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Relay-Supplied DHCP Options

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 6422.
Authors Qin Wu , Ted Lemon
Last updated 2023-09-08 (Latest revision 2011-09-06)
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Additional resources Mailing list discussion
Stream WG state WG Document
Document shepherd (None)
IESG IESG state Became RFC 6422 (Proposed Standard)
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Consensus boilerplate Unknown
Telechat date (None)
Responsible AD Ralph Droms
IESG note
Send notices to (None)
dhc                                                             T. Lemon
Internet-Draft                                                   Nominum
Updates: 3315 (if approved)                                        Q. Wu
Intended status: Standards Track                                  Huawei
Expires: March 9, 2012                                 September 6, 2011

                      Relay-Supplied DHCP Options


   DHCPv6 relay agents can not 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 RFC3315 (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 Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 9, 2012.

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
   ( in effect on the date of

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   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  . . . . . . . . . . . . . . . . . . . . . . . . . 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  . . . . . . . . . . . . . . . . . . 8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 8

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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 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 a Relay-Supplied Options
   option.  The DHCP server can then choose to place those options in
   the response it sends to the client.

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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

   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

   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

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      Option" maintained at

5.  DHCP Relay Agent Behavior

   Relay agents MAY include a Relay-Supplied Options option 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 to whether or not they will relay RSOO-containing Relay-
   Forward messages.

   Relay agents that have this configuration parameter and that are
   configured to disable forwarding of Relay-Forward messages that
   contain a Relay-Supplied Options option 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 Relay-Supplied DHCP Options 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 DHCPv6 [RFC3315].

   If the server implementing Relay-Supplied DHCP Options 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

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   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 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 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.

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   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 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 is requested to assign one new DHCPv6 option code from the
   registry of DHCP Option Codes maintained at  The option code
   OPTION_RSOO will be assigned to the Relay-Supplied Options option.

   IANA is also requested to create a new registry on the same
   assignments page, titled "Options Permitted in the Relay-Supplied
   Options Option".  This registry will contain a list of DHCPv6 option
   codes from the DHCP Option Codes list at as suboptions of
   the Relay-Supplied Options option.  Currently, the list is empty.
   Options may be added to this list after IETF Review[RFC5226].

   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., 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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   [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

              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

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


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