Network Working Group R. Troll

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

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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].

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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

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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.

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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".

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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.

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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

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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: [email protected]

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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.

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