draft-ietf-dnsext-dhcid-rr-12.txt

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   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 in Section 3.5).

     chi.example.com.      A       10.0.12.99
     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 allocates the IPv6 address 2000::1234:5678 to a client
   which 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 in Section 3.5).

     chi6.example.com.     AAAA    2000::1234:5678
     chi6.example.com.     DHCID   ( AAIBY2/AuCccgoJbsaxcQc9TUapptP69l
                                     OjxfNuVAA2kjEA= )

   If the DHCID RR type is not supported, the RDATA would be encoded
   [13] as:

     \# 35 ( 000201636fc0b8271c82825bb1ac5c41cf5351aa69b4febd94e8f17cd
             b95000da48c40 )


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.



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   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.  And, 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., TSIG [11]) when performing DNS updates.


7.  IANA Considerations




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   IANA is requested to allocate 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 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 identifier
   type codes are assigned through Standards Action, as defined in RFC
   2434 [5].

   This specification defines a new number-space for the 1-octet digest
   type codes associated with the DHCID RR.  IANA is requested to
   establish a registry of the values for this number-space.  Two
   initial values are assigned in Section 3.4.  New DHCID RR digest type
   codes are assigned through Standards Action, as defined in RFC 2434
   [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.  References

9.1.  Normative References

   [1]  Stapp, M. and B. Volz, "Resolution of DNS Name Conflicts Among
        DHCP Clients (draft-ietf-dhc-dns-resolution-*)", February 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.







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9.2.  Informative References

   [6]   Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
         March 1997.

   [7]   Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
         RFC 3548, July 2003.

   [8]   Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
         "Resource Records for the DNS Security Extensions", RFC 4034,
         March 2005.

   [9]   Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
         Extensions", RFC 2132, March 1997.

   [10]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M.
         Carney, "Dynamic Host Configuration Protocol for IPv6
         (DHCPv6)", RFC 3315, July 2003.

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

   [12]  Lemon, T. and B. Sommerfeld, "Node-specific Client Identifiers
         for Dynamic Host Configuration Protocol Version Four (DHCPv4)",
         RFC 4361, February 2006.

   [13]  Gustafsson, A., "Handling of Unknown DNS Resource Record (RR)
         Types", RFC 3597, September 2003.






















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Authors' Addresses

   Mark Stapp
   Cisco Systems, Inc.
   1414 Massachusetts Ave.
   Boxborough, MA  01719
   USA

   Phone: [phone]
   Email: [e-mail]


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

   Email: [e-mail]


   Andreas Gustafsson
   Araneus Information Systems Oy
   Ulappakatu 1
   02320 Espoo
   Finland

   Email: [e-mail]























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