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Versions: 00 01 02 03 draft-ietf-radext-fixes

Network Working Group                                       David Nelson
INTERNET-DRAFT                                        Enterasys Networks
Updates: 2865, 2866, 2869, 3576, 3579                         Alan DeKok
Category: Proposed Standard                               Infoblox, Inc.
<draft-aboba-radext-fixes-03.txt>                          Bernard Aboba
3 June 2006                                                    Microsoft


        Common RADIUS Implementation Issues and Suggested Fixes

   By submitting this Internet-Draft, each author represents that any
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   This Internet-Draft will expire on December 10, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document describes common issues seen in RADIUS implementations
   and suggests some fixes.  Where applicable, ambiguities and errors in
   previous RADIUS specifications are clarified.









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Table of Contents

1.     Introduction ..........................................    3
   1.1       Terminology .....................................    3
   1.2       Requirements Language ...........................    3
2.     Issues ................................................    4
   2.1       Session Definition ..............................    4
   2.2       Overload Conditions .............................    6
   2.3       Accounting Issues ...............................    7
   2.4       Multiple Filter-ID Attributes ...................    8
   2.5       Mandatory and Optional Attributes ...............    9
   2.6       Interpretation of Access-Reject .................   10
   2.7       Addressing ......................................   12
   2.8       Idle Timeout ....................................   13
   2.9       Calculation of Message-Authenticator ............   14
  2.10       Unknown Identity ................................   15
3.     IANA Considerations ...................................   16
4.     Security Considerations ...............................   16
5.     References ............................................   16
  5.1  Informative References ................................   16
ACKNOWLEDGMENTS ..............................................   17
AUTHORS' ADDRESSES ...........................................   17
Intellectual Property Statement ..............................   18
Disclaimer of Validity .......................................   19
Copyright Statement ..........................................   19


























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1.  Introduction

   The last few years have seen an increase in the deployment of RADIUS
   clients and servers.  This document describes common issues seen in
   RADIUS implementations and suggests some fixes.  Where applicable,
   ambiguities and errors in previous RADIUS specifications are
   clarified.

1.1.  Terminology

   This document uses the following terms:

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 network access via
     802.11 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.

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.

1.2.  Requirements Language

   In this document, several words are used to signify the requirements
   of the specification.  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].










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2.  Issues

2.1.  Session Definition

2.1.1.  State Attribute

   Regarding the State attribute, [RFC2865] Section 5.24 states:

      This Attribute is available to be sent by the server to the client
      in an Access-Challenge and MUST be sent unmodified from the client
      to the server in the new Access-Request reply to that challenge,
      if any.

      This Attribute is available to be sent by the server to the client
      in an Access-Accept that also includes a Termination-Action
      Attribute with the value of RADIUS-Request.  If the NAS performs
      the Termination-Action by sending a new Access-Request upon
      termination of the current session, it MUST include the State
      attribute unchanged in that Access-Request.

   Some RADIUS client implementations do not properly use the State
   attribute in order to distinguish a restarted EAP authentication
   process from the continuation of an ongoing process (by the same user
   on the same NAS and port).

   Where an EAP-Message attribute is included in an Access-Challenge or
   Access-Accept attribute, RADIUS servers SHOULD also include a State
   attribute.

   An Access-Request sent as a result of a new or restarted
   authentication run MUST NOT include the State attribute, even if the
   State attribute has previously been received in an Access-Challenge
   for the same user and port.

   Since a State attribute is always initially provided by the server in
   an Access-Accept, Access-Challenge, CoA-Request or Disconnect-
   Request, a RADIUS client MUST NOT insert a State attribute that it
   has not previously received from the server.

   A State attribute is REQUIRED in Access-Request packets neither
   including an authentication attribute nor a Service-Type attribute
   with the value Call Check (10).  [RFC2865] Section 5.44 states:

      An Access-Request MUST contain either a User-Password or a CHAP-
      Password or State.  An Access-Request MUST NOT contain both a
      User-Password and a CHAP-Password.  If future extensions allow
      other kinds of authentication information to be conveyed, the
      attribute for that can be used in an Access-Request instead of



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      User-Password or CHAP-Password.

