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DHC Working Group                                           Carl Smith
Internet Draft                                  Sun Microsystems, Inc.
                                                             Ted Lemon
                                                         Nominum, Inc.
Updates:  RFC 2132                                       February 2002
                                                   Expires August 2002


           Considerations for the use of the Host Name option
           <draft-ietf-dhc-host-option-considerations-00.txt>


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.  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".

     The list of current Internet-Drafts can be accessed at
     http://www.ietf.org/1id-abstracts.html

     The list of Internet-Draft Shadow Directories can be accessed at
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Abstract

   This document clarifies the use of the DHCP Host Name option.  The
   primary point of this clarification addresses the use of the option
   by clients to request proxy DNS updates by DHCP servers.

1. Introduction

   The initial concept of the Host Name option, as documented in RFC
   1533[1] and duplicated in RFC 2132 [2], was simply to allow a DHCP
   server to supply a client with its name (``This option specifies the
   name of the client'').  The DHCP client was merely a consumer of the
   option information.  Even in this case, confusion has been reported
   in interactions with various domain name options.

   Behavior of client and server when the client supplies the option
   was, and still is, unspecified.  In the intervening years, the



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   ability to easily update Domain Name Service information [3] has
   encouraged the use of this option by DHCP clients as a way to request
   that DHCP servers issue proxy DNS updates on their behalf.  Lack of a
   document describing its exact usage has led, as one would surely
   expect, to interoperability problems.  It is the purpose of this
   document to outline the expectations that clients and servers should
   have when using the Host Name option.

2. Definitions

   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 RFC 2119 [4].  This
   document also uses the following terms:

      "DHCP client"

         DHCP client or "client" is an Internet host using DHCP to
         obtain configuration parameters such as a network address.

      "DHCP server"

         A DHCP server or "server" is an Internet host that returns
         configuration parameters to DHCP clients.

      "FQDN"

         A fully-qualified name, including the host part and Domain Name
         system domain.

3. Interactions with Name Services

   A DHCP client's use of the Host Name option should be fairly
   straightforward, but users report problems, particularly with the
   interactions between it and Domain Name option and between the DNS
   (RFC 1034 [5] and RFC 1035 [6]) and other naming services so we
   reiterate and expand the description of the expected behavior:

      if a DHCP server supplies both Host Name and Domain Name options
      to a client, the host name SHOULD NOT be fully-qualified

      if a DHCP server supplies only a Host Name option, the host name
      SHOULD be fully qualified; the server MUST append only DNS domain
      names in forming a fully-qualified name

      a client MUST check to see whether a Host Name option contains a
      fully-qualified name and if so, MUST NOT append the value of the
      Domain Name option (if present) in forming its fully-qualified



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      domain name

      since a Host Name option's value may be fully-qualified only by
      supplying the DNS domain name, a client that receives a fully-
      qualified name in the Host Name option MAY infer the DNS domain
      name from the suffix of the supplied host name.  This inference
      remains valid even in the presence of client configuration infor-
      mation or policies that prefer other name services in favor of, or
      in place of, DNS.

4. DNS Updates for DHCP Clients

   DNS maintains Resource Records (RRs) for mapping between IP addresses
   FQDNs.  Specifically, A records map FQDNs to IP addresses and PTR
   records map IP addresses to FQDNs.  Several options are available to
   DHCP clients interested in updating A and PTR records:

      issuing direct updates to DNS

      using the DHCP client FQDN option [7]

      using the DHCP Host Name option

   Either of the first two methods has the advantage of offering the
   client a number of approaches for fine-tuning DNS update requests as
   well as direct feedback on the success or failure of the intended
   operations.  Both are to be preferred over the latter:  use of the
   Host Name option does not even guarantee that the DHCP server will
   attempt any DNS updates on a client's behalf.  Nevertheless, support
   for this method of requesting proxy DNS updates is widespread and it
   may be viewed as appropriate for situations in which there are no
   requirements for other finely-tunable methods.

4.1 DHCP Client Considerations and Behavior

   A DHCP client that uses the Host Name option to request a DNS update
   MUST be prepared to independently verify the success or failure of
   the request before using the name in a manner that would imply its
   validity.  If a DHCP server returns the requested name in the
   DHCPACK's Host Name option, the client MAY infer that the server has
   honored its request.

   There are a number of reasons that a DHCP server may fail to return a
   Host Name option, so nothing should be inferred from the option's
   absence in the DHCPACK.  The client MAY supply the option on subse-
   quent RENEW operations as a method of retrying the request.  However,
   if the Host Name option is absent in the DHCPACK, the client MUST NOT
   use the requested name until it has verified the validity of the



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   association between it and the IP address supplied in the yiaddr
   field.  Moreover, if the name returned in the DHCPACK is different
   from the one requested, the client MUST use the new name.

   A DHCP client MAY send either an unqualified or fully-qualified name
   in the Host Name option.  Clients sending unqualified names are
   implicitly relying on DHCP servers to associate the clients with the
   appropriate zone before issuing any updates to DNS.  A DHCP client in
   INIT state SHOULD fill in the requested host name in the DHCPDISCOVER
   packet.  It MUST do so in its subsequent DHCPREQUEST packet.

   Clients in states other than INIT SHOULD avoid ambiguity in their
   requests by supplying the same Host Name option value on subsequent
   DHCPREQUESTs as was supplied on their original (INIT state) DHCPRE-
   QUEST.

