Network Working Group R. Droms
Internet-Draft J. Schnizlein
Expires: October 11, 2004 Cisco Systems
April 12, 2004
RADIUS Attributes Sub-option for the DHCP Relay Agent Information
Option
draft-ietf-dhc-agentopt-radius-06.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
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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.
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 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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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 |
|(802.1X and DHCP |
| relay agent} |
+-----------------+
| ^
| |
(2) Request for authentication
| |
| (3) Access-Accept/Reject
v |
+-----------------+
| RADIUS |
| Server |
+-----------------+
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. 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.
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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
2.3 802.1X 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
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in RFC 2865, in octets b1...bN.
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 few 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:
# Attribute
--- ---------
1 User-Name (RFC 2865 [3])
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)
44 Acct-Session-Id (RFC 2866 [5])
50 Acct-Multi-Session-Id (RFC 2866)
87 NAS-Port-Id (RFC 2869 [6])
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
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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.
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 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.
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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.
[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
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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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