   [RFC3576] defines the Service-Type value of "Authorize-Only".  This
   Service-Type can be included within Disconnect or CoA-Request
   packets.  A NAS receiving such a Disconnect or CoA-Request will send
   an Access-Request packet containing a Service-Type attribute with the
   value "Authorize-Only" and no authentication attributes.  In order to
   satisfy the requirements of [RFC2865] Section 5.44, an Access-Request
   with Service-Type="Authorize-Only" MUST contain a State attribute.

   In order to provide a State attribute to the NAS, Disconnect and CoA-
   Request packets containing a Service-Type of value "Authorize-Only"
   MUST also contain a State attribute, and the NAS MUST include the
   State attribute unchanged in the Access-Request.  A NAS receiving a
   Disconnect or CoA-Request containing a Service-Type value of
   "Authorize-Only" but lacking a State attribute MUST send a Disconnect
   or CoA-NAK and SHOULD include an Error-Cause attribute with value 402
   (Missing Attribute).

2.1.2.  Request-ID Supplementation

   [RFC3579] Section 2.6.1 states:

      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.

   There are issues with the suggested algorithm.  Since proxies may
   translate Access-Request attributes such as NAS-IP-Address, depending
   on any attribute under control of the NAS to distinguish request
   identifiers can result in deployment problems.

   The FreeRADIUS implementation does not track EAP identifiers by NAS-
   IP-Address or other attributes sent by the NAS.  Instead, it uses the
   EAP identifier, source IP address, and the State attribute. Since the
   State attribute is under the control of the RADIUS server, this means
   that the uniqueness of each session is controlled by the server, not
   the NAS.  The algorithm used in FreeRADIUS is as follows:

      if (EAP start, or EAP identity) {
      allocate unique State Attribute



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      insert into "active" queue, with key (EAP identifier, State, source IP)
      } else {
      look up active session in queue, with above key
      }

   This algorithm appears to work well in variety of situations,
   including situations where home servers receive messages via
   intermediate RADIUS proxies.

2.2.  Overload Conditions

2.2.1.  Retransmission Behavior

   [RFC2865] Section 2.4 describes the retransmission requirements for
   RADIUS clients:

      At one extreme, RADIUS does not require a "responsive" detection
      of lost data.  The user is willing to wait several seconds for the
      authentication to complete.  The generally aggressive TCP
      retransmission (based on average round trip time) is not required,
      nor is the acknowledgment overhead of TCP.

      At the other extreme, the user is not willing to wait several
      minutes for authentication.  Therefore the reliable delivery of
      TCP data two minutes later is not useful.  The faster use of an
      alternate server allows the user to gain access before giving up.

   Some existing RADIUS clients implement excessively aggressive
   retransmission behavior, utilizing default retransmission timeouts of
   one second or less without support for congestive backoff.   When
   deployed at large scale, these implementations are susceptible to
   congestive collapse.  For example, as the result of a power failure,
   a network with 3000 NAS devices with a fixed retransmission timer of
   one second will continuously generate 3000 RADIUS Access-Requests per
   second.  This is sufficient to overwhelm most RADIUS servers.

   Suggested solutions include:

[b]  Jitter.  To avoid synchronization, a RADIUS client SHOULD
     incorporate jitter within its retransmission algorithm.

[a]  Congestive backoff.  While it is not necessary for RADIUS client
     implementations to implement complex retransmission algorithms,
     implementations SHOULD support congestive backoff within the limits
     suggested by [RFC2865] Section 2.4.  For example, an implementation
     SHOULD double the initial retransmission timer until a maximum
     retransmission time is reached, after which the client will
     failover to another RADIUS server.  For example, if the initial



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     retransmission timer is one second, a maximum retransmission timer
     of 16 seconds might be used.

