Network Working Group                                      Yakov Rekhter
Internet Draft                                             Cisco Systems
Expiration Date: August 1998                               February 1998


                    Interaction between DHCP and DNS
                     draft-ietf-dhc-dhcp-dns-07.txt


. 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
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   munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
   ftp.isi.edu (US West Coast).


. 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-07.txt        February 1998


. Terminology

   Throughout this document, the words that are used to define the
   significance of particular requirements are capitalized.  These words
   are:

      - "MUST"
         This word or the adjective "REQUIRED" means that the item is an
         absolute requirement of this specification.

      - "MUST NOT"
         This phrase means that the item is an absolute prohibition of
         this specification.

      - "SHOULD"
         This word or the adjective "RECOMMENDED" means that there may
         exist valid reasons in particular circumstances to ignore this
         item, but the full implications should be understood and the
         case carefully weighed before choosing a different course.

      - "SHOULD NOT"
         This phrase means that there may exist valid reasons in
         particular circumstances when the listed behavior is acceptable
         or even useful, but the full implications should be understood
         and the case carefully weighed before implementing any behavior
         described with this label.

      - "MAY"
         This word or the adjective "OPTIONAL" means that this item is
         truly optional. One vendor may choose to include the item
         because a particular marketplace requires it or because it
         enhances the product, for example; another vendor may omit the
         same item.



. 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



Yakov Rekhter                                                   [Page 2]

 
Internet Draft       draft-ietf-dhc-dhcp-dns-07.txt        February 1998


   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 [RFC2136] 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.


. 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

         (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.









Yakov Rekhter                                                   [Page 3]

 
Internet Draft       draft-ietf-dhc-dhcp-dns-07.txt        February 1998


5.1. 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 81. Its minimum length is 4.



         Code   Len    Flags  RCODE1 RCODE2   Domain Name
        +------+------+------+------+------+------+--
        |  81  |   n  |      |      |      |       ...
        +------+------+------+------+------+------+--



   The Flags field allows a DHCP client to indicate to a DHCP server
   whether (a) the client wants to be responsible for updating the FQDN
   to IP address mapping (if Flags is set to 0), or (b) the client wants
   the server to be responsible for updating the FQDN to IP address
   mapping (if Flags is set to 1). The Flags field also allows a DHCP
   server to indicate to a DHCP client that the server assumes the
   responsibility for updating the FQDN to IP address mapping, even if
   the client wants to be responsible for this update (if Flags is set
   to 3).

   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.


5.2. DHCP Client behavior

   The following describes behavior of a DHCP client that implements the
   Client FQDN option.

   If a client that owns/maintains is own FQDN 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 MUST include the
   Client FQDN option in the DHCPREQUEST message originated by the
   client. The Flags field in the option MUST 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 MUST originate an
   update for the A RR (associated with the client's FQDN). The update



Yakov Rekhter                                                   [Page 4]

 
Internet Draft       draft-ietf-dhc-dhcp-dns-07.txt        February 1998


   MUST be originated following the procedures described in [RFC2136].

   A client that owns/maintains its own FQDN can choose to delegate the
   responsibility for updating the FQDN to IP address mapping for the
   FQDN and address(es) used by the client to the server.  In order to
   inform the server of this choice, the client MUST include the Client
   FQDN option in the DHCPREQUEST message originated by the client. The
   Flags field in the option MUST be set to 1.  In this case, the client
   MAY supply an FQDN in the Client FQDN option, or it MAY leave that
   field empty as a signal to the server to determine an FQDN for the
   client in a local to the server manner.

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

   A client MUST set the RCODE1 and RCODE2 fields in the Client FQDN
   option to 0 when sending the option.

   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
   [RFC2136]) associated with the leased address before sending DHCP
   RELEASE message.


5.3. 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 SHOULD originate an
   update for the PTR RR (associated with the address leased to the
   client). The update MUST be originated following the procedures
   described in Section 5.4. The server MAY originate the update before
   the server sends the DHCPACK message to the client. In this case the
   RCODE from the update [RFC2136] MUST be carried to the client in the
   RCODE1 field of the Client FQDN option in the DHCPACK message and the
   RCODE2 field MUST be set to 0. Alternatively, the server MAY send the
   DHCPACK message to the client without waiting for the update to be
   completed.  In this case the RCODE1 field of the Client FQDN option
   in the DHCPACK message MUST be set to 255, and the RCODE2 field MUST
   be set to 0.  The choice between the two alternatives is a local to a
   DHCP server matter.

   In addition, if the Client FQDN option carried in the DHCPREQUEST
   message has its Flags field set to 1, then the server MUST originate



Yakov Rekhter                                                   [Page 5]

 
Internet Draft       draft-ietf-dhc-dhcp-dns-07.txt        February 1998


   an update for the A RR (associated with the FQDN carried in the
   option). The update MUST be originated following the procedures
   described in Section 5.4. The server MAY originate the update before
   the server sends the DHCPACK message to the client. In this case the
   RCODE from the update [RFC2136] MUST be carried to the client in the
   RCODE2 field of the Client FQDN option in the DHCPACK message.
   Alternatively the server MAY send the DHCPACK message to the client
   without waiting for the update to be completed. In this case the
   RCODE2 field of the Client FQDN option in the DHCKACK message MUST be
   set to 255. The choice between the two alternatives is a local to the
   server matter.

