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RFC 2865 - Remote Authentication Dial In User Service (RADIUS)


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Network Working Group                                          C. Rigney
Request for Comments: 2865                                    S. Willens
Obsoletes: 2138                                               Livingston
Category: Standards Track                                      A. Rubens
                                                                   Merit
                                                              W. Simpson
                                                              Daydreamer
                                                               June 2000

          Remote Authentication Dial In User Service (RADIUS)

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

IESG Note:

   This protocol is widely implemented and used.  Experience has shown
   that it can suffer degraded performance and lost data when used in
   large scale systems, in part because it does not include provisions
   for congestion control.  Readers of this document may find it
   beneficial to track the progress of the IETF's AAA working group,
   which may develop a successor protocol that better addresses the
   scaling and congestion control issues.

Abstract

   This document describes a protocol for carrying authentication,
   authorization, and configuration information between a Network Access
   Server which desires to authenticate its links and a shared
   Authentication Server.

Implementation Note

   This memo documents the RADIUS protocol.  The early deployment of
   RADIUS was done using UDP port number 1645, which conflicts with the
   "datametrics" service.  The officially assigned port number for
   RADIUS is 1812.

Table of Contents

   1.     Introduction ..........................................    3
      1.1       Specification of Requirements ...................    4
      1.2       Terminology .....................................    5
   2.     Operation .............................................    5
      2.1       Challenge/Response ..............................    7
      2.2       Interoperation with PAP and CHAP ................    8
      2.3       Proxy ...........................................    8
      2.4       Why UDP? ........................................   11
      2.5       Retransmission Hints ............................   12
      2.6       Keep-Alives Considered Harmful ..................   13
   3.     Packet Format .........................................   13
   4.     Packet Types ..........................................   17
      4.1       Access-Request ..................................   17
      4.2       Access-Accept ...................................   18
      4.3       Access-Reject ...................................   20
      4.4       Access-Challenge ................................   21
   5.     Attributes ............................................   22
      5.1       User-Name .......................................   26
      5.2       User-Password ...................................   27
      5.3       CHAP-Password ...................................   28
      5.4       NAS-IP-Address ..................................   29
      5.5       NAS-Port ........................................   30
      5.6       Service-Type ....................................   31
      5.7       Framed-Protocol .................................   33
      5.8       Framed-IP-Address ...............................   34
      5.9       Framed-IP-Netmask ...............................   34
      5.10      Framed-Routing ..................................   35
      5.11      Filter-Id .......................................   36
      5.12      Framed-MTU ......................................   37
      5.13      Framed-Compression ..............................   37
      5.14      Login-IP-Host ...................................   38
      5.15      Login-Service ...................................   39
      5.16      Login-TCP-Port ..................................   40
      5.17      (unassigned) ....................................   41
      5.18      Reply-Message ...................................   41
      5.19      Callback-Number .................................   42
      5.20      Callback-Id .....................................   42
      5.21      (unassigned) ....................................   43
      5.22      Framed-Route ....................................   43
      5.23      Framed-IPX-Network ..............................   44
      5.24      State ...........................................   45
      5.25      Class ...........................................   46
      5.26      Vendor-Specific .................................   47
      5.27      Session-Timeout .................................   48
      5.28      Idle-Timeout ....................................   49
      5.29      Termination-Action ..............................   49

      5.30      Called-Station-Id ...............................   50
      5.31      Calling-Station-Id ..............................   51
      5.32      NAS-Identifier ..................................   52
      5.33      Proxy-State .....................................   53
      5.34      Login-LAT-Service ...............................   54
      5.35      Login-LAT-Node ..................................   55
      5.36      Login-LAT-Group .................................   56
      5.37      Framed-AppleTalk-Link ...........................   57
      5.38      Framed-AppleTalk-Network ........................   58
      5.39      Framed-AppleTalk-Zone ...........................   58
      5.40      CHAP-Challenge ..................................   59
      5.41      NAS-Port-Type ...................................   60
      5.42      Port-Limit ......................................   61
      5.43      Login-LAT-Port ..................................   62
      5.44      Table of Attributes .............................   63
   6.     IANA Considerations ...................................   64
      6.1       Definition of Terms .............................   64
      6.2       Recommended Registration Policies ...............   65
   7.     Examples ..............................................   66
      7.1       User Telnet to Specified Host ...................   66
      7.2       Framed User Authenticating with CHAP ............   67
      7.3       User with Challenge-Response card ...............   68
   8.     Security Considerations ...............................   71
   9.     Change Log ............................................   71
   10.    References ............................................   73
   11.    Acknowledgements ......................................   74
   12.    Chair's Address .......................................   74
   13.    Authors' Addresses ....................................   75
   14.    Full Copyright Statement ..............................   76

