Network Working Group R. Droms
Internet-Draft J. Schnizlein
Expires: August 4, 2004 Cisco Systems
February 4, 2004
RADIUS Attributes Sub-option for the DHCP Relay Agent Information
Option
draft-ietf-dhc-agentopt-radius-04.txt
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Copyright (C) The Internet Society (2004). 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 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",
"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: 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
code
+-------+-----+------+------+------+------+--...-+------+
| 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. RADIUS Server Behavior
The RADIUS server MUST return the User-Name and Class attributes to
the access device, and MAY return other attributes.
To avoid dependencies between the address allocation and other state
information between the RADIUS server and the DHCP server, only the
attributes in the table below SHOULD be included in this sub-option.
Because RADIUS servers rely on the directive in section 1.1 or RFC
2865 that "A NAS MUST treat a RADIUS access-accept authorizing an
unavailable service as an access-reject instead.", a RADIUS server
SHOULD send only those attributes for which the relay agent can
ensure that either the relay agent or the DHCP server will provide
the associated service. The following table, based on the analysis
in RFC 3580 [10], lists attributes that MAY be included:
# Attribute
--- ---------
1 User-Name (RFC 2865)
4 NAS-IP-Address (RFC 2865)
6 Service-Type (RFC 2865)
25 Class (RFC 2865)
26 Vendor-Specific (RFC 2865)
27 Session-Timeout (RFC 2865)
30 Called-Station-Id (RFC 2865)
31 Calling-Station-Id (RFC 2865)
32 NAS-Identifier (RFC 2865)
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44 Acct-Session-Id (RFC 2866)
50 Acct-Multi-Session-Id (RFC 2866)
87 NAS-Port-Id (RFC 2869)
88 Framed-Pool (RFC 2869)
100 Framed-IPv6-Pool (RFC 3162)
5. 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.
6. DHCP Server Behavior
When the DHCP server receives a message from a 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. Even if the relay agent forwards other
RADIUS attributes from the RADIUS server, the DHCP server SHOULD
ignore any attributes it receives for which it cannot ensure that the
associated service will be provided either by the DHCP server or the
relay agent.
7. DHCP Client Behavior
Relay agent options are exchanged only between relay agents and DHCP
server, so DHCP clients are never aware of their use.
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 [8]. 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
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option for relay agent options [11] or IPsec [12] SHOULD be deployed
as well.
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.
11. Acknowledgments
Bernard Aboba's expert advice on avoiding RADIUS entanglements is
gratefully acknowledged.
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
2000.
[5] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046,
January 2001.
Informative References
[6] Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.
[7] Rigney, C., Willats, W. and P. Calhoun, "RADIUS Extensions",
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RFC 2869, June 2000.
[8] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
RFC 3118, June 2001.
[9] Aboba, B., Zorn, G. and D. Mitton, "RADIUS and IPv6", RFC 3162,
August 2001.
[10] 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.
[11] 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
2002.
[12] 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
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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