home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
Internet Info 1997 December
/
Internet_Info_CD-ROM_Walnut_Creek_December_1997.iso
/
drafts
/
draft_ietf_q_t
/
draft-ietf-radius-eap-02.txt
< prev
next >
Wrap
Text File
|
1997-05-22
|
32KB
|
855 lines
RADIUS Working Group Pat Calhoun
INTERNET-DRAFT US Robotics Access Corp.
Updates: RFC 2058 Allan C. Rubens
Category: Standards Track Merit Network, Inc.
<draft-ietf-radius-eap-02.txt> Bernard Aboba
May 22, 1997 Microsoft
Extensible Authentication Protocol Support in RADIUS
1. Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working docu-
ments of the Internet Engineering Task Force (IETF), its areas, and
its working groups. Note that other groups may also distribute work-
ing documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as ``work in progress.''
To learn the current status of any Internet-Draft, please check the
``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow
Directories on ds.internic.net (US East Coast), nic.nordu.net
(Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim).
The distribution of this memo is unlimited. It is filed as <draft-
ietf-radius-eap-02.txt>, and expires January 1, 1998. Please send
comments to the authors.
2. Abstract
The Extensible Authentication Protocol (EAP) is a PPP extension that
provides support for additional authentication methods within PPP.
This document describes how the EAP-Message and Signature attributes
may be used for providing EAP support within RADIUS.
3. Introduction
The Extensible Authentication Protocol (EAP), described in [1], pro-
vides a standard mechanism for support of additional authentication
methods within PPP. Through the use of EAP, support for a number of
authentication schemes may be added, including smart cards, Kerberos,
Public Key, One Time Passwords, and others. In order to provide for
support of EAP within RADIUS, two new attributes, EAP-Message and Sig-
nature, were introduced as RADIUS extensions in [4]. This document
describes how these new attributes may be used for providing EAP sup-
port within RADIUS.
In the proposed scheme, the RADIUS server is used to shuttle RADIUS-
Calhoun, Rubens & Aboba [Page 1]
INTERNET-DRAFT May 22, 1997
encapsulated EAP Packets between the NAS and a backend security
server. While the conversation between the RADIUS server and the
backend security server will typically occur using a proprietary pro-
tocol developed by the backend security server vendor, it is also pos-
sible to use RADIUS-encapsulated EAP via the EAP-Message attribute.
This has the advantage of allowing the RADIUS server to support EAP
without the need for authentication-specific code, which can instead
reside on a backend security server.
3.1. Requirements language
This specification uses the same words as [6] for defining the signi-
ficance of each particular requirement. These words are:
MUST This word, or the adjectives "REQUIRED" or "SHALL", means
that the definition is an absolute requirement of the
specification.
MUST NOT This phrase, or the phrase "SHALL NOT", means that the
definition is an absolute prohibition of the specification.
SHOULD This word, or the adjective "RECOMMENDED", means that there
may exist valid reasons in particular circumstances to
ignore a particular item, but the full implications must be
understood and carefully weighed before choosing a different
course.
SHOULD NOTThis phrase means that there may exist valid reasons in par-
ticular circumstances when the particular behavior is
acceptable or even useful, but the full implications should
be understood and the case carefully weighed before imple-
menting any behavior described with this label.
MAY This word, or the adjective "AL", means that an item is
truly optional. One vendor may choose to include the item
because a particular marketplace requires it or because the
vendor feels that it enhances the product while another ven-
dor may omit the same item. An implementation which does
not include a particular option MUST be prepared to intero-
perate with another implementation which does include the
option, though perhaps with reduced functionality. In the
same vein an implementation which does include a particular
option MUST be prepared to interoperate with another imple-
mentation which does not include the option.(except, of
course, for the feature the option provides)
An implementation is not compliant if it fails to satisfy one or more
of the must or must not requirements for the protocols it implements.
An implementation that satisfies all the must, must not, should and
should not requirements for its protocols is said to be "uncondition-
ally compliant"; one that satisfies all the must and must not require-
ments but not all the should or should not requirements for its
Calhoun, Rubens & Aboba [Page 2]
INTERNET-DRAFT May 22, 1997
protocols is said to be "conditionally compliant."
