dhc T. Lemon
Internet-Draft Nominum
Intended status: Standards Track Q. Wu
Expires: April 2, 2011 Huawei
September 29, 2010
Relay-Supplied DHCP Options
draft-ietf-dhc-dhcpv6-relay-supplied-options-02
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.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 2, 2011.
Copyright Notice
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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. RSOO-enabled options . . . . . . . . . . . . . . . . . . . . . 4
5. DHCP Relay Agent Behavior . . . . . . . . . . . . . . . . . . . 4
6. DHCP Server Behavior . . . . . . . . . . . . . . . . . . . . . 4
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6
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1. Introduction
The DHCPv6 specification [RFC3315] allows DHCP relay agents to
forward DHCPv6 messages between clients and servers that are not on
the same IPv6 link. In some cases the DHCP relay agent has
information the DHCP server does not have that would be useful to
provide to a DHCP client. For example, the DHCP client may need to
learn the EAP local domain name [I.D-ietf-hokey-ldn-discovery] for
use in EAP re-authentication [RFC5296], which is known to the relay
agent but not the server. The DHCPv6 protocol specification does not
provide a mechanism whereby the relay agent can provide options to
the client. This document extends DHCP with a mechanism that allows
DHCP relay agents to propose options for the server to send to DHCP
clients.
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
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.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_RSOO | option-length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| suboptions...
+-+-+-+-+-+-+-+-+-+-+-+
OPTION_RSOO Relay-Supplied Options code (TBD).
option-length Length of Relay-Supplied Options option.
suboptions One or more DHCPv6 options.
4. RSOO-enabled options
Unless specifically called out as an RSOO-enabled option, no DHCP
option should appear in an RSOO. Specifications that describe RSOO-
enabled options MUST reference this specification, and MUST state
that the option they define is RSOO-enabled.
5. 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.
6. DHCP Server Behavior
DHCP servers that implement this specification MUST examine each
option contained in an RSOO to see if it is an RSOO-enabled option.
DHCP servers MUST silently discard any option contained in an RSOO
that is not RSOO-enabled. DHCP server implementations SHOULD have a
user-configurable list of RSOO-enabled options, so that new RSOO-
enabled options do not require software to be updated.
DHCP servers normally construct a list of options that are candidates
to send to the DHCP client, and then construct 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
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appear in the RSOO for which it already has one or more candidates.
If the server receives more than one RSOO, it SHOULD not foward
multiple versions of the same option from different RSOOs to the DHCP
client.
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].
DHCP Server implementations MAY discard options deemed inappropriate
to forward. For example, it would never be appropriate for the DHCP
server to forward an IA option. The list of options that will be
discarded SHOULD be configurable by the administrator.
7. 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 relay agent to supply additional
options to the DHCP client.
Because the relay agent is supplying options which the DHCP server
might then sign, this provides a mechanism whereby an attacker could
get the DHCP server to authenticate a message that the attacker could
not itself forge to the client.
For this reason, DHCP servers in environments where a rogue relay
could interpose itself into the packet flow SHOULD authenticate the
relay agent as described in section 21.1 of DHCPv6 [RFC3315].
Note, however, that this attack is only useful if the DHCP server is
using the DHCPv6 authentication mechanism to authenticate the message
that it sends to the DHCP client. If the message from the server to
the client is not authenticated, the relay agent can simply add
whatever options it wants to the message for the client, rather than
using this more complicated mechanism to provide the same option by
way of the server.
Furthermore, because DHCP servers will discard non-RSOO-enabled
options provided by relay agents, the risk is limited to options that
are specifically designed to be provided by relay agents, so the set
of options that could be provided in such an attack is very
restricted.
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8. IANA Considerations
We request that IANA assign one new option code from the registry of
DHCP Option Codes maintained at
http://www.iana.org/assignments/dhcpv6-parameters. This option code
will be assigned to the Relay-Supplied Options option.
9. References
9.1. 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.
9.2. Informative References
[I.D-ietf-hokey-ldn-discovery]
Zorn, G., Wu, Q., and Y. Wang, "The Local Domain Name
DHCPv6 Option", draft-ietf-hokey-ldn-discovery-05 (work in
progress), September 2010.
[RFC5296] Narayanan, V. and L. Dondeti, "EAP Extensions for EAP Re-
authentication Protocol (ERP)", RFC 5296, August 2008.
Authors' Addresses
Ted Lemon
Nominum
2000 Seaport Blvd
Redwood City, CA 94063
USA
Phone: +1 650 381 6000
Email: mellon@nominum.com
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Qin Wu
Huawei Technologies Co., Ltd.
101 Software Avenue, Yuhua District
Nanjing, Jiangsu 210012
China
Email: sunseawq@huawei.com
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