💾 Archived View for gmi.noulin.net › rfc › rfc3579.gmi captured on 2021-12-05 at 23:47:19. Gemini links have been rewritten to link to archived content
-=-=-=-=-=-=-
Updates:
Updated by:
Keywords: RADIUS, encryption, NAS, Network, Access, Server
Network Working Group B. Aboba
Request for Comments: 3579 Microsoft
Updates: 2869 P. Calhoun
Category: Informational Airespace
September 2003
RADIUS (Remote Authentication Dial In User Service)
Support For Extensible Authentication Protocol (EAP)
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document defines Remote Authentication Dial In User Service
(RADIUS) support for the Extensible Authentication Protocol (EAP), an
authentication framework which supports multiple authentication
mechanisms. In the proposed scheme, the Network Access Server (NAS)
forwards EAP packets to and from the RADIUS server, encapsulated
within EAP-Message attributes. This has the advantage of allowing
the NAS to support any EAP authentication method, without the need
for method-specific code, which resides on the RADIUS server. While
EAP was originally developed for use with PPP, it is now also in use
with IEEE 802.
This document updates RFC 2869.
Aboba & Calhoun Informational [Page 1]
RFC 3579 RADIUS & EAP September 2003
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Specification of Requirements. . . . . . . . . . . . . . 3
1.2. Terminology. . . . . . . . . . . . . . . . . . . . . . . 3
2. RADIUS Support for EAP . . . . . . . . . . . . . . . . . . . . 4
2.1. Protocol Overview. . . . . . . . . . . . . . . . . . . . 5
2.2. Invalid Packets. . . . . . . . . . . . . . . . . . . . . 9
2.3. Retransmission . . . . . . . . . . . . . . . . . . . . . 10
2.4. Fragmentation. . . . . . . . . . . . . . . . . . . . . . 10
2.5. Alternative uses . . . . . . . . . . . . . . . . . . . . 11
2.6. Usage Guidelines . . . . . . . . . . . . . . . . . . . . 11
3. Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.1. EAP-Message. . . . . . . . . . . . . . . . . . . . . . . 15
3.2. Message-Authenticator. . . . . . . . . . . . . . . . . . 16
3.3. Table of Attributes. . . . . . . . . . . . . . . . . . . 18
4. Security Considerations. . . . . . . . . . . . . . . . . . . . 19
4.1. Security Requirements. . . . . . . . . . . . . . . . . . 19
4.2. Security Protocol. . . . . . . . . . . . . . . . . . . . 20
4.3. Security Issues. . . . . . . . . . . . . . . . . . . . . 22
5. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 30
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
6.1. Normative References . . . . . . . . . . . . . . . . . . 30
6.2. Informative References . . . . . . . . . . . . . . . . . 32
Appendix A - Examples. . . . . . . . . . . . . . . . . . . . . . . 34
Appendix B - Change Log. . . . . . . . . . . . . . . . . . . . . . 43
Intellectual Property Statement. . . . . . . . . . . . . . . . . . 44
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . 44
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 45
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 46
1. Introduction
The Remote Authentication Dial In User Service (RADIUS) is an
authentication, authorization and accounting protocol used to control
network access. RADIUS authentication and authorization is specified
in [RFC2865], and RADIUS accounting is specified in [RFC2866]; RADIUS
over IPv6 is specified in [RFC3162].
The Extensible Authentication Protocol (EAP), defined in [RFC2284],
is an authentication framework which supports multiple authentication
mechanisms. EAP may be used on dedicated links, switched circuits,
and wired as well as wireless links.
To date, EAP has been implemented with hosts and routers that connect
via switched circuits or dial-up lines using PPP [RFC1661]. It has
also been implemented with bridges supporting [IEEE802]. EAP
encapsulation on IEEE 802 wired media is described in [IEEE8021X].
Aboba & Calhoun Informational [Page 2]
RFC 3579 RADIUS & EAP September 2003
RADIUS attributes are comprised of variable length Type-Length-Value
3-tuples. New attribute values can be added without disturbing
existing implementations of the protocol. This specification
describes RADIUS attributes supporting the Extensible Authentication
Protocol (EAP): EAP-Message and Message-Authenticator. These
attributes now have extensive field experience. The purpose of this
document is to provide clarification and resolve interoperability
issues.
As noted in [RFC2865], a Network Access Server (NAS) that does not
implement a given service MUST NOT implement the RADIUS attributes
for that service. This implies that a NAS that is unable to offer
EAP service MUST NOT implement the RADIUS attributes for EAP. A NAS
MUST treat a RADIUS Access-Accept requesting an unavailable service
as an Access-Reject instead.
1.1. Specification of Requirements
In this document, several words are used to signify the requirements
of the specification. These words are often capitalized. 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 [RFC2119].
1.2. Terminology
This document frequently uses the following terms:
authenticator
The end of the link requiring the authentication. Also
known as the Network Access Server (NAS) or RADIUS client.
Within IEEE 802.1X terminology, the term Authenticator is
used.
peer The other end of the point-to-point link (PPP),
point-to-point LAN segment (IEEE 802.1X) or wireless link,
which is being authenticated by the authenticator. In IEEE
802.1X, this end is known as the Supplicant.
authentication server
An authentication server is an entity that provides an
authentication service to an authenticator (NAS). This
service verifies from the credentials provided by the peer,
the claim of identity made by the peer; it also may provide
credentials allowing the peer to verify the identity of the
authentication server. Within this document it is assumed
that the NAS operates as a pass-through, forwarding EAP
packets between the RADIUS server and the EAP peer.
Aboba & Calhoun Informational [Page 3]
RFC 3579 RADIUS & EAP September 2003
Therefore the RADIUS server operates as an authentication
server.
silently discard
This means the implementation discards the packet without
further processing. The implementation SHOULD provide the
capability of logging the error, including the contents of
the silently discarded packet, and SHOULD record the event
in a statistics counter.
displayable message
This is interpreted to be a human readable string of
characters, and MUST NOT affect operation of the protocol.
The message encoding MUST follow the UTF-8 transformation
format [RFC2279].
Network Access Server (NAS)
The device providing access to the network. Also known as
the Authenticator (IEEE 802.1X or EAP terminology) or
RADIUS client.
service The NAS provides a service to the user, such as IEEE 802 or
PPP.
session Each service provided by the NAS to a peer constitutes a
session, with the beginning of the session defined as the
point where service is first provided and the end of the
session defined as the point where service is ended. A
peer may have multiple sessions in parallel or series if
the NAS supports that, with each session generating a
separate start and stop accounting record.
2. RADIUS Support for EAP
The Extensible Authentication Protocol (EAP), described in [RFC2284],
provides a standard mechanism for support of additional
authentication methods without the NAS to be upgraded to support each
new method. Through the use of EAP, support for a number of
authentication schemes may be added, including smart cards, Kerberos
[RFC1510], Public Key [RFC2716], One Time Passwords [RFC2284], and
others.
One of the advantages of the EAP architecture is its flexibility.
EAP is used to select a specific authentication mechanism. Rather
than requiring the NAS to be updated to support each new
authentication method, EAP permits the use of an authentication
server implementing authentication methods, with the NAS acting as a
pass-through for some or all methods and peers.
Aboba & Calhoun Informational [Page 4]
RFC 3579 RADIUS & EAP September 2003
A NAS MAY authenticate local peers while at the same time acting as a
pass-through for non-local peers and authentication methods it does
not implement locally. A NAS implementing this specification is not
required to use RADIUS to authenticate every peer. However, once the
NAS begins acting as a pass-through for a particular session, it can
no longer perform local authentication for that session.
In order to support EAP within RADIUS, two new attributes,
EAP-Message and Message-Authenticator, are introduced in this
document. This section describes how these new attributes may be
used for providing EAP support within RADIUS.
2.1. Protocol Overview
In RADIUS/EAP, RADIUS is used to shuttle RADIUS-encapsulated EAP
Packets between the NAS and an authentication server.
The authenticating peer and the NAS begin the EAP conversation by
negotiating use of EAP. Once EAP has been negotiated, the NAS SHOULD
send an initial EAP-Request message to the authenticating peer. This
will typically be an EAP-Request/Identity, although it could be an
EAP-Request for an authentication method (Types 4 and greater). A
NAS MAY be configured to initiate with a default authentication
method. This is useful in cases where the identity is determined by
another means (such as Called-Station-Id, Calling-Station-Id and/or
Originating-Line-Info); where a single authentication method is
required, which includes its own identity exchange; where identity
hiding is desired, so that the identity is not requested until after
a protected channel has been set up.
The peer replies with an EAP-Response. The NAS MAY determine from
the Response that it should proceed with local authentication.
Alternatively, the NAS MAY act as a pass-through, encapsulating the
EAP-Response within EAP-Message attribute(s) sent to the RADIUS
server within a RADIUS Access-Request packet. If the NAS sends an
EAP-Request/Identity message as the initial packet, the peer responds
with an EAP-Response/Identity. The NAS may determine that the peer
is local and proceed with local authentication. If no match is found
against the list of local users, the NAS encapsulates the
EAP-Response/Identity message within an EAP-Message attribute,
enclosed within an Access-Request packet.
On receiving a valid Access-Request packet containing EAP-Message
attribute(s), a RADIUS server compliant with this specification and
wishing to authenticate with EAP MUST respond with an
Access-Challenge packet containing EAP-Message attribute(s). If the
RADIUS server does not support EAP or does not wish to authenticate
with EAP, it MUST respond with an Access-Reject.
Aboba & Calhoun Informational [Page 5]
RFC 3579 RADIUS & EAP September 2003
EAP-Message attribute(s) encapsulate a single EAP packet which the
NAS decapsulates and passes on to the authenticating peer. The peer
then responds with an EAP-Response packet, which the NAS encapsulates
within an Access-Request containing EAP-Message attribute(s). EAP is
a 'lock step' protocol, so that other than the initial Request, a new
Request cannot be sent prior to receiving a valid Response.