4. Protocol overview
The EAP conversation between the authenticating peer (dial-in user)
and the NAS begins with the negotiation of EAP within LCP. Once EAP
has been negotiated, the NAS will typically send to the RADIUS server
a RADIUS Access-Request packet containing an EAP-Message attribute
signifying EAP-Start. EAP-Start is indicated by sending an EAP-Message
attribute with a length of 2 (no data). NAS-Port SHOULD be included in
the attributes issued by the NAS in the Access-Request packet; either
NAS-Identifier or NAS-IP-Address MUST be included.
If the RADIUS server supports EAP, it MUST respond with an Access-
Challenge packet containing an EAP-Message attribute. If the RADIUS
server does not support EAP, it MUST respond with an Access-Reject.
The EAP-Message attribute includes an encapsulated EAP packet which is
then passed on to the authenticating peer. The Access-Challenge typi-
cally will contain an EAP-Message attribute encapsulating an EAP-
Request/Identity message, requesting the dial-in user to identify
themself. The NAS will then respond with a RADIUS Access-Request
packet containing an EAP-Message attribute encapsulating an EAP-
Response. The conversation continues until either a RADIUS Access-
Reject or Access-Accept packet is received.
Reception of a RADIUS Access-Reject packet, with or without an EAP-
Message attribute encapsulating EAP-Failure, MUST result in the NAS
issuing an LCP Terminate Request to the authenticating peer. A RADIUS
Access-Accept packet with an EAP-Message attribute encapsulating EAP-
Success successfully ends the authentication phase. The RADIUS
Access-Accept/EAP-Message/EAP-Success packet MUST contain all of the
expected attributes which are currently returned in an Access-Accept
packet.
The above scenario creates a situation in which the NAS never needs to
manipulate an EAP packet. An alternative may be used in situations
where an EAP-Request/Identity message will always be sent by the NAS
to the authenticating peer. This involves having the NAS send an EAP-
Request/Identity message to the authenticating peer, and forwarding
the EAP-Response/Identity packet to the RADIUS server in the EAP-
Message attribute of a RADIUS Access-Request packet. While this
approach will save a round-trip, it cannot be universally employed.
There are circumstances in which the user's identity may not be needed
(such as when authentication and accounting is handled based on
Called-Station-Id or Calling-Station-Id), and therefore an EAP-
Request/Identity packet may not necessarily be issued by the NAS to
the authenticating peer.
Unless the NAS interprets the EAP-Response/Identity packet returned by
the authenticating peer, it will not have access to the user's iden-
tity. Therefore, the RADIUS Server SHOULD return the user's identity
by inserting it in the User-Name attribute of subsequent Access-
Calhoun, Rubens & Aboba [Page 3]
INTERNET-DRAFT May 22, 1997
Challenge and Access-Accept packets. Without the user's identity,
accounting and billing becomes very difficult to manage.
For proxied RADIUS requests there are two methods of processing. If
the domain is determined based on the Called-Station-Id, the RADIUS
Server may proxy the initial RADIUS Access-Request/EAP-Start. If the
domain is determined based on the user's identity, the local RADIUS
Server MUST respond with a RADIUS Access-Challenge/EAP-Identity
packet. The response from the authenticating peer MUST be proxied to
the final authentication server.
For proxied RADIUS requests, the NAS may receive an Access-Reject
packet in response to its Access-Request/EAP-Identity packet. This
would occur if the message was proxied to a RADIUS Server which does
not support the EAP-Message extension. On receiving an Access-Reject,
the NAS MUST send an LCP Terminate Request to the authenticating peer,
and disconnect.
The NAS is not responsible for the retransmission of any AP messages.
The authenticating peer and the RADIUS Server are responsible for any
retransmissions.
The example below shows the conversation between the authenticating
peer, NAS, and RADIUS server, for the case of a One Time Password
(OTP) authentication. OTP is used only for illustrative purposes;
other authentication protocols could also have been used, although
they might show somewhat different behavior.