1.  Introduction

   This document obsoletes RFC 2138 [1].  A summary of the changes
   between this document and RFC 2138 is available in the "Change Log"
   appendix.

   Managing dispersed serial line and modem pools for large numbers of
   users can create the need for significant administrative support.
   Since modem pools are by definition a link to the outside world, they
   require careful attention to security, authorization and accounting.
   This can be best achieved by managing a single "database" of users,
   which allows for authentication (verifying user name and password) as
   well as configuration information detailing the type of service to
   deliver to the user (for example, SLIP, PPP, telnet, rlogin).

   Key features of RADIUS are:

   Client/Server Model

      A Network Access Server (NAS) operates as a client of RADIUS.  The
      client is responsible for passing user information to designated
      RADIUS servers, and then acting on the response which is returned.

      RADIUS servers are responsible for receiving user connection
      requests, authenticating the user, and then returning all
      configuration information necessary for the client to deliver
      service to the user.

      A RADIUS server can act as a proxy client to other RADIUS servers
      or other kinds of authentication servers.

   Network Security

      Transactions between the client and RADIUS server are
      authenticated through the use of a shared secret, which is never
      sent over the network.  In addition, any user passwords are sent
      encrypted between the client and RADIUS server, to eliminate the
      possibility that someone snooping on an unsecure network could
      determine a user's password.

   Flexible Authentication Mechanisms

      The RADIUS server can support a variety of methods to authenticate
      a user.  When it is provided with the user name and original
      password given by the user, it can support PPP PAP or CHAP, UNIX
      login, and other authentication mechanisms.

   Extensible Protocol

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

1.1.  Specification of Requirements

   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 BCP 14 [2].  These key
   words mean the same thing whether capitalized or not.

   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".

   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.

1.2.  Terminology

   This document frequently uses the following terms:

   service   The NAS provides a service to the dial-in user, such as PPP
             or Telnet.

   session   Each service provided by the NAS to a dial-in user
             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 user may have multiple sessions in parallel or
             series if the NAS supports that.

   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.

2.  Operation

   When a client is configured to use RADIUS, any user of the client
   presents authentication information to the client.  This might be
   with a customizable login prompt, where the user is expected to enter
   their username and password.  Alternatively, the user might use a
   link framing protocol such as the Point-to-Point Protocol (PPP),
   which has authentication packets which carry this information.

   Once the client has obtained such information, it may choose to
   authenticate using RADIUS.  To do so, the client creates an "Access-
   Request" containing such Attributes as the user's name, the user's
   password, the ID of the client and the Port ID which the user is
   accessing.  When a password is present, it is hidden using a method
   based on the RSA Message Digest Algorithm MD5 [3].

   The Access-Request is submitted to the RADIUS server via the network.
   If no response is returned within a length of time, the request is
   re-sent a number of times.  The client can also forward requests to
   an alternate server or servers in the event that the primary server
   is down or unreachable.  An alternate server can be used either after
   a number of tries to the primary server fail, or in a round-robin
   fashion.  Retry and fallback algorithms are the topic of current
   research and are not specified in detail in this document.

   Once the RADIUS server receives the request, it validates the sending
   client.  A request from a client for which the RADIUS server does not
   have a shared secret MUST be silently discarded.  If the client is
   valid, the RADIUS server consults a database of users to find the
   user whose name matches the request.  The user entry in the database
   contains a list of requirements which must be met to allow access for
   the user.  This always includes verification of the password, but can
   also specify the client(s) or port(s) to which the user is allowed
   access.