The conversation continues until either a RADIUS Access-Reject or
Access-Accept packet is received from the RADIUS server. Reception
of a RADIUS Access-Reject packet MUST result in the NAS denying
access to the authenticating peer. A RADIUS Access-Accept packet
successfully ends the authentication phase. The NAS MUST NOT
"manufacture" a Success or Failure packet as the result of a timeout.
After a suitable number of timeouts have elapsed, the NAS SHOULD
instead end the EAP conversation.
Using RADIUS, the NAS can act as a pass-through for an EAP
conversation between the peer and authentication server, without
needing to implement the EAP method used between them. Where the NAS
initiates the conversation by sending an EAP-Request for an
authentication method, it may not be required that the NAS fully
implement the EAP method reflected in the initial EAP-Request.
Depending on the initial method, it may be sufficient for the NAS to
be configured with the initial packet to be sent to the peer, and for
the NAS to act as a pass-through for subsequent messages. Note that
since the NAS only encapsulates the EAP-Response in its initial
Access-Request, the initial EAP-Request within the authentication
method is not available to the RADIUS server. For the RADIUS server
to be able to continue the conversation, either the initial
EAP-Request is vestigial, so that the RADIUS server need not be aware
of it, or the relevant information from the initial EAP-Request (such
as a nonce) is reflected in the initial EAP-Response, so that the
RADIUS server can obtain it without having received the initial
EAP-Request.
Where the initial EAP-Request sent by the NAS is for an
authentication Type (4 or greater), the peer MAY respond with a Nak
indicating that it would prefer another authentication method that is
not implemented locally. In this case, the NAS SHOULD send
Access-Request encapsulating the received EAP-Response/Nak. This
provides the RADIUS server with a hint about the authentication
method(s) preferred by the peer, although it does not provide
information on the Type of the original Request. It also provides
the server with the Identifier used in the initial EAP-Request, so
that Identifier conflicts can be avoided.
Aboba & Calhoun Informational [Page 6]
RFC 3579 RADIUS & EAP September 2003
In order to evaluate whether the alternatives preferred by the
authenticating peer are allowed, the RADIUS server will typically
respond with an Access-Challenge containing EAP-Message attribute(s)
encapsulating an EAP-Request/Identity (Type 1). This allows the
RADIUS server to determine the peer identity, so as to be able to
retrieve the associated authentication policy. Alternatively, an
EAP-Request for an authentication method (Type 4 or greater) could be
sent. Since the RADIUS server may not be aware of the Type of the
initial EAP-Request, it is possible for the RADIUS server to choose
an unacceptable method, and for the peer to respond with another Nak.
In order to permit non-EAP aware RADIUS proxies to forward the
Access-Request packet, if the NAS initially sends an
EAP-Request/Identity message to the peer, the NAS MUST copy the
contents of the Type-Data field of the EAP-Response/Identity received
from the peer into the User-Name attribute and MUST include the
Type-Data field of the EAP-Response/Identity in the User-Name
attribute in every subsequent Access-Request. Since RADIUS proxies
are assumed to act as a pass-through, they cannot be expected to
parse an EAP-Response/Identity encapsulated within EAP-Message
attribute(s). If the NAS initially sends an EAP-Request for an
authentication method, and the peer identity cannot be determined
from the EAP-Response, then the User-Name attribute SHOULD be
determined by another means. As noted in [RFC2865] Section 5.6, it
is recommended that Access-Requests use the value of the
Calling-Station-Id as the value of the User-Name attribute.
Having the NAS send the initial EAP-Request packet has a number of
advantages:
[1] It saves a round trip between the NAS and RADIUS server.
[2] An Access-Request is only sent to the RADIUS server if the
authenticating peer sends an EAP-Response, confirming that it
supports EAP. In situations where peers may be EAP unaware,
initiating a RADIUS Access-Request on a "carrier sense" or
"media up" indication may result in many authentication
exchanges that cannot complete successfully. For example, on
wired networks [IEEE8021X] Supplicants typically do not initiate
the 802.1X conversation with an EAPOL-Start. Therefore an IEEE
802.1X-enabled bridge may not be able to determine whether the
peer supports EAP until it receives a Response to the initial
EAP-Request.
[3] It allows some peers to be authenticated locally.
Aboba & Calhoun Informational [Page 7]
RFC 3579 RADIUS & EAP September 2003
Although having the NAS send the initial EAP-Request packet has
substantial advantages, this technique cannot be universally
employed. There are circumstances in which the peer identity is
already known (such as when authentication and accounting is handled
based on Called-Station-Id, Calling-Station-Id and/or
Originating-Line-Info), but where the appropriate EAP method may vary
based on that identity.
Rather than sending an initial EAP-Request packet to the
authenticating peer, on detecting the presence of the peer, the NAS
MAY send an Access-Request packet to the RADIUS server containing an
EAP-Message attribute signifying EAP-Start. The RADIUS server will
typically respond with an Access-Challenge containing EAP-Message
attribute(s) encapsulating an EAP-Request/Identity (Type 1).
However, an EAP-Request for an authentication method (Type 4 or
greater) can also be sent by the server.
EAP-Start is indicated by sending an EAP-Message attribute with a
length of 2 (no data). The Calling-Station-Id SHOULD be included in
the User-Name attribute. This may result in a RADIUS Access-Request
being sent by the NAS to the RADIUS server without first confirming
that the peer supports EAP. Since this technique can result in a
large number of uncompleted RADIUS conversations, in situations where
EAP unaware peers are common, or where peer support for EAP cannot be
determined on initial contact (e.g. [IEEE8021X] Supplicants not
initiating the conversation with an EAPOL-Start) it SHOULD NOT be
employed by default.
For proxied RADIUS requests, there are two methods of processing. If
the domain is determined based on the Calling-Station-Id,
Called-Station-Id and/or Originating-Line-Info, the RADIUS server may
proxy the initial RADIUS Access-Request/EAP-Start. If the realm is
determined based on the peer identity, the local RADIUS server MUST
respond with a RADIUS Access-Challenge including an EAP-Message
attribute encapsulating an EAP-Request/Identity packet. The response
from the authenticating peer SHOULD be proxied to the final
authentication server.
If an Access-Request is sent to a RADIUS server which does not
support the EAP-Message attribute, then an Access-Reject MUST be sent
in response. On receiving an Access-Reject, the NAS MUST deny access
to the authenticating peer.
Aboba & Calhoun Informational [Page 8]
RFC 3579 RADIUS & EAP September 2003
2.2. Invalid Packets
While acting as a pass-through, the NAS MUST validate the EAP header
fields (Code, Identifier, Length) prior to forwarding an EAP packet
to or from the RADIUS server. On receiving an EAP packet from the
peer, the NAS checks the Code (2) and Length fields, and matches the
Identifier value against the current Identifier, supplied by the
RADIUS server in the most recently validated EAP-Request. On
receiving an EAP packet from the RADIUS server (encapsulated within
an Access-Challenge), the NAS checks the Code (1) and Length fields,
then updates the current Identifier value. Pending EAP Responses
that do not match the current Identifier value are silently discarded
by the NAS.
Since EAP method fields (Type, Type-Data) are typically not validated
by a NAS operating as a pass-through, despite these checks it is
possible for a NAS to forward an invalid EAP packet to or from the
RADIUS server. A RADIUS server receiving EAP-Message attribute(s) it
does not understand SHOULD make the determination of whether the
error is fatal or non-fatal based on the EAP Type. A RADIUS server
determining that a fatal error has occurred MUST send an
Access-Reject containing an EAP-Message attribute encapsulating
EAP-Failure.
A RADIUS server determining that a non-fatal error has occurred MAY
send an Access-Challenge to the NAS including EAP-Message
attribute(s) as well as an Error-Cause attribute [RFC3576] with value
202 (decimal), "Invalid EAP Packet (Ignored)". The Access-Challenge
SHOULD encapsulate within EAP-Message attribute(s) the most recently
sent EAP-Request packet (including the same Identifier value). On
receiving such an Access-Challenge, a NAS implementing previous
versions of this specification will decapsulate the EAP-Request and
send it to the peer, which will retransmit the EAP-Response.
A NAS compliant with this specification, on receiving an
Access-Challenge with an Error-Cause attribute of value 202 (decimal)
SHOULD discard the EAP-Response packet most recently transmitted to
the RADIUS server and check whether additional EAP-Response packets
have been received matching the current Identifier value. If so, a
new EAP-Response packet, if available, MUST be sent to the RADIUS
server within an Access-Request, and the EAP-Message attribute(s)
included within the Access-Challenge are silently discarded. If no
EAP-Response packet is available, then the EAP-Request encapsulated
within the Access-Challenge is sent to the peer, and the
retransmission timer is reset.
Aboba & Calhoun Informational [Page 9]
RFC 3579 RADIUS & EAP September 2003
In order to provide protection against Denial of Service (DoS)
attacks, it is advisable for the NAS to allocate a finite buffer for
EAP packets received from the peer, and to discard packets according
to an appropriate policy once that buffer has been exceeded. Also,
the RADIUS server is advised to permit only a modest number of
invalid EAP packets within a single session, prior to terminating the
session with an Access-Reject. By default a value of 5 invalid EAP
packets is recommended.
2.3. Retransmission
As noted in [RFC2284], if an EAP packet is lost in transit between
the authenticating peer and the NAS (or vice versa), the NAS will
retransmit.
It may be necessary to adjust retransmission strategies and
authentication timeouts in certain cases. For example, when a token
card is used additional time may be required to allow the user to
find the card and enter the token. Since the NAS will typically not
have knowledge of the required parameters, these need to be provided
by the RADIUS server. This can be accomplished by inclusion of
Session-Timeout attribute within the Access-Challenge packet.