Authenticating Peer NAS RADIUS Server
------------------- --- -------------
<- PPP LCP Request-EAP
auth
PPP LCP ACK-EAP
auth ->
RADIUS
Access-Request/
EAP-Message/Start ->
<- RADIUS
Access-Challenge/
EAP-Message/Identity
<- PPP EAP-Request/
Identity
PPP EAP-Response/
Identity (MyID) ->
RADIUS
Access-Request/
EAP-Message/
EAP-Response/
(MyID) ->
<- RADIUS
Access-Challenge/
EAP-Message/EAP-Request
Calhoun, Rubens & Aboba [Page 4]
INTERNET-DRAFT May 22, 1997
OTP/OTP Challenge
<- PPP EAP-Request/
OTP/OTP Challenge
PPP EAP-Response/
OTP, OTPpw ->
RADIUS
Access-Request/
EAP-Message/
EAP-Response/
OTP, OTPpw ->
<- RADIUS
Access-Accept/
EAP-Message/EAP-Success
(other attributes)
<- PPP EAP-Success
PPP Authentication
Phase complete,
NCP Phase starts
In the case where the NAS sends the authenticating peer an EAP-
Request/Identity packet without first sending an EAP-Start packet to
the RADIUS server, the conversation would appear as follows:
Authenticating Peer NAS RADIUS Server
------------------- --- -------------
<- PPP LCP Request-EAP
auth
PPP LCP ACK-EAP
auth ->
<- PPP EAP-Request/
Identity
PPP EAP-Response/
Identity (MyID) ->
RADIUS
Access-Request/
EAP-Message/
EAP-Response/
(MyID) ->
<- RADIUS
Access-Challenge/
EAP-Message/EAP-Request
OTP/OTP Challenge
<- PPP EAP-Request/
OTP/OTP Challenge
PPP EAP-Response/
OTP, OTPpw ->
RADIUS
Access-Request/
EAP-Message/
EAP-Response/
OTP, OTPpw ->
<- RADIUS
Access-Accept/
Calhoun, Rubens & Aboba [Page 5]
INTERNET-DRAFT May 22, 1997
EAP-Message/EAP-Success
(other attributes)
<- PPP EAP-Success
PPP Authentication
Phase complete,
NCP Phase starts
In the case where the client fails EAP authentication, the conversa-
tion would appear as follows:
Autheticating Peer NAS RADIUS Server
------------------- --- -------------
<- PPP LCP Request-EAP
auth
PPP LCP ACK-EAP
auth ->
Access-Request/
EAP-Message/Start ->
<- RADIUS
Access-Challenge/
EAP-Message/Identity
<- PPP EAP-Request/
Identity
PPP EAP-Response/
Identity (MyID) ->
RADIUS
Access-Request/
EAP-Message/
EAP-Response/
(MyID) ->
<- RADIUS
Access-Challenge/
EAP-Message/EAP-Request
OTP/OTP Challenge
<- PPP EAP-Request/
OTP/OTP Challenge
PPP EAP-Response/
OTP, OTPpw ->
RADIUS
Access-Request/
EAP-Message/
EAP-Response/
OTP, OTPpw ->
<- RADIUS
Access-Reject/
EAP-Message/EAP-Failure
<- PPP EAP-Failure
(client disconnected)
In the case that the RADIUS server or proxy does not support EAP-
Message, the conversation would appear as follows:
Calhoun, Rubens & Aboba [Page 6]
INTERNET-DRAFT May 22, 1997
Authenticating Peer NAS RADIUS Server
------------------- --- -------------
<- PPP LCP Request-EAP
auth
PPP LCP ACK-EAP
auth ->
RADIUS
Access-Request/
EAP-Message/Start ->
<- RADIUS
Access-Reject
<- PPP LCP Terminate
(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
------------------- --- -------------
<- PPP LCP Request-EAP
auth
PPP LCP ACK-EAP
auth ->
RADIUS
Access-Request/
EAP-Message/Start ->
<- RADIUS
Access-Challenge/
EAP-Message/Identity
<- PPP EAP-Request/
Identity
PPP EAP-Response/
Identity
(MyID) ->
RADIUS
Access-Request/
EAP-Message/EAP-Response/
(MyID) ->
<- RADIUS
Access-Reject
(proxied from remote
RADIUS Server)
<- PPP LCP Terminate
(User Disconnected)
In the case where 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
------------------- --- -------------
Calhoun, Rubens & Aboba [Page 7]
INTERNET-DRAFT May 22, 1997
<- 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 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)
5. Alternative uses
Currently the conversation between the backend security server and the
RADIUS server is proprietary because of lack of standardization. In
order to increase standardization and provide interoperability between
Radius vendors and backend 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 backend
security server instead.