   The RADIUS server MAY make requests of other servers in order to
   satisfy the request, in which case it acts as a client.

   If any Proxy-State attributes were present in the Access-Request,
   they MUST be copied unmodified and in order into the response packet.
   Other Attributes can be placed before, after, or even between the
   Proxy-State attributes.

   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.

   If all conditions are met and the RADIUS server wishes to issue a
   challenge to which the user must respond, the RADIUS server sends an
   "Access-Challenge" response.  It MAY include a text message to be
   displayed by the client to the user prompting for a response to the
   challenge, and MAY include a State attribute.

   If the client receives an Access-Challenge and supports
   challenge/response it MAY display the text message, if any, to the
   user, and then prompt the user for a response.  The client then re-
   submits its original Access-Request with a new request ID, with the
   User-Password Attribute replaced by the response (encrypted), and
   including the State Attribute from the Access-Challenge, if any.
   Only 0 or 1 instances of the State Attribute SHOULD be

   present in a request.  The server can respond to this new Access-
   Request with either an Access-Accept, an Access-Reject, or another
   Access-Challenge.

   If all conditions are met, the list of configuration values for the
   user are placed into an "Access-Accept" response.  These values
   include the type of service (for example: SLIP, PPP, Login User) and
   all necessary values to deliver the desired service.  For SLIP and
   PPP, this may include values such as IP address, subnet mask, MTU,
   desired compression, and desired packet filter identifiers.  For
   character mode users, this may include values such as desired
   protocol and host.

2.1.  Challenge/Response

   In challenge/response authentication, the user is given an
   unpredictable number and challenged to encrypt it and give back the
   result. Authorized users are equipped with special devices such as
   smart cards or software that facilitate calculation of the correct
   response with ease. Unauthorized users, lacking the appropriate
   device or software and lacking knowledge of the secret key necessary
   to emulate such a device or software, can only guess at the response.

   The Access-Challenge packet typically contains a Reply-Message
   including a challenge to be displayed to the user, such as a numeric
   value unlikely ever to be repeated. Typically this is obtained from
   an external server that knows what type of authenticator is in the
   possession of the authorized user and can therefore choose a random
   or non-repeating pseudorandom number of an appropriate radix and
   length.

   The user then enters the challenge into his device (or software) and
   it calculates a response, which the user enters into the client which
   forwards it to the RADIUS server via a second Access-Request.  If the
   response matches the expected response the RADIUS server replies with
   an Access-Accept, otherwise an Access-Reject.

   Example: The NAS sends an Access-Request packet to the RADIUS Server
   with NAS-Identifier, NAS-Port, User-Name, User-Password (which may
   just be a fixed string like "challenge" or ignored).  The server
   sends back an Access-Challenge packet with State and a Reply-Message
   along the lines of "Challenge 12345678, enter your response at the
   prompt" which the NAS displays.  The NAS prompts for the response and
   sends a NEW Access-Request to the server (with a new ID) with NAS-
   Identifier, NAS-Port, User-Name, User-Password (the response just
   entered by the user, encrypted), and the same State Attribute that

   came with the Access-Challenge.  The server then sends back either an
   Access-Accept or Access-Reject based on whether the response matches
   the required value, or it can even send another Access-Challenge.

2.2.  Interoperation with PAP and CHAP

   For PAP, the NAS takes the PAP ID and password and sends them in an
   Access-Request packet as the User-Name and User-Password. The NAS MAY
   include the Attributes Service-Type = Framed-User and Framed-Protocol
   = PPP as a hint to the RADIUS server that PPP service is expected.

