative Support Activity (IASA).Stapp, et al.               Standards Track                    [Page 12]