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Network Working Group                                      Yakov Rekhter
Internet Draft                                             Cisco Systems
Expiration Date: January 1997                                  July 1996


                    Interaction between DHCP and DNS
                     draft-ietf-dhc-dhcp-dns-01.txt


1. Status of this Memo

   This document is an Internet-Draft.  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.

   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.''

   To learn the current status of any Internet-Draft, please check the
   ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow
   Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
   munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
   ftp.isi.edu (US West Coast).


2. Abstract

   DHCP provides a powerful mechanism for IP host autoconfiguration.
   However, the autoconfiguration provided by DHCP does not include
   updating DNS, and specifically updating the name to address and
   address to name mappings maintained by DNS.

   This document specifies how DHCP clients and servers should use the
   Dynamic DNS Updates mechanism to update the DNS name to address and
   address to name mapping, so that the mappings for DHCP clients would
   be consistent with the IP addresses that the clients acquire via
   DHCP.











Yakov Rekhter                                                   [Page 1]


Internet Draft       draft-ietf-dhc-dhcp-dns-01.txt            July 1996


3. Interaction between DHCP and DNS

   DNS [RFC1034, RFC1035] maintains (among other things) the information
   about mapping between hosts' Fully Qualified Domain Names (FQDNs)
   [RFC1594] and IP addresses assigned to the hosts.  The information is
   maintained in two types of Resource Records (RRs): A and PTR. The A
   RR contains mapping from a FQDN to an IP address; the PTR RR contains
   mapping from an IP address to a FQDN.

   DHCP [RFC1541] provides a mechanism by which a host (a DHCP client)
   could acquire certain configuration information, and specifically its
   IP address(es). However, DHCP does not provide any mechanisms to
   update the DNS RRs that contain the information about mapping between
   the host's FQDN and its IP address(es) (A and PTR RRs). Thus the
   information maintained by DNS for a DHCP client may be incorrect -  a
   host (the client) could acquire its address by using DHCP, but the A
   RR for the host's FQDN wouldn't reflect the address that the host
   acquired, and the PTR RR for the acquired address wouldn't reflect
   the host's FQDN.

   Dynamic DNS Updates [DynDNS] is a mechanism that enables DNS
   information to be updated DNS over a network.

   The Dynamic DNS Update protocol can be used to maintain consistency
   between the information stored in the A and PTR RRs and the actual
   address assignment done via DHCP.  When a host with a particular FQDN
   acquires its IP address via DHCP, the A RR associated with the host's
   FQDN would be updated (by using the Dynamic DNS Updates protocol) to
   reflect the new address.  Likewise, when an IP address gets assigned
   to a host with a particular FQDN, the PTR RR associated with this
   address would be updated (using the Dynamic DNS Updates protocol) to
   reflect the new FQDN.


4. Models of operations

   When a DHCP client acquires a new address, both the A RR (for the
   client's FQDN) and the PTR RR (for the acquired address) have to be
   updated. Therefore, we have two separate Dynamic DNS Update
   transactions. Acquiring an address via DHCP involves two entities: a
   DHCP client and a DHCP server. In principle each of these entities
   could perform none, one, or both of the transactions. However, upon
   some introspection one could realize that not all permutations make
   sense.  This document covers the possible design permutations:

         (1) DHCP client updates the A RR, DHCP server updates the PTR
               RR




Yakov Rekhter                                                   [Page 2]


Internet Draft       draft-ietf-dhc-dhcp-dns-01.txt            July 1996


         (2) DHCP server updates both the A and the PTR RRs

   One could observe that the only difference between these two cases is
   whether the FQDN to IP address mapping is updated by a DHCP client or
   by a DHCP server. The IP address to FQDN mapping is updated by a DHCP
   server in both cases.


4.1. Client FQDN Option

   To update the IP address to FQDN mapping a DHCP server needs to know
   FQDN of the client to which the server leases the address. To allow
   the client to convey its FQDN to the server this document defines a
   new option, called "Client FQDN".

   The code for this option is TBD. Its minimum length is 2.



         Code   Len    Flags  RCODE1 RCODE2   Domain Name
        +------+------+------+------+------+------+--
        | TBD  |   n  |  0/1 |      |      |       ...
        +------+------+------+------+------+------+--



   The Flags field allows a DHCP client to indicate to a DHCP server
   whether the client wants the server to be responsible for updating
   the FQDN to IP address mapping (if Flags is set to 1), or whether the
   client wants to take this responsibility (if Flags is set to 0).

   The RCODE1 and RCODE2 fields are used by a DHCP server to indicate to
   a DHCP client the Response Code from Dynamic DNS Updates.

   The Domain Name part of the option carries FQDN of a client.



4.2. DHCP Client behavior

   If a client wants to be responsible for updating the FQDN to IP
   address mapping for the FQDN and address(es) used by the client, then
   the client shall include the Client FQDN option in the DHCPREQUEST
   message originated by the client. The Flags field in the option shall
   be set to 0. Once the client's DHCP configuration is completed (the
   client receives a DHCPACK message, and successfully completed a final
   check on the parameters passed in the message), the client shall
   originate an update for the A RR (associated with the client's FQDN).