Calhoun, Rubens & Aboba [Page 8]
INTERNET-DRAFT May 22, 1997
In the case where RADIUS-encapsulated EAP is used in a conversation
between a RADIUS server and a backend security server, the security
server will typically return an Access-Accept/EAP-Success message
without inclusion of the expected attributes currently returned in an
Access-Accept. This means that the RADIUS server MUST add these attri-
butes prior to sending an Access-Accept/EAP-Success message to the
NAS.
6. Attributes
6.1. EAP-Message
Description
This attribute encapsulates Extensible Authentication Protocol [1]
packets so as to allow the NAS to authenticate dial-in users via
EAP without having to understand the protocol.
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. The RADIUS Server
may return one or more EAP-Message attributes in Access-Challenge,
Access-Accept and Access-Reject packets.
Access-Request packets including one or more EAP-Message attributes
MUST also contain a Signature attribute, described in [4], in order
to provide for authentication of the shuttled EAP packets. Access-
Request packets including an EAP-Message attribute without a Signa-
ture attribute SHOULD be silently discarded by the RADIUS server. A
RADIUS Server supporting EAP-Message MUST calculate the correct
value of the Signature and silently discard the packet if it does
not match the value sent. A RADIUS Server not supporting EAP-
Message MUST return an Access-Reject if it receives an Access-
Reqeust containing an EAP-Message attribute. A RADIUS Server
receiving an EAP-Message attribute that it does not understand MUST
return an Access-Reject.
A summary of the EAP-Message attribute format is shown below. The
fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | String...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
67 for EAP-Message
Length
Calhoun, Rubens & Aboba [Page 9]
INTERNET-DRAFT May 22, 1997
>=3 (EAP packet enclosed)
=2 (EAP-Start message)
String
The String field includes EAP packets, as defined in [1]. If multi-
ple EAP-Message attributes are present in a packet their values
should be concatenated; this allows EAP packets longer than 253
octets to be passed by RADIUS.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ /
/ Data /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
7. Security considerations
Since the purpose of EAP is to provide enhanced security for PPP
authentication, it is critical that RADIUS support for EAP be secure.
In particular, protection must be provided against the following
attacks:
Connection hijacking
Man in the middle attacks
Multiple databases
Negotiation attacks
7.1. Connection hijacking
In this form of attack, the attacker attempts to inject packets into
the conversation between the NAS and the RADIUS server, or between the
RADIUS server and the backend security server. RADIUS does not support
encryption, and as described in [2], only Access-Reply and Access-
Challenge packets are authenticated.
In order to provide for authentication of all packets in the EAP
exchange, all Access-Request/EAP-Message packets MUST be authenticated
using the Signature attribute, as described in [4]. The RADIUS server
MUST silently discard all Access-Request packets failing authentica-
tion.
7.2. Man in the middle attacks
Since RADIUS security is based on shared secrets, end-to-end security
is not provided in the case where authentication or accounting packets
are forwarded along a proxy chain. As a result, attackers gaining con-
trol of a RADIUS proxy will be able to modify EAP packets in transit
Calhoun, Rubens & Aboba [Page 10]
INTERNET-DRAFT May 22, 1997
without fear of detection.
This represents a weakness of RADIUS which can be remedied by imple-
menting RADIUS on top of IP Security.
