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Versions: 00 01 02 03 04 05 06 07 08 RFC 4014

Network Working Group                                           R. Droms
Internet-Draft                                             J. Schnizlein
Expires: January 5, 2005                                   Cisco Systems
                                                            July 7, 2004

   RADIUS Attributes Sub-option for the DHCP Relay Agent Information

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Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.


   A NAS (network access server) may choose to authenticate the identity
   of a device before granting that device access to the network.  The
   IEEE 802.1X protocol is an example of a mechanism for providing
   authenticated layer 2 network access.  A network element using RADIUS
   as an authentication authority will receive attributes from a RADIUS
   server that may be used by a DHCP server in the selection of
   configuration parameters to be delivered to the device through its
   DHCP client. The RADIUS Attributes sub-option enables a network
   element to pass along attributes for the user of a device received

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   during RADIUS authentication to a DHCP server.

1. Introduction and Background

   The RADIUS Attributes sub-option for the DHCP Relay Agent option
   provides a way in which a NAS can pass attributes obtained from a
   RADIUS server to a DHCP server [1].  IEEE 802.1X [2] is an example of
   a mechanism through which a NAS such as a switch or a wireless LAN
   access point can authenticate the identity of the user of a device
   before providing layer 2 network access using RADIUS as the
   Authentication Service specified in RFC3580 [10].  In IEEE 802.1X
   authenticated access, a device must first exchange some
   authentication credentials with the NAS.  The NAS then supplies these
   credentials to a RADIUS server, which eventually sends either an
   Access-Accept or an Access-Reject in response to an Access-Request.
   The NAS, based on the reply of the RADIUS server, then allows or
   denies network access to the requesting device.

   Figure 1 summarizes the message exchange among the participants in
   IEEE 802.1X authentication.

         |Device requesting|
         | network access  |
          |         ^
          |         |
         (1) Request for access
          |         |
          |        (4) Success/Failure
          v         |
         |       NAS       |
         |(IEEE 802.1X and |
         |DHCP relay agent}|
            |     ^
            |     |
           (2) Request for authentication
            |     |
            |    (3) Access-Accept/Reject
            v     |
         |     RADIUS      |
         |     Server      |

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                                Figure 1

   In the application described in this document, the access device acts
   as an IEEE 802.1X Authenticator and adds DHCP relay agent options to
   DHCP messages. At the successful conclusion of IEEE 802.1X
   authentication, a RADIUS Access-Accept provides attributes for
   service authorizations to the NAS.  The NAS stores these attributes
   locally. When the NAS subsequently forwards DHCP messages from the
   network device, the NAS adds these attributes in a RADIUS Attributes
   sub-option.  The RADIUS Attributes sub-option is another suboption of
   the Relay Agent Information option [5].

   This document uses IEEE 802.1X as an example to motivate the use of
   RADIUS by a NAS.  The RADIUS Attributes sub-option described in this
   document is not limited to use in conjunction with IEEE 802.1X and
   can be used to carry RADIUS attributes obtained by the relay agent
   for any reason.  That is, the option is not limited to use with IEEE
   802.1X, but is constrained by RADIUS semantics (see Section 4).

2. Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [3].

2.1 DHCP Terminology

   The following terms are used as defined in RFC2131 and RFC3046: DHCP
   relay agent, DHCP server, DHCP client.

2.2 RADIUS Terminology

   The following terms are used in conjunction with RADIUS:
   RADIUS server: A RADIUS server is responsible for receiving user
      connection requests, authenticating the user, and then returning
      all configuration information necessary for the client to deliver
      service to the user.
   Attribute: A Type-Length-Value tuple encapsulating data elements as
      defined in RFC 2865 [4].
   NAS:  A Network Access Server (NAS) provides access to the network
      and operates as a client of RADIUS. The client is responsible for
      passing user information to designated RADIUS servers, and then
      acting on the response which is returned.  Unlike a traditional
      dial NAS, the NAS considered here may not have a protocol like PPP
      through which it can pass configuration information from the
      RADIUS attributes to the client

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2.3 IEEE 802.1X Terminology

   The following terms are used as defined in the IEEE 802.1X protocol:
   Authenticator, Supplicant.

3. RADIUS Attributes sub-option format

   The RADIUS Attributes Sub-option is a new sub-option for the DHCP
   Relay Agent option.

   The format of the RADIUS Attributes sub-option is:

        SubOpt   Len     RADIUS attributes
       |  TBD  |  N  |  o1  |  o2  |  o3  |  o4  |      |  oN  |

   The RADIUS attributes are encoded according to the encoding rules in
   RFC 2865, in octets o1...oN.