2.2.2.  Server Response to Overload

   Some RADIUS server implementations are not robust in response to
   overload, dropping packets with even probability across multiple
   sessions.  In an overload situation, this results in a high failure
   rate for multi-round authentication protocols such as EAP [RFC3579].
   Typically, users will continually retry in an attempt to gain access,
   increasing the load even further.

   A more sensible approach is for a RADIUS server to preferentially
   accept RADIUS Access-Request packets containing a valid State
   attribute, so that multi-round authentication conversations, once
   begun, will be more likely to succeed.  This will allow some users to
   gain access to the network, reducing the load created by ongoing
   access attempts.

2.3.  Accounting Issues

2.3.1.  Interim Update

   [RFC2866] indicates that Acct-Input-Octets, Acct-Output-Octets, Acct-
   Session-Time, Acct-Input-Packets, Acct-Output-Packets and Acct-
   Terminate-Cause attributes "can only be present in Accounting-Request
   records where the Acct-Status-Type is set to Stop."

   However [RFC2869] Section 2.1 states:

      It is envisioned that an Interim Accounting record (with Acct-
      Status-Type = Interim-Update (3)) would contain all of the
      attributes normally found in an Accounting Stop message with the
      exception of the Acct-Term-Cause attribute.

   Although [RFC2869] does not indicate that it updates [RFC2866], this
   is an oversight, and the above attributes are allowable in an Interim
   Accounting record.

2.3.2.  Acct-Session-Id and Acct-Multi-Session-Id

   [RFC2866] Section 5.5 describes Acct-Session-Id as Text within the
   description, but also states that "The String field SHOULD be a
   string of UTF-8 encoded 10646 characters."

   Since Acct-Multi-Session-Id is consistently described as a String, it
   appears that this is a typographical error, and that Acct-Session-Id
   is of type String.



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   The implication is that a robust implementation SHOULD support the
   String fields within Acct-Session-Id and Acct-Multi-Session-Id as
   undistinguished octets.

2.3.3.  Request Authenticator

   [RFC2866] Section 4.1 states:

      The Request Authenticator of an Accounting-Request contains a
      16-octet MD5 hash value calculated according to the method
      described in "Request Authenticator" above.

   However, the text does not indicate any action to take when an
   Accounting-Request packet contains an invalid Request Authenticator.
   The following text should be considered to be part of the above
   description:

      The Request Authenticator field MUST contain the correct data, as
      given by the above calculation.  Invalid packets are silently
      discarded.  Note that some early implementations always set the
      Request Authenticator to all zeros.  New implementations of RADIUS
      clients MUST use the above algorithm to calculate the Request
      Authenticator field.  New RADIUS server implementations MUST
      silently discard invalid packets.

2.4.  Multiple Filter-ID Attributes

   [RFC2865] Section 5.11 states:

      Zero or more Filter-Id attributes MAY be sent in an Access-Accept
      packet.

   In practice the behavior of a RADIUS client receiving multiple
   Filter-ID attributes is implementation dependent.  For example, some
   implementations treat multiple instances of the Filter-ID attribute
   as alternative filters; the first Filter-ID attribute having a name
   matching a locally defined filter is used, and the remaining ones are
   discarded.  Other implementations may combine matching filters.

   As a result, the interpretation of multiple Filter-ID attributes is
   undefined within RADIUS.  The sending of multiple Filter-ID
   attributes within an Access-Accept SHOULD be avoided within
   heterogeneous deployments and roaming scenarios, where it is likely
   to produce unpredictable results.







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2.5.  Mandatory and Optional Attributes

   RADIUS attributes do not explicitly state whether they are optional
   or mandatory.  Nevertheless there are instances where RADIUS
   attributes need to be treated as mandatory.

   [RFC2865] Section 1.1 states:

      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 ARAP service MUST NOT implement the RADIUS
      attributes for ARAP.  A NAS MUST treat a RADIUS access-accept
      authorizing an unavailable service as an access-reject instead.

