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RFC 6422 - Relay-Supplied DHCP Options

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Internet Engineering Task Force (IETF)                          T. Lemon
Request for Comments: 6422                                       Nominum
Updates: 3315                                                      Q. Wu
Category: Standards Track                                         Huawei
ISSN: 2070-1721                                            December 2011

                      Relay-Supplied DHCP Options


   DHCPv6 relay agents cannot communicate with DHCPv6 clients directly.
   However, in some cases, the relay agent possesses some information
   that would be useful to the DHCPv6 client.  This document describes a
   mechanism whereby the DHCPv6 relay agent can provide such information
   to the DHCPv6 server, which can, in turn, pass this information on to
   the DHCP client.

   This document updates RFC 3315 (DHCPv6) by making explicit the
   implicit requirement that relay agents not modify the content of
   encapsulation payloads as they are relayed back toward clients.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................2
      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 ............................................5
   7. Security Considerations .........................................6
   8. IANA Considerations .............................................7
   9. References ......................................................7
      9.1. Normative References .......................................7
      9.2. Informative References .....................................7

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 Re-authentication Protocol (ERP) local domain name
   [RFC6440] 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:

   o  DHCP: Dynamic Host Configuration Protocol Version 6 [RFC3315]

   o  RSOO: Relay-Supplied Options option

2.  Protocol Summary

   DHCP clients do not support a mechanism for receiving options from
   relay agents -- the relay agent is required to deliver the payload
   from the DHCP server to the DHCP client without changing it.  In
   order for the DHCP relay agent to provide options to the client, it
   sends those options to the DHCP server, encapsulated in an RSOO.  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 an RSOO 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 (66).


      Length of the RSOO.


      One or more DHCPv6 options.

4.  RSOO-Enabled Options

   The RSOO MUST NOT contain any option that is not specifically called
   out as an RSOO-enabled option.  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/.

   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/.

5.  DHCP Relay Agent Behavior

   Relay agents MAY include an RSOO in the option payload of a Relay-
   Forward message being sent toward a DHCP server.  When relaying the
   payload of Relay-Reply messages toward clients, relay agents MUST NOT
   modify the payload.

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

   In order to allow network administrators to control the flow of RSOO
   options onto the network, relay agents that implement the Relay-
   Supplied Options option need to have a configuration parameter that
   determines whether or not they will relay Relay-Forward messages
   containing RSOOs.

   Relay agents that have this configuration parameter and that are
   configured to disable forwarding of a Relay-Forward message
   containing an RSOO MUST silently discard any such message.

   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 protocol 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 an administrator-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 the DHCPv6 specification [RFC3315].

   If the server implementing this protocol specification 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

   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 [RFC3315].

   DHCP servers may receive multiply-nested Relay-Forward messages
   containing conflicting values for options contained in RSOOs in these

   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 currently known use cases -- for example, the ERP Local Domain
   Option [RFC6440] -- RSOO-enabled options are options that will only
   ever originate on a relay agent, and do not make sense when
   originating on a DHCP server.

   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 network side of this interface
   are considered trusted.

   It is possible for a malicious node acting as a relay agent on the
   untrusted side of this interface to supply an RSOO 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 RSOO, and are advised to configure
   those relay agents 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 [RFC3315], or using some similar mechanism.  Without
   this authentication, the malicious node 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

   IANA has assigned one new DHCPv6 option code from the registry of
   DHCP Option Codes maintained at http://www.iana.org/.  The option
   code 66 (OPTION_RSOO) has been assigned to the Relay-Supplied Options

   IANA has created a new registry on the same assignments page, titled
   "Options Permitted in the Relay-Supplied Options Option".  This
   registry will enumerate the set of all code points from the DHCP
   Option Codes table for options that may appear in the RSOO.  Options
   may be added to this list after IETF Review [RFC5226].  When adding
   options to the list, please ensure that the description for the code
   added matches the description in the DHCP Option Codes table for that
   code.  Option codes that have not been requested to be added
   according to the stated procedure should not be mentioned at all in
   the table, and should not be listed as "reserved" or "unassigned".

   IETF Review should include careful consideration of the security
   implications of allowing a relay agent to provide a value for the
   option being considered for addition to this registry.  In the case
   where an IETF working group chartered to review DHCP protocol
   extensions exists, it is not sufficient for some other working group
   to review the registry addition.

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., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
              C., and M. Carney, "Dynamic Host Configuration Protocol
              for IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

9.2.  Informative References

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

   [RFC6440]  Zorn, G., Wu, Q., and Y. Wang, "The EAP Re-authentication
              Protocol (ERP) Local Domain Name DHCPv6 Option", RFC 6440,
              December 2011.

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

   Phone: +86-25-56623633
   EMail: sunseawq@huawei.com


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