If Session-Timeout is present in an Access-Challenge packet that also
contains an EAP-Message, the value of the Session-Timeout is used to
set the EAP retransmission timer for that EAP Request, and that
Request alone. Once the EAP-Request has been sent, the NAS sets the
retransmission timer, and if it expires without having received an
EAP-Response corresponding to the Request, then the EAP-Request is
retransmitted.
2.4. Fragmentation
Using the EAP-Message attribute, it is possible for the RADIUS server
to encapsulate an EAP packet that is larger than the MTU on the link
between the NAS and the peer. Since it is not possible for the
RADIUS server to use MTU discovery to ascertain the link MTU, the
Framed-MTU attribute may be included in an Access-Request packet
containing an EAP-Message attribute so as to provide the RADIUS
server with this information. A RADIUS server having received a
Framed-MTU attribute in an Access-Request packet MUST NOT send any
subsequent packet in this EAP conversation containing EAP-Message
attributes whose values, when concatenated, exceed the length
specified by the Framed-MTU value, taking the link type (specified by
the NAS-Port-Type attribute) into account. For example, as noted in
[RFC3580] Section 3.10, for a NAS-Port-Type value of IEEE 802.11, the
Aboba & Calhoun Informational [Page 10]
RFC 3579 RADIUS & EAP September 2003
RADIUS server may send an EAP packet as large as Framed-MTU minus
four (4) octets, taking into account the additional overhead for the
IEEE 802.1X Version (1), Type (1) and Body Length (2) fields.
2.5. Alternative Uses
Currently the conversation between security servers and the RADIUS
server is often proprietary because of lack of standardization. In
order to increase standardization and provide interoperability
between RADIUS vendors and security vendors, it is recommended that
RADIUS- encapsulated EAP be used for this conversation.
This has the advantage of allowing the RADIUS server to support EAP
without the need for authentication-specific code within the RADIUS
server. Authentication-specific code can then reside on a security
server instead.
In the case where RADIUS-encapsulated EAP is used in a conversation
between a RADIUS server and a security server, the security server
will typically return an Access-Accept message without inclusion of
the expected attributes currently returned in an Access-Accept. This
means that the RADIUS server MUST add these attributes prior to
sending an Access-Accept message to the NAS.
2.6. Usage Guidelines
2.6.1. Identifier Space
In EAP, each session has its own unique Identifier space. RADIUS
server implementations MUST be able to distinguish between EAP
packets with the same Identifier existing within distinct sessions,
originating on the same NAS. For this purpose, sessions can be
distinguished based on NAS and session identification attributes.
NAS identification attributes include NAS-Identifier,
NAS-IPv6-Address and NAS-IPv4-Address. Session identification
attributes include User-Name, NAS-Port, NAS-Port-Type, NAS-Port-Id,
Called-Station-Id, Calling-Station-Id and Originating-Line-Info.
2.6.2. Role Reversal
Since EAP is a peer-to-peer protocol, an independent and simultaneous
authentication may take place in the reverse direction. Both peers
may act as authenticators and authenticatees at the same time.
However, role reversal is not supported by this specification. A
RADIUS server MUST respond to an Access-Request encapsulating an
EAP-Request with an Access-Reject. In order to avoid retransmissions
Aboba & Calhoun Informational [Page 11]
RFC 3579 RADIUS & EAP September 2003
by the peer, the Access-Reject SHOULD include an EAP-Response/Nak
packet indicating no preferred method, encapsulated within
EAP-Message attribute(s).
2.6.3. Conflicting Messages
The NAS MUST make its access control decision based solely on the
RADIUS Packet Type (Access-Accept/Access-Reject). The access control
decision MUST NOT be based on the contents of the EAP packet
encapsulated in one or more EAP-Message attributes, if present.
Access-Accept packets SHOULD have only one EAP-Message attribute in
them, containing EAP Success; similarly, Access-Reject packets SHOULD
have only one EAP-Message attribute in them, containing EAP Failure.
Where the encapsulated EAP packet does not match the result implied
by the RADIUS Packet Type, the combination is likely to cause
confusion, because the NAS and peer will arrive at different
conclusions as to the outcome of the authentication.
For example, if the NAS receives an Access-Reject with an
encapsulated EAP Success, it will not grant access to the peer.
However, on receiving the EAP Success, the peer will be lead to
believe that it authenticated successfully.
If the NAS receives an Access-Accept with an encapsulated EAP
Failure, it will grant access to the peer. However, on receiving an
EAP Failure, the peer will be lead to believe that it failed
authentication. If no EAP-Message attribute is included within an
Access-Accept or Access-Reject, then the peer may not be informed as
to the outcome of the authentication, while the NAS will take action
to allow or deny access.
As described in [RFC2284], the EAP Success and Failure packets are
not acknowledged, and these packets terminate the EAP conversation.
As a result, if these packets are encapsulated within an
Access-Challenge, no response will be received, and therefore the NAS
will send no further Access-Requests to the RADIUS server for the
session. As a result, the RADIUS server will not indicate to the NAS
whether to allow or deny access, while the peer will be informed as
to the outcome of the authentication.
Aboba & Calhoun Informational [Page 12]
RFC 3579 RADIUS & EAP September 2003
To avoid these conflicts, the following combinations SHOULD NOT be
sent by a RADIUS server:
Access-Accept/EAP-Message/EAP Failure
Access-Accept/no EAP-Message attribute
Access-Accept/EAP-Start
Access-Reject/EAP-Message/EAP Success
Access-Reject/no EAP-Message attribute
Access-Reject/EAP-Start
Access-Challenge/EAP-Message/EAP Success
Access-Challenge/EAP-Message/EAP Failure
Access-Challenge/no EAP-Message attribute
Access-Challenge/EAP-Start
Since the responsibility for avoiding conflicts lies with the RADIUS
server, the NAS MUST NOT "manufacture" EAP packets in order to
correct contradictory messages that it receives. This behavior,
originally mandated within [IEEE8021X], will be deprecated in the
future.
2.6.4. Priority
A RADIUS Access-Accept or Access-Reject packet may contain EAP-
Message attribute(s). In order to ensure the correct processing of
RADIUS packets, the NAS MUST first process the attributes, including
the EAP-Message attribute(s), prior to processing the Accept/Reject
indication.
2.6.5. Displayable Messages
The Reply-Message attribute, defined in [RFC2865], Section 5.18,
indicates text which may be displayed to the peer. This is similar
in concept to EAP Notification, defined in [RFC2284]. When sending a
displayable message to a NAS during an EAP conversation, the RADIUS
server MUST encapsulate displayable messages within
EAP-Message/EAP-Request/Notification attribute(s). Reply-Message
attribute(s) MUST NOT be included in any RADIUS message containing an
EAP-Message attribute. An EAP-Message/EAP-Request/Notification
SHOULD NOT be included within an Access-Accept or Access-Reject
packet.
In some existing implementations, a NAS receiving Reply-Message
attribute(s) copies the Text field(s) into the Type-Data field of an
EAP-Request/Notification packet, fills in the Identifier field, and
sends this to the peer. However, several issues arise from this:
Aboba & Calhoun Informational [Page 13]
RFC 3579 RADIUS & EAP September 2003
[1] Unexpected Responses. On receiving an EAP-Request/Notification,
the peer will send an EAP-Response/Notification, and the NAS
will pass this on to the RADIUS server, encapsulated within
EAP-Message attribute(s). However, the RADIUS server may not be
expecting an Access-Request containing an
EAP-Message/EAP-Response/Notification attribute.
For example, consider what happens when a Reply-Message is
included within an Access-Accept or Access-Reject packet with no
EAP-Message attribute(s) present. If the value of the
Reply-Message attribute is copied into the Type-Data of an
EAP-Request/Notification and sent to the peer, this will result
in an Access-Request containing an
EAP-Message/EAP-Response/Notification attribute being sent by
the NAS to the RADIUS server. Since an Access-Accept or
Access-Reject packet terminates the RADIUS conversation, such an
Access-Request would not be expected, and could be interpreted
as the start of another conversation.
[2] Identifier conflicts. While the EAP-Request/Notification is an
EAP packet containing an Identifier field, the Reply-Message
attribute does not contain an Identifier field. As a result, a
NAS receiving a Reply-Message attribute and wishing to translate
this to an EAP-Request/Notification will need to choose an
Identifier value. It is possible that the chosen Identifier
value will conflict with a value chosen by the RADIUS server for
another packet within the EAP conversation, potentially causing
confusion between a new packet and a retransmission.
To avoid these problems, a NAS receiving a Reply-Message attribute
from the RADIUS server SHOULD silently discard the attribute, rather
than attempting to translate it to an EAP Notification Request.
3. Attributes
The NAS-Port or NAS-Port-Id attributes SHOULD be included by the NAS
in Access-Request packets, and either NAS-Identifier, NAS-IP-Address
or NAS-IPv6-Address attributes MUST be included. In order to permit
forwarding of the Access-Reply by EAP-unaware proxies, if a User-Name
attribute was included in an Access-Request, the RADIUS server MUST
include the User-Name attribute in subsequent Access-Accept packets.
Without the User-Name attribute, accounting and billing becomes
difficult to manage. The User-Name attribute within the Access-
Accept packet need not be the same as the User-Name attribute in the
Access-Request.
Aboba & Calhoun Informational [Page 14]
RFC 3579 RADIUS & EAP September 2003
3.1. EAP-Message
Description
This attribute encapsulates EAP [RFC2284] packets so as to allow
the NAS to authenticate peers via EAP without having to understand
the EAP method it is passing through.