   With respect to the Service-Type attribute, [RFC2865] Section 5.6
   says:

      This Attribute indicates the type of service the user has
      requested, or the type of service to be provided. It MAY be used
      in both Access-Request and Access-Accept packets.  A NAS is not
      required to implement all of these service types, and MUST treat
      unknown or unsupported Service-Types as though an Access-Reject
      had been received instead.

   [RFC2865] Section 5 states:

      A RADIUS server MAY ignore Attributes with an unknown Type.
      A RADIUS client MAY ignore Attributes with an unknown Type.

   With respect to Vendor-Specific Attributes (VSAs), [RFC2865] Section
   5.26 states:

      Servers not equipped to interpret the vendor-specific information
      sent by a client MUST ignore it (although it may be reported).
      Clients which do not receive desired vendor-specific information
      SHOULD make an attempt to operate without it, although they may do
      so (and report they are doing so) in a degraded mode.

   It is possible for either a standard attribute or VSA to represent a
   request for an unavailable service.  However, where the Type or
   Vendor-ID is unknown, a RADIUS client will not know whether the
   attribute defines a service or not.

   In general, it is best for RADIUS clients to err on the side of
   caution.  On receiving an Access-Accept including an attribute of
   unknown Type, a RADIUS client SHOULD assume that it is a potential
   service definition, and treat it as an Access-Reject.  Unknown VSAs
   SHOULD be ignored by RADIUS clients.



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   RADIUS authentication server implementations SHOULD ignore attributes
   of unknown Type.  Since RADIUS accounting server implementations
   typically do not need to understand attributes in order to write them
   to stable storage or pass them to the billing engine, accounting
   server implementations SHOULD be equipped to handle unknown
   attributes.

   To avoid misinterpretation of service requests encoded within VSAs,
   RADIUS servers SHOULD NOT send VSAs containing service requests to
   RADIUS clients that are not known to understand them.  For example, a
   RADIUS server should not send a VSA encoding a filter without
   knowledge that the RADIUS client supports the VSA.

2.6.  Interpretation of Access-Reject

2.6.1.  Improper Use of Access-Reject

   The intent of an Access-Reject is to deny access to the requested
   service.  [RFC2865] Section 2 states:

      If any condition is not met, the RADIUS server sends an "Access-
      Reject" response indicating that this user request is invalid.  If
      desired, the server MAY include a text message in the Access-
      Reject which MAY be displayed by the client to the user.  No other
      Attributes (except Proxy-State) are permitted in an Access-Reject.

   This text makes it clear that RADIUS does not allow the provisioning
   of services within an Access-Reject.  If the desire is to allow
   limited access, then an Access-Accept can be sent with attributes
   provisioning limited access.  Attributes within an Access-Reject are
   restricted to those necessary to route the message (e.g. Proxy-
   State), attributes providing the user with an indication that access
   has been denied (e.g. an EAP-Message attribute containing an EAP-
   Failure) or attributes conveying an error message (e.g. a Reply-
   Message or Error-Cause attribute).

   Unfortunately, there are examples where this requirement has been
   misunderstood.  [RFC2869] Section 2.2 states:

      If that authentication fails, the RADIUS server should return an
      Access-Reject packet to the NAS, with optional Password-Retry and
      Reply-Messages attributes.  The presence of Password-Retry
      indicates the ARAP NAS MAY choose to initiate another challenge-
      response cycle,

   This paragraph is problematic from two perspectives.  Firstly, a
   Password-Retry attribute is being returned in an Access-Reject; this
   attribute does not fit into the categories established in [RFC2865].



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   Secondly, an Access-Reject packet is being sent in the context of a
   continuing authentication conversation; [RFC2865] requires use of an
   Access-Challenge for this.  [RFC2869] uses the phrase "challenge-
   response" to describe this use of Access-Reject, indicating that the
   semantics of Access-Challenge are being used.

