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rfc:rfc2563

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 --|
                        .               .               .
                        .               .               .
                        |               |               |
                        |      Graceful shutdown        |
                        |               |               |
                        |               |               |
                        v               v               v

2.3. DHCP Server Behavior

 When a DHCP server receives a DHCPDISCOVER, it MUST be processed as
 described in [DHCP], section 4.3.1.  However, if no address is chosen
 for the host, a few additional steps MUST be taken.
 If the DHCPDISCOVER does not contain the Auto-Configure option, it is
 not answered.
 If the DHCPDISCOVER contains the Auto-Configure option, and the site
 administrator has specified that Auto-Configuration should be
 disabled on the subnet the DHCPDISCOVER is originating from, or for
 the client originating the request, then a DHCPOFFER MUST be sent to
 the DHCP client.  This offer MUST be for the address 0x00000000, and
 the Auto-Configure option MUST be set to "DoNotAutoConfigure".

Troll Standards Track [Page 5] RFC 2563 DHCP Auto-Configuration Option May 1999

 If the site administrator allows auto-configuration on the
 originating subnet, the DHCPDISCOVER is not answered as before.

2.4. Mixed Environments

 Environments containing a mixture of clients and servers that do and
 do not support the Auto-Configure option will not be a problem.
 Every DHCP transaction is between a Server and a Client, and the
 possible mixed scenarios between these two are listed below.

2.4.1. Client Supports, Server Does Not

 If a DHCP client sends a request that contains the Auto-Configure
 tag, a DHCP server that does not know what this tag is will respond
 normally.  According to [DHCP] Section 4.3.1, the server MUST NOT
 return a value for that parameter.
 In this case, the server will either respond with a valid DHCPOFFER,
 or it will not respond at all.  In both cases, a DHCP client that
 supports this option will never care what the state of the option is,
 and may auto-configure.

2.4.2. Servers Supports, Client Does Not

 If the Auto-Configure option is not present in the DHCPDISCOVER, the
 server will do nothing about it.  The client will auto-configure if
 it doesn't receive a response and believes that's what it should do.
 This scenario SHOULD not occur, as any stacks that implement an
 auto-configuration mechanism MUST implement this option as well.

2.5. Interaction With Other DHCP Messages

 As this option only affects the initial IP address selection, it does
 not apply to subsequent DHCP messages.  If the DHCP client received a
 lease from a DHCP server, future DHCP messages (RENEW, INFORM, ACK,
 etc.) have no need to fall over into an auto- configuration state.
 If the DHCP client's lease expires, the client falls back into the
 INIT state, and the initial DHCPDISCOVER is sent as before.

2.5.1. DHCPRELEASE Messages

 DHCPRELEASEs occur exactly as described in [DHCP], section 4.4.6.
 When a DHCP client is done with a lease, it MAY notify the server
 that it is finished.  For this to occur, the DHCP client already
 received a DHCP lease, and the state of Auto-Configuration on the
 local wire does not matter.

Troll Standards Track [Page 6] RFC 2563 DHCP Auto-Configuration Option May 1999

2.5.2. DHCPDECLINE Messages

 A DHCPDECLINE is sent by the DHCP client when it determines the
 network address it is attempting to use is already in use.  As a
 network address has been tested, it must have been offered by the
 DHCP Server, and the state of Auto-Configuration on the local wire
 does not matter.

2.5.3. DHCPINFORM Messages

 DHCPINFORMs should be handled as described in [DHCP], section 4.4.3.
 No changes are necessary.

2.6. Message Option

 If the DHCP server would like to tell a client why it is not allowed
 to auto-configure, it MAY add the Message option to the response.
 This option is defined in [DHCPOPT], Section 9.9.
 If the DHCP client receives a response with the Message option set,
 it MUST provide this information to the administrator of the DHCP
 client.  How this information is provided is implementation
 dependant.

3. Security Considerations

 DHCP per se currently provides no authentication or security
 mechanisms.  Potential exposures to attack are discussed in section 7
 of the DHCP protocol specification [DHCP].
 This mechanism does add one other potential attack.  Malicious users
 on a subnet may respond to all DHCP requests with responses telling
 DHCP clients that they should NOT auto-configure on the local wire.
 On a network where Auto-Configuration is required, this will cause
 all DHCP clients to not choose an address.

4. Acknowledgments

 This idea started at a joint Common Solutions Group / Microsoft
 meeting at Microsoft in May, 1998.  The IP stacks in Win98 and NT5
 assign themselves an IP address (in a specific subnet) in the absence
 of a responding DHCP server, and this is causing headaches for many
 sites that actually rely on machines not getting IP addresses when
 the DHCP servers do not know them.
 Walter Wong proposed a solution that would allow the DHCP servers to
 tell clients not to do this.  His initial solution would not work
 without slight modifications to DHCP itself.  This document describes

Troll Standards Track [Page 7] RFC 2563 DHCP Auto-Configuration Option May 1999

 those modifications.

5. IANA Considerations

 The IANA has assigned option number 116 for this option.

6. References

 [DHCP]     Droms, R., "Dynamic Host Configuration Protocol", RFC
            2131, March 1997.
 [DHCPOPT]  Alexander, S. and  R. Droms, "DHCP  Options  and  BOOTP
            Vendor Extension", RFC 2132, March 1997.
 [IPv6SAC]  Thomson, S. and T. Narten, "IPv6 Stateless Address
            Autoconfiguration", RFC 2462, December 1998.
 [KEYWORDS] Bradner, S., "Key  words  for  use  in  RFCs  to  Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.

7. Author's Address

 Ryan Troll
 @Home Network
 425 Broadway
 Redwood City, CA 94063
 Phone: (650) 556-6031
 EMail: rtroll@corp.home.net

Troll Standards Track [Page 8] RFC 2563 DHCP Auto-Configuration Option May 1999

8. Full Copyright Statement

 Copyright (C) The Internet Society (1999).  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.

Troll Standards Track [Page 9]

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