   Even, if the Client FQDN option carried in the DHCPREQUEST message
   has its Flags field set to 0 (indicating that the client wants to
   update the A RR), the server MAY (under configuration control) update
   the A RR. The update MUST be originated following the procedures
   described in Section 5.4. The server MAY originate the update before
   the server sends the DHCPACK message to the client. In this case the
   RCODE from the update [RFC2136] MUST be carried to the client in the
   RCODE2 field of the Client FQDN option in the DHCPACK message, and
   the Flags field in the Client FQND option MUST be set to 3.
   Alternatively, the server MAY send the DHCPACK message to the client
   without waiting for the update to be completed. In this case the
   RCODE2 field of the Client FQDN option in the DHCKACK message MUST be
   set to 255, and the Flags field in the Client FQDN option MUST be set
   to 3. The choice between the two alternatives is a local to the
   server matter.

   When a server receives a DHCPREQUEST message from a client, and the
   message contains the Client FQDN option, the server MUST ignore the
   value carried in the RCODE1 and RCODE2 fields of the option.

   When a DHCP server sends the Client FQDN option to a client in the
   DHCPACK message, the server MUST copy the Domain Name fields from the
   Client FQDN option that the client sent to the server in the
   DHCPREQUEST message.

   If the DHCPREQUST message received by a DHCP server from a DHCP
   client doesn't carry the Client FQDN option (e.g., the client doesn't
   implement the Client FQDN option), and the DHCP client acquires its
   FQDN from a DHCP server (as part of a normal DHCP transaction), then
   the server MAY be configured to update both A and PTR RRs. The
   updates MUST be originated following the procedures described in
   Section 5.4.

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




Yakov Rekhter                                                   [Page 6]

 
Internet Draft       draft-ietf-dhc-dhcp-dns-07.txt        February 1998


   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
   MUST follow the procedures described in [RFC2136].

   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 MUST follow the procedures described in
   [RFC2136].


5.4. Procedures for performing DNS updates

   When a DHCP server needs to update the PTR RR for a particular IP
   address, the server just adds a new PTR RR for that address.

   When a DHCP server needs to update the A RR for a particular FQDN,
   the server first has to delete all the A RRs associated with that
   FQDN, and then add a new A RR for that FQDN. Note that this rule
   precludes the ability to support multi-homed hosts in the scenario
   where A RRs are updated by a DHCP server. Therefore, multi-homed
   hosts SHOULD perform updates to their A RRs by themselves.

   Procedures for deleting and adding RRs are described in [RFC2136].


. 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.


. Security Considerations

   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).

   Whether a DHCP server is always responsible for updating the FQDN to
   IP address mapping (in addition to updating the IP to FQDN mapping),



Yakov Rekhter                                                   [Page 7]

 
Internet Draft       draft-ietf-dhc-dhcp-dns-07.txt        February 1998


   regarless of the wishes of a DHCP client, is a local to the server
   matter. The choice between the two alternatives may be based on a
   particular security model.

   The client SHOULD use some form of data origin authentication
   procedures (e.g., DNSSEC [DNSSEC]) when performing DNS updates.

   While the DHCP client SHOULD  be the one to update the DNS A record,
   in certain specialized cases a DHCP server MAY do so instead.  In
   this case, the DHCP server MUST be sure of both the name to use for
   the client, as well as the identity of the client.

   In the general case, both of these conditions are not satisfied --
   one needs to be mindful of the possibility of MAC address spoofing in
   a DHCP packet.  It is not difficult for a DHCP server to know
   unambiguously the DNS name to use for a client, but only in certain
   relatively unusual circumstances will the DHCP server know for sure
   the identity of the client.  One example of such a circumstance is
   where the DHCP client is connected to a network through an MCNS cable
   modem, and the CMTS (head-end) of the cable modem ensures that MAC
   address spoofing simply does not occur.

   Another example where the DHCP server would know the identity of the
   client would be in a case where it was interacting with a remote
   access server which encoded a client identification into the DHCP
   client-id option.  In this case, the remote access server as well as
   the DHCP server would be operating within a trusted environment, and
   the DHCP server could trust that the user authentication and
   authorization procedure of the remote access server was sufficient,
   and would therefore trust the client identification encoded within
   the DHCP client-id.

   In either of these cases, a DHCP server would be able to correctly
   enter the DNS A record on behalf of a client, since it would know the
   name associated with a client (through some administrative procedure
   outside the scope of this protocol), and it would also know the
   client's identity in a secure manner.














Yakov Rekhter                                                   [Page 8]

 
Internet Draft       draft-ietf-dhc-dhcp-dns-07.txt        February 1998


. 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

   [RFC2131] R. Droms, "Dynamic Host Configuration Protocol", RFC2131,
   March 1997

   [RFC1594] A. Marine, J. Reynolds, G. Malkin, "FYI on Questions and
   Answer Answers to Commonly asked ``New Internet User'' Questions",
   RFC1594, 03/11/1994

   [DNSSEC]

   [RFC2136] P. Vixie, S. Thomson, Y. Rekhter, J. Bound, "Dynamic
   Updates in the Domain Name System (DNS UPDATE)", RFC2136, April 1997



. Acknowledgements

   Many thanks to Mark Beyer, Jim Bound, Ralph Droms, Peter Ford, Edie
   Gunter, Kim Kinnear, Stuart Kwan, Ted Lemon, Michael Lewis, Michael
   Patton, Mark Stapp, and Glenn Stump for their review and comments.


. Author Information


   Yakov Rekhter
   cisco Systems, Inc.
   170 Tasman Dr.
   San Jose, CA 95134
   Phone: (914) 235-2128
   email: yakov@cisco.com













Yakov Rekhter                                                   [Page 9]