   [RFC2865] Section 4.4, addresses the semantics of Access-Challenge
   being equivalent to Access-Reject in some cases:

      If the NAS does not support challenge/response, it MUST treat an
      Access-Challenge as though it had received an Access-Reject
      instead.

   While it is difficult to correct existing deployments of [RFC2869],
   we make the following recommendations:

[1]  New RADIUS specifications and implementations MUST NOT use Access-
     Reject where the semantics of Access-Challenge are intended.

[2]  Access-Reject MUST mean denial of access to the requested service.
     In response to an Access-Reject, the NAS MUST NOT send any
     additional Access-Request packets for that user session.

[3]  New deployments of ARAP [RFC2869] SHOULD use Access-Challenge
     instead of Access-Reject packets in the conversations described in
     [RFC2869] Section 2.2.

   We also note that the table of attributes [RFC2869] Section 5.19 has
   an error for the Password-Retry attribute.  It says:

   Request  Accept  Reject  Challenge   #    Attribute
   0        0       0-1     0           75   Password-Retry

   However, the text in [RFC2869] Section 2.3.2 says that Password-Retry
   can be included within an Access-Challenge packet, for EAP
   authentication sessions.  We recommend a correction to the table:

   Request  Accept  Reject  Challenge   #    Attribute
   0        0       0       0-1         75   Password-Retry [Note 2]

   [Note 2] As per RFC 3579, the use of the Password-Retry in EAP
   authentications is deprecated.  The Password-Retry attribute can be
   used only for ARAP authentication.

2.6.2.  Service Request Denial

   RADIUS has been deployed for purposes outside network access
   authentication, authorization and accounting.  For example, RADIUS



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   has been deployed as a "back-end" for authenticating VOIP connections
   [digest], HTTP sessions [apache], FTP sessions [proftpd], and machine
   logins for multiple operating systems [bsdi, pam, gina].  In those
   contexts, an Access-Reject sent to the RADIUS client MUST be
   interpreted as a rejection of the request for service, and the RADIUS
   client MUST NOT offer that service to the user.

   For example, when an authentication failure occurs in the context of
   an FTP session, the normal semantics for rejecting FTP services
   apply.  The rejection does not necessarily cause the FTP server to
   terminate the underlying TCP connection, but the FTP server MUST NOT
   offer any services protected by user authentication.

   Users may request multiple services from the NAS.  Where those
   services are independent, the deployment MUST treat the RADIUS
   sessions as being independent.

   For example, a NAS may offer multi-link services, where a user may
   have multiple simultaneous network connections. In that case, an
   Access-Reject for a later multi-link connection request does not
   necessarily mean that earlier multi-link connections are torn down.
   Similarly, if a NAS offers both dialup and VOIP services, the
   rejection of a VOIP attempt does not mean that the dialup session is
   torn down.

   Where a NAS offers multiple services, confusion may result with
   respect to interpretation of a Disconnect-Request [RFC3576].  In
   order to prevent confusion a RADIUS Server SHOULD identify the
   session that it desires to terminate as specifically as possible.
   For example, an Acct-Session-Id attribute SHOULD be included in
   Disconnect-Request and CoA-Request packets, rather than just the
   User-Name attribute.

2.7.  Addressing

2.7.1.  Link-Local Addresses

   Since Link-Local addresses are unique only on the local link, if the
   NAS and RADIUS server are not on the same link, then an IPv6 Link-
   Local address [RFC2462] or an IPv4 Link-Local Address [RFC3927]
   cannot be used to uniquely identify the NAS.  A RADIUS server
   receiving a NAS-IPv6-Address or NAS-IP-Address attribute containing a
   Link-Local address SHOULD NOT count such an attribute toward
   satisfying the requirements of [RFC3162] Section 2.1:

         NAS-IPv6-Address and/or NAS-IP-Address MAY be present in an
         Access-Request packet; however, if neither attribute is present
         then NAS-Identifier MUST be present.