The NAS places EAP messages received from the authenticating peer
into one or more EAP-Message attributes and forwards them to the
RADIUS server within an Access-Request message. If multiple
EAP-Message attributes are contained within an Access-Request or
Access-Challenge packet, they MUST be in order and they MUST be
consecutive attributes in the Access-Request or Access-Challenge
packet. The RADIUS server can return EAP-Message attributes in
Access-Challenge, Access-Accept and Access-Reject packets.
When RADIUS is used to enable EAP authentication, Access-Request,
Access-Challenge, Access-Accept, and Access-Reject packets SHOULD
contain one or more EAP-Message attributes. Where more than one
EAP-Message attribute is included, it is assumed that the
attributes are to be concatenated to form a single EAP packet.
Multiple EAP packets MUST NOT be encoded within EAP-Message
attributes contained within a single Access-Challenge,
Access-Accept, Access-Reject or Access-Request packet.
It is expected that EAP will be used to implement a variety of
authentication methods, including methods involving strong
cryptography. In order to prevent attackers from subverting EAP
by attacking RADIUS/EAP, (for example, by modifying EAP Success or
EAP Failure packets) it is necessary that RADIUS provide
per-packet authentication and integrity protection.
Therefore the Message-Authenticator attribute MUST be used to
protect all Access-Request, Access-Challenge, Access-Accept, and
Access-Reject packets containing an EAP-Message attribute.
Access-Request packets including EAP-Message attribute(s) without
a Message-Authenticator attribute SHOULD be silently discarded by
the RADIUS server. A RADIUS server supporting the EAP-Message
attribute MUST calculate the correct value of the
Message-Authenticator and MUST silently discard the packet if it
does not match the value sent. A RADIUS server not supporting the
EAP-Message attribute MUST return an Access-Reject if it receives
an Access-Request containing an EAP-Message attribute.
Aboba & Calhoun Informational [Page 15]
RFC 3579 RADIUS & EAP September 2003
Access-Challenge, Access-Accept, or Access-Reject packets
including EAP-Message attribute(s) without a Message-Authenticator
attribute SHOULD be silently discarded by the NAS. A NAS
supporting the EAP-Message attribute MUST calculate the correct
value of the Message-Authenticator and MUST silently discard the
packet if it does not match the value sent.
A summary of the EAP-Message attribute format is shown below. The
fields are transmitted from left to right.
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | String...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
79 for EAP-Message
Length
>= 3
String
The String field contains an EAP packet, as defined in [RFC2284].
If multiple EAP-Message attributes are present in a packet their
values should be concatenated; this allows EAP packets longer than
253 octets to be transported by RADIUS.
3.2. Message-Authenticator
Description
This attribute MAY be used to authenticate and integrity-protect
Access-Requests in order to prevent spoofing. It MAY be used in
any Access-Request. It MUST be used in any Access-Request,
Access-Accept, Access-Reject or Access-Challenge that includes an
EAP-Message attribute.
A RADIUS server receiving an Access-Request with a
Message-Authenticator attribute present MUST calculate the correct
value of the Message-Authenticator and silently discard the packet
if it does not match the value sent.
Aboba & Calhoun Informational [Page 16]
RFC 3579 RADIUS & EAP September 2003
A RADIUS client receiving an Access-Accept, Access-Reject or
Access-Challenge with a Message-Authenticator attribute present
MUST calculate the correct value of the Message-Authenticator and
silently discard the packet if it does not match the value sent.
This attribute is not required in Access-Requests which include
the User-Password attribute, but is useful for preventing attacks
on other types of authentication. This attribute is intended to
thwart attempts by an attacker to setup a "rogue" NAS, and perform
online dictionary attacks against the RADIUS server. It does not
afford protection against "offline" attacks where the attacker
intercepts packets containing (for example) CHAP challenge and
response, and performs a dictionary attack against those packets
offline.
A summary of the Message-Authenticator attribute format is shown
below. The fields are transmitted from left to right.
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | String...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
80 for Message-Authenticator
Length
18
String
When present in an Access-Request packet, Message-Authenticator is
an HMAC-MD5 [RFC2104] hash of the entire Access-Request packet,
including Type, ID, Length and Authenticator, using the shared
secret as the key, as follows.
Message-Authenticator = HMAC-MD5 (Type, Identifier, Length,
Request Authenticator, Attributes)
When the message integrity check is calculated the signature
string should be considered to be sixteen octets of zero.
Aboba & Calhoun Informational [Page 17]
RFC 3579 RADIUS & EAP September 2003
For Access-Challenge, Access-Accept, and Access-Reject packets,
the Message-Authenticator is calculated as follows, using the
Request-Authenticator from the Access-Request this packet is in
reply to:
Message-Authenticator = HMAC-MD5 (Type, Identifier, Length,
Request Authenticator, Attributes)
When the message integrity check is calculated the signature
string should be considered to be sixteen octets of zero. The
shared secret is used as the key for the HMAC-MD5 message
integrity check. The Message-Authenticator is calculated and
inserted in the packet before the Response Authenticator is
calculated.
3.3. Table of Attributes
The following table provides a guide to which attributes may be found
in packets including EAP-Message attribute(s), and in what quantity.
The EAP-Message and Message-Authenticator attributes specified in
this document MUST NOT be present in an Accounting-Request. If a
table entry is omitted, the values found in [RFC2548], [RFC2865],
[RFC2868], [RFC2869] and [RFC3162] should be assumed.
Request Accept Reject Challenge # Attribute
0-1 0-1 0 0 1 User-Name
0 0 0 0 2 User-Password [Note 1]
0 0 0 0 3 CHAP-Password [Note 1]
0 0 0 0 18 Reply-Message
0 0 0 0 60 CHAP-Challenge
0 0 0 0 70 ARAP-Password [Note 1]
0 0 0 0 75 Password-Retry
1+ 1+ 1+ 1+ 79 EAP-Message [Note 1]
1 1 1 1 80 Message-Authenticator [Note 1]
0-1 0 0 0 94 Originating-Line-Info [Note 3]
0 0 0-1 0-1 101 Error-Cause [Note 2]
Request Accept Reject Challenge # Attribute
[Note 1] An Access-Request that contains either a User-Password or
CHAP-Password or ARAP-Password or one or more EAP-Message attributes
MUST NOT contain more than one type of those four attributes. If it
does not contain any of those four attributes, it SHOULD contain a
Message-Authenticator. If any packet type contains an EAP-Message
attribute it MUST also contain a Message-Authenticator. A RADIUS
server receiving an Access-Request not containing any of those four
attributes and also not containing a Message-Authenticator attribute
SHOULD silently discard it.
Aboba & Calhoun Informational [Page 18]
RFC 3579 RADIUS & EAP September 2003
[Note 2] The Error-Cause attribute is defined in [RFC3576].
[Note 3] The Originating-Line-Info attribute is defined in [NASREQ].
The following table defines the meaning of the above table entries.
0 This attribute MUST NOT be present.
0+ Zero or more instances of this attribute MAY be present.
0-1 Zero or one instance of this attribute MAY be present.
1 Exactly one instance of this attribute MUST be present.
1+ One or more of these attributes MUST be present.
4. Security Considerations
4.1. Security Requirements
RADIUS/EAP is used in order to provide authentication and
authorization for network access. As a result, both the RADIUS and
EAP portions of the conversation are potential targets of an attack.
Threats are discussed in [RFC2607], [RFC2865], and [RFC3162].
Examples include:
[1] An adversary may attempt to acquire confidential data and
identities by snooping RADIUS packets.
[2] An adversary may attempt to modify packets containing RADIUS
messages.
[3] An adversary may attempt to inject packets into a RADIUS
conversation.
[4] An adversary may launch a dictionary attack against the RADIUS
shared secret.
[5] An adversary may launch a known plaintext attack, hoping to
recover the key stream corresponding to a Request Authenticator.
[6] An adversary may attempt to replay a RADIUS exchange.
[7] An adversary may attempt to disrupt the EAP negotiation, in
order to weaken the authentication, or gain access to peer
passwords.
[8] An authenticated NAS may attempt to forge NAS or session
identification attributes,
[9] A rogue (unauthenticated) NAS may attempt to impersonate a
legitimate NAS.
Aboba & Calhoun Informational [Page 19]
RFC 3579 RADIUS & EAP September 2003
[10] An attacker may attempt to act as a man-in-the-middle.
To address these threats, it is necessary to support confidentiality,
data origin authentication, integrity, and replay protection on a
per-packet basis. Bi-directional authentication between the RADIUS
client and server also needs to be provided. There is no requirement
that the identities of RADIUS clients and servers be kept
confidential (e.g., from a passive eavesdropper).
4.2. Security Protocol
To address the security vulnerabilities of RADIUS/EAP,
implementations of this specification SHOULD support IPsec [RFC2401]
along with IKE [RFC2409] for key management. IPsec ESP [RFC2406]
with non-null transform SHOULD be supported, and IPsec ESP with a
non-null encryption transform and authentication support SHOULD be
used to provide per-packet confidentiality, authentication, integrity
and replay protection. IKE SHOULD be used for key management.
Within RADIUS [RFC2865], a shared secret is used for hiding of
attributes such as User-Password, as well as in computation of the
Response Authenticator. In RADIUS accounting [RFC2866], the shared
secret is used in computation of both the Request Authenticator and
the Response Authenticator.
Since in RADIUS a shared secret is used to provide confidentiality as
well as integrity protection and authentication, only use of IPsec
ESP with a non-null transform can provide security services
sufficient to substitute for RADIUS application-layer security.
Therefore, where IPSEC AH or ESP null is used, it will typically
still be necessary to configure a RADIUS shared secret.
