RFC2937 - The Name Service Search Option for DHCP

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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

Full Copyright Statement

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

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Acknowledgement

Funding for the RFCEditor function is currently provided by the

Internet Society.

 
 
 
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