draft-ietf-dhc-autoconfig-03.txt   draft-ietf-dhc-autoconfig-04.txt 
Dynamic Host Configuration WG Ryan Troll Dynamic Host Configuration WG Ryan Troll
Document: draft-ietf-dhc-autoconfig-03.txt Carnegie Mellon Document: draft-ietf-dhc-autoconfig-04.txt Carnegie Mellon
Expires July 29, 1999 January 24, 1999 Expires August 28, 1999 February 23, 1999
DHCP Option to Disable Stateless Auto-Configuration in IPv4 Clients DHCP Option to Disable Stateless Auto-Configuration in IPv4 Clients
<draft-ietf-dhc-autoconfig-03.txt> <draft-ietf-dhc-autoconfig-04.txt>
Status of this Memo Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working This document is an Internet-Draft and is in full conformance with
documents of the Internet Engineering Task Force (IETF), its areas, all provisions of Section 10 of RFC2026. Internet-Drafts are
and its working groups. Note that other groups may also distribute working documents of the Internet Engineering Task Force (IETF), its
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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
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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 generate a link-local IP address if appropriate. However, if MAY generate a link-local IP address if appropriate. However, if
the server responds with "DoNotAutoConfigure", the client MUST not the 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-Configure option to the list of options included in its initial Auto-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, and the DHCP client MAY generate a link-local IP address. ignored, and the DHCP client MAY generate a link-local IP address.
However, if this option is set to "DoNotAutoConfigure", then the However, if this option is set to "DoNotAutoConfigure", then the
DHCPOFFER MUST be ignored, and the client MUST not generate a link- DHCPOFFER MUST be ignored, and the client MUST NOT generate a link-
local IP address. local IP 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 the absence of a DHCPOFFER with a valid 'yiaddr', the DHCP client 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 MUST NOT generate a link-local IP address. The amount of time a
DHCP client waits to collect any other DHCPOFFERs is implementation DHCP 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
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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, and may auto-configure. is, 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
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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 of the DHCP protocol specification [DHCP]. 7 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 that 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 of a responding DHCP server, and this is causing headaches absence of a responding DHCP server, and this is causing headaches
for many sites that actually rely on machines not getting IP for many sites that actually rely on machines not getting IP
addresses when the DHCP servers do not know them. addresses when the DHCP servers do not know them.
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