Where RADIUS is run over IPsec ESP with a non-null transform, the
secret shared between the NAS and the RADIUS server MAY NOT be
configured. In this case, a shared secret of zero length MUST be
assumed. However, a RADIUS server that cannot know whether incoming
traffic is IPsec-protected MUST be configured with a non-null RADIUS
shared secret.
When IPsec ESP is used with RADIUS, per-packet authentication,
integrity and replay protection MUST be used. 3DES-CBC MUST be
supported as an encryption transform and AES-CBC SHOULD be supported.
AES-CBC SHOULD be offered as a preferred encryption transform if
supported. HMAC-SHA1-96 MUST be supported as an authentication
transform. DES-CBC SHOULD NOT be used as the encryption transform.
Aboba & Calhoun Informational [Page 20]
RFC 3579 RADIUS & EAP September 2003
A typical IPsec policy for an IPsec-capable RADIUS client is
"Initiate IPsec, from me to any destination port UDP 1812". This
causes an IPsec SA to be set up by the RADIUS client prior to sending
RADIUS traffic. If some RADIUS servers contacted by the client do
not support IPsec, then a more granular policy will be required:
"Initiate IPsec, from me to IPsec-Capable-RADIUS-Server, destination
port UDP 1812".
For an IPsec-capable RADIUS server, a typical IPsec policy is "Accept
IPsec, from any to me, destination port 1812". This causes the
RADIUS server to accept (but not require) use of IPsec. It may not
be appropriate to require IPsec for all RADIUS clients connecting to
an IPsec-enabled RADIUS server, since some RADIUS clients may not
support IPsec.
Where IPsec is used for security, and no RADIUS shared secret is
configured, it is important that the RADIUS client and server perform
an authorization check. Before enabling a host to act as a RADIUS
client, the RADIUS server SHOULD check whether the host is authorized
to provide network access. Similarly, before enabling a host to act
as a RADIUS server, the RADIUS client SHOULD check whether the host
is authorized for that role.
RADIUS servers can be configured with the IP addresses (for IKE
Aggressive Mode with pre-shared keys) or FQDNs (for certificate
authentication) of RADIUS clients. Alternatively, if a separate
Certification Authority (CA) exists for RADIUS clients, then the
RADIUS server can configure this CA as a trust anchor [RFC3280] for
use with IPsec.
Similarly, RADIUS clients can be configured with the IP addresses
(for IKE Aggressive Mode with pre-shared keys) or FQDNs (for
certificate authentication) of RADIUS servers. Alternatively, if a
separate CA exists for RADIUS servers, then the RADIUS client can
configure this CA as a trust anchor for use with IPsec.
Since unlike SSL/TLS, IKE does not permit certificate policies to be
set on a per-port basis, certificate policies need to apply to all
uses of IPsec on RADIUS clients and servers. In IPsec deployments
supporting only certificate authentication, a management station
initiating an IPsec-protected telnet session to the RADIUS server
would need to obtain a certificate chaining to the RADIUS client CA.
Issuing such a certificate might not be appropriate if the management
station was not authorized as a RADIUS client.
Where RADIUS clients may obtain their IP address dynamically (such as
an Access Point supporting DHCP), IKE Main Mode with pre-shared keys
[RFC2409] SHOULD NOT be used, since this requires use of a group
Aboba & Calhoun Informational [Page 21]
RFC 3579 RADIUS & EAP September 2003
pre-shared key; instead, Aggressive Mode SHOULD be used. IKEv2, a
work in progress, may address this issue in the future. Where RADIUS
client addresses are statically assigned, either Aggressive Mode or
Main Mode MAY be used. With certificate authentication, Main Mode
SHOULD be used.
Care needs to be taken with IKE Phase 1 Identity Payload selection in
order to enable mapping of identities to pre-shared keys even with
Aggressive Mode. Where the ID_IPV4_ADDR or ID_IPV6_ADDR Identity
Payloads are used and addresses are dynamically assigned, mapping of
identities to keys is not possible, so that group pre-shared keys are
still a practical necessity. As a result, the ID_FQDN identity
payload SHOULD be employed in situations where Aggressive mode is
utilized along with pre-shared keys and IP addresses are dynamically
assigned. This approach also has other advantages, since it allows
the RADIUS server and client to configure themselves based on the
fully qualified domain name of their peers.
Note that with IPsec, security services are negotiated at the
granularity of an IPsec SA, so that RADIUS exchanges requiring a set
of security services different from those negotiated with existing
IPsec SAs will need to negotiate a new IPsec SA. Separate IPsec SAs
are also advisable where quality of service considerations dictate
different handling RADIUS conversations. Attempting to apply
different quality of service to connections handled by the same IPsec
SA can result in reordering, and falling outside the replay window.
For a discussion of the issues, see [RFC2983].
4.3. Security Issues
This section provides more detail on the vulnerabilities identified
in Section 4.1., and how they may be mitigated. Vulnerabilities
include:
Privacy issues
Spoofing and hijacking
Dictionary attacks
Known plaintext attacks
Replay attacks
Negotiation attacks
Impersonation
Man in the middle attacks
Separation of authenticator and authentication server
Multiple databases
Aboba & Calhoun Informational [Page 22]
RFC 3579 RADIUS & EAP September 2003
4.3.1. Privacy Issues
Since RADIUS messages may contain the User-Name attribute as well as
NAS-IP-Address or NAS-Identifier attributes, an attacker snooping on
RADIUS traffic may be able to determine the geographic location of
peers in real time. In wireless networks, it is often assumed that
RADIUS traffic is physically secure, since it typically travels over
the wired network and that this limits the release of location
information.
However, it is possible for an authenticated attacker to spoof ARP
packets [RFC826] so as to cause diversion of RADIUS traffic onto the
wireless network. In this way an attacker may obtain RADIUS packets
from which it can glean peer location information, or which it can
subject to a known plaintext or offline dictionary attack. To
address these vulnerabilities, implementations of this specification
SHOULD use IPsec ESP with non-null transform and per-packet
encryption, authentication, integrity and replay protection to
protect both RADIUS authentication [RFC2865] and accounting [RFC2866]
traffic, as described in Section 4.2.
4.3.2. Spoofing and Hijacking
Access-Request packets with a User-Password attribute establish the
identity of both the user and the NAS sending the Access-Request,
because of the way the shared secret between the NAS and RADIUS
server is used. Access-Request packets with CHAP-Password or
EAP-Message attributes do not have a User-Password attribute. As a
result, the Message-Authenticator attribute SHOULD be used in
Access-Request packets that do not have a User-Password attribute, in
order to establish the identity of the NAS sending the request.
An attacker may attempt to inject packets into the conversation
between the NAS and the RADIUS server, or between the RADIUS server
and the security server. RADIUS [RFC2865] does not support
encryption other than attribute hiding. As described in [RFC2865],
only Access-Reply and Access-Challenge packets are integrity
protected. Moreover, the per-packet authentication and integrity
protection mechanism described in [RFC2865] has known weaknesses
[MD5Attack], making it a tempting target for attackers looking to
subvert RADIUS/EAP.
To provide stronger security, the Message-Authenticator attribute
MUST be used in all RADIUS packets containing an EAP-Message
attribute. Implementations of this specification SHOULD use IPsec
ESP with non-null transform and per-packet encryption,
authentication, integrity and replay protection, as described in
Section 4.2.
Aboba & Calhoun Informational [Page 23]
RFC 3579 RADIUS & EAP September 2003
4.3.3. Dictionary Attacks
The RADIUS shared secret is vulnerable to offline dictionary attack,
based on capture of the Response Authenticator or
Message-Authenticator attribute. In order to decrease the level of
vulnerability, [RFC2865] recommends:
The secret (password shared between the client and the RADIUS
server) SHOULD be at least as large and unguessable as a
well-chosen password. It is preferred that the secret be at least
16 octets.
The risk of an offline dictionary attack can be further reduced by
employing IPsec ESP with non-null transform in order to encrypt the
RADIUS conversation, as described in Section 4.2.
4.3.4. Known Plaintext Attacks
Since EAP [RFC2284] does not support PAP, the RADIUS User-Password
attribute is not used to carry hidden user passwords within
RADIUS/EAP conversations. The User-Password hiding mechanism,
defined in [RFC2865] utilizes MD5, defined in [RFC1321], in order to
generate a key stream based on the RADIUS shared secret and the
Request Authenticator. Where PAP is in use, it is possible to
collect key streams corresponding to a given Request Authenticator
value, by capturing RADIUS conversations corresponding to a PAP
authentication attempt, using a known password. Since the
User-Password is known, the key stream corresponding to a given
Request Authenticator can be determined and stored.
Since the key stream may have been determined previously from a known
plaintext attack, if the Request Authenticator repeats, attributes
encrypted using the RADIUS attribute hiding mechanism should be
considered compromised. In addition to the User-Password attribute,
which is not used with EAP, this includes attributes such as
Tunnel-Password [RFC2868, section 3.5] and MS-MPPE-Send-Key and
MS-MPPE-Recv-Key attributes [RFC2548, section 2.4], which include a
Salt field as part of the hiding algorithm.
To avoid this, [RFC2865], Section 3 advises:
Since it is expected that the same secret MAY be used to
authenticate with servers in disparate geographic regions, the
Request Authenticator field SHOULD exhibit global and temporal
uniqueness.
Aboba & Calhoun Informational [Page 24]
RFC 3579 RADIUS & EAP September 2003
Where the Request Authenticator repeats, the Salt field defined in
[RFC2548], Section 2.4 does not provide protection against
compromise. This is because MD5 [RFC1321], rather than HMAC-MD5
[RFC2104], is used to generate the key stream, which is calculated
from the 128-bit RADIUS shared secret (S), the 128-bit Request
Authenticator (R), and the Salt field (A), using the formula b(1) =
MD5(S + R + A). Since the Salt field is placed at the end, if the
Request Authenticator were to repeat on a network where PAP is in
use, then the salted keystream could be calculated from the
User-Password keystream by continuing the MD5 calculation based on
the Salt field (A), which is sent in the clear.
