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Versions: (RFC 2869) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 RFC 3579

Network Working Group                                           B. Aboba
INTERNET-DRAFT                                                 Microsoft
Category: Informational                                       P. Calhoun
<draft-aboba-radius-rfc2869bis-01.txt>              Black Storm Networks
9 April 2002
Updates: RFC 2869


      RADIUS Support For Extensible Authentication Protocol (EAP)

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC 2026.

Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups.  Note that other groups
may also distribute working 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."

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

Copyright Notice

Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

This document defines RADIUS support for the Extensible Authentication
Protocol (EAP), an authentication protocol which supports multiple
authentication mechanisms.  While EAP was originally developed for use
with PPP, it is also now in use with IEEE 802.

This document updates RFC 2869.











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Table of Contents

1.     Introduction ..........................................    3
   1.1       Specification of Requirements ...................    3
   1.2       Terminology .....................................    4
2.     RADIUS support for EAP ................................    5
   2.1       Protocol overview ...............................    5
   2.2       Retransmission ..................................    7
   2.3       Fragmentation ...................................    7
   2.4       Alternative uses ................................    7
   2.5       Usage guidelines ................................    8
3.     Attributes ............................................   10
   3.1       Password-Retry ..................................   10
   3.2       EAP-Message .....................................   11
   3.3       Message-Authenticator ...........................   12
   3.4       Table of attributes .............................   14
4.     Security considerations ...............................   14
   4.1       Message-Authenticator Security ..................   15
   4.2       EAP Security ....................................   15
5.     Normative references ..................................   18
6.     Informative references ................................   19
Appendix A - Examples ........................................   21
ACKNOWLEDGMENTS ..............................................   25
AUTHORS' ADDRESSES ...........................................   25
Full Copyright Statement .....................................   25


























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1.  Introduction

[RFC2865] describes the RADIUS Protocol as it is implemented and
deployed today, and [RFC2866] describes how Accounting can be performed
with RADIUS.

The Extensible Authentication Protocol (EAP)  is a general protocol for
authentication which supports multiple authentication mechanisms.  EAP
may be used on dedicated links as well as 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 switches and access points using IEEE 802
[IEEE802].  EAP encapsulation on IEEE 802 media is described in
[IEEE8021X].

This memo suggests several additional Attributes that can be added to
RADIUS to support the Extensible Authentication Protocol (EAP).  These
Attributes now have extensive field experience, and so the purpose of
this document is to clarify interoperability issues.

The Extensible Authentication Protocol (EAP) [RFC2284bis] is an
extension that provides support for additional authentication methods.
This memo describes how the EAP-Message and Message- Authenticator
attributes may be used for providing EAP support within RADIUS.

All attributes are comprised of variable length Type-Length-Value 3-
tuples.  New attribute values can be added without disturbing existing
implementations of the protocol.

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].

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 "unconditionally
compliant"; one that satisfies all the must and must not requirements
but not all the should or should not requirements for its protocols is
said to be "conditionally compliant."





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A NAS that does not implement a given service MUST NOT implement the
RADIUS attributes for that service.  For example, a NAS that is unable
to offer EAP service MUST NOT implement the RADIUS attributes for EAP.
A NAS MUST treat a RADIUS access-request requesting an unavailable
service as an access-reject instead.

1.2.  Terminology

This document frequently uses the following terms:

Authenticator
          The end of the link requiring the authentication.

Peer      The other end of the point-to-point link (PPP), point-to-point
          LAN segment (IEEE 802.1x) or 802.11 wireless link, which 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. This service
          verifies from the credentials provided by the peer, the claim
          of identity made by the peer.

Port Access Entity (PAE)
          The protocol entity associated with a physical or virtual
          (802.11) Port.  A given PAE may support the protocol
          functionality associated with the Authenticator, Peer or both.

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 [RFC2044].

Service   The NAS provides a service to the user, such as IEEE 802 or
          PPP.

