1. RFC 4243
Network Working Group                                           M. Stapp
Request for Comments: 4243                                    R. Johnson
Category: Standards Track                                 T. Palaniappan
                                                     Cisco Systems, Inc.
                                                           December 2005

             Vendor-Specific Information Suboption for the
     Dynamic Host Configuration Protocol (DHCP) Relay Agent Option

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).


   This memo defines a new Vendor-Specific Information suboption for the
   Dynamic Host Configuration Protocol's (DHCP) relay agent information
   option.  The suboption allows a DHCP relay agent to include vendor-
   specific information in the DHCP messages it forwards, as configured
   by its administrator.

Table of Contents

   1. Introduction ....................................................2
   2. Requirements Terminology ........................................2
   3. The Vendor-Specific Suboption ...................................2
   4. Relay Agent Behavior ............................................4
   5. DHCP Server Behavior ............................................4
   6. Security Considerations .........................................4
   7. IANA Considerations .............................................5
   8. Acknowledgements ................................................5
   Normative References ...............................................5
   Informative References .............................................5

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RFC 4243            Vendor-Specific Relay Suboption        December 2005

1.  Introduction

   DHCP (RFC 2131 [2]) provides IP addresses and configuration
   information for IPv4 clients.  It includes a relay agent capability,
   in which processes within the network infrastructure receive
   broadcast messages from clients and forward them to DHCP servers as
   unicast messages.  In network environments like DOCSIS data-over-
   cable and xDSL, for example, it has proven useful for the relay agent
   to add information to the DHCP message before forwarding it, using
   the relay agent information option (RFC 3046 [3]).

   Servers that recognize the relay agent option echo it back in their
   replies, and some of the information that relays add may be used to
   help an edge device efficiently return replies to clients.  The
   information that relays supply can also be used in the server's
   decision making about the addresses and configuration parameters that
   the client should receive.

   In many environments, it's desirable to associate some vendor- or
   provider-specific information with the clients' DHCP messages.  This
   is often done using the relay agent information option.  RFC 3046
   defines Remote-ID and Circuit-ID sub-options that are used to carry
   such information.  The values of those suboptions, however, are
   usually based on some network resource, such as an IP address of a
   network access device, an ATM Virtual Circuit identifier, or a DOCSIS
   cable-modem identifier.  As a result, the values carried in these
   suboptions are dependent on the physical network configuration.  The
   Vendor-Specific suboption allows administrators to associate other
   useful data with relayed DHCP messages.

2.   Requirements Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [1].

3.  The Vendor-Specific Suboption

   This memo defines a new DHCP relay agent option suboption that
   carries vendor-defined data.  The suboption takes a form similar to
   the Vendor-Identifying, Vendor-Specific Option [7].

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RFC 4243            Vendor-Specific Relay Suboption        December 2005

       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
      |     Code      |    Length     |        Enterprise Number1     |
      |                               |  DataLen1     |               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +
      \                         Suboption Data1                       \
      |                      Enterprise Number2                       |
      |  DataLen2     |             Suboption Data2                   |
      \                                                               \
      .                                                               .
      .                                                               .

   The Code for the suboption is 9.

   The one-byte Length field is the length of the data carried in the
   suboption, in bytes.  The length includes the length of the first
   Enterprise Number; the minimum length is 4 bytes.

   "Enterprise NumberN" is a vendor's Enterprise Number as registered
   with IANA [4].  It is a four-byte integer value in network byte-

   DataLenN is the length of the data associated with the Enterprise

   The Suboption Data is an opaque sequence of bytes.

   The Vendor-Specific suboption includes at least one Enterprise Number
   and carries opaque data defined by the organization identified by the
   Enterprise Number.  A relay may include data associated with more
   than one vendor's Enterprise Number within a single instance of the

   Of course, the Vendor-Specific data are vendor-specific.  This
   specification does not establish any requirements on the data in the
   suboption.  Vendors who make use of this suboption are encouraged to
   document their usage in order to make interoperability possible.

