Relay-Supplied DHCP Options
draft-ietf-dhc-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.

Status of this Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on March 18, 2011.

Copyright Notice

Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

1.  Introduction
    1.1.  Requirements Language
    1.2.  Terminology
2.  Protocol Summary
3.  Encoding
4.  DHCP Relay Agent Behavior
5.  DHCP Server Behavior
6.  Security Considerations
7.  IANA Considerations
8.  Normative References
§  Authors' Addresses

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 (Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, “Dynamic Host Configuration Protocol for IPv6 (DHCPv6),” July 2003.) [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 initial motivation for this draft came from a proposal from the Mobile IPv6 working group that proposed a single-use mechanism whereby a particular relay option would be forwarded to the client. Subsequent independent effort in another working group has confirmed the need for a general mechanism to do this.

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 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].

1.2.  Terminology

The following terms and acronyms are used in this document:

DHCP

- Dynamic Host Configuration Protocol Version 6 [RFC3315] (Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, “Dynamic Host Configuration Protocol for IPv6 (DHCPv6),” July 2003.)

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.

+----+----+----+----+--------------+
+   TBD   |  length | suboptions...|
+----+----+----+----+--------------+

TBD

Relay-Supplied Options code

length

Length of Relay-Supplied Options option

suboptions

One or more DHCPv6 options

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 (Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, “Dynamic Host Configuration Protocol for IPv6 (DHCPv6),” July 2003.) [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 (Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, “Dynamic Host Configuration Protocol for IPv6 (DHCPv6),” July 2003.) [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.

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 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 (Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, “Dynamic Host Configuration Protocol for IPv6 (DHCPv6),” July 2003.) [RFC3315].

Note, however, that this attack is only useful if the DHCP server is using the DHCPv6 authentication mechanism; in the absence of DHCPv6 authentication, the relay agent could more easily forge a message to the client, rather than using this mechanism to cause the server to produce a message containing forged information.

7.  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.

8. Normative References

Authors' Addresses