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2.7.2.  Multiple Addresses

   There are situations in which a RADIUS client or server may have
   multiple addresses.  For example, a dual stack host can have both
   IPv4 and IPv6 addresses; a host that is a member of multiple VLANs
   could have IPv4 and/or IPv6 addresses on each VLAN; a host can have
   multiple IPv4 or IPv6 addresses on a single interface.  However,
   [RFC2865] Section 5.44 only  permits zero or one NAS-IP-Address
   attribute within an Access-Request and [RFC3162] Section 3 only
   permits zero or one NAS-IPv6-Address attribute within an Access-
   Request.  When a NAS has more than one global address and no ability
   to determine which is used for identification in a particular
   request, it is RECOMMENDED that the NAS include the NAS-Identifier
   attribute in an Access-Request in order to identify itself to the
   RADIUS server.

   [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.

   Therefore if a RADIUS client sends packets from more than one source
   address, a shared secret will need to be configured on both the
   client and server for each source address.

2.8.  Idle-Timeout

   With respect to the Idle-Timeout attribute, [RFC2865] Section 5.28
   states:

      This Attribute sets the maximum number of consecutive seconds of
      idle connection allowed to the user before termination of the
      session or prompt.  This Attribute is available to be sent by the
      server to the client in an Access-Accept or Access-Challenge.

   [RFC3580] Section 3.12 states:

      The Idle-Timeout attribute is described in [RFC2865].  For IEEE
      802 media other than 802.11 the media are always on.  As a result
      the Idle-Timeout attribute is typically only used with wireless
      media such as IEEE 802.11.  It is possible for a wireless device
      to wander out of range of all Access Points.  In this case, the
      Idle-Timeout attribute indicates the maximum time that a wireless
      device may remain idle.

   In the above paragraphs "idle" may not necessarily mean "no traffic";
   the NAS may support filters defining what traffic is included in the



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   idle time determination.  As a result, an "idle connection" is
   defined by local policy in the absence of other attributes.

2.9.  Calculation of Message-Authenticator

   [RFC3579] Section 3.3 indicates that the Message-Authenticator
   attribute may be included in Access-Request, Accept, Reject and
   Challenge packets,  and [RFC3579] Section 3.2 describes how the
   Message-Authenticator attribute is calculated for these packets:

      When present in an Access-Request packet, Message-Authenticator is
      an HMAC-MD5 [RFC2104] checksum 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.

      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 is calculated and inserted in the packet
      before the Response Authenticator is calculated.

   [RFC3576] Section 3.2 indicates that a Message-Authenticator
   attribute may be included within a Disconnect or CoA-Request, ACK or
   NAK.  However [RFC3579] does not describe how a Message-Authenticator
   attribute is calculated for these RADIUS packets, or for Accounting-
   Request or Response packets.

   The algorithm described in [RFC3579] Section 3.2 cannot be applied
   because in a Disconnect, CoA or Accounting-Request packet the Request
   Authenticator is not a nonce, but rather represents a keyed MD5 hash
   over the packet.  This creates a circular dependency -- the Request
   Authenticator depends on Message-Authenticator and Message-
   Authenticator depends on the Response Authenticator.




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   As a result, in Accounting, CoA or Disconnect-Request or Response
   packets the Message-Authenticator attribute is calculated as follows:

      Message-Authenticator = HMAC-MD5 (Type, Identifier, Length,
      Authenticator, Attributes)

      When the HMAC-MD5 hash is calculated the Authenticator field and
      Message-Authenticator attribute should be considered to be sixteen
      octets of zero.  The Message-Authenticator is calculated and
      inserted in the packet before the Authenticator is calculated.

2.10.  Unknown Identity

   [RFC3748] Section 5.1 states:

        If the Identity is unknown, the Identity Response field
        should be zero bytes in length.

   However, [RFC2865] Section 5.1 describes the User-Name attribute as
   follows:

        The String field is one or more octets.

