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. RFC2132 [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.

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 RFC2119 [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 RFC2132, 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

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.

References

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

[2] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC2132, March 1997.

[3] Bradner, S., "Key words for use in RFCs to indicate requirement
levels", BCP 14, RFC2119, 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: cs@Eng.Sun.COM

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