draft-ietf-dhc-autoconfig-04.txt   rfc2563.txt 
Dynamic Host Configuration WG Ryan Troll Network Working Group R. Troll
Document: draft-ietf-dhc-autoconfig-04.txt Carnegie Mellon Request for Comments: 2563 @Home Network
Expires August 28, 1999 February 23, 1999 Category: Standards Track May 1999
DHCP Option to Disable Stateless Auto-Configuration in IPv4 Clients
<draft-ietf-dhc-autoconfig-04.txt> DHCP Option to Disable Stateless Auto-Configuration in IPv4 Clients
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document specifies an Internet standards track protocol for the
all provisions of Section 10 of RFC2026. Internet-Drafts are Internet community, and requests discussion and suggestions for
working documents of the Internet Engineering Task Force (IETF), its improvements. Please refer to the current edition of the "Internet
areas, and its working groups. Note that other groups may also Official Protocol Standards" (STD 1) for the standardization state
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Internet-Drafts are draft documents valid for a maximum of six
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To view the list Internet-Draft Shadow Directories, see Copyright Notice
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Distribution of this memo is unlimited. Copyright (C) The Internet Society (1999). All Rights Reserved.
Abstract Abstract
Operating Systems are now attempting to support ad-hoc networks of Operating Systems are now attempting to support ad-hoc networks of
two or more systems, while keeping user configuration at a minimum. two or more systems, while keeping user configuration at a minimum.
To accommodate this, in the absence of a central configuration To accommodate this, in the absence of a central configuration
mechanism (DHCP), some OS's are automatically choosing a link-local mechanism (DHCP), some OS's are automatically choosing a link-local
IP address which will allow them to communicate only with other IP address which will allow them to communicate only with other hosts
hosts on the same link. This address will not allow the OS to on the same link. This address will not allow the OS to communicate
communicate with anything beyond a router. However, some sites with anything beyond a router. However, some sites depend on the
depend on the fact that a host with no DHCP response will have no IP fact that a host with no DHCP response will have no IP address. This
address. This document describes a mechanism by which DHCP servers document describes a mechanism by which DHCP servers are able to tell
are able to tell clients that they do not have an IP address to clients that they do not have an IP address to offer, and that the
offer, and that the client should not generate an IP address it's client should not generate an IP address it's own.
own.
1. Introduction 1. Introduction
With computers becoming a larger part of everyday life, operating With computers becoming a larger part of everyday life, operating
systems must be able to support a larger range of operating systems must be able to support a larger range of operating
environments. One aspect of this support is the selection of an IP environments. One aspect of this support is the selection of an IP
address. The Dynamic Host Configuration Protocol [DHCP] provides a address. The Dynamic Host Configuration Protocol [DHCP] provides a
superb method by which site administrators may supply IP addresses superb method by which site administrators may supply IP addresses
(and other network parameters) to network devices. However, some (and other network parameters) to network devices. However, some
operating environments are not centrally maintained, and operating operating environments are not centrally maintained, and operating
systems must now be able to handle this quickly and easily. systems must now be able to handle this quickly and easily.
IPv6 accounts for this, and allows an IPv6 stack to assign itself a IPv6 accounts for this, and allows an IPv6 stack to assign itself a
global address in the absence of any other mechanism for global address in the absence of any other mechanism for
configuration [IPv6SAC]. However, Operating System designers can't configuration [IPv6SAC]. However, Operating System designers can't
wait for IPv6 support everywhere. They need to be able to assume wait for IPv6 support everywhere. They need to be able to assume
they will have IPv4 addresses, so that they may communicate with one they will have IPv4 addresses, so that they may communicate with one
another even in the smallest networks. another even in the smallest networks.
This document looks at three types of network nodes, and how IPv4 This document looks at three types of network nodes, and how IPv4
address auto-configuration may be disabled on a per-subnet (or even address auto-configuration may be disabled on a per-subnet (or even
per-node) basis. The three types of network nodes are: per-node) basis. The three types of network nodes are:
* A node for which the site administrator will hand out * A node for which the site administrator will hand out configuration
configuration information, information,
* A node on a network segment for which there is no site * A node on a network segment for which there is no site
administrator, and administrator, and
* A node on a network segment that has a central site administrator, * A node on a network segment that has a central site administrator,
and that administrator chooses not to hand out any configuration and that administrator chooses not to hand out any configuration
information to the node. information to the node.
