Network Working Group R. Droms

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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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",
"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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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
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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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.

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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
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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RFC 4014 RADIUS Attributes Suboption February 2005

Authors' Addresses

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

EMail: [email protected]

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

EMail: [email protected]

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RFC 4014 RADIUS Attributes Suboption February 2005

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Acknowledgement

Funding for the RFC Editor function is currently provided by the
Internet Society.

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