dhc                                                             T. Lemon
Internet-Draft                                                   Nominum
Intended status: Standards Track                                   Q. Wu
Expires: November 10, 2011                                        Huawei
                                                             May 9, 2011

                      Relay-Supplied DHCP Options


   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

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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  . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
   4.  RSOO-enabled options  . . . . . . . . . . . . . . . . . . . . . 4
   5.  DHCP Relay Agent Behavior . . . . . . . . . . . . . . . . . . . 5
   6.  DHCP Server Behavior  . . . . . . . . . . . . . . . . . . . . . 5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     9.2.  Informative References  . . . . . . . . . . . . . . . . . . 7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 8

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 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] 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.

   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",
   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 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...


      Relay-Supplied Options code (TBD).


      Length of Relay-Supplied Options option.


      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-

   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

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

   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 [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 [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 [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.
   Options may be added to this list either as a result of protocol
   action or expert review.  Expert review may either be in the form of
   a working group review and consensus call, or IESG review in
   consultation with the DHC working group chair or chairs.

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

              Zorn, G., Wu, Q., and Y. Wang, "The ERP Local Domain Name
              DHCPv6 Option", draft-ietf-hokey-ldn-discovery-10 (work in
              progress), April 2011.

   [RFC5296]  Narayanan, V. and L. Dondeti, "EAP Extensions for EAP Re-
              authentication Protocol (ERP)", RFC 5296, August 2008.

Authors' Addresses

   Ted Lemon
   2000 Seaport Blvd
   Redwood City, CA  94063

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

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

   Email: sunseawq@huawei.com