RFC2563

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

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].

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

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.

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".

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.

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

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", RFC2132, March 1997.

[IPv6SAC]  Thomson, S. and T. Narten, "IPv6 Stateless Address

Autoconfiguration", RFC2462, December 1998.

[KEYWORDS] Bradner, S., "Key  words  for  use  in  RFCs  to  Indicate

Requirement Levels", BCP 14, RFC2119, 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

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 RFCEditor function is currently provided by the

Internet Society.

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