   For CHAP, the NAS generates a random challenge (preferably 16 octets)
   and sends it to the user, who returns a CHAP response along with a
   CHAP ID and CHAP username.  The NAS then sends an Access-Request
   packet to the RADIUS server with the CHAP username as the User-Name
   and with the CHAP ID and CHAP response as the CHAP-Password
   (Attribute 3).  The random challenge can either be included in the
   CHAP-Challenge attribute or, if it is 16 octets long, it can be
   placed in the Request Authenticator field of the Access-Request
   packet.  The NAS MAY include the Attributes Service-Type = Framed-
   User and Framed-Protocol = PPP as a hint to the RADIUS server that
   PPP service is expected.

   The RADIUS server looks up a password based on the User-Name,
   encrypts the challenge using MD5 on the CHAP ID octet, that password,
   and the CHAP challenge (from the CHAP-Challenge attribute if present,
   otherwise from the Request Authenticator), and compares that result
   to the CHAP-Password.  If they match, the server sends back an
   Access-Accept, otherwise it sends back an Access-Reject.

   If the RADIUS server is unable to perform the requested
   authentication it MUST return an Access-Reject.  For example, CHAP
   requires that the user's password be available in cleartext to the
   server so that it can encrypt the CHAP challenge and compare that to
   the CHAP response.  If the password is not available in cleartext to
   the RADIUS server then the server MUST send an Access-Reject to the
   client.

2.3.  Proxy

   With proxy RADIUS, one RADIUS server receives an authentication (or
   accounting) request from a RADIUS client (such as a NAS), forwards
   the request to a remote RADIUS server, receives the reply from the
   remote server, and sends that reply to the client, possibly with
   changes to reflect local administrative policy.  A common use for
   proxy RADIUS is roaming.  Roaming permits two or more administrative
   entities to allow each other's users to dial in to either entity's
   network for service.

   The NAS sends its RADIUS access-request to the "forwarding server"
   which forwards it to the "remote server".  The remote server sends a
   response (Access-Accept, Access-Reject, or Access-Challenge) back to
   the forwarding server, which sends it back to the NAS.  The User-Name
   attribute MAY contain a Network Access Identifier [8] for RADIUS
   Proxy operations.  The choice of which server receives the forwarded
   request SHOULD be based on the authentication "realm". The
   authentication realm MAY be the realm part of a Network Access
   Identifier (a "named realm").  Alternatively, the choice of which
   server receives the forwarded request MAY be based on whatever other
   criteria the forwarding server is configured to use, such as Called-
   Station-Id (a "numbered realm").

   A RADIUS server can function as both a forwarding server and a remote
   server, serving as a forwarding server for some realms and a remote
   server for other realms.  One forwarding server can act as a
   forwarder for any number of remote servers.  A remote server can have
   any number of servers forwarding to it and can provide authentication
   for any number of realms.  One forwarding server can forward to
   another forwarding server to create a chain of proxies, although care
   must be taken to avoid introducing loops.

   The following scenario illustrates a proxy RADIUS communication
   between a NAS and the forwarding and remote RADIUS servers:

   1. A NAS sends its access-request to the forwarding server.

   2. The forwarding server forwards the access-request to the remote
      server.

   3. The remote server sends an access-accept, access-reject or
      access-challenge back to the forwarding server.  For this example,
      an access-accept is sent.

   4. The forwarding server sends the access-accept to the NAS.

   The forwarding server MUST treat any Proxy-State attributes already
   in the packet as opaque data.  Its operation MUST NOT depend on the
   content of Proxy-State attributes added by previous servers.

   If there are any Proxy-State attributes in the request received from
   the client, the forwarding server MUST include those Proxy-State
   attributes in its reply to the client.  The forwarding server MAY
   include the Proxy-State attributes in the access-request when it
   forwards the request, or MAY omit them in the forwarded request.  If
   the forwarding server omits the Proxy-State attributes in the
   forwarded access-request, it MUST attach them to the response before
   sending it to the client.

   We now examine each step in more detail.

   1. A NAS sends its access-request to the forwarding server.  The
      forwarding server decrypts the User-Password, if present, using
      the shared secret it knows for the NAS.  If a CHAP-Password
      attribute is present in the packet and no CHAP-Challenge attribute
      is present, the forwarding server MUST leave the Request-
      Authenticator untouched or copy it to a CHAP-Challenge attribute.

