Network Working Group R. Droms
Internet-Draft J. Schnizlein
Expires: September 30, 2004 Cisco Systems
April 1, 2004
RADIUS Attributes Sub-option for the DHCP Relay Agent Information
Option
draft-ietf-dhc-agentopt-radius-05.txt
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Copyright (C) The Internet Society (2004). All Rights Reserved.
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
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 in which network elements can pass information
obtained through layer 2 authentication to a DHCP server [2]. IEEE
802.1X 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 [9]. 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 either confirms or denies the
identity of the user of the device requesting network access. 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 |
|(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. 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 [4].
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.
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 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: An entity that provides RADIUS service through the
exchange of RADIUS protocol messages
Attribute: Data value carried in a RADIUS protocol message
NAS: Network access server; unlike 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.
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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 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 [9], 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])
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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. 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. 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 [7]. 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 [10] or IPsec [11] SHOULD be deployed
as well.
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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] 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] Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote
Authentication Dial In User Service (RADIUS)", RFC 2865, June
2000.
[4] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046,
January 2001.
Informative References
[5] Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.
[6] Rigney, C., Willats, W. and P. Calhoun, "RADIUS Extensions",
RFC 2869, June 2000.
[7] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
RFC 3118, June 2001.
[8] Aboba, B., Zorn, G. and D. Mitton, "RADIUS and IPv6", RFC 3162,
August 2001.
[9] 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.
[10] 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.
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[11] 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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