rfc2563.txt

RFC 的详细文档！

Network Working Group                                           R. Troll
Request for Comments: 2563                                 @Home Network
Category: Standards Track                                       May 1999


  DHCP Option to Disable Stateless Auto-Configuration in IPv4 Clients

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 (1999).  All Rights Reserved.

Abstract

   Operating Systems are now attempting to support ad-hoc networks of
   two or more systems, while keeping user configuration at a minimum.
   To accommodate this, in the absence of a central configuration
   mechanism (DHCP), some OS's are automatically choosing a link-local
   IP address which will allow them to communicate only with other hosts
   on the same link.  This address will not allow the OS to communicate
   with anything beyond a router.  However, some sites depend on the
   fact that a host with no DHCP response will have no IP address.  This
   document describes a mechanism by which DHCP servers are able to tell
   clients that they do not have an IP address to offer, and that the
   client should not generate an IP address it's own.

1.  Introduction

   With computers becoming a larger part of everyday life, operating
   systems must be able to support a larger range of operating
   environments.  One aspect of this support is the selection of an IP
   address.  The Dynamic Host Configuration Protocol [DHCP] provides a
   superb method by which site administrators may supply IP addresses
   (and other network parameters) to network devices.  However, some
   operating environments are not centrally maintained, and operating
   systems must now be able to handle this quickly and easily.

   IPv6 accounts for this, and allows an IPv6 stack to assign itself a
   global address in the absence of any other mechanism for
   configuration [IPv6SAC].  However, Operating System designers can't
   wait for IPv6 support everywhere.  They need to be able to assume



Troll                       Standards Track                     [Page 1]

RFC 2563             DHCP Auto-Configuration Option             May 1999


   they will have IPv4 addresses, so that they may communicate with one
   another even in the smallest networks.

   This document looks at three types of network nodes, and how IPv4
   address auto-configuration may be disabled on a per-subnet (or even
   per-node) basis.  The three types of network nodes are:

   * A node for which the site administrator will hand out configuration
     information,

   * A node on a network segment for which there is no site
     administrator, and

   * A node on a network segment that has a central site administrator,
     and that administrator chooses not to hand out any configuration
     information to the node.

   The difference between the second and third cases is the clients
   behavior.

   In one case, the node may assign itself an IP address, and have full
   connectivity with other nodes on the local wire.  In the last case,
   the node is not told what to do, and while it may assign itself a
   network address in the same way as case #2, this may not be what the
   central administrator wants.

   The first scenario is handled by the current DHCP standard.  However,
   the current DHCP specification [DHCP] says servers must silently
   ignore requests from hosts they do not know.  Because of this, DHCP
   clients are unable to determine whether they are on a subnet with no
   administration, or with administration that is choosing not to hand
   out addresses.

   This document describes a method by which DHCP clients will be able
   to determine whether or not the network is being centrally
   administrated, allowing it to intelligently determine whether or not
   it should assign itself a "link-local" address.

1.1.  Conventions Used in the Document

   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 [KEYWORDS].








Troll                       Standards Track                     [Page 2]

RFC 2563             DHCP Auto-Configuration Option             May 1999


1.2.  Terminology

   DHCP client    A DHCP client is an Internet host using DHCP to obtain
                  configuration parameters such as a network address.

   DHCP server    A DHCP server is an Internet host that returns
                  configuration parameters to DHCP clients.

2.  The Auto-Configure Option

   This option code is used to ask whether, and be notified if, auto-
   configuration should be disabled on the local subnet.  The auto-
   configure option is an 8-bit number.

        Code   Len   Value
       +-----+-----+-----+
       | 116 |  1  |  a  |
       +-----+-----+-----+

   The code for this option is 116, and its length is 1.

   This code, along with the IP address assignment, will allow a DHCP
   client to determine whether or not it should generate a link-local IP
   address.

2.1.  Auto-Configure Values

   The auto-configure option uses the following values:

            DoNotAutoConfigure     0
            AutoConfigure          1

   When a server responds with the value "AutoConfigure", the client MAY
   generate a link-local IP address if appropriate.  However, if the
   server responds with "DoNotAutoConfigure", the client MUST NOT
   generate a link-local IP address, possibly leaving it with no IP
   address.

2.2.  DHCP Client Behavior

   Clients that have auto-configuration capabilities MUST add the Auto-
   Configure option to the list of options included in its initial
   DHCPDISCOVER message.  ([DHCP] Section 4.4.1)  At this time, the
   option's value should be set to "AutoConfigure".

   When a DHCPOFFER is received, it is handled as described in [DHCP],
   section 4.4.1, with one exception.  If the 'yiaddr' field is
   0x00000000, the Auto-Configure option must be consulted.  If this



Troll                       Standards Track                     [Page 3]

RFC 2563             DHCP Auto-Configuration Option             May 1999


   option is set to "AutoConfigure", then the DHCPOFFER MUST be ignored,
   and the DHCP client MAY generate a link-local IP address.  However,
   if this option is set to "DoNotAutoConfigure", then the DHCPOFFER
   MUST be ignored, and the client MUST NOT generate a link-local IP
   address.

   If a DHCP client receives any DHCPOFFER which contains a 'yiaddr' of
   0x00000000, and the Auto-Configure flag says "DoNotAutoConfigure", in
   the absence of a DHCPOFFER with a valid 'yiaddr', the DHCP client
   MUST NOT generate a link-local IP address.  The amount of time a DHCP
   client waits to collect any other DHCPOFFERs is implementation
   dependant.

   DHCPOFFERs with a 'yiaddr' of 0x00000000 will only be sent by DHCP
   servers supporting the Auto-Configure option when the DHCPDISCOVER
   contained the Auto-Configure option.  Since the DHCPDISCOVER will
   only contain the Auto-Configure option when a DHCP client knows how
   to handle it, there will be no inter-operability problems.

   If the DHCP server does have an address to offer, the message states
   are the same as those described in [DHCP], section 3.

   The following depicts the difference in responses for non-registered
   DHCP clients that support the "Auto-Configure" option on networks
   that have DHCP servers that support auto-configuration and networks
   with DHCP servers that do not.

























Troll                       Standards Track                     [Page 4]

RFC 2563             DHCP Auto-Configuration Option             May 1999


                        Network         Client          Network
                  (no auto-configure)               (auto-configure)

                          v               v               v
                          |               |               |
                          |     Begins initialization     |
                          |               |               |
                          | _____________/|\____________  |
                          |/DHCPDISCOVER  | DHCPDISCOVER \|
                          |               |               |
                      Determines          |          Determines
                     configuration        |         configuration
                          |               |               |
                          |               |  ____________/|
                          |               | /DHCPOFFER    |
                          |               |/              |
                          |               |               |
                          |       Collects replies        |
                          |               |               |
                          |     Selects configuration     |
                          |               |               |
                          |--AutoConfigs--|- NO IP ADDR --|
                          .               .               .
                          .               .               .