Yakov Rekhter                                                   [Page 3]


Internet Draft       draft-ietf-dhc-dhcp-dns-01.txt            July 1996


   The update shall be originated following the procedures described in
   [DynDNS].


   If a client does not want to be responsible for updating the FQDN to
   IP address mapping for the FQDN and address(es) used by the client,
   then the client shall include the Client FQDN option in the
   DHCPREQUEST message originated by the client. The Flags field in the
   option shall be set to 1.


   Whether the client wants to be responsible for updating the FQDN to
   IP address mapping, or whether the client wants to delegate this
   responsibility to a server is a local to the client matter. The
   choice between the two alternatives may be based on a particular
   security model that is used with the Dynamic DNS Update protocol
   (e.g., only a client may have sufficient credentials to perform
   updates to the FQDN to IP address mapping for its FQDN).

   If a client releases its address lease prior to the lease expiration
   time, and the client is responsible for updating its A RR(s), the
   client should delete the A RR (following the procedures described in
   [DynDNS]) associated with the leased address before sending DHCP
   RELEASE message.


4.3. DHCP Server behavior

   When a server receives a DHCPREQUEST message from a client, if the
   message contains the Client FQDN option, and the server replies to
   the message with a DHCPACK message, the server shall originate an
   update for the PTR RR (associated with the address leased to the
   client).  The server shall originate the update before the server
   sends the DHCPACK message to the client. The update shall be
   originated following the procedures described in [DynDNS].  The RCODE
   from the update [DynDNS] should be carried to the client in the
   RCODE1 field of the Client FQDN option in the DHCPACK message. The
   RCODE2 field should be set to 0.

   In addition, if the Client FQDN option carried in the DHCPREQUEST
   message has its Flags field set to 1, then the server shall originate
   an update for the A RR (associated with the FQDN carried in the
   option).  The server shall originate the update before the server
   sends the DHCPACK message to the client.  The update shall be
   originated following the procedures described in [DynDNS].  The RCODE
   from the update [DynDNS] should be carried to the client in the
   RCODE2 field of the Client FQDN option in the DHCPACK message.




Yakov Rekhter                                                   [Page 4]


Internet Draft       draft-ietf-dhc-dhcp-dns-01.txt            July 1996


   When a server receives a DHCPREQUEST message from a client, and the
   message contains the Client FQDN option, the server shall ignore the
   value carried in the RCODE field of the option.


   If a server originates updates for both the A and PTR RRs, then the
   order in which the updates are generated is not significant.


   If a server detects that a lease on an address that the server leases
   to a client expires, the server should delete the PTR RR associated
   with the address. In addition, if the client authorized the server to
   update its A RR, the server should also delete the A RR. The deletion
   should follow the procedures described in [DynDNS].

   If a server terminates a lease on an address prior to the lease
   expiration time, the server should delete the PTR RR associated with
   the address. In addition, if the client (that leased the address)
   authorized the server to update its A RR, the server should also
   delete the A RR.  The deletion should follow the procedures described
   in [DynDNS].


5. Updating other RRs

   The procedures described in this document cover updates only to the A
   and PTR RRs. Updating other types of RRs is outside the scope of this
   document.



6. Security Considerations

   Security issues are not discussed in this document.


7. References

   [RFC1034] P. Mockapetris, "Domain names - concepts and facilities",
   RFC1034, 11/01/1987

   [RFC1035] P. Mockapetris, "Domain names - implementation and
   specification", RFC1035, 11/01/1987

   [RFC1541] R. Droms, "Dynamic Host Configuration Protocol", RFC1541,
   10/27/1993

   [RFC1594] A. Marine, J. Reynolds, G. Malkin, "FYI on Questions and



Yakov Rekhter                                                   [Page 5]


Internet Draft       draft-ietf-dhc-dhcp-dns-01.txt            July 1996


   Answer Answers to Commonly asked ``New Internet User'' Questions",
   RFC1594, 03/11/1994

   [DynDNS] P. Vixie, S. Thomson, Y. Rekhter, J. Bound, "Dynamic Updates
   in the Domain Name System (DNS UPDATE)", draft-ietf-dnsind-dynDNS-
   09.txt



8. Acknowledgements

   Many thanks to Mark Beyer (Tandem), Jim Bound (DEC), Ralph Droms
   (Bucknell University), Edie Gunter (IBM), Michael Lewis (Chevron),
   and Michael Patton (BBN) for their review and comments.


9. Author Information


   Yakov Rekhter
   cisco Systems, Inc.
   170 Tasman Dr.
   San Jose, CA 95134
   Phone: (914) 528-0090
   email: yakov@cisco.com


























Yakov Rekhter                                                   [Page 6]


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