Network Working Group R. Droms
Internet-Draft J. Schnizlein
Expires: August 15, 2002 Cisco Systems
February 14, 2002
RADIUS Attributes Sub-option for the DHCP Relay Agent Information
Option
draft-ietf-dhc-agentopt-radius-00.txt
Status of this Memo
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Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
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 allows 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. IEEE
802.1X [2] is an example of a mechanism through which a 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. 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 [3],
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 |
+-----------------+
Figure 1
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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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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 or attribute-value pair (AVP): Data value carried in a
RADIUS protocol message
2.4 802.1X Terminology
The following terms are used as defined in the IEEE 802.1X protocol:
Authenticator, Supplicant.
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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
code
+-------+-----+------+------+------+------+--...-+------+
| TBD | N | b1 | b2 | b3 | b4 | | bN |
+-------+-----+------+------+------+------+--...-+------+
The RADIUS attributes are carried as attribute-value pairs (AVPs),
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 recieved 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, Calling-Station-ID 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
The RADIUS server MUST return the User-Name, Calling-Station-ID and
Class attributes to the access device, and MAY return other
attributes.
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8. Security Considerations
DHCP as currently defined provides no authentication or security
mechanisms. 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. Because the RADIUS attributes are not encrypted or protected
against modification in any way, the contents can be spoofed or
modifed by hostile devices in an unsecured network.
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.
References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Institute of Electrical and Electronics Engineers, "Port based
Network Access Control", IEEE Standard 802.1X, March 2001.
[3] Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote
Authentication Dial In User Service (RADIUS)", RFC 2865, June
2000.
[4] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[5] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046,
January 2001.
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Authors' Addresses
Ralph Droms
Cisco Systems
250 Apollo Drive
Chelmsford, MA 01824
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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