Session   Each service provided by the NAS to a user constitutes a
          session, with the beginning of the session defined as the
          point where service is first provided and the end of the



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          session defined as the point where service is ended.  A user
          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 [RFC2284bis],
provides a standard mechanism for support of additional authentication
methods.  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, and others.  In order to provide for
support of 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.

In the proposed scheme, the RADIUS server is used to shuttle RADIUS-
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 protocol
developed by the backend security server vendor, it is also possible 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 the
backend security server.

2.1.  Protocol Overview

The EAP conversation between the authenticating peer and the NAS begins
with the negotiation of EAP.  Once EAP has been negotiated, the NAS
SHOULD send an EAP-Request/Identity message to the authenticating peer,
unless the identity exchange is postponed until later or the identity is
determined via some other means such as Called-Station-Id or Calling-
Station-Id.  The peer will then respond with an EAP-Response/Identity
which the the NAS will then forward to the RADIUS server in the EAP-
Message attribute of a RADIUS Access-Request packet. The RADIUS Server
will typically use the EAP-Response/Identity to determine which EAP type
is to be applied to the user.

In order to permit non-EAP aware RADIUS proxies to forward the Access-
Request packet, if the NAS sends the EAP-Request/Identity, the NAS MUST
copy the contents of the EAP-Response/Identity into the User-Name
attribute and MUST include the EAP-Response/Identity in the User-Name
attribute in every subsequent Access-Request. NAS-Port or NAS-Port-Id
SHOULD be included in the attributes issued by the NAS in the Access-
Request packet, and either NAS-Identifier or NAS-IP- Address MUST be
included.  In order to permit forwarding of the Access-Reply by EAP-



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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 very difficult to manage.

If identity is determined via another means such as Called-Station-Id or
Calling-Station-Id, the NAS MUST include these identifying attributes in
every Access-Request.

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. In cases where an EAP-Request/Identity packet will
not be sent, the NAS will 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). However, it should be noted that since no User-
Name attribute is included in the Access- Request, this approach is not
compatible with RADIUS as specified in [RFC2865], nor can it easily be
applied in situations where proxies are deployed, such as roaming or
shared use networks.

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.  In the case where the NAS does
not initially send an EAP-Request/Identity message to the peer, the
Access-Challenge typically 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 MUST result in the NAS
denying access to the authenticating peer.  A RADIUS Access-Accept
packet successfully ends the authentication phase.

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.

For proxied RADIUS requests there are two methods of processing.  If the
domain is determined based on the Called-Station-Id, the RADIUS Server



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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 deny
access to the authenticating peer.

2.2.  Retransmission

As noted in [RFC2284bis], the EAP authenticator (NAS) is responsible for
retransmission of packets between the authenticating peer and the NAS.
Thus if an EAP packet is lost in transit between the authenticating peer
and the NAS (or vice versa), the NAS will retransmit. As in RADIUS
[RFC2865], the RADIUS client is responsible for retransmission of
packets between the RADIUS client and the RADIUS server.

Note that 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
and Password-Retry attributes 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 provides the
NAS with the maximum number of seconds the NAS should wait for an EAP-
Response before retransmitting the EAP-Message to the dial-in user.

2.3.  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.

2.4.  Alternative uses

Currently the conversation between the backend security server and the
RADIUS server is proprietary because of lack of standardization.  In



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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.

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
attributes prior to sending an Access-Accept/EAP-Success message to the
NAS.

2.5.  Usage guidelines

2.5.1.  Conflicting messages

In some cases, the authentication result implied by the encapsulated EAP
packet may not match the result communicated in the RADIUS message. For
example, and EAP Failure packet may be encapsulated within an Access-
Accept message and an EAP Success packet may be encapsulated within an
Access-Reject. Alternatively, no EAP-Message attribute may be included
within an Access-Accept or Access-Reject.