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RFC 4243            Vendor-Specific Relay Suboption        December 2005

4.  Relay Agent Behavior

   DHCP relay agents MAY be configured to include Vendor-Specific
   suboptions if they include a relay agent information option in
   relayed DHCP messages.  The suboptions' types and data are assigned
   and configured through mechanisms that are outside the scope of this

   Relay implementors are encouraged to offer their administrators a
   means of configuring what data can be included in this suboption, and
   to document what they are capable of.

5.  DHCP Server Behavior

   This suboption provides additional information to the DHCP server.
   The DHCP server, if it is configured to support this suboption, may
   use this information, in addition to other relay agent option data
   and other options included in the DHCP client messages, in order to
   assign an IP address and/or other configuration parameters to the
   client.  There is no special additional processing for this

6.  Security Considerations

   Message authentication in DHCP for intradomain use, where the out-
   of-band exchange of a shared secret is feasible, is defined in RFC
   3118 [5].  Potential exposures to attack are discussed in section 7
   of the DHCP protocol specification in RFC 2131 [2].

   The DHCP relay agent option depends on a trusted relationship between
   the DHCP relay agent and the server, as described in section 5 of RFC
   3046.  Fraudulent relay agent option data could potentially lead to
   theft-of-service or exhaustion of limited resources (like IP
   addresses) by unauthorized clients.  A host that tampered with relay
   agent data associated with another host's DHCP messages could deny
   service to that host, or interfere with its operation by leading the
   DHCP server to assign it inappropriate configuration parameters.

   While the introduction of fraudulent relay agent options can be
   prevented by a perimeter defense that blocks these options unless the
   relay agent is trusted, a deeper defense using authentication for
   relay agent options via the Authentication Suboption [6] SHOULD be
   deployed as well.

   There are several data in a DHCP message that convey information that
   may identify an individual host on the network.  These include the
   chaddr, the client-id option, and the hostname and client-fqdn
   options.  Depending on the type of data included, the Vendor-Specific
   suboption may also convey information that identifies a specific host
   or a specific user on the network.  In practice, this information
   isn't exposed outside the internal service-provider network, where
   DHCP messages are usually confined.  Administrators who configure
   data that will be used in DHCP Vendor-Specific suboptions should be
   careful to use data that are appropriate for the types of networks

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RFC 4243            Vendor-Specific Relay Suboption        December 2005

   they administer.  If DHCP messages travel outside the service-
   provider's own network, or if the suboption values may become visible
   to other users, it may raise privacy concerns for the access provider
   or service provider.

7.  IANA Considerations

   The IANA has assigned the suboption number 9 for the Vendor-Specific
   Information Suboption from the DHCP Relay Agent Information Option
   [3] suboption number space.

8.  Acknowledgements

   The authors are grateful to Andy Sudduth, Josh Littlefield, and Kim
   Kinnear for their review and comments.

Normative References

   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [2]  Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
        March 1997.

   [3]  Patrick, M., "DHCP Relay Agent Information Option", RFC 3046,
        January 2001.

   [4]  IANA, "Private Enterprise Numbers (http://www.iana.org/

Informative References

   [5]  Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
        RFC 3118, June 2001.

   [6]  Stapp, M. and T. Lemon, "The Authentication Suboption for the
        Dynamic Host Configuration Protocol (DHCP) Relay Agent Option",
        RFC 4030, March 2005.

   [7]  Littlefield, J., "Vendor-Identifying Vendor Options for Dynamic
        Host Configuration Protocol version 4 (DHCPv4)", RFC 3925,
        October 2004.

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RFC 4243            Vendor-Specific Relay Suboption        December 2005

Authors' Addresses

   Mark Stapp
   Cisco Systems, Inc.
   1414 Massachusetts Ave.
   Boxborough, MA  01719

   Phone: 978.936.0000
   EMail: mjs@cisco.com

   Richard Johnson
   Cisco Systems, Inc.
   170 W. Tasman Dr.
   San Jose, CA  95134

   Phone: 408.526.4000
   EMail: raj@cisco.com

   Theyn Palaniappan
   Cisco Systems, Inc.
   170 W. Tasman Dr.
   San Jose, CA  95134

   Phone: 408.526.4000
   EMail: athenmoz@cisco.com

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RFC 4243            Vendor-Specific Relay Suboption        December 2005

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  1. RFC 4243