Dynamic Host Configuration WG                                   R. Troll
Internet Draft: DHC-AUTOCONFIG                           Carnegie Mellon
Document: draft-ietf-dhc-autoconfig-00.txt                   August 1998
Expires: February 1999

  DHCP Option to Disable Stateless Auto-Configuration in IPv4 Clients


Status of this memo

     This document is an Internet-Draft.  Internet-Drafts are working
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     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 automaticly choosing an IP address
     which will allow them to communicate with other hosts running the
     same OS.  However, some sites depend on the fact that a host with
     no DHCP response will have no IP address.  This draft 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
     auto-configure 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

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     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
     link-local address in the absence of any other mechanism for
     configuration.  This ability, described in [IPv6SAC], describes
     three types of network nodes:

     * 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 a link-local IP
     address, and have full connectivity with other nodes on the local
     wire.  In the last case, the node is specificly told not to do
     this, and the node winds up with no IP address at all.

     Operating System designers can't wait for IPv6 support everywhere.
     They need to be able to handle these same three network scenarios
     under IPv4.  The first scenario is handled by the current DHCP
     standard.  However, at this time, 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", "SHOULD", "SHOULD NOT", and "MAY"
     in this document are to be interpreted as defined in "Key words for
     use in RFCs to Indicate Requirement Levels" [KEYWORDS]

1.2 Terminology

          DHCP client    A DHCP client is an Internet host using DHCP to

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                         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
          | TBD |  1  |  a  |

     The code for this option is TBD (To Be Determined), 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 auto-configure.

2.1. Auto-Configure Values

     The auto-configure option uses the following values:

               DoNOTAutoConfigure     1
               AutoConfigure          2

     When a server responds with the value "AutoConfigure", the client
     MAY auto-configure if appropriate.  However, if the server responds
     with "DoNOTAutoConfigure", the client MUST not auto-configure,
     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 requested parameters 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 'ciaddr' field is
     0x00000000, the AutoConfigure option must be consulted.  If this

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     option is set to "AutoConfigure", then the DHCPOFFER MUST be
     ignored, and the DHCP Client MAY auto-configure.  However, if this
     option is set to "DoNOTAutoConfigure", then the DHCPOFFER MUST be
     ignored, and the client MUST not auto-configure.

     If a DHCP Client receives any DHCPOFFER which contains a 'ciaddr'
     of 0x00000000, and the Auto-Configure flag says
     'DoNOTAutoConfigure', in the absence of a DHCPOFFER with a valid
     'ciaddr', the DHCP Client MUST not Auto-Configure.

     DHCPOFFERs with a 'ciaddr' of 0x00000000 will only be sent by DHCP
     servers supporting the AutoConfigure 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.

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                          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 AutoConfigure option, it
     is not answered.

     If the DHCPDISCOVER contains the AutoConfigure option, and the site
     administrator has specified that Auto-Configuration should be
     disabled on the subnet the DHCPDISCOVER is originating from, then a
     DHCPOFFER MUST be sent to the DHCP Client.  This offer MUST be for
     the address 0x00000000, and the AutoConfigure option MUST be set to

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     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.

2.4.2 Servers Supports, Client Does Not

     If the AutoConfigure 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

     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

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     received a DHCP lease, and the state of Auto-Configuration on the
     local wire does not matter.

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.

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 autoconfigure on the
     local wire.  On a network that 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 those modifications.

5. Copyright

     Copyright (C) The Internet Society 1998. All Rights Reserved.

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     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

6. References

     [DHCP] Droms, R. "Dynamic Host Configuration Protocol", RFC 2131,
     Bucknell University, March 1997.


     [DHCPOPT] Alexander, S. and Droms, R., "DHCP Options and BOOTP
     Vendor Extension", RFC 2132, March 1997.


     [KEYWORDS] Bradner, "Key words for use in RFCs to Indicate
     Requirement Levels", RFC 2119, Harvard University, March 1997.


     [IPv6SAC] Thomson, S. and Narten, T. "IPv6 Stateless Address
     Autoconfiguration", RFC 1971, August 1996


7. Author's Address

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     Ryan Troll
     Network Development
     Carnegie Mellon
     5000 Forbes Avenue
     Pittsburgh, PA 15213

     Phone: (412) 268-8691
     EMail: ryan@andrew.cmu.edu

     This document will expire February 1999

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