The difference between the second and third cases is the clients The difference between the second and third cases is the clients
behavior. behavior.
In one case, the node may assign itself an IP address, and have full 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, 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 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 network address in the same way as case #2, this may not be what the
central administrator wants. central administrator wants.
The first scenario is handled by the current DHCP standard. The first scenario is handled by the current DHCP standard. However,
However, the current DHCP specification [DHCP] says servers must the current DHCP specification [DHCP] says servers must silently
silently ignore requests from hosts they do not know. Because of ignore requests from hosts they do not know. Because of this, DHCP
this, DHCP clients are unable to determine whether they are on a clients are unable to determine whether they are on a subnet with no
subnet with no administration, or with administration that is administration, or with administration that is choosing not to hand
choosing not to hand out addresses. out addresses.
This document describes a method by which DHCP clients will be able This document describes a method by which DHCP clients will be able
to determine whether or not the network is being centrally to determine whether or not the network is being centrally
administrated, allowing it to intelligently determine whether or not administrated, allowing it to intelligently determine whether or not
it should assign itself a "link-local" address. it should assign itself a "link-local" address.
1.1. Conventions Used in the Document 1.1. Conventions Used in the Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [KEYWORDS]. document are to be interpreted as described in [KEYWORDS].
1.2. Terminology 1.2. Terminology
DHCP client A DHCP client is an Internet host using DHCP to DHCP client A DHCP client is an Internet host using DHCP to obtain
obtain configuration parameters such as a network configuration parameters such as a network address.
address.
DHCP server A DHCP server is an Internet host that returns DHCP server A DHCP server is an Internet host that returns
configuration parameters to DHCP clients. configuration parameters to DHCP clients.
2. The Auto-Configure Option 2. The Auto-Configure Option
This option code is used to ask whether, and be notified if, auto- This option code is used to ask whether, and be notified if, auto-
configuration should be disabled on the local subnet. The auto- configuration should be disabled on the local subnet. The auto-
configure option is an 8-bit number. configure option is an 8-bit number.
Code Len Value Code Len Value
+-----+-----+-----+ +-----+-----+-----+
| TBD | 1 | a | | 116 | 1 | a |
+-----+-----+-----+ +-----+-----+-----+
The code for this option is TBD (To Be Determined), and its length The code for this option is 116, and its length is 1.
is 1.
This code, along with the IP address assignment, will allow a DHCP This code, along with the IP address assignment, will allow a DHCP
client to determine whether or not it should generate a link-local client to determine whether or not it should generate a link-local IP
IP address. address.
2.1. Auto-Configure Values 2.1. Auto-Configure Values
The auto-configure option uses the following values: The auto-configure option uses the following values:
DoNotAutoConfigure 0 DoNotAutoConfigure 0
AutoConfigure 1 AutoConfigure 1
When a server responds with the value "AutoConfigure", the client When a server responds with the value "AutoConfigure", the client MAY
MAY generate a link-local IP address if appropriate. However, if generate a link-local IP address if appropriate. However, if the
the server responds with "DoNotAutoConfigure", the client MUST NOT server responds with "DoNotAutoConfigure", the client MUST NOT
generate a link-local IP address, possibly leaving it with no IP generate a link-local IP address, possibly leaving it with no IP
address. address.
2.2. DHCP Client Behavior 2.2. DHCP Client Behavior
Clients that have auto-configuration capabilities MUST add the Clients that have auto-configuration capabilities MUST add the Auto-
Auto-Configure option to the list of options included in its initial Configure option to the list of options included in its initial
DHCPDISCOVER message. ([DHCP] Section 4.4.1) At this time, the DHCPDISCOVER message. ([DHCP] Section 4.4.1) At this time, the
option's value should be set to "AutoConfigure". option's value should be set to "AutoConfigure".