   '' The forwarding server MAY add one Proxy-State attribute to the
      packet.  (It MUST NOT add more than one.)  If it adds a Proxy-
      State, the Proxy-State MUST appear after any other Proxy-States in
      the packet.  The forwarding server MUST NOT modify any other
      Proxy-States that were in the packet (it may choose not to forward
      them, but it MUST NOT change their contents).  The forwarding
      server MUST NOT change the order of any attributes of the same
      type, including Proxy-State.

   2. The forwarding server encrypts the User-Password, if present,
      using the secret it shares with the remote server, sets the
      Identifier as needed, and forwards the access-request to the
      remote server.

   3. The remote server (if the final destination) verifies the user
      using User-Password, CHAP-Password, or such method as future
      extensions may dictate, and returns an access-accept, access-
      reject or access-challenge back to the forwarding server.  For
      this example, an access-accept is sent.  The remote server MUST
      copy all Proxy-State attributes (and only the Proxy-State
      attributes) in order from the access-request to the response
      packet, without modifying them.

   4. The forwarding server verifies the Response Authenticator using
      the secret it shares with the remote server, and silently discards
      the packet if it fails verification.  If the packet passes
      verification, the forwarding server removes the last Proxy-State
      (if it attached one), signs the Response Authenticator using the
      secret it shares with the NAS, restores the Identifier to match
      the one in the original request by the NAS, and sends the access-
      accept to the NAS.

   A forwarding server MAY need to modify attributes to enforce local
   policy.  Such policy is outside the scope of this document, with the
   following restrictions.  A forwarding server MUST not modify existing
   Proxy-State, State, or Class attributes present in the packet.

   Implementers of forwarding servers should consider carefully which
   values it is willing to accept for Service-Type.  Careful
   consideration must be given to the effects of passing along Service-
   Types of NAS-Prompt or Administrative in a proxied Access-Accept, and
   implementers may wish to provide mechanisms to block those or other
   service types, or other attributes.  Such mechanisms are outside the
   scope of this document.

2.4.  Why UDP?

   A frequently asked question is why RADIUS uses UDP instead of TCP as
   a transport protocol.  UDP was chosen for strictly technical reasons.

   There are a number of issues which must be understood.  RADIUS is a
   transaction based protocol which has several interesting
   characteristics:

   1. If the request to a primary Authentication server fails, a
      secondary server must be queried.

      To meet this requirement, a copy of the request must be kept above
      the transport layer to allow for alternate transmission.  This
      means that retransmission timers are still required.

   2. The timing requirements of this particular protocol are
      significantly different than TCP provides.

      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 acknowledgement 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.

   3. The stateless nature of this protocol simplifies the use of UDP.

      Clients and servers come and go.  Systems are rebooted, or are
      power cycled independently.  Generally this does not cause a
      problem and with creative timeouts and detection of lost TCP
      connections, code can be written to handle anomalous events.  UDP
      however completely eliminates any of this special handling.  Each
      client and server can open their UDP transport just once and leave
      it open through all types of failure events on the network.

   4. UDP simplifies the server implementation.

      In the earliest implementations of RADIUS, the server was single
      threaded.  This means that a single request was received,
      processed, and returned.  This was found to be unmanageable in
      environments where the back-end security mechanism took real time
      (1 or more seconds).  The server request queue would fill and in
      environments where hundreds of people were being authenticated
      every minute, the request turn-around time increased to longer
      than users were willing to wait (this was especially severe when a
      specific lookup in a database or over DNS took 30 or more
      seconds).  The obvious solution was to make the server multi-
      threaded.  Achieving this was simple with UDP.  Separate processes
      were spawned to serve each request and these processes could
      respond directly to the client NAS with a simple UDP packet to the
      original transport of the client.

   It's not all a panacea.  As noted, using UDP requires one thing which
   is built into TCP: with UDP we must artificially manage
   retransmission timers to the same server, although they don't require
   the same attention to timing provided by TCP.  This one penalty is a
   small price to pay for the advantages of UDP in this protocol.