Even though EAP does not support PAP authentication, a security
vulnerability can still exist where the same RADIUS shared secret is
used for hiding User-Password as well as other attributes. This can
occur, for example, if the same RADIUS proxy handles authentication
requests for both EAP and PAP.
The threat can be mitigated by protecting RADIUS with IPsec ESP with
non-null transform, as described in Section 4.2. Where RADIUS shared
secrets are configured, the RADIUS shared secret used by a NAS
supporting EAP MUST NOT be reused by a NAS utilizing the
User-Password attribute, since improper shared secret hygiene could
lead to compromise of hidden attributes.
4.3.5. Replay Attacks
The RADIUS protocol provides only limited support for replay
protection. RADIUS Access-Requests include liveness via the 128-bit
Request Authenticator. However, the Request Authenticator is not a
replay counter. Since RADIUS servers may not maintain a cache of
previous Request Authenticators, the Request Authenticator does not
provide replay protection.
RADIUS accounting [RFC2866] does not support replay protection at the
protocol level. Due to the need to support failover between RADIUS
accounting servers, protocol-based replay protection is not
sufficient to prevent duplicate accounting records. However, once
accepted by the accounting server, duplicate accounting records can
be detected by use of the the Acct-Session-Id [RFC2866, section 5.5]
and Event-Timestamp [RFC2869, section 5.3] attributes.
Unlike RADIUS authentication, RADIUS accounting does not use the
Request Authenticator as a nonce. Instead, the Request Authenticator
contains an MD5 hash calculated over the Code, Identifier, Length,
and request attributes of the Accounting Request packet, plus the
shared secret. The Response Authenticator also contains an MD5 hash
calculated over the Code, Identifier and Length, the Request
Aboba & Calhoun Informational [Page 25]
RFC 3579 RADIUS & EAP September 2003
Authenticator field from the Accounting-Request packet being replied
to, the response attributes and the shared secret.
Since the Accounting Response Authenticator depends in part on the
Accounting Request Authenticator, it is not possible to replay an
Accounting-Response unless the Request Authenticator repeats. While
it is possible to utilize EAP methods such as EAP TLS [RFC2716] which
include liveness checks on both sides, not all EAP messages will
include liveness so that this provides incomplete protection.
Strong replay protection for RADIUS authentication and accounting can
be provided by enabling IPsec replay protection with RADIUS, as
described in Section 4.2.
4.3.6. Negotiation Attacks
In a negotiation attack a rogue NAS, tunnel server, RADIUS proxy or
RADIUS server attempts to cause the authenticating peer to choose a
less secure authentication method. For example, a session that would
normally be authenticated with EAP would instead be authenticated via
CHAP or PAP; alternatively, a connection that would normally be
authenticated via a more secure EAP method such as EAP-TLS [RFC2716]
might be made to occur via a less secure EAP method, such as
MD5-Challenge. The threat posed by rogue devices, once thought to be
remote, has gained currency given compromises of telephone company
switching systems, such as those described in [Masters].
Protection against negotiation attacks requires the elimination of
downward negotiations. The RADIUS exchange may be further protected
by use of IPsec, as described in Section 4.2. Alternatively, where
IPsec is not used, the vulnerability can be mitigated via
implementation of per-connection policy on the part of the
authenticating peer, and per-peer policy on the part of the RADIUS
server. For the authenticating peer, authentication policy should be
set on a per-connection basis. Per-connection policy allows an
authenticating peer to negotiate a strong EAP method when connecting
to one service, while negotiating a weaker EAP method for another
service.
With per-connection policy, an authenticating peer will only attempt
to negotiate EAP for a session in which EAP support is expected. As
a result, there is a presumption that an authenticating peer
selecting EAP requires that level of security. If it cannot be
provided, it is likely that there is some kind of misconfiguration,
or even that the authenticating peer is contacting the wrong server.
Should the NAS not be able to negotiate EAP, or should the
EAP-Request sent by the NAS be of a different EAP type than what is
expected, the authenticating peer MUST disconnect. An authenticating
Aboba & Calhoun Informational [Page 26]
RFC 3579 RADIUS & EAP September 2003
peer expecting EAP to be negotiated for a session MUST NOT negotiate
a weaker method, such as CHAP or PAP. In wireless networks, the
service advertisement itself may be spoof-able, so that an attacker
could fool the peer into negotiating an authentication method
suitable for a less secure network.
For a NAS, it may not be possible to determine whether a peer is
required to authenticate with EAP until the peer's identity is known.
For example, for shared-uses NASes it is possible for one reseller to
implement EAP while another does not. Alternatively, some peer might
be authenticated locally by the NAS while other peers are
authenticated via RADIUS. In such cases, if any peers of the NAS
MUST do EAP, then the NAS MUST attempt to negotiate EAP for every
session. This avoids forcing a peer to support more than one
authentication type, which could weaken security.
If CHAP is negotiated, the NAS will pass the User-Name and
CHAP-Password attributes to the RADIUS server in an Access-Request
packet. If the peer is not required to use EAP, then the RADIUS
server will respond with an Access-Accept or Access-Reject packet as
appropriate. However, if CHAP has been negotiated but EAP is
required, the RADIUS server MUST respond with an Access-Reject,
rather than an Access-Challenge/EAP-Message/EAP-Request packet. The
authenticating peer MUST refuse to renegotiate authentication, even
if the renegotiation is from CHAP to EAP.
If EAP is negotiated but is not supported by the RADIUS proxy or
server, then the server or proxy MUST respond with an Access-Reject.
In these cases, a PPP NAS MUST send an LCP-Terminate and disconnect
the peer. This is the correct behavior since the authenticating peer
is expecting EAP to be negotiated, and that expectation cannot be
fulfilled. An EAP-capable authenticating peer MUST refuse to
renegotiate the authentication protocol if EAP had initially been
negotiated. Note that problems with a non-EAP capable RADIUS proxy
could prove difficult to diagnose, since a peer connecting from one
location (with an EAP-capable proxy) might be able to successfully
authenticate via EAP, while the same peer connecting at another
location (and encountering an EAP-incapable proxy) might be
consistently disconnected.
4.3.7. Impersonation
[RFC2865] Section 3 states:
A RADIUS server MUST use the source IP address of the RADIUS UDP
packet to decide which shared secret to use, so that RADIUS
requests can be proxied.
Aboba & Calhoun Informational [Page 27]
RFC 3579 RADIUS & EAP September 2003
When RADIUS requests are forwarded by a proxy, the NAS-IP-Address or
NAS-IPv6-Address attributes may not match the source address. Since
the NAS-Identifier attribute need not contain an FQDN, this attribute
also may not correspond to the source address, even indirectly, with
or without a proxy present.
As a result, the authenticity check performed by a RADIUS server or
proxy does not verify the correctness of NAS identification
attributes. This makes it possible for a rogue NAS to forge
NAS-IP-Address, NAS-IPv6-Address or NAS-Identifier attributes within
a RADIUS Access-Request in order to impersonate another NAS. It is
also possible for a rogue NAS to forge session identification
attributes such as Called-Station-Id, Calling-Station-Id, and
Originating-Line-Info.
This could fool the RADIUS server into subsequently sending
Disconnect or CoA-Request messages [RFC3576] containing forged
session identification attributes to a NAS targeted by an attacker.
To address these vulnerabilities RADIUS proxies SHOULD check whether
NAS identification attributes (NAS-IP-Address, NAS-IPv6-Address,
NAS-Identifier) match the source address of packets originating from
the NAS. Where a match is not found, an Access-Reject SHOULD be
sent, and an error SHOULD be logged.
However, such a check may not always be possible. Since the
NAS-Identifier attribute need not correspond to an FQDN, it may not
be resolvable to an IP address to be matched against the source
address. Also, where a NAT exists between the RADIUS client and
proxy, checking the NAS-IP-Address or NAS-IPv6-Address attributes may
not be feasible.
To allow verification of NAS and session identification parameters,
EAP methods can support the secure exchange of these parameters
between the EAP peer and EAP server. NAS identification attributes
include NAS-IP-Address, NAS-IPv6-Address and Called-Station-Id;
session identification attributes include User-Name and
Calling-Station-Id. The secure exchange of these parameters between
the EAP peer and server enables the RADIUS server to check whether
the attributes provided by the NAS match those provided by the peer;
similarly, the peer can check the parameters provided by the NAS
against those provided by the EAP server. This enables detection of
a rogue NAS.
Aboba & Calhoun Informational [Page 28]
RFC 3579 RADIUS & EAP September 2003
4.3.8. Man in the Middle Attacks
RADIUS only provides security on a hop-by-hop basis, even where IPsec
is used. As a result, an attacker gaining control of a RADIUS proxy
could attempt to modify EAP packets in transit. To protect against
this, EAP methods SHOULD incorporate their own per-packet integrity
protection and authentication mechanisms.
4.3.9. Separation of Authenticator and Authentication Server
As noted in [RFC2716], it is possible for the EAP peer and
authenticator to mutually authenticate, and derive a Master Session
Key (MSK) for a ciphersuite used to protect subsequent data traffic.
This does not present an issue on the peer, since the peer and EAP
client reside on the same machine; all that is required is for the
EAP client module to derive and pass a Transient Session Key (TSK) to
the ciphersuite module.
The situation is more complex when EAP is used with RADIUS, since the
authenticator and authentication server may not reside on the same
host.
In the case where the authenticator and authentication server reside
on different machines, there are several implications for security.
First, mutual authentication will occur between the peer and the
authentication server, not between the peer and the authenticator.
This means that it is not possible for the peer to validate the
identity of the NAS or tunnel server that it is speaking to, using
EAP alone.