Such combinations are 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 Success, the Peer will be lead to believe that
it authenticated successfully. Similarly, if the NAS receives an Access-
Accept with an encapsulated EAP Failure, it will grant access to the
Peer. However, on receiving a 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 [RFC2284bis], 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 no further Access-Requests will
be sent to the RADIUS server. As a result, the NAS will not be given an
indication of whether to Allow or Deny access while the Peer will be
informed as to the outcome of the authentication.



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To avoid these conflicts, the RADIUS server SHOULD check to make sure
that the results implied by an  encapsulated EAP-Message attribute and
the RADIUS message are in agreement. The following combinations SHOULD
NOT be sent by a RADIUS server as part of an EAP conversation:

   Access-Accept/EAP-Message/EAP-Failure
   Access-Accept/no EAP-Message attribute
   Access-Reject/EAP-Message/EAP-Success
   Access-Reject/no EAP-Message attribute
   Access-Challenge/EAP-Message/EAP-Success
   Access-Challenge/EAP-Message/EAP-Failure

Since the responsibility for avoiding these conflicts lies with the
RADIUS server, the NAS MUST NOT "manufacture" EAP packets in order to
correct contradictory messages that it receives.

2.5.2.  Priority

In addition to containing EAP-Message attributes, RADIUS messages may
also contain other attributes. In order to ensure the correct processing
of RADIUS messages, the NAS SHOULD process EAP-Message attributes last.

2.5.3.  Displayable messages

The Reply-Message attribute, defined in section 5.18 of [RFC2865],
indicates text which MAY be displayed to the user. This is similar in
concept to the EAP Notification Type, defined in [RFC2284bis].  However,
when sent during an EAP conversation, displayable messages SHOULD be
encapsulated within an EAP-Message/Notification-Request rather than
within a Reply-Message attribute.

While a NAS receiving a Reply-Message attribute MAY send an EAP
Notification-Request to the peer, several issues may arise from this:

[1]  Unexpected Notification-Responses. On receiving an EAP
     Notification-Request, the EAP Peer will send a Notification-
     Response, and the NAS will pass this on to the RADIUS server,
     encapsulated within an EAP-Message attribute.  Since the RADIUS
     server may not be expecting a Notification-Response, it may
     silently discard the packet.

[2]  Identifier conflicts. While the Notification-Request contains an an
     Identifier, a Reply-Message attribute does not. As a result, a NAS
     receiving a Reply-Message and wishing to translate it to a
     Notification-Request will need to make up an Identifier. Since the
     NAS cannot make assumptions about how the RADIUS server chooses
     Identifiers, it is possible that the chosen Identifier will
     conflict with a value chosen by the RADIUS server for another



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     packet within the EAP conversation. As noted in [RFC2284bis] this
     would violate the requirement that Identifier values be unique
     within an EAP conversation.

2.5.4.  Multiple EAP-Message attributes

An Access-Challenge, Access-Accept, Access-Reject or Access-Request
message MAY contain zero or more EAP-Message attributes. However, where
more than one EAP-Message attribute is included, it is assumed that the
attributes are to be concatenated to to form a single EAP packet. Since
EAP is a "lockstep" protocol, a new EAP-Request cannot be sent until an
EAP-Response is received to an outstanding request and only a single
Request can be outstanding at a given time.  As a result, multiple EAP
packets MUST NOT be encoded within EAP-Message attributes contained
within a single Access-Challenge, Access-Accept, Access-Reject or
Access-Request message.

In addition, since a Reply-Message attribute MAY be translated to a
Notification-Request by some NASes, a Reply-Message attribute MUST NOT
be included in a RADIUS message containing an EAP-Message attribute.

3.  Attributes

3.1.  Password-Retry

Description

   This attribute MAY be included in an Access-Reject to indicate how
   many authentication attempts a user may be allowed to attempt before
   being disconnected.

   A summary of the Password-Retry 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |             Value
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Value (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

   75 for Password-Retry.