   Without TCP we would still probably be using tin cans connected by
   string.  But for this particular protocol, UDP is a better choice.

2.5.  Retransmission Hints

   If the RADIUS server and alternate RADIUS server share the same
   shared secret, it is OK to retransmit the packet to the alternate
   RADIUS server with the same ID and Request Authenticator, because the
   content of the attributes haven't changed.  If you want to use a new
   Request Authenticator when sending to the alternate server, you may.

   If you change the contents of the User-Password attribute (or any
   other attribute), you need a new Request Authenticator and therefore
   a new ID.

   If the NAS is retransmitting a RADIUS request to the same server as
   before, and the attributes haven't changed, you MUST use the same
   Request Authenticator, ID, and source port.  If any attributes have
   changed, you MUST use a new Request Authenticator and ID.

   A NAS MAY use the same ID across all servers, or MAY keep track of
   IDs separately for each server, it is up to the implementer.  If a
   NAS needs more than 256 IDs for outstanding requests, it MAY use

   additional source ports to send requests from, and keep track of IDs
   for each source port.  This allows up to 16 million or so outstanding
   requests at one time to a single server.

2.6.  Keep-Alives Considered Harmful

   Some implementers have adopted the practice of sending test RADIUS
   requests to see if a server is alive.  This practice is strongly
   discouraged, since it adds to load and harms scalability without
   providing any additional useful information.  Since a RADIUS request
   is contained in a single datagram, in the time it would take you to
   send a ping you could just send the RADIUS request, and getting a
   reply tells you that the RADIUS server is up.  If you do not have a
   RADIUS request to send, it does not matter if the server is up or
   not, because you are not using it.

   If you want to monitor your RADIUS server, use SNMP.  That's what
   SNMP is for.

3.  Packet Format

   Exactly one RADIUS packet is encapsulated in the UDP Data field [4],
   where the UDP Destination Port field indicates 1812 (decimal).

   When a reply is generated, the source and destination ports are
   reversed.

   This memo documents the RADIUS protocol.  The early deployment of
   RADIUS was done using UDP port number 1645, which conflicts with the
   "datametrics" service.  The officially assigned port number for
   RADIUS is 1812.

   A summary of the RADIUS data 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                         Authenticator                         |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Attributes ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-

   Code

      The Code field is one octet, and identifies the type of RADIUS
      packet.  When a packet is received with an invalid Code field, it
      is silently discarded.

      RADIUS Codes (decimal) are assigned as follows:

        1       Access-Request
        2       Access-Accept
        3       Access-Reject
        4       Accounting-Request
        5       Accounting-Response
       11       Access-Challenge
       12       Status-Server (experimental)
       13       Status-Client (experimental)
      255       Reserved

   Codes 4 and 5 are covered in the RADIUS Accounting document [5].
   Codes 12 and 13 are reserved for possible use, but are not further
   mentioned here.

   Identifier

      The Identifier field is one octet, and aids in matching requests
      and replies.  The RADIUS server can detect a duplicate request if
      it has the same client source IP address and source UDP port and
      Identifier within a short span of time.

   Length

      The Length field is two octets.  It indicates the length of the
      packet including the Code, Identifier, Length, Authenticator and
      Attribute fields.  Octets outside the range of the Length field
      MUST be treated as padding and ignored on reception.  If the
      packet is shorter than the Length field indicates, it MUST be
      silently discarded.  The minimum length is 20 and maximum length
      is 4096.

   Authenticator

      The Authenticator field is sixteen (16) octets.  The most
      significant octet is transmitted first.  This value is used to
      authenticate the reply from the RADIUS server, and is used in the
      password hiding algorithm.

      Request Authenticator

         In Access-Request Packets, the Authenticator value is a 16
         octet random number, called the Request Authenticator.  The
         value SHOULD be unpredictable and unique over the lifetime of a
         secret (the password shared between the client and the RADIUS
         server), since repetition of a request value in conjunction
         with the same secret would permit an attