Network Working Group                                           R. Droms
Internet-Draft                                             J. Schnizlein
Expires: April 6, 2004                                     Cisco Systems
                                                         October 7, 2003

   RADIUS Attributes Sub-option for the DHCP Relay Agent Information

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   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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   Copyright (C) The Internet Society (2003). All Rights Reserved.


   A network access device 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 an IP
   address for assignment 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 during RADIUS
   authentication to a DHCP server.

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1. Introduction and Background

   The RADIUS Attributes sub-option for the DHCP Relay Agent option
   provides a way through which network elements can pass information
   obtained through layer 2 authentication to a DHCP server [2].  IEEE
   802.1X [3] is an example of a mechanism through which a network
   access device 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 [4] as the Authentication Service
   specified in 802.1X.  In 802.1X authenticated access, a device must
   first exchange some authentication credentials with the network
   access device.  The access device then supplies these credentials to
   a RADIUS server, which either confirms or denies the identity of the
   user of the device requesting network access.  The access device,
   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) Access granted
          v         |
         |  Access Device  |
         |(802.1X and DHCP |
         |   relay agent}  |
         |     ^
         |     |
        (2) Request for authentication
         |     |
         |    (3) Authentication confirm/deny
         v     |
         |     RADIUS      |
         |     Service     |

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   Figure 1

                                Figure 1

   In the application described in this document, the access device acts
   as an 802.1X authenticator and adds DHCP relay agent options to DHCP
   messages. During 802.1X authentication, the reply message from the
   RADIUS server carries additional identification information as
   attributes to the access device. The access device stores these
   attributes locally. When the access device subsequently forwards DHCP
   messages from the network device, the access device adds the
   identification information in an 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 an access device.  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.

2. Terminology

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

2.1 General Terminology

   Access device:  A network element providing network access to a host

2.2 DHCP Terminology

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

2.3 RADIUS Terminology

   The following terms are used in conjunction with RADIUS:

   RADIUS server:                           An entity that provides
      RADIUS service through the exchange of RADIUS protocol messages

   Attribute: Data value carried in a RADIUS protocol message

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2.4 802.1X 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  |  b1  |  b2  |  b3  |  b4  |      |  bN  |

   The RADIUS attributes are encoded according to the encoding rules in
   RFC 2865, in bytes b1...bN.

4. 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 contain
   the attributes received in response to the client's authentication
   with the RADIUS service. 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, and MAY include other attributes.

5. DHCP Server Behavior

   When the DHCP server receives a message from an relay agent
   containing a RADIUS Attributes sub-option, it extracts the contents
   of the of the sub-option and uses that information in selecting
   configuration parameters for the client.

6. DHCP Client Behavior

   The host need not make any special provision for the use of the
   RADIUS Attributes sub-option.

7. RADIUS Server Behavior

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   The RADIUS server MUST return the User-Name and Class attributes to
   the access device, and MAY return other attributes.

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

   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.  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 [7] or IPsec [8] SHOULD be deployed as

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. Terms of Use

   Cisco has a pending patent which relates to the subject matter of
   this Internet Draft. If a standard relating to this subject matter is
   adopted by IETF and any claims of any issued Cisco patents are
   necessary for practicing this standard, any party will be able to
   obtain a license from Cisco to use any such patent claims under
   openly specified, reasonable, non-discriminatory terms to implement
   and fully comply with the standard.

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]  Institute of Electrical and Electronics Engineers, "Port based
        Network Access Control", IEEE Standard 802.1X, March 2001.

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

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

Informative References

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

   [7]  Stapp, M., Lemon, T. and R. Droms, "The Authentication Suboption
        for the DHCP Relay Agent Option",
        draft-ietf-dhc-auth-suboption-01 (work in progress), November

   [8]  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
   250 Apollo Drive
   Chelmsford, MA  01824


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


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