As described in Section 4.2, when RADIUS/EAP is used to encapsulate
EAP packets, IPsec SHOULD be used to provide per-packet
authentication, integrity, replay protection and confidentiality.
The Message-Authenticator attribute is also required in RADIUS
Access-Requests containing an EAP-Message attribute sent from the NAS
or tunnel server to the RADIUS server. Since the
Message-Authenticator attribute involves an HMAC-MD5 message
integrity check, it is possible for the RADIUS server to verify the
integrity of the Access-Request as well as the NAS or tunnel server's
identity, even where IPsec is not used. Similarly, Access-Challenge
packets containing an EAP-Message attribute sent from the RADIUS
server to the NAS are also authenticated and integrity protected
using an HMAC-MD5 message integrity check, enabling the NAS or tunnel
server to determine the integrity of the packet and verify the
identity of the RADIUS server, even where IPsec is not used.
Moreover, EAP packets sent using methods that contain their own
integrity protection cannot be successfully modified by a rogue NAS
or tunnel server.
Aboba & Calhoun Informational [Page 29]
RFC 3579 RADIUS & EAP September 2003
The second issue that arises where the authenticator and
authentication server reside on separate hosts is that the EAP Master
Session Key (MSK) negotiated between the peer and authentication
server will need to be transmitted to the authenticator. Therefore a
mechanism needs to be provided to transmit the MSK from the
authentication server to the NAS or tunnel server that needs it. The
specification of the key transport and wrapping mechanism is outside
the scope of this document. However, it is expected that the
wrapping mechanism will provide confidentiality, integrity and replay
protection, and data origin authentication.
4.3.10. Multiple Databases
In many cases a security server will be deployed along with a RADIUS
server in order to provide EAP services. Unless the security server
also functions as a RADIUS server, two separate user databases will
exist, each containing information about the security requirements
for the user. This represents a weakness, since security may be
compromised by a successful attack on either of the servers, or their
databases. With multiple user databases, adding a new user may
require multiple operations, increasing the chances for error. The
problems are further magnified in the case where user information is
also being kept in an LDAP server. In this case, three stores of
user information may exist.
In order to address these threats, consolidation of databases is
recommended. This can be achieved by having both the RADIUS server
and security server store information in the same database; by having
the security server provide a full RADIUS implementation; or by
consolidating both the security server and the RADIUS server onto
the same machine.
5. IANA Considerations
This specification does not create any new registries, or define any
new RADIUS attributes or values.
6. References
6.1. Normative References
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC
1321, April 1992.
[RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC:
Keyed-Hashing for Message Authentication", RFC 2104,
February 1997.
Aboba & Calhoun Informational [Page 30]
RFC 3579 RADIUS & EAP September 2003
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, January 1998.
[RFC2284] Blunk, L. and J. Vollbrecht, "PPP Extensible
Authentication Protocol (EAP)", RFC 2284, March 1998.
[RFC2401] Atkinson, R. and S. Kent, "Security Architecture for
the Internet Protocol", RFC 2401, November 1998.
[RFC2406] Kent, S. and R. Atkinson, "IP Encapsulating Security
Payload (ESP)", RFC 2406, November 1998.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[RFC2486] Aboba, B. and M. Beadles, "The Network Access
Identifier", RFC 2486, January 1999.
[RFC2865] Rigney, C., Willens, S., Rubens, A. and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, June 2000.
[RFC2988] Paxson, V. and M. Allman, "Computing TCP's
Retransmission Timer", RFC 2988, November 2000.
[RFC3162] Aboba, B., Zorn, G. and D. Mitton, "RADIUS and IP6",
RFC 3162, August 2001.
[RFC3280] Housley, R., Polk, W., Ford, W. and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002.
[RFC3576] Chiba, M., Dommety, G., Eklund, M., Mitton, D. and B.
Aboba, "Dynamic Authorization Extensions to Remote
Authentication Dial In User Service (RADIUS)", RFC
3576, July 2003.
Aboba & Calhoun Informational [Page 31]
RFC 3579 RADIUS & EAP September 2003
6.2. Informative References
[RFC826] Plummer, D., "An Ethernet Address Resolution
Protocol", STD 37, RFC 826, November 1982.
[RFC1510] Kohl, J. and C. Neuman, "The Kerberos Network
Authentication Service (V5)", RFC 1510, September
1993.
[RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD
51, RFC 1661, July 1994.
[RFC2548] Zorn, G., "Microsoft Vendor-specific RADIUS
Attributes", RFC 2548, March 1999.
[RFC2607] Aboba, B. and J. Vollbrecht, "Proxy Chaining and
Policy Implementation in Roaming", RFC 2607, June
1999.
[RFC2716] Aboba, B. and D. Simon,"PPP EAP TLS Authentication
Protocol", RFC 2716, October 1999.
[RFC2866] Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.
[RFC2867] Zorn, G., Aboba, B. and D. Mitton, "RADIUS Accounting
Modifications for Tunnel Protocol Support", RFC 2867,
June 2000.
[RFC2868] Zorn, G., Leifer, D., Rubens, A., Shriver, J.,
Holdrege, M. and I. Goyret, "RADIUS Attributes for
Tunnel Protocol Support", RFC 2868, June 2000.
[RFC2869] Rigney, C., Willats, W. and P. Calhoun, "RADIUS
Extensions", RFC 2869, June 2000.
[RFC2983] Black, D. "Differentiated Services and Tunnels", RFC
2983, October 2000.
[RFC3580] 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.
[IEEE802] IEEE Standards for Local and Metropolitan Area
Networks: Overview and Architecture, ANSI/IEEE Std
802, 1990.
Aboba & Calhoun Informational [Page 32]
RFC 3579 RADIUS & EAP September 2003
[IEEE8021X] IEEE Standards for Local and Metropolitan Area
Networks: Port based Network Access Control, IEEE Std
802.1X-2001, June 2001.
[MD5Attack] Dobbertin, H., "The Status of MD5 After a Recent
Attack", CryptoBytes Vol.2 No.2, Summer 1996.
[Masters] Slatalla, M. and J. Quittner, "Masters of Deception."
HarperCollins, New York, 1995.
[NASREQ] Calhoun, P., et al., "Diameter Network Access Server
Application", Work in Progress.
Aboba & Calhoun Informational [Page 33]
RFC 3579 RADIUS & EAP September 2003
Appendix A - Examples
The examples below illustrate conversations between an authenticating
peer, NAS, and RADIUS server. The OTP and EAP-TLS protocols are used
only for illustrative purposes; other authentication protocols could
also have been used, although they might show somewhat different
behavior.
Where the NAS sends an EAP-Request/Identity as the initial packet,
the exchange appears as follows:
Authenticating peer NAS RADIUS server
------------------- --- -------------
<- EAP-Request/
Identity
EAP-Response/
Identity (MyID) ->
RADIUS Access-Request/
EAP-Message/EAP-Response/
(MyID) ->
<- RADIUS
Access-Challenge/
EAP-Message/EAP-Request
OTP/OTP Challenge
<- EAP-Request/
OTP/OTP Challenge
EAP-Response/
OTP, OTPpw ->
RADIUS Access-Request/
EAP-Message/EAP-Response/
OTP, OTPpw ->
<- RADIUS
Access-Accept/
EAP-Message/EAP-Success
(other attributes)
<- EAP-Success
Aboba & Calhoun Informational [Page 34]
RFC 3579 RADIUS & EAP September 2003
In the case where the NAS initiates with an EAP-Request for EAP TLS
[RFC2716], and the identity is determined based on the contents of
the client certificate, the exchange will appear as follows:
Authenticating peer NAS RADIUS server
------------------- --- -------------
<- EAP-Request/
EAP-Type=EAP-TLS
(TLS Start, S bit set)
EAP-Response/
EAP-Type=EAP-TLS
(TLS client_hello)->
RADIUS Access-Request/
EAP-Message/EAP-Response/
EAP-Type=EAP-TLS->
<-RADIUS Access-Challenge/
EAP-Message/
EAP-Request/
EAP-Type=EAP-TLS
<- EAP-Request/
EAP-Type=EAP-TLS
(TLS server_hello,
TLS certificate,
[TLS server_key_exchange,]
[TLS certificate_request,]
TLS server_hello_done)
EAP-Response/
EAP-Type=EAP-TLS
(TLS certificate,
TLS client_key_exchange,
[TLS certificate_verify,]
TLS change_cipher_spec,
TLS finished)->
RADIUS Access-Request/
EAP-Message/EAP-Response/
EAP-Type=EAP-TLS->
<-RADIUS Access-Challenge/
EAP-Message/
EAP-Request/
EAP-Type=EAP-TLS
<- EAP-Request/
EAP-Type=EAP-TLS
(TLS change_cipher_spec,
TLS finished)
Aboba & Calhoun Informational [Page 35]
RFC 3579 RADIUS & EAP September 2003
EAP-Response/
EAP-Type=EAP-TLS ->
RADIUS Access-Request/
EAP-Message/EAP-Response/
EAP-Type=EAP-TLS->
<-RADIUS Access-Accept/
EAP-Message/EAP-Success
(other attributes)
<- EAP-Success
In the case where the NAS first sends an EAP-Start packet to the
