Network Working Group                                        C. Smith
Request for Comments: 2937                     Sun Microsystems, Inc.
Category: Standards Track                              September 2000


               The Name Service Search Option for DHCP

Status of this Memo

  This document specifies an Internet standards track protocol for the
  Internet community, and requests discussion and suggestions for
  improvements.  Please refer to the current edition of the "Internet
  Official Protocol Standards" (STD 1) for the standardization state
  and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

  Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

  This document defines a new Dynamic Host Configuration Protocol
  (DHCP) option which is passed from the DHCP Server to the DHCP Client
  to specify the order in which name services should be consulted when
  resolving hostnames and other information.

Introduction

  The Dynamic Host Configuration Protocol (DHCP)[1] provides a
  framework for passing configuration information to hosts on a TCP/IP
  network.  RFC 2132 [2] allows DHCP servers to specify configuration
  information for various kinds of name services to be passed to DHCP
  clients.  Many clients use multiple name services and have crafted
  their own conventions that allow an individual host to express the
  order among the various name services with which lookups are done.
  However, no search order can be specified via DHCP.  The purpose of
  this document is to allow DHCP servers to specify the search order to
  be used by DHCP clients.  To avoid the need for inventing and
  maintaining a separate name space for this option, we rely on the
  existence of previously-defined DHCP options that specify the IP
  address(es) of servers which provide name services whose order we
  wish to express.









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RFC 2937        The Name Service Search Option for DHCP   September 2000


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 [3].  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.

  Name Service Search Option Format

    The code for this option is 117, and its minimum length is 2 bytes.
    A DHCP server SHOULD return, in its preferred order, the 16-bit,
    network byte order (big-endian [4]) integer option code for the
    name services (the earlier in the list, the more preferred the name
    service).

      Code            Length      Name Service Search Order in Sequence
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     117       |     Len       |             ns1               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             ns2               |             ...               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    In the above diagram, ns1 and ns2 are 16-bit integers corresponding
    to two DHCP options which specify the IP addresses of two different
    types of name server.  The current list of name services and their
    DHCP option codes, taken from RFC 2132, includes

      Name Service                                  Value

      Domain Name Server Option                       6
      Network Information Servers Option             41
      NetBIOS over TCP/IP Name Server Option         44
      Network Information Service+ Servers Option    65






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RFC 2937        The Name Service Search Option for DHCP   September 2000


      A name service option code of 0 is used to indicate that the
      client should refer to local naming information (e.g., an
      /etc/hosts file on a UNIX machine).

    A DHCP server wishing to express that a client should first search
    DNS, then NIS+, would send

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     117       |      4        |              6                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |              65               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  DHCP Client Behavior

    The DHCP client will use this option to create a search list for
    name resolution.  The client may receive name services in this
    option that it does not support or has not been configured to
    access.  Likewise, a client may receive an option that lists name
    services for which no corresponding DHCP option was supplied.
    Clients will interpret this option in a system-specific manner
    whose specification is outside the scope of this document.

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].

IANA Considerations

  IANA has assigned a value of 117 for the DHCP option code described
  in this document.
















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RFC 2937        The Name Service Search Option for DHCP   September 2000


References

  [1] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March
      1997.

  [2] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
      Extensions", RFC 2132, March 1997.

  [3] Bradner, S., "Key words for use in RFCs to indicate requirement
      levels", BCP 14, RFC 2119, March 1997.

  [4] Cohen, D., "On Holy Wars and a Plea for Peace", Computer, IEEE,
      October 1981.

Author's Address

  Carl Smith
  Sun Microsystems, Inc.
  901 San Antonio Road
  Palo Alto, CA 94043

  EMail:  [email protected]





























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RFC 2937        The Name Service Search Option for DHCP   September 2000


Full Copyright Statement

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Acknowledgement

  Funding for the RFC Editor function is currently provided by the
  Internet Society.



















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