Relay-Supplied DHCP Options
draft-ietf-dhc-dhcpv6-relay-supplied-options-04

Abstract

This document describes a mechanism whereby a DHCPv6 relay agent can provide options to a DHCPv6 server that the DHCPv6 server can then provide to the DHCPv6 client in certain restricted cases where this is necessary.

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 April 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.  RSOO-enabled options
5.  DHCP Relay Agent Behavior
6.  DHCP Server Behavior
7.  Security Considerations
8.  IANA Considerations
9.  References
    9.1.  Normative References
    9.2.  Informative References
§  Authors' Addresses

1.  Introduction

The DHCPv6 specification [RFC3315] (Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, “Dynamic Host Configuration Protocol for IPv6 (DHCPv6),” July 2003.) 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 not available to the DHCP server 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] (Zorn, G., Wu, Q., and Y. Wang, “The Local Domain Name DHCPv6 Option,” September 2010.) for use in EAP re-authentication [RFC5296] (Narayanan, V. and L. Dondeti, “EAP Extensions for EAP Re-authentication Protocol (ERP),” August 2008.), 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.

This document is not intended to provide a general mechanism for storing client configuration information in the relay agent. Rather, it is intended to address specific use cases where only the relay agent has information needed by the client. This extension is not applicable to DHCP options in general, but rather provided as a mechanism for new specifications that require this functionality.

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 to send to the client, 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.

   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         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | options...
   +-+-+-+-+-+-+-+-+-+-+-+

OPTION_RSOO

Relay-Supplied Options code (TBD).

option-length

Length of Relay-Supplied Options option.

options

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. No DHCP option specified prior to the issuance of this specification is RSOO-enabled.

A current list of RSOO-enabled options can be found in the list titled "Options Permitted in the Relay-Supplied Options option" maintained at http://www.iana.org/assignments/dhcpv6-parameters.

DHCP option specifications that define RSOO-enabled options MUST add text similar to the following to their IANA considerations section; "random relay option" should be replaced with the name of the option being defined in the specification:

We request that IANA add the name "random relay option" to the registry titled "Options Permitted in the Relay-Supplied Options Option" maintained at http://www.iana.org/assignments/dhcpv6-parameters.

5.  DHCP Relay Agent Behavior

DHCP relay agents that implement this specification MUST be configurable not to send the Relay-Supplied Options option.

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.

Relay agents MUST NOT send non-RSOO-enabled options in the Relay-Supplied Options option.

Relay agents implementing this specification SHOULD have a configuration parameter that determines whether or not they will relay a Relay-Forward message toward the DHCP server if it contains a Relay-Supplied Options option.

Relay agents that have this configuration parameter and that are configured to enable this behavior MUST silently discard any Relay-Forward packet that contains a Relay-Supplied Options option.

Implementations that can be configured in this way MUST examine all Relay-Forward encapsulations, not just the outer encapsulation.

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 (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 implementing this specificaton receives an RSOO, 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 servers may receive multiply-nested Relay-Forward messages containing conflicting values for options contained in Relay Supplied Options options in these messages.

When such a conflict exists, the DHCP server MUST choose no more than one of these options to forward to the client. The DHCP server MUST NOT forward more than one of these options to the client.

By default, the DHCP server MUST choose the innermost value—the value supplied by the relay agent closest to the DHCP client, to forward to the DHCP client.

DHCP server implementations MAY provide other heuristics for choosing which one of a set of such conflicting options to forward to the client, as long as the specified behavior is the default behavior.

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. In general it is expected that RSOO-enabled options will be specified because they only make sense when originating from the relay agent. This is true of existing use cases.

In the event that some new RSOO-enabled option is specified that can originate from either the server or the relay agent, this should be addressed in the security considerations section of the document that specifies the use of that option.

In some environments, there is an interface on one side of which is the client, and zero or more routers, and on the other side of which is a network managed by a monolithic or effectively monolithic administrative entity. Nodes and routers on the client side of the interface are not controlled by this entity, and are considered "untrusted." Nodes and routers on the other side of this interface are considered trusted.

It is possible for a relay agent on the untrusted side of this interface to supply a Relay-Supplied Options option containing one or more RSOO-enabled options that would override the same option or options that were provided by a relay agent on the trusted side of the interface.

In environments where this is a possibility, network administrators are advised to use relay agents that are capable of dropping Relay-Forward messages containing the Relay-Supplied Options option, and are advised to configure those relays to drop such messages.

Note, however, that this will only be effective if the message from the DHCP server to the DHCP client is authenticated as specified in Section 21 of DHCP Version 6 (Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, “Dynamic Host Configuration Protocol for IPv6 (DHCPv6),” July 2003.) [RFC3315], or using some similar mechanism. Without this authentication, the relay agent on the untrusted portion of the network can simply modify the DHCP server's response in transit back to the DHCP client, and there is no way for the client to detect that this has happened.

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.

We request that the IANA create a new registry on the same assignments page, titled "Options Permitted in the Relay-Supplied Options Option". This option will contain a list of names of options from the DHCP Option Codes list. Currently, the list is empty.

9.  References

9.1. Normative References

9.2. Informative References

Authors' Addresses