Length




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   6

Value

   The Value field is four octets, containing an integer specifying the
   number of password retry attempts to permit the user.

3.2.  EAP-Message

Description

   This attribute encapsulates Extended Access Protocol [RFC2284bis]
   packets so as to allow the NAS to authenticate dial-in users via EAP
   without having to understand the EAP protocol.

   The NAS places any EAP messages received from the user into one or
   more EAP attributes and forwards them to the RADIUS Server as part of
   the Access-Request, which can return EAP messages in Access-
   Challenge, Access-Accept and Access-Reject packets.

   A RADIUS Server receiving EAP messages that it does not understand
   SHOULD return an Access-Reject.

   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-
   Messages 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.  Access-Accept and
   Access-Reject packets SHOULD only have ONE EAP-Message attribute in
   them, containing EAP-Success or EAP- Failure.

   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 the EAP-Success or
   EAP-Failure packets) it is necessary that RADIUS/EAP provide
   integrity protection at least as strong as those used in the EAP
   methods themselves.

   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 an EAP-Message attribute without a
   Message-Authenticator attribute SHOULD be silently discarded by the
   RADIUS server.  A RADIUS Server supporting EAP-Message MUST calculate
   the correct value of the Message-Authenticator and silently discard



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   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-Request containing an EAP-Message attribute. A RADIUS
   Server receiving an EAP-Message attribute that it does not understand
   MUST return an Access-Reject.

   Access-Challenge, Access-Accept, or Access-Reject packets including
   an EAP-Message attribute without a Message-Authenticator attribute
   SHOULD be silently discarded by the NAS. A NAS supporting EAP-Message
   MUST calculate the correct value of the Message-Authenticator and
   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 EAP packets, as defined in [3].  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 passed by RADIUS.

3.3.  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



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   the Message-Authenticator and silently discard the packet if it does
   not match the value sent.

   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.

   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 hash is calculated the signature string should be
      considered to be sixteen octets of zero.

      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 hash is calculated the signature string should be
      considered to be sixteen octets of zero.  The shared secret is



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      used as the key for the HMAC-MD5 hash.  The is calculated and
      inserted in the packet before the Response Authenticator is
      calculated.

      This attribute is not needed if the User-Password attribute is
      present, 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.

      IP Security will eventually make this attribute unnecessary, so it
      should be considered an interim measure.

3.4.  Table of Attributes

The following table provides a guide to which attributes may be found in
which kind of packets.  The attributes added in this document must not
be present in an Accounting-Request.

Request  Accept  Reject  Challenge   #    Attribute
0        0       0-1     0           75   Password-Retry
0+       0+      0+      0+          79   EAP-Message [Note 1]
0-1      0-1     0-1     0-1         80   Message-Authenticator [Note 1]
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.

The following table defines 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.

4.  Security Considerations

The attributes other than Message-Authenticator and EAP-Message in this
document have no additional security considerations beyond those already
identified in [RFC2865].




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4.1.  Message-Authenticator Security

Access-Request packets with a User-Password establish the identity of
both the user and the NAS sending the Access-Request, because of the way
the shared secret between NAS and RADIUS server is used.  Access-Request
packets with CHAP-Password or EAP-Message do not have a User-Password
attribute, so the Message-Authenticator attribute should be used in
access-request packets that do not have a User- Password, in order to
establish the identity of the NAS sending the request.

Note that the Message-Authenticator attribute may be subjected to an
offline dictionary attack in order to recover the RADIUS shared secret.
As noted in [RFC2645]:

   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.

4.2.  EAP Security

Since the purpose of EAP is to provide enhanced security for
authentication, it is critical that RADIUS support for EAP be secure.
In particular, the following issues must be addressed:

   Separation of EAP server and PPP authenticator
   Connection hijacking
   Man in the middle attacks
   Multiple databases
   Negotiation attacks

4.2.1.  Separation of EAP server and authenticator

It is possible for the EAP endpoints to mutually authenticate, negotiate
a ciphersuite, and derive a session key for use in a ciphersuite.

