dhc T. Lemon
Internet-Draft Nominum
Intended status: Standards Track March 24, 2010
Expires: September 25, 2010
Relay-Supplied DHCP Options
draft-lemon-dhcpv6-relay-supplied-options-00
Abstract
This document describes a general mechanism whereby a DHCPv6 relay
agent can provide options to a DHCPv6 server that the DHCPv6 server
can then provide to the DHCPv6 client.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Protocol Summary . . . . . . . . . . . . . . . . . . . . . . . 3
3. Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. DHCP Relay Agent Behavior . . . . . . . . . . . . . . . . . . . 4
5. DHCP Server Behavior . . . . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 5
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1. Introduction
There are some cases where a DHCP relay agent has information that
would be useful to provide to a DHCP client, and the DHCP server does
not have that information. The DHCPv6 specification [RFC3315] does
not provide a mechanism whereby the DHCP relay can provide options to
the DHCP client. This document defines an extension to DHCP that
allows DHCP relay agents to propose options to be sent to DHCP
clients.
The motivation for this draft comes from a proposal from the Mobile
IPv6 working group, DHCP Options for Home Information Discovery in
MIPv6 [hiopt]. This draft initially proposed a one-off mechanism
whereby the relay agent can provide a home agent option to be sent
back to the DHCP client. It is our belief that there may be other
uses cases for this functionality, so rather than requiring special
code in the server for each such use case, we propose to provide a
general mechanism that will satisfy any such use case.
1.1. Requirements Language
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 RFC 2119 [RFC2119].
1.2. Terminology
The following terms and acronyms are used in this document:
DHCP - Dynamic Host Configuration Protocol Version 6 [RFC3315]
RSOO - Relay-Supplied Options option
2. Protocol Summary
DHCP clients do not support a mechanism for receiving options from
relay agents--the function of the relay agent is simply to deliver
the payload from the server. Consequently, in order for the DHCP
relay agent to provide options to the client, it sends those options
to the DHCP server, encapsulated in a Relay-Supplied Options option.
The DHCP server can then choose to place those options in the
response it sends to the client.
3. Encoding
In order to supply options for the DHCP server, the relay agent sends
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a Relay-Supplied Options option in the Relay-Forward message. This
option encapsulates whatever options the relay agent wishes to
provide to the DHCPv6 server.
+------+--------+----------+-----+----------+
+ code | length | option 1 | ... | option n |
+------+--------+----------+-----+----------+
4. DHCP Relay Agent Behavior
Relay agents MAY include a Relay-Supplied Options option in the
option payload of a Relay-Forward message. Relay agents MUST NOT
modify the contents of any message before forwarding it to the DHCP
client.
5. DHCP Server Behavior
A DHCP server that implements this spec must have a user-configurable
setting which determines whether or not it accepts a Relay-Supplied
Options option. If the DHCP server is configured not to accept the
RSOO, it MUST discard any such options that it receives.
DHCP servers normally construct a list of options that are candidates
to send to the DHCP client, and then constructs the DHCP packet
according to section 17.2.2 of DHCPv6 [RFC3315].
If the server receives an RSOO and is configured to accept it, it
SHOULD add any options that appear in the RSOO for which it has no
internal candidate to the list of options that are candidates to send
to the DHCP client. The server SHOULD discard any options that
appear in the RSOO for which it already has one or more candidates.
Aside from the addition of options from the RSOO, the DHCP server
should then construct a DHCP packet as it normally would, and
transmit it to the DHCP client as described in DHCPv6 [RFC3315].
6. Security Considerations
This document provides a mechanism whereby a relay agent can inject
options into the response the DHCP server sends to the DHCP client.
Because the DHCP server prefers its own configured options to those
supplied by the relay agent, this can't be used as a means for
overriding server-supplied options. However, it is still possible in
some configurations for a rogue DHCP server to supply additional
options to the DHCP client.
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For this reason, DHCP servers in environments where a rogue relay
could interject itself into the packet flow SHOULD authenticate the
relay agent as described in section 21.1 of DHCPv6 [RFC3315].
Note, however, that in any environment where this is possible, it
would also possible for the attacker to simply supply a bogus DHCPv6
packet, so unless the packet from the server is authenticated, the
same risk exists even in environments where the RSOO is not supported
by or enabled on the DHCP server.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
7.2. Informative References
[hiopt] Jang, H., Yegin, A., Chowdhury, K., and J. Choi, "DHCP
Options for Home Information Discovery in MIPv6",
May 2008.
Author's Address
Ted Lemon
Nominum
2000 Seaport Blvd
Redwood City, CA 94063
USA
Phone: +1 650 381 6000
Email: mellon@nominum.com
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