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Versions: 00 01 02 03                                                   
Network Working Group                       Yakov Rekhter, cisco Systems
INTERNET DRAFT                          Ralph Droms, Bucknell University
Obsoletes: draft-ietf-dhc-fqdn-opt-01.txt                     March 1997
                                                  Expires September 1997

                  An option for FQDNs in DHCP options

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.

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   ftp.isi.edu (US West Coast).


   DHCP [DHCP] can be used to automate the process of configuring TCP/IP
   host computers.  However, some of the DHCP options carry IP addresses
   rather than Fully Qualified Domain Names (FQDN). Use of IP addresses
   constrains the DHCP client to use the addresses that were in use at
   the time the client received its configuration information; these
   addresses may change over time, (e.g., a server may be assigned a new
   IP address), so that the IP addresses used by the client may become

   An alternative to passing IP addresses is to pass FQDNs instead of
   (numeric) IP addresses.  Doing this allows to defer binding between a
   particular network entity (e.g., a server) and its IP address until
   run time.  As stated in [Carpenter:96], "Deferring the binding avoids
   the risk of changed mapping between IP addresses and specific network
   entities (due to changing addressing information).  Moreover,
   reliance on FQDNs (rather than IP addresses) also localizes to the
   DNS the changes needed to deal with changing addressing information
   due to renumbering."

Rekhter, Droms                                                  [Page 1]

DRAFT             An option for FQDNs in DHCP options         March 1997

   This document defines a new DHCP option that allows the use of FQDNs
   instead of IP addresses in DHCP options.

1. FQDN Option

   The FQDN option allows the use of FQDNs rather than IP addresses in
   DHCP options.  The FQDN option contains other DHCP options, which
   then carry FQDNs rather than IP addresses as data.

   The code for the FQDN option is 89.  The Len field gives the total
   length of all of the DHCP options contained in the FQDN option.  The
   Code, Len, Subcode and Sublen are all one octet long.  The FQDN field
   is variable length.

   For each subcode carried in the FQDN option, the IP address in the
   option represented by the subcode is replaced by a FQDN.

   The Sublen field shall be set to the length (in octets) of the FQDN
   carried in the option.  The FQDN field carries the FQDN itself.

    |   Code   |   Len    |
    | Subcode  |  Sublen  |             FQDN


    | Subcode  |  Sublen  |             FQDN

1.1 DHCP options containing a list of parameters

   More that one triple with a given subcode may appear within a single
   FQDN option.  The FQDNs contained in triples with the same subcode
   should be treated as a list of parameters for the DHCP option
   represented by the subcode.

   Because FQDNs are variable length, lists of FQDNs cannot be encoded
   in DHCP options within the FQDN option.  DHCP Options that can carry
   a list of IP addresses should be coded as multiple subcodes in the
   FQDN option, to differentiate among the variable-length FQDNs.

Rekhter, Droms                                                  [Page 2]

DRAFT             An option for FQDNs in DHCP options         March 1997

   This option only allows the use of FQDNs for options that have been
   elsewhere defined to carry IP addresses.

1.2 Example

   The following illustrates how the FQDN option could be used to carry
   FQDNs for 2 LPR Servers with FQDNs lpr1.xxx.org and lpr2.yy.org, and
   one Network Information Server with FQDN nis.zzzz.org.

    |xx |41 |
    |41 |12 | n | i | s | . | z | z | z | z | . | o | r | g |
    | 9 |12 | l | p | r | 1 | . | x | x | x | . | o | r | g |
    | 9 |11 | l | p | r | 2 | . | y | y | . | o | r | g |

2. Security Considerations

   Security issues are not discussed in this document.

3. References

   [Carpenter:96] Carpenter, B., Rekhter, Y., "Renumbering needs work",
   RFC1900, February 1996.

   [DHCP] Droms, R., "Dynamic Host Configuration Protocol", RFC1541,
   October 1993.

4. Acknowledgments

   The authors gratefully acknowledge the input and review of the
   Dynamic Host Configuration working group.  They also thank cisco
   Systems and Bucknell University for their support in the development
   of this document.

Rekhter, Droms                                                  [Page 3]

DRAFT             An option for FQDNs in DHCP options         March 1997

5. Author Information

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

   Ralph Droms
   Computer Science Department
   323 Dana Engineering
   Bucknell University
   Lewisburg, PA 17837
   Phone: (717) 524-1145
   email: droms@bucknell.edu

Rekhter, Droms                                                  [Page 4]