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 pass the session key to the ciphersuite module.

The situation is more complex when EAP is used with RADIUS, since the
authenticator will typically not reside on the same machine as the EAP
server. For example, the EAP server may be a backend security server, or
a module residing on the RADIUS server.

In the case where the EAP server and authenticator reside on different
machines, there are several implications for security.  Firstly, mutual
authentication will occur between the peer and the EAP server, not
between the peer and the authenticator. This means that it is not



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possible for the peer to validate the identity of the NAS or tunnel
server that it is speaking to.

As described earlier, when EAP/RADIUS is used to encapsulate EAP
packets, the Message-Authenticator attribute is required in EAP/RADIUS
Access-Requests sent from the NAS or tunnel server to the RADIUS server.
Since the Message-Authenticator attribute involves a HMAC-MD5 hash, 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.  Similarly,
Access-Challenge packets sent from the RADIUS server to the NAS are also
authenticated and integrity protected using an HMAC-MD5 hash, enabling
the NAS or tunnel server to determine the integrity of the packet and
verify the identity of the RADIUS server.  Moreover, EAP packets sent
via methods that contain their own integrity protection cannot be
successfully modified by a rogue NAS or tunnel server.

The second issue that arises in the case of an EAP server and
authenticator residing on different machines is that the session key
negotiated between the peer and EAP server will need to be transmitted
to the authenticator.  Therefore a mechanism needs to be provided to
transmit the session key from the EAP server to the authenticator or
tunnel server that needs to use the key. The specification of this
transit mechanism is outside the scope of this document.

4.2.2.  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 [RFC2865], only Access-Reply and Access-
Challenge packets are integrity protected. Moreover, the integrity
protection mechanism described in [RFC2865] is weaker than that likely
to be used by some EAP methods, making it possible to subvert those
methods by attacking EAP/RADIUS.

In order to provide for authentication of all packets in the EAP
exchange, all EAP/RADIUS packets MUST be authenticated using the
Message-Authenticator attribute, as described previously.

4.2.3.  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 control
of a RADIUS proxy will be able to modify EAP packets in transit.






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4.2.4.  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 database;
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.

4.2.5.  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 [Masters].

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
result, there is a presumption that an authenticating peer selecting EAP



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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 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 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 encountering an EAP-
incapable proxy) might be consistently disconnected.

5.  Normative references

[RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC
          1661, July 1994.

[RFC1994] Simpson, W., "PPP Challenge Handshake Authentication Protocol
          (CHAP)", RFC 1994, August 1996.





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[RFC2044] Yergeau, F., "UTF-8, a transformation format of Unicode and
          ISO 10646", RFC 2044, October 1996.

[RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing
          for Message Authentication", RFC 2104, February 1997.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
          Requirement Levels", RFC 2119, March 1997.]

[RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO 10646",
          RFC 2279, January 1998.

[RFC2434] Alvestrand, H. and Narten, T., "Guidelines for Writing an IANA
          Considerations Section in RFCs", BCP 26, RFC 2434, October
          1998.

[RFC2865] Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote
          Authentication Dial In User Service (RADIUS)", RFC 2865, June
          2000.

[RFC2866] Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.

[RFC2284bis]
          Blunk, L., J. Vollbrecht, and Aboba, B., "Extensible
          Authentication Protocol (EAP)", Internet draft (work in
          progress), draft-ietf-pppext-rfc2284bis-04.txt, March 2002.

[IEEE802] IEEE Standards for Local and Metropolitan Area Networks:
          Overview and Architecture, ANSI/IEEE Std 802, 1990.

[IEEE8021X]
          IEEE Standards for Local and Metropolitan Area Networks: Port
          based Network Access Control, IEEE Std 802.1X-2001, June 2001.

