dhc                                                             T. Lemon
Internet-Draft                                                   Nominum
Intended status: Standards Track                                   Q. Wu
Expires: March 18, 2011                                           Huawei
                                                      September 14, 2010


                      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
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   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on March 18, 2011.

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   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  . . . . . . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . . . . . 5
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   8.  Normative References  . . . . . . . . . . . . . . . . . . . . . 5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 5






































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.  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 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 [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


.  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.


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

   TBD  Relay-Supplied Options code

   length  Length of Relay-Supplied Options option

   suboptions  One or more DHCPv6 options


.  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.


.  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].

   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.







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.  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; 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.


.  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.


.  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.









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Authors' Addresses

   Ted Lemon
   Nominum
   2000 Seaport Blvd
   Redwood City, CA  94063
   USA

   Phone: +1 650 381 6000
   Email: mellon@nominum.com


   Qin Wu
   Huawei Technologies Co., Ltd.
   101 Software Avenue, Yuhua District
   Nanjing, Jiangsu  210012
   China

   Email: sunseawq@huawei.com
































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