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Network Working Group                                      Yakov Rekhter
Internet Draft                                             Cisco Systems
Expiration Date: August 1996                               February 1996


                    Interaction between DHCP and DNS
                     draft-ietf-dhc-dhcp-dns-00.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-00.txt        February 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 to update
   DNS information over a network.

   Use of the Dynamic DNS Updates protocol enables 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 restricts the possible design permutations to
   the following cases:

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



Yakov Rekhter                                                   [Page 2]


Internet Draft       draft-ietf-dhc-dhcp-dns-00.txt        February 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     Domain Name
        +-----+-----+-----+-----+-----+-----+--
        | TBD |  n  | 0/1 |  d1 |  d2 |  d3 |  ...
        +-----+-----+-----+-----+-----+-----+--



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





Yakov Rekhter                                                   [Page 3]


Internet Draft       draft-ietf-dhc-dhcp-dns-00.txt        February 1996


   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.

   A client that delegates the responsibility for updating the FQDN to
   IP address mapping to a server does not receive any indications
   (either positive or negative) from the server whether the server was
   able to perform the update. The client may use DNS query to check
   whether the mapping is updated.

   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.



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 only after the server
   sends the DHCPACK message to the client.  The update shall be
   originated following the procedures described in [DynDNS].

   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 only after the server
   sends the DHCPACK message to the client.  The update shall be
   originated following the procedures described in [DynDNS].


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


   [Discussion: should it be possible to configure a server to perform
   updates for the FQDN to IP address mapping, even when a client
   indicates to the server that the client wants to update this mapping
   ?]

   [Discussion: how should the duration of the lease be reflected in the
   DNS updates ? At the minimum we can set TTL on the A and PTR RRs to
   the value of the lease time. What else ?]



Yakov Rekhter                                                   [Page 4]


Internet Draft       draft-ietf-dhc-dhcp-dns-00.txt        February 1996


   [Discussion: when a server detects that a lease on an address that
   the server leases to a client expires, should the server delete the
   PTR RR associated with the address ?]

   [Discussion: if a server terminates a lease prior to the lease
   expiration time, should the server update the associated PTR RR ?
   Should the A RR be updated, and if yes, then by whom ? ]


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. Applicability to IPv6

   The procedures described above are directly applicable to an IPv6
   client. The only difference is that instead of updating its A RR(s)
   the client has to update its AAAA RR(s).


7. Security Considerations

   Security issues are not discussed in this document.


8. 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
   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-
   06.txt, Feb 1996





Yakov Rekhter                                                   [Page 5]


Internet Draft       draft-ietf-dhc-dhcp-dns-00.txt        February 1996


9. Acknowledgements

   Many thanks to Ralph Droms for his review and comments.


10. 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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