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Network Working Group                                        C. SmithRequest for Comments: 2937                     Sun Microsystems, Inc.Category: Standards Track                              September 2000The Name Service Search Option for DHCPStatus 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.Smith                       Standards Track                     [Page 1]

RFC 2937        The Name Service Search Option for DHCP   September 2000Definitions   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 inRFC 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 fromRFC 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    65Smith                       Standards Track                     [Page 2]

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 insection 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.Smith                       Standards Track                     [Page 3]

RFC 2937        The Name Service Search Option for DHCP   September 2000References   [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:  cs@Eng.Sun.COMSmith                       Standards Track                     [Page 4]

RFC 2937        The Name Service Search Option for DHCP   September 2000Full Copyright Statement   Copyright (C) The Internet Society (2000).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Smith                       Standards Track                     [Page 5]