7.3. Multiple databases
In many cases a backend security server will be deployed along with a
RADIUS server in order to provide EAP services. Unless the backend
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 backend 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
backend security server store information in the same backend data-
base; by having the backend security server provide a full RADIUS
implementation; or by consolidating both the backend security server
and the RADIUS server onto the same machine.
7.4. Negotiation attacks
In a negotiation attack, a rogue NAS, tunnel server, RADIUS proxy or
RADIUS server causes the authenticating peer to choose a less secure
authentication method so as to make it easier to obtain the user's
password. For example, a session that would normally be authenticated
with EAP would instead authenticated via CHAP or PAP; alternatively, a
connection that would normally be authenticated via one EAP type
occurs via a less secure EAP type, such as MD5. 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 [7].
Protection against negotiation attacks requires the elimination of
downward negotiations. This can be achieved via implementation of
per-connection policy on the part of the authenticating peer, and
per-user 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 EAP when calling one service, while negotiating CHAP
for another service, even if both services are accessible via the same
phone number.
With per-connection policy, an authenticating peer will only attempt
to negotiate EAP for a session in which EAP support is expected. As a
Calhoun, Rubens & Aboba [Page 11]
INTERNET-DRAFT May 22, 1997
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 peer expecting EAP to be negotiated
for a session MUST NOT negotiate CHAP or PAP.
For a NAS, it may not be possible to determine whether a user is
required to authenticate with EAP until the user's identity is known.
For example, for shared-uses NASes it is possible for one reseller to
implement EAP while another does not. In such cases, if any users of
the NAS MUST do EAP, then the NAS MUST attempt to negotiate EAP for
every call. This avoids forcing an EAP-capable client to do more than
one authentication, which weakens 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 user 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, the NAS MUST send an LCP-Terminate and disconnect the
user. This is the correct behavior since the authenticating peer is
expecting EAP to be negotiated, and that expectation cannot be ful-
filled. 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 user dialing in from one location (with
an EAP-capable proxy) might be able to successfully authenticate via
EAP, while the same user dialing into another location (and encounter-
ing an EAP-incapable proxy) might be consistently disconnected.
8. Acknowledgments
Thanks to Dave Dawson and Karl Fox of Ascend, and Glen Zorn and Naren-
dra Gidwani of Microsoft for useful discussions of this problem space.
9. References
[1] L. J. Blunk, J. R. Vollbrecht. "PPP Extensible Authentication
Protocol (EAP)." Work in progress, draft-ietf-pppext-eap-auth-02.txt,
Merit Network, Inc., June, 1996.
[2] C. Rigney, A. Rubens, W. Simpson, S. Willens. "Remote
Calhoun, Rubens & Aboba [Page 12]
INTERNET-DRAFT May 22, 1997
Authentication Dial In User Service (RADIUS)." RFC 2058, Livingston,
Merit, Daydreamer, January, 1997.
[3] C. Rigney. "RADIUS Accounting." RFC 2059, Livingston, January,
1997.
[4] C. Rigney, W. Willats. "RADIUS Extensions." Work in progress,
draft-ietf-radius-ext-00.txt, Livingston, January, 1997.
[5] R. Rivest, S. Dusse. "The MD5 Message-Digest Algorithm." RFC
1321, MIT Laboratory for Computer Science, RSA Data Security Inc.,
April 1992.
[6] S. Bradner. "Key words for use in RFCs to Indicate Requirement
Levels." RFC 2119, Harvard University, March, 1997.
[7] M. Slatalla, J. Quittner. "Masters of Deception." HarperCollins,
New York, 1995.
10. Authors' Addresses
Pat R. Calhoun
US Robotics Access Corp.
8100 N. McCormick Blvd.
Skokie, IL 60076-2999
Phone: 847-342-6898
EMail: pcalhoun@usr.com
Allan C. Rubens
Merit Network, Inc.
4251 Plymouth Rd.
Ann Arbor, MI 48105-2785
Phone: 313-647-0417
EMail: acr@merit.edu
Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
Phone: 206-936-6605
EMail: bernarda@microsoft.com
Calhoun, Rubens & Aboba [Page 13]