   A client that wishes to change its host name MAY request it by sup-
   plying a Host Name option with the new name in a subsequent RENEW
   request.  As with the initial request, a client MUST NOT use the
   newly-requested name until it has verified that it is now valid.

4.2 DHCP Server Considerations and Behavior

   Use of the FQDN option makes it possible to easily separate update
   operations into pieces corresponding to what are thought of as the
   traditional ownership boundaries:  DHCP servers own the addresses
   they lease, while the clients own their names.  This boundary is not
   present when the Host Name option is used:  the implied proxy update
   request assumes that the DHCP server has sufficient privilege to
   change both the A and PTR records.  That is, it ``owns'' both.

   For this and other reasons, use of the FQDN option is preferred:  a
   DHCP server that receives both a Host Name option and a client FQDN
   option MUST prefer the FQDN option.  In such a case, the server
   SHOULD behave as if the Host Name option is not present.

   A DHCP server MAY use the value of the Host Name option in a DHCPDIS-
   COVER packet in some limited ways:  it may check to see whether the
   requested name belongs to an address that ls leaseable to the client,
   saving the need for a DNS update, or it may begin preparation of an
   update request.  The server MUST wait for the DHCPREQUEST before ini-
   tiating any update operations.

   DNS updates may not complete in a timely manner, forcing the DHCP
   server to reply to a client before the update has finished.  Alterna-
   tively, an error may be reported in response to the update request.
   It is not possible to distinguish these cases for the client's bene-
   fit, and the the DHCP server simply omits the Host Name option from



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   its DHCPACK.  For simplicity of implementation, servers may choose to
   ``orphan'' any outstanding requests, taking no note of subsequent
   reports of success or failure.  Servers that choose to keep track of
   the results of update requests SHOULD use successful completion
   reports to avoid subsequent unnecessary work; those servers SHOULD
   ignore reports of soft, transitory errors.  Hard errors SHOULD be
   logged by the server so that corrective action, if any, may be taken
   by an administrator.  Servers MAY choose to not cache hard failures,
   retrying on subsequent DHCPREQUESTs in the hope that the errors
   logged have led to a remedy.

   Issuing DNS updates on behalf of DHCP clients is an inherently state-
   ful operation.  A DHCP server MUST commit to stable storage the
   necessary information regarding any updates it successfully makes on
   behalf of its clients.  This state may be needed

      when a lease expires

      when communicating with a failover partner

      on subsequent lease renewals

   and may need to be recovered when the server is restarted.

   When a lease expires, a DHCP server MAY use this stored information
   to expunge the name-to-address association it created on the client's
   behalf.  Because the use of the Host Name option cedes the ownership
   of the name to the server, a server MAY instead choose to allow the
   association to continue, saving itself work now and possibly sparing
   the need for a future update.

   A server MAY choose to reevaluate the Host Name option each time a
   client sends a RENEW request via a DHCPREQUEST, or the server MAY
   choose to view the update request as an action to be taken once, upon
   initial lease of an address.  Servers that take the former view offer
   their clients the possibility of changing the name associated with a
   currently valid lease, but may incur additional processing costs
   because of it.  Servers taking the latter view do not afford clients
   the opportunity to change names, but more importantly do not allow
   them to retry failed requests, possibly even with different host
   names.  For this reason, the former behavior is preferred:  servers
   SHOULD reevaluate the Host Name option on each RENEW.

   Some servers interpret a Host Name option on the initial DHCPREQUEST,
   followed by the absence of the option on subsequent RENEW DHCPRE-
   QUESTs as a request by the client to delete a name-to-address associ-
   ation.  Here we invoke the principle of least surprise:  it is more
   reasonable for a client to expect that DNS updates apply for the



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   duration of a lease than it is to expect that a client will wish to
   delete an update but retain the lease.  Because clients that expect
   DNS updates to apply for the duration of a lease may not send a Host
   Name option when RENEWing, servers SHOULD NOT interpret the absence
   of the option as a request for deletion of the association.

5. References

   [1] Alexander, S. and Droms, R., "DHCP Options and BOOTP Vendor
   Extensions", RFC 1533, October 1993.
   [2] Alexander, S. and Droms, R., "DHCP Options and BOOTP Vendor
   Extensions", RFC 2132, March 1997.
   [3] Vixie, P., Thomson, S., Rekhter, Y., Bound, J., "Dynamic Updates
   in the Domain Name System (DNS UPDATE)", RFC 2136, April 1997.
   [4] Bradner, S., "Key words for use in RFCs to indicate requirement
   levels", RFC 2119, March 1997.
   [5] Mockapetris, P., "Domain names - Concepts and Facilities", RFC
   1034, November 1987.
   [6] Mockapetris, P., "Domain names - Implementation and Specifica-
   tion", RFC 1035, November 1987.
   [7] Stapp, M. and Rekhter, Y., "The DHCP Client FQDN Option", draft-
   ietf-dhc-fqdn-option-03.txt, November 2001.

6. Author Information

   Carl Smith
   Sun Microsystems, Inc.
   901 San Antonio Road
   Palo Alto, CA 94043
   USA
   email:  cs@Eng.Sun.COM

   Ted Lemon
   Nominum, Inc.
   950 Charter Street
   Redwood City, CA 94043
   USA
   email:  Ted.Lemon@nominum.com

7. Expiration

   This document will expire on August 22, 2002.

8. Full Copyright Statement

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

   This document and translations of it may be copied and furnished to



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   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of develop-
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   The limited permissions granted above are perpetual and will not be
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   This document and the information contained herein is provided on an
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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MER-
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