RADIUS server, the conversation would appear as follows:
Authenticating peer NAS RADIUS server
------------------- --- -------------
RADIUS Access-Request/
EAP-Message/Start ->
<- RADIUS
Access-Challenge/
EAP-Message/EAP-Request/
Identity
<- EAP-Request/
Identity
EAP-Response/
Identity (MyID) ->
RADIUS Access-Request/
EAP-Message/EAP-Response/
Identity (MyID) ->
<- RADIUS
Access-Challenge/
EAP-Message/EAP-Request/
OTP/OTP Challenge
<- EAP-Request/
OTP/OTP Challenge
EAP-Response/
OTP, OTPpw ->
RADIUS Access-Request/
EAP-Message/EAP-Response/
OTP, OTPpw ->
<- RADIUS
Access-Accept/
EAP-Message/EAP-Success
(other attributes)
<- EAP-Success
Aboba & Calhoun Informational [Page 36]
RFC 3579 RADIUS & EAP September 2003
In the case where the NAS initiates with an EAP-Request for EAP TLS
[RFC2716], but the peer responds with a Nak, indicating that it would
prefer another method not implemented locally on the NAS, the
exchange will appear as follows:
Authenticating peer NAS RADIUS server
------------------- --- -------------
<- EAP-Request/
EAP-Type=EAP-TLS
(TLS Start, S bit set)
EAP-Response/
EAP-Type=Nak
(Alternative(s))->
RADIUS Access-Request/
EAP-Message/EAP-Response/
Nak ->
<- RADIUS
Access-Challenge/
EAP-Message/EAP-Request/
Identity
<- EAP-Request/
Identity
EAP-Response/
Identity (MyID) ->
RADIUS Access-Request/
EAP-Message/EAP-Response/
(MyID) ->
<- RADIUS
Access-Challenge/
EAP-Message/EAP-Request
OTP/OTP Challenge
<- EAP-Request/
OTP/OTP Challenge
EAP-Response/
OTP, OTPpw ->
RADIUS Access-Request/
EAP-Message/EAP-Response/
OTP, OTPpw ->
<- RADIUS
Access-Accept/
EAP-Message/EAP-Success
(other attributes)
<- EAP-Success
Aboba & Calhoun Informational [Page 37]
RFC 3579 RADIUS & EAP September 2003
In the case where the authenticating peer attempts to authenticate
the NAS, the conversation would appear as follows:
Authenticating peer NAS RADIUS Server
------------------- --- -------------
EAP-Request/
Challenge, MD5 ->
RADIUS Access-Request/
EAP-Message/EAP-Request/
Challenge, MD5 ->
<- RADIUS
Access-Reject/
EAP-Message/
EAP-Response/
Nak (no alternative)
<- EAP-Response/Nak
(no alternative)
EAP-Failure ->
Aboba & Calhoun Informational [Page 38]
RFC 3579 RADIUS & EAP September 2003
In the case where an invalid EAP Response is inserted by an attacker,
the conversation would appear as follows:
Authenticating peer NAS RADIUS server
------------------- --- -------------
<- EAP-Request/
EAP-Type=Foo
EAP-Response/
EAP-Type=Foo ->
RADIUS Access-Request/
EAP-Message/EAP-Response/
EAP-Type=Foo ->
<- RADIUS
Access-Challenge/
EAP-Message/EAP-Request/
EAP-Type=Foo
<- EAP-Request/
EAP-Type=Foo
Attacker spoof:
EAP-Response/
EAP-Type=Bar ->
Good guy:
EAP-Response/
EAP-Type=Foo ->
RADIUS Access-Request/
EAP-Message/EAP-Response/
EAP-Type=Bar ->
<- RADIUS
Access-Challenge/
EAP-Message/EAP-Request/
EAP-Type=Foo,
Error-Cause="Invalid EAP
Packet (Ignored)"
RADIUS Access-Request/
EAP-Message/EAP-Response/
EAP-Type=Foo ->
<- Access-Accept/
EAP-Message/Success
<- EAP Success
Aboba & Calhoun Informational [Page 39]
RFC 3579 RADIUS & EAP September 2003
In the case where the client fails EAP authentication, and an error
message is sent prior to disconnection, the conversation would appear
as follows:
Authenticating peer NAS RADIUS server
------------------- --- -------------
RADIUS Access-Request/
EAP-Message/Start ->
<- RADIUS
Access-Challenge/
EAP-Message/EAP-Response/
Identity
<- EAP-Request/
Identity
EAP-Response/
Identity (MyID) ->
RADIUS Access-Request/
EAP-Message/EAP-Response/
(MyID) ->
<- RADIUS
Access-Challenge/
EAP-Message/EAP-Request
OTP/OTP Challenge
<- EAP-Request/
OTP/OTP Challenge
EAP-Response/
OTP, OTPpw ->
RADIUS Access-Request/
EAP-Message/EAP-Response/
OTP, OTPpw ->
<- RADIUS
Access-Challenge/
EAP-Message/EAP-Request/
Notification
<- EAP-Request/
Notification
EAP-Response/
Notification ->
RADIUS Access-Request/
EAP-Message/EAP-Response/
Notification ->
<- RADIUS
Access-Reject/
EAP-Message/EAP-Failure
<- EAP-Failure
(client disconnected)
Aboba & Calhoun Informational [Page 40]
RFC 3579 RADIUS & EAP September 2003
In the case that the RADIUS server or proxy does not support EAP-
Message, but no error message is sent, the conversation would appear
as follows:
Authenticating peer NAS RADIUS server
------------------- --- -------------
RADIUS Access-Request/
EAP-Message/Start ->
<- RADIUS
Access-Reject
(User Disconnected)
In the case where the local RADIUS server does support EAP-Message, but
the remote RADIUS server does not, the conversation would appear as
follows:
Authenticating peer NAS RADIUS server
------------------- --- -------------
RADIUS Access-Request/
EAP-Message/Start ->
<- RADIUS
Access-Challenge/
EAP-Message/
EAP-Response/
Identity
<- EAP-Request/
Identity
EAP-Response/
Identity
(MyID) ->
RADIUS Access-Request/
EAP-Message/EAP-Response/
(MyID) ->
<- RADIUS
Access-Reject
(proxied from remote
RADIUS server)
(User Disconnected)
Aboba & Calhoun Informational [Page 41]
RFC 3579 RADIUS & EAP September 2003
In the case where PPP is the link and the authenticating peer does
not support EAP, but where EAP is required for that user, the
conversation would appear as follows:
Authenticating peer NAS RADIUS server
------------------- --- -------------
<- PPP LCP Request-EAP
auth
PPP LCP NAK-EAP
auth ->
<- PPP LCP Request-CHAP
auth
PPP LCP ACK-CHAP
auth ->
<- PPP CHAP Challenge
PPP CHAP Response ->
RADIUS Access-Request/
User-Name,
CHAP-Password ->
<- RADIUS
Access-Reject
<- PPP LCP Terminate
(User Disconnected)
In the case where PPP is the link, the NAS does not support EAP, but
where EAP is required for that user, the conversation would appear as
follows:
Authenticating peer NAS RADIUS server
------------------- --- -------------
<- PPP LCP Request-CHAP
auth
PP LCP ACK-CHAP
auth ->
<- PPP CHAP Challenge
PPP CHAP Response ->
RADIUS Access-Request/
User-Name,
CHAP-Password ->
<- RADIUS
Access-Reject
<- PPP LCP Terminate
(User Disconnected)
Aboba & Calhoun Informational [Page 42]
RFC 3579 RADIUS & EAP September 2003
Appendix B - Change Log
The following changes have been made from RFC 2869:
A NAS may simultaneously support both local authentication and
pass-through; once the NAS enters pass-through mode within a session,
it cannot revert back to local authentication. Also EAP is
explicitly described as a 'lock step' protocol. (Section 2).
The NAS may initiate with an EAP-Request for an authentication Type.
If the Request is NAK'd, the NAS should send an initial
Access-Request with an EAP-Message attribute containing an
EAP-Response/Nak.
The RADIUS server may treat an invalid EAP Response as a non-fatal
error (Section 2.2)
For use with RADIUS/EAP, the Password-Retry (Section 2.3) and
Reply-Message (2.6.5) attributes are deprecated.
Each EAP session has a unique Identifier space (Section 2.6.1).
Role reversal is not supported (Section 2.6.2).
Message combinations (e.g. Access-Accept/EAP-Failure) that conflict
are discouraged (Section 2.6.3).
Only a single EAP packet may be encapsulated within a RADIUS message
(Section 3.1).
An Access-Request lacking explicit authentication as well as a
Message- Authenticator attribute SHOULD be silently discarded
(Section 3.3).
The Originating-Line-Info attribute is supported (Section 3.3).
IPsec ESP with non-null transform SHOULD be used and the usage model
is described in detail (Section 4.2).
Additional discussion of security vulnerabilities (Section 4.1) and
potential fixes (Section 4.3).
Separated normative (Section 6.1) and informative (Section 6.2)
references.
Aboba & Calhoun Informational [Page 43]
RFC 3579 RADIUS & EAP September 2003
Added additional examples (Appendix A): a NAS initiating with an
EAP-Request for an authentication Type; attempted role reversal.
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
Acknowledgments
Thanks to Dave Dawson and Karl Fox of Ascend, Glen Zorn of Cisco
Systems, Jari Arkko of Ericsson and Ashwin Palekar, Tim Moore and
Narendra Gidwani of Microsoft for useful discussions of this problem
space. The authors would also like to acknowledge Tony Jeffree,
Chair of IEEE 802.1 for his assistance in resolving RADIUS/EAP issues
in IEEE 802.1X-2001.
Aboba & Calhoun Informational [Page 44]
RFC 3579 RADIUS & EAP September 2003
Authors' Addresses
Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
Phone: +1 425 706 6605
Fax: +1 425 936 7329
EMail: bernarda@microsoft.com
Pat R. Calhoun
Airespace
110 Nortech Parkway
San Jose, California, 95134
USA
Phone: +1 408 635 2023
Fax: +1 408 635 2020
EMail: pcalhoun@airespace.com
Aboba & Calhoun Informational [Page 45]
RFC 3579 RADIUS & EAP September 2003
Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assignees.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Aboba & Calhoun Informational [Page 46]