Remote Authentication Dial-In User Service (RADIUS) Attributes Suboption for the Dynamic Host Configuration Protocol (DHCP) Relay Agent Information Option
RFC 4014

Network Working Group                                           R. Droms
Request for Comments: 4014                                 J. Schnizlein
Category: Standards Track                                  Cisco Systems
                                                           February 2005

          Remote Authentication Dial-In User Service (RADIUS)
                     Attributes Suboption for the
              Dynamic Host Configuration Protocol (DHCP)
                     Relay Agent Information 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).

Abstract

   The RADIUS Attributes suboption enables a network element to pass
   identification and authorization attributes received during RADIUS
   authentication to a DHCP server.  When the DHCP server receives a
   message from a relay agent containing a RADIUS Attributes suboption,
   it extracts the contents of the suboption and uses that information
   in selecting configuration parameters for the client.

1.  Introduction and Background

   The RADIUS Attributes suboption 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 with RADIUS as the
   Authentication Service, as specified in RFC 3580 [8].  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.

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RFC 4014              RADIUS Attributes Suboption          February 2005

   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      |
                        +-----------------+

                             Figure 1

   The access device acts as an IEEE 802.1X Authenticator and adds a
   DHCP relay agent option that includes a RADIUS Attributes suboption
   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 relays DHCP messages from the
   network device, the NAS adds these attributes in a RADIUS Attributes
   suboption.  The RADIUS Attributes suboption is another suboption of
   the Relay Agent Information option [5].

   The RADIUS Attributes suboption 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).

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   The scope of applicability of this specification is such that robust
   interoperability is only guaranteed for RADIUS service
   implementations that exist within the same scope as does the DHCP
   service implementation, i.e., within a single, localized
   administrative domain.  Global interoperability of this
   specification, across administrative domains, is not required.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [3].

   Within this specification, the use of the key words "MUST", "MUST
   NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
   "RECOMMENDED", "MAY", and "OPTIONAL" is with respect to RADIUS
   clients and servers that implement the optional features of this
   specification.  The use of these key words does not create any
   normative requirements outside of that scope, and does not modify the
   base RADIUS specifications, such as RFC 2865 [4].

2.1.  DHCP Terminology

   The following terms are used as defined in RFC 2131 and RFC 3046:
   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 that is returned.  Unlike a traditional
      dial NAS, the NAS considered here may not have a protocol such as
      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 Suboption Format

   The RADIUS Attributes suboption is a new suboption for the DHCP Relay
   Agent option.

   The format of the RADIUS Attributes suboption is as follows:

        SubOpt   Len     RADIUS attributes
         code
       +-------+-----+------+------+------+------+--...-+------+
       |   7   |  N  |  o1  |  o2  |  o3  |  o4  |      |  oN  |
       +-------+-----+------+------+------+------+--...-+------+

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

   The DHCP relay agent truncates the RADIUS attributes to fit in the
   RADIUS Attributes suboption.

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 suboption, along with any other suboptions it is
   configured to supply.  The RADIUS Attributes suboption MUST only
   contain the attributes provided in the RADIUS Access/Accept message.
   The DHCP relay agent MUST NOT add more than one RADIUS Attributes
   suboption in a message.

   The relay agent MUST include the User-Name and Framed-Pool attributes
   in the RADIUS Attributes suboption, if they are available, and MAY
   include other attributes.

   To avoid dependencies between the address allocation and other state
   information between the RADIUS server and the DHCP server, the DHCP
   relay agent SHOULD include only the attributes in the table below in
   an instance of the RADIUS Attributes suboption.  The table, based on
   the analysis in RFC 3580 [8], lists attributes that MAY be included:

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           #   Attribute
         ---   ---------
           1   User-Name (RFC 2865 [3])
           6   Service-Type (RFC 2865)
          26   Vendor-Specific (RFC 2865)
          27   Session-Timeout (RFC 2865)
          88   Framed-Pool (RFC 2869)
         100   Framed-IPv6-Pool (RFC 3162 [7])

5.  DHCP Server Behavior

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

6.  DHCP Client Behavior

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

7.  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 [1].

   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].  Although 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 [9] or IPsec [10]
   SHOULD be deployed as well.

8.  IANA Considerations

   IANA has assigned the value of 7 for the DHCP Relay Agent Information
   option suboption code for this suboption.  This document does not
   define any new namespaces or other constants for which IANA must
   maintain a registry.

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9.  Acknowledgements

   Expert advice from Bernard Aboba, Paul Funk, David Nelson, Ashwin
   Palekar, and Greg Weber on avoiding RADIUS entanglements is
   gratefully acknowledged.

10.  References

10.1.  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
        2000.

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

10.2.  Informative References

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

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

   [8]  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.

   [9]  Stapp, M. and T. Lemon, "The Authentication Suboption for the
        DHCP Relay Agent Option", Work in Progress, October 2003.

   [10] Droms, R., "Authentication of DHCP Relay Agent Options Using
        IPsec", Work in Progress, September 2003.

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Authors' Addresses

   Ralph Droms
   Cisco Systems
   1414 Massachusetts Avenue
   Boxborough, MA  01719
   USA

   EMail: rdroms@cisco.com

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

   EMail: jschnizl@cisco.com

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RFC 4014              RADIUS Attributes Suboption          February 2005

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