When a DHCPOFFER is received, it is handled as described in [DHCP], When a DHCPOFFER is received, it is handled as described in [DHCP],
section 4.4.1, with one exception. If the 'yiaddr' field is section 4.4.1, with one exception. If the 'yiaddr' field is
0x00000000, the Auto-Configure option must be consulted. If this 0x00000000, the Auto-Configure option must be consulted. If this
option is set to "AutoConfigure", then the DHCPOFFER MUST be option is set to "AutoConfigure", then the DHCPOFFER MUST be ignored,
ignored, and the DHCP client MAY generate a link-local IP address. and the DHCP client MAY generate a link-local IP address. However,
However, if this option is set to "DoNotAutoConfigure", then the if this option is set to "DoNotAutoConfigure", then the DHCPOFFER
DHCPOFFER MUST be ignored, and the client MUST NOT generate a link- MUST be ignored, and the client MUST NOT generate a link-local IP
local IP address. address.
If a DHCP client receives any DHCPOFFER which contains a 'yiaddr' of If a DHCP client receives any DHCPOFFER which contains a 'yiaddr' of
0x00000000, and the Auto-Configure flag says "DoNotAutoConfigure", 0x00000000, and the Auto-Configure flag says "DoNotAutoConfigure", in
in the absence of a DHCPOFFER with a valid 'yiaddr', the DHCP client 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 MUST NOT generate a link-local IP address. The amount of time a DHCP
DHCP client waits to collect any other DHCPOFFERs is implementation client waits to collect any other DHCPOFFERs is implementation
dependant. dependant.
DHCPOFFERs with a 'yiaddr' of 0x00000000 will only be sent by DHCP DHCPOFFERs with a 'yiaddr' of 0x00000000 will only be sent by DHCP
servers supporting the Auto-Configure option when the DHCPDISCOVER servers supporting the Auto-Configure option when the DHCPDISCOVER
contained the Auto-Configure option. Since the DHCPDISCOVER will contained the Auto-Configure option. Since the DHCPDISCOVER will
only contain the Auto-Configure option when a DHCP client knows how only contain the Auto-Configure option when a DHCP client knows how
to handle it, there will be no inter-operability problems. to handle it, there will be no inter-operability problems.
If the DHCP server does have an address to offer, the message states If the DHCP server does have an address to offer, the message states
are the same as those described in [DHCP], section 3. are the same as those described in [DHCP], section 3.
The following depicts the difference in responses for non-registered The following depicts the difference in responses for non-registered
DHCP clients that support the "Auto-Configure" option on networks DHCP clients that support the "Auto-Configure" option on networks
that have DHCP servers that support auto-configuration and networks that have DHCP servers that support auto-configuration and networks
with DHCP servers that do not. with DHCP servers that do not.
Network Client Network Network Client Network
(no auto-configure) (auto-configure) (no auto-configure) (auto-configure)
v v v v v v
| | | | | |
| Begins initialization | | Begins initialization |
| | | | | |
| _____________/|\____________ | | _____________/|\____________ |
|/DHCPDISCOVER | DHCPDISCOVER \| |/DHCPDISCOVER | DHCPDISCOVER \|
skipping to change at page 5, line 37 skipping to change at page 5, line 37
. . . . . .
. . . . . .
| | | | | |
| Graceful shutdown | | Graceful shutdown |
| | | | | |
| | | | | |
v v v v v v
2.3. DHCP Server Behavior 2.3. DHCP Server Behavior
When a DHCP server receives a DHCPDISCOVER, it MUST be processed as When a DHCP server receives a DHCPDISCOVER, it MUST be processed as
described in [DHCP], section 4.3.1. However, if no address is described in [DHCP], section 4.3.1. However, if no address is chosen
chosen for the host, a few additional steps MUST be taken. for the host, a few additional steps MUST be taken.
If the DHCPDISCOVER does not contain the Auto-Configure option, it If the DHCPDISCOVER does not contain the Auto-Configure option, it is
is not answered. not answered.
If the DHCPDISCOVER contains the Auto-Configure option, and the site If the DHCPDISCOVER contains the Auto-Configure option, and the site
administrator has specified that Auto-Configuration should be administrator has specified that Auto-Configuration should be
disabled on the subnet the DHCPDISCOVER is originating from, or for disabled on the subnet the DHCPDISCOVER is originating from, or for
the client originating the request, then a DHCPOFFER MUST be sent to 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 DHCP client. This offer MUST be for the address 0x00000000, and
the Auto-Configure option MUST be set to "DoNotAutoConfigure". the Auto-Configure option MUST be set to "DoNotAutoConfigure".
