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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).


 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.

Droms & Schnizlein Standards Track [Page 1] 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).

Droms & Schnizlein Standards Track [Page 2] RFC 4014 RADIUS Attributes Suboption February 2005

 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",
 document are to be interpreted as described in RFC 2119 [3].
 Within this specification, the use of the key words "MUST", "MUST
 "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.

Droms & Schnizlein Standards Track [Page 3] RFC 4014 RADIUS Attributes Suboption February 2005

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
     |   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:

Droms & Schnizlein Standards Track [Page 4] RFC 4014 RADIUS Attributes Suboption February 2005

         #   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.

Droms & Schnizlein Standards Track [Page 5] RFC 4014 RADIUS Attributes Suboption February 2005

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
 [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.

Droms & Schnizlein Standards Track [Page 6] RFC 4014 RADIUS Attributes Suboption February 2005

Authors' Addresses

 Ralph Droms
 Cisco Systems
 1414 Massachusetts Avenue
 Boxborough, MA  01719
 John Schnizlein
 Cisco Systems
 9123 Loughran Road
 Fort Washington, MD  20744

Droms & Schnizlein Standards Track [Page 7] RFC 4014 RADIUS Attributes Suboption February 2005

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Droms & Schnizlein Standards Track [Page 8]

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