   The NAS truncates the RADIUS attributes to fit in the RADIUS
   Attributes sub-option.  For predictable behavior, the RADIUS server
   should be configured to return fewer than 255 octets of RADIUS

4. RADIUS Server Behavior

   The RADIUS server that implements this specification MUST be
   configured to return the User-Name and Class attributes to the NAS,
   and MAY return other attributes.

   To avoid dependencies between the address allocation and other state
   information between the RADIUS server and the DHCP server, only the
   attributes in the table below SHOULD be included in this sub-option.
   Because RADIUS servers rely on the directive in section 1.1 or RFC
   2865 that "A NAS MUST treat a RADIUS access-accept authorizing an
   unavailable service as an access-reject instead.", a RADIUS server
   SHOULD send only those attributes for which the relay agent can
   ensure that either the relay agent or the DHCP server will provide
   the associated service.  The following table, based on the analysis
   in RFC 3580 [10], lists attributes that MAY be included:

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            #   Attribute
          ---   ---------
            1   User-Name (RFC 2865 [3])
            6   Service-Type (RFC 2865)
           25   Class (RFC 2865)
           26   Vendor-Specific (RFC 2865)
           27   Session-Timeout (RFC 2865)
           88   Framed-Pool (RFC 2869)
          100   Framed-IPv6-Pool (RFC 3162 [8])

5. DHCP Relay Agent Behavior

   When the DHCP relay agent receives a DHCP message from the client, it
   MAY append a DHCP Relay Agent Information option containing the
   RADIUS Attributes sub-option, along with any other sub-options it is
   configured to supply.  The RADIUS Attributes sub-option MUST only
   contain the attributes provided in the RADIUS Access/Accept message.
   The DHCP relay agent MUST NOT add more than one RADIUS Attributes
   sub-option in a message.

   The relay agent SHOULD include the User-Name and Class attributes in
   the RADIUS Attributes sub-option if available, and MAY include other

6. DHCP Server Behavior

   When the DHCP server receives a message from a relay agent containing
   a RADIUS Attributes sub-option, it extracts the contents of the
   sub-option and uses that information in selecting configuration
   parameters for the client.  If the relay agent forwards RADIUS
   attributes not included in the table in Section 4, the DHCP server
   SHOULD ignore them.  If the DHCP server uses attributes not specified
   here, it might result in side effects not anticipated in the existing
   RADIUS specifications.

7. DHCP Client Behavior

   Relay agent options are exchanged only between relay agents and DHCP
   server, so DHCP clients are never aware of their use.

8. 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 [8].  Potential exposures to attack are discussed in section 7
   of the DHCP protocol specification in RFC 2131 [1].

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   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 [5].  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 the
   authentication option for relay agent options [11] or IPsec [12]
   SHOULD be deployed as well.

9. IANA Considerations

   IANA has assigned the value of TBD for the DHCP Relay Agent
   Information option sub-option code for this sub-option.  This
   document does not define any new namespaces or other constants for
   which IANA must maintain a registry.

10. Acknowledgments

   Bernard Aboba's expert advice on avoiding RADIUS entanglements is
   gratefully acknowledged.

Normative References

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

   [2]  Institute of Electrical and Electronics Engineers, "Local and
        Metropolitan Area Networks: Port based Network Access Control",
        IEEE Standard 802.1X, March 2001.

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

   [4]  Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote
        Authentication Dial In User Service (RADIUS)", RFC 2865, June

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

Informative References

   [6]   Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.

   [7]   Rigney, C., Willats, W. and P. Calhoun, "RADIUS Extensions",
         RFC 2869, June 2000.

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

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   [9]   Aboba, B., Zorn, G. and D. Mitton, "RADIUS and IPv6", RFC 3162,
         August 2001.

   [10]  Congdon, P., Aboba, B., Smith, A., Zorn, G. and J. Roese, "IEEE
         802.1X Remote Authentication Dial In User Service (RADIUS)
         Usage Guidelines", RFC 3580, September 2003.

   [11]  Stapp, M. and T. Lemon, "The Authentication Suboption for the
         DHCP Relay Agent Option", draft-ietf-dhc-auth-suboption-02
         (work in progress), October 2003.

   [12]  Droms, R., "Authentication of DHCP Relay Agent Options Using
         IPsec", draft-ietf-dhc-relay-agent-ipsec-00 (work in progress),
         September 2003.

Authors' Addresses

   Ralph Droms
   Cisco Systems
   1414 Massachusetts Avenue
   Boxborough, MA  01719

   EMail: rdroms@cisco.com

   John Schnizlein
   Cisco Systems
   9123 Loughran Road
   Fort Washington, MD  20744

   EMail: jschnizl@cisco.com

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