If the site administrator allows auto-configuration on the If the site administrator allows auto-configuration on the
originating subnet, the DHCPDISCOVER is not answered as before. originating subnet, the DHCPDISCOVER is not answered as before.
2.4. Mixed Environments 2.4. Mixed Environments
Environments containing a mixture of clients and servers that do and Environments containing a mixture of clients and servers that do and
do not support the Auto-Configure option will not be a problem. do not support the Auto-Configure option will not be a problem.
Every DHCP transaction is between a Server and a Client, and the Every DHCP transaction is between a Server and a Client, and the
possible mixed scenarios between these two are listed below. possible mixed scenarios between these two are listed below.
2.4.1. Client Supports, Server Does Not 2.4.1. Client Supports, Server Does Not
If a DHCP client sends a request that contains the Auto-Configure 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 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 normally. According to [DHCP] Section 4.3.1, the server MUST NOT
return a value for that parameter. return a value for that parameter.
In this case, the server will either respond with a valid DHCPOFFER, 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 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 supports this option will never care what the state of the option is,
is, and may auto-configure. and may auto-configure.
2.4.2. Servers Supports, Client Does Not 2.4.2. Servers Supports, Client Does Not
If the Auto-Configure option is not present in the DHCPDISCOVER, the If the Auto-Configure option is not present in the DHCPDISCOVER, the
server will do nothing about it. The client will auto-configure if 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. 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 This scenario SHOULD not occur, as any stacks that implement an
auto-configuration mechanism MUST implement this option as well. auto-configuration mechanism MUST implement this option as well.
2.5. Interaction With Other DHCP Messages 2.5. Interaction With Other DHCP Messages
As this option only affects the initial IP address selection, it As this option only affects the initial IP address selection, it does
does not apply to subsequent DHCP messages. If the DHCP client not apply to subsequent DHCP messages. If the DHCP client received a
received a lease from a DHCP server, future DHCP messages (RENEW, lease from a DHCP server, future DHCP messages (RENEW, INFORM, ACK,
INFORM, ACK, etc.) have no need to fall over into an auto- etc.) have no need to fall over into an auto- configuration state.
configuration state.
If the DHCP client's lease expires, the client falls back into the If the DHCP client's lease expires, the client falls back into the
INIT state, and the initial DHCPDISCOVER is sent as before. INIT state, and the initial DHCPDISCOVER is sent as before.
2.5.1. DHCPRELEASE Messages 2.5.1. DHCPRELEASE Messages
DHCPRELEASEs occur exactly as described in [DHCP], section 4.4.6. 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 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 that it is finished. For this to occur, the DHCP client already
received a DHCP lease, and the state of Auto-Configuration on the received a DHCP lease, and the state of Auto-Configuration on the
local wire does not matter. local wire does not matter.
2.5.2. DHCPDECLINE Messages 2.5.2. DHCPDECLINE Messages
A DHCPDECLINE is sent by the DHCP client when it determines the 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 it is attempting to use is already in use. As a
network address has been tested, it must have been offered by the network address has been tested, it must have been offered by the
DHCP Server, and the state of Auto-Configuration on the local wire DHCP Server, and the state of Auto-Configuration on the local wire
does not matter. does not matter.
2.5.3. DHCPINFORM Messages 2.5.3. DHCPINFORM Messages
DHCPINFORMs should be handled as described in [DHCP], section 4.4.3. DHCPINFORMs should be handled as described in [DHCP], section 4.4.3.
No changes are necessary. No changes are necessary.
2.6. Message Option 2.6. Message Option
If the DHCP server would like to tell a client why it is not allowed 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. to auto-configure, it MAY add the Message option to the response.
This option is defined in [DHCPOPT], Section 9.9. This option is defined in [DHCPOPT], Section 9.9.
