Network Working Group R. Droms
Internet-Draft J. Schnizlein
Expires: January 5, 2005 Cisco Systems
July 7, 2004
RADIUS Attributes Sub-option for the DHCP Relay Agent Information
Option
draft-ietf-dhc-agentopt-radius-07.txt
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Abstract
A NAS (network access server) 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
configuration parameters to be delivered 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
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during RADIUS authentication to a DHCP server.
1. Introduction and Background
The RADIUS Attributes sub-option 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 using RADIUS as the
Authentication Service specified in RFC3580 [10]. 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.
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 |
+-----------------+
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Figure 1
In the application described in this document, the access device acts
as an IEEE 802.1X Authenticator and adds DHCP relay agent options 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 forwards DHCP messages from the
network device, the NAS adds these attributes in a 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 a NAS. 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. That is, the option is not limited to use with IEEE
802.1X, but is constrained by RADIUS semantics (see Section 4).
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].
2.1 DHCP Terminology
The following terms are used as defined in RFC2131 and RFC3046: 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 which is returned. Unlike a traditional
dial NAS, the NAS considered here may not have a protocol like 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 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 | o1 | o2 | o3 | o4 | | oN |
+-------+-----+------+------+------+------+--...-+------+
The RADIUS attributes are encoded according to the encoding rules in
RFC 2865, in octets o1...oN.
The NAS truncates the RADIUS attributes to fit in the RADIUS
Attributes sub-option. For predictable behavior, the RADIUS server
should be configured to return fewer than 255 octets of RADIUS
attributes.
4. RADIUS Server Behavior
The RADIUS server that implements this specification MUST be
configured to return the User-Name and Class attributes to the NAS,
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:
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# Attribute
--- ---------
1 User-Name (RFC 2865 [3])
6 Service-Type (RFC 2865)
25 Class (RFC 2865)
26 Vendor-Specific (RFC 2865)
27 Session-Timeout (RFC 2865)
88 Framed-Pool (RFC 2869)
100 Framed-IPv6-Pool (RFC 3162 [8])
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 only
contain the attributes provided in the RADIUS Access/Accept message.
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 if available, 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
sub-option and uses that information in selecting configuration
parameters for the client. If the relay agent forwards 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.
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 [1].
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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]. 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 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. Acknowledgments
Bernard Aboba's expert advice on avoiding RADIUS entanglements is
gratefully acknowledged.
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.
Informative References
[6] Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.
[7] Rigney, C., Willats, W. and P. Calhoun, "RADIUS Extensions",
RFC 2869, June 2000.
[8] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
RFC 3118, June 2001.
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[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. and T. Lemon, "The Authentication Suboption for the
DHCP Relay Agent Option", draft-ietf-dhc-auth-suboption-02
(work in progress), October 2003.
[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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