Network Working Group                       Yakov Rekhter, cisco Systems
INTERNET DRAFT                          Ralph Droms, Bucknell University
Obsoletes: draft-ietf-dhc-fqdn-opt-02.txt                      July 1997
                                                    Expires January 1998

                  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,
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   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 a client 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          July 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 length specified by the Sublen field does
   not include the Subcode and Sublen fields.  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 than 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

Rekhter, Droms                                                  [Page 2]

DRAFT             An option for FQDNs in DHCP options          July 1997

   FQDN option, to differentiate among the variable-length FQDNs.  If
   the order of the IP addresses in the option identified by the subcode
   was meaningful, e.g., representing a priority or preference order,
   the order retains that same meaning in multiple instances of the same
   subcode in the FQDN option.  DHCP options that carry pairs of IP
   addresses, e.g., the static route option (code 33), MUST NOT be
   encoded in the FQDN option.

   This option only allows the use of FQDNs for options that have been
   elsewhere defined to carry IP addresses.  If the FQDN option is used,
   the DNS server option (code 6) SHOULD be specified before any FQDN
   options, and the client's protocol software MUST initialize its DNS
   resolver with that DNS server address before resolving any FQDNs in
   subsequent options. Not all DHCP options that specify IP addresses
   may be sensibly transmitted as FQDNs; for example, options that
   specify an IP address-subnet mask pair MUST NOT be encoded in the
   FQDN option.  The DNS server option SHOULD NOT be encoded in the FQDN
   option because, under most circumstances, the FQDN of a DNS server
   cannot be resolved until the IP address fo a server is available.
   The router option SHOULD NOT be encoded as an FQDN because queries to
   the DNS server may require that the client's protocol software be
   initialized with the router's IP address; e.g., the DNS server may be
   on a different subnet.

1.2 Example

   The following illustrates how the FQDN option could be used to carry
   FQDNs for 2 LPR Servers with FQDNs and, and
   one Network Information Server with FQDN

    |89 |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

   DHCP currently provides no authentication or security mechanisms.
   Potential exposures to attack are discussed in section 7 of the DHCP
   protocol specification [1].

Rekhter, Droms                                                  [Page 3]

DRAFT             An option for FQDNs in DHCP options          July 1997

   The DHCP FQDN option introduces DNS into the client configuration
   process, so that compromises to the DNS system may compromise the
   security of client configuration.

3. References

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

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

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.

5. Author Information

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

   Ralph Droms
   Computer Science Department
   323 Dana Engineering
   Bucknell University
   Lewisburg, PA 17837
   Phone: (717) 524-1145

Rekhter, Droms                                                  [Page 4]