   How should the RADIUS client behave if it receives an EAP-
   Response/Identity that is zero octets in length?

   [RFC2865] Section 5.1 states:

         This Attribute indicates the name of the user to be authenticated.
         It MUST be sent in Access-Request packets if available.

   This suggests that the User-Name attribute may be ommitted if it is
   unavailable.

   However, [RFC3579] Section 2.1 states:

      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.

   This suggests that the User-Name attribute should contain the
   contents of the Type-Data field of the EAP-Response/Identity, even if
   it is zero octets in length.




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   Note that [RFC4282] does not permit an NAI of zero octets, so that an
   EAP-Response/Identity with a Type-Data field of zero octets MUST NOT
   be construed as a request for privacy (e.g. anonymous NAI).

   When a NAS receives an EAP-Response/Identity with a Type-Data field
   that is zero octets in length, it is RECOMMENDED that it either omit
   a User-Name attribute in the Access-Request or include the Calling-
   Station-Id in the User-Name attribute, along with a Calling-Station-
   Id attribute.

3.  IANA Considerations

   This specification does not create any new registries, nor does it
   require assignment of any protocol parameters.

4.  Security Considerations

   Since this document describes the use of RADIUS for purposes of
   authentication, authorization, and accounting in WLANs, it is
   vulnerable to all of the threats that are present in other RADIUS
   applications.  For a discussion of these threats, see [RFC2865], [RFC
   2607], [RFC3162], [RFC3576], [RFC3579], and [RFC3580].

5.  References

5.1.  Informative references

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

[RFC2462] Thomson, S. and T. Narten, "IPv6 Stateless Address
          Autoconfiguration", RFC 2462, December 1998.

[RFC2607] Aboba, B. and J. Vollbrecht, "Proxy Chaining and Policy
          Implementation in Roaming", RFC 2607, June 1999.

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

[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



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          Support", RFC 2868, June 2000.

[RFC2869] Rigney, C., Willats, W. and P. Calhoun, "RADIUS Extensions",
          RFC 2869, June 2000.

[RFC2882] Mitton, D., "Network Access Servers Requirements: Extended
          RADIUS Practices", RFC 2882, July 2000.

[RFC3162] Aboba, B., Zorn, G. and D. Mitton, "RADIUS and IPv6", RFC
          3162, August 2001.

[RFC3575] Aboba, B., "IANA Considerations for RADIUS", RFC 3575, July
          2003.

[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.

[RFC3579] Aboba, B. and P. Calhoun, "RADIUS Support for Extensible
          Authentication Protocol (EAP)", RFC 3579, September 2003.

[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.

[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J. and H.
          Levkowetz, "Extensible Authentication Protocol (EAP)", RFC
          3748, June 2004.

[RFC3927] Cheshire, S., Aboba, B. and E. Guttman, "Dynamic Configuration
          of IPv4 Link-Local Addresses",  RFC 3927, May 2005.

[RFC4282] Aboba, B., Beadles, M., Arkko, J. and P. Eronen, "The Network
          Access Identifier", RFC 4282, December 2005.

Acknowledgments

   The authors would like to acknowledge Glen Zorn for contributions to
   this document.

Authors' Addresses

   David B. Nelson
   Enterasys Networks
   50 Minuteman Road
   Andover, MA  01810

   EMail: dnelson@enterasys.com



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   Alan DeKok
   Infoblox, Inc.
   475 Potrero Ave
   Sunnyvale, CA 94087

   Email: adekok@infoblox.com
   Phone: +1 408 716 4386
   Fax:   +1 408 716 4400

   Bernard Aboba
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052

   EMail: bernarda@microsoft.com
   Phone: +1 425 706 6605
   Fax:   +1 425 936 7329

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   ipr@ietf.org.










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Disclaimer of Validity

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM 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.

Copyright Statement

   Copyright (C) The Internet Society (2006).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.

Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.

Open issues

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

   http://www.drizzle.com/~aboba/RADEXT/
























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