6.  Informative references

[Masters] Slatalla, M., and  Quittner, J., "Masters of Deception."
          HarperCollins, New York, 1995.

[RFC1510] Kohl, J., Neuman, C., "The Kerberos Network Authentication
          Service (V5)", RFC 1510, September 1993.

[RFC2246] Dierks, T. and  C. Allen, "The TLS Protocol Version 1.0", RFC
          2246, November 1998.

[RFC2486] Beadles, M., Aboba, B., "The Network Access Identifier", RFC
          2486, January 1999.




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INTERNET-DRAFT                RADIUS & EAP                  9 April 2002


[RFC2401] Atkinson, R., Kent, S., "Security Architecture for the
          Internet Protocol", RFC 2401, November 1998.

[RFC2408] Maughan, D., Schertler, M., Schneider, M., Turner, J.,
          "Internet Security Association and Key Management Protocol
          (ISAKMP)", RFC 2408, November 1998.

[RFC2716] Aboba, B., Simon, D.,"PPP EAP TLS Authentication Protocol",
          RFC 2716, October 1999.

[IEEE80211]
          Information technology - Telecommunications and information
          exchange between systems - Local and metropolitan area
          networks - Specific Requirements Part 11:  Wireless LAN Medium
          Access Control (MAC) and Physical Layer (PHY) Specifications,
          IEEE Std. 802.11-1997, 1997.



































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Appendix A - Examples

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
-------------------     ---                    -------------
                        <- 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

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



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                                               Access-Challenge/
                                               EAP-Message/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

In the case where the client fails EAP authentication, the conversation
would appear as follows:

Authenticating Peer     NAS                    RADIUS Server
-------------------     ---                    -------------
                        Access-Request/
                        EAP-Message/Start ->
                                               <- RADIUS
                                               Access-Challenge/
                                               EAP-Message/Identity
                        <- EAP-Request/
                        Identity
EAP-Response/
Identity (MyID) ->
                        RADIUS
                        Access-Request/
                        EAP-Message/



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                        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-Reject/
                                                 EAP-Message/EAP-Failure

                        <- EAP-Failure
                        (client disconnected)

In the case that the RADIUS server or proxy does not support EAP-
Message, 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/Identity
                        <- EAP-Request/
                        Identity



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EAP-Response/
Identity
(MyID) ->
                        RADIUS
                        Access-Request/
                        EAP-Message/EAP-Response/
                        (MyID) ->
                                                  <- RADIUS
                                                  Access-Reject
                                                  (proxied from remote
                                                   RADIUS Server)
                        (User Disconnected)

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 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 ->



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                        <- PPP CHAP Challenge
PPP CHAP Response ->
                        RADIUS
                        Access-Request/
                        User-Name,
                        CHAP-Password ->

                                                 <- RADIUS
                                                 Access-Reject
                        <-  PPP LCP Terminate
                        (User Disconnected)

Acknowledgments

Thanks also to Dave Dawson and Karl Fox of Ascend, Glen Zorn of Cisco
Systems and Ashwin Palekar, Tim Moore and Narendra Gidwani of Microsoft
for useful discussions of this problem space.

Author's Addresses

Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052

Phone: +1 425 706 6605
EMail: bernarda@microsoft.com

Pat R. Calhoun
Black Storm Networks
250 Cambridge Avenue, Suite 200
Palo Alto, California, 94306
USA

Phone:  +1 650-617-2932
Fax:    +1 650-786-6445
E-mail:  pcalhoun@bstormnetworks.com

Full Copyright Statement

Copyright (C) The Internet Society (2002).  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



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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 assigns.  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."

Open issues

Open issues relating to this specification are tracked on the following
web site:

http://www.drizzle.com/~aboba/AAA/issues.html

Expiration Date

This memo is filed as <draft-aboba-radius-rfc2869bis-01.txt>, and
expires November 24, 2002.



























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