If the DHCP client receives a response with the Message option set, If the DHCP client receives a response with the Message option set,
it MUST provide this information to the administrator of the DHCP it MUST provide this information to the administrator of the DHCP
client. How this information is provided is implementation client. How this information is provided is implementation
dependant. dependant.
3. Security Considerations 3. Security Considerations
DHCP per se currently provides no authentication or security DHCP per se currently provides no authentication or security
mechanisms. Potential exposures to attack are discussed in section mechanisms. Potential exposures to attack are discussed in section 7
7 of the DHCP protocol specification [DHCP]. of the DHCP protocol specification [DHCP].
This mechanism does add one other potential attack. Malicious users This mechanism does add one other potential attack. Malicious users
on a subnet may respond to all DHCP requests with responses telling on a subnet may respond to all DHCP requests with responses telling
DHCP clients that they should NOT auto-configure on the local wire. DHCP clients that they should NOT auto-configure on the local wire.
On a network where Auto-Configuration is required, this will cause On a network where Auto-Configuration is required, this will cause
all DHCP clients to not choose an address. all DHCP clients to not choose an address.
4. Acknowledgments 4. Acknowledgments
This idea started at a joint Common Solutions Group / Microsoft This idea started at a joint Common Solutions Group / Microsoft
meeting at Microsoft in May, 1998. The IP stacks in Win98 and NT5 meeting at Microsoft in May, 1998. The IP stacks in Win98 and NT5
assign themselves an IP address (in a specific subnet) in the assign themselves an IP address (in a specific subnet) in the absence
absence of a responding DHCP server, and this is causing headaches of a responding DHCP server, and this is causing headaches for many
for many sites that actually rely on machines not getting IP sites that actually rely on machines not getting IP addresses when
addresses when the DHCP servers do not know them. the DHCP servers do not know them.
Walter Wong proposed a solution that would allow the DHCP servers to Walter Wong proposed a solution that would allow the DHCP servers to
tell clients not to do this. His initial solution would not work tell clients not to do this. His initial solution would not work
without slight modifications to DHCP itself. This document without slight modifications to DHCP itself. This document describes
describes those modifications. those modifications.
5. IANA Considerations 5. IANA Considerations
The IANA has assigned option number TBD for this option. The IANA has assigned option number 116 for this option.
6. Copyright
Copyright (C) The Internet Society 1999. All Rights Reserved.
This document and translations of it may be copied and furnished to 6. References
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 [DHCP] Droms, R., "Dynamic Host Configuration Protocol", RFC
revoked by the Internet Society or its successors or assigns. 2131, March 1997.
This document and the information contained herein is provided on an [DHCPOPT] Alexander, S. and R. Droms, "DHCP Options and BOOTP
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING Vendor Extension", RFC 2132, March 1997.
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.
7. References [IPv6SAC] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[DHCP] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
March 1997 Requirement Levels", BCP 14, RFC 2119, March 1997.
<ftp://ds.internic.net/rfc/rfc2131.txt> 7. Author's Address
[DHCPOPT] Alexander, S. and Droms, R., "DHCP Options and BOOTP Ryan Troll
Vendor Extension", RFC 2132, March 1997 @Home Network
425 Broadway
Redwood City, CA 94063
<ftp://ds.internic.net/rfc/rfc2132.txt> Phone: (650) 556-6031
EMail: rtroll@corp.home.net
[IPv6SAC] Thomson, S. and Narten, T., "IPv6 Stateless Address 8. Full Copyright Statement
Autoconfiguration", RFC 2462, December 1998
<ftp://ds.internic.net/rfc/rfc2462.txt> Copyright (C) The Internet Society (1999). All Rights Reserved.
[KEYWORDS] S. Bradner, "Key words for use in RFCs to Indicate This document and translations of it may be copied and furnished to
Requirement Levels", RFC 2119, March 1997 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.
<ftp://ds.internic.net/rfc/rfc2119.txt> The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
8. Author's Address 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.
Ryan Troll Acknowledgement
Network Development
Carnegie Mellon
5000 Forbes Avenue
Pittsburgh, PA 15213
Phone: (412) 268-8691 Funding for the RFC Editor function is currently provided by the
EMail: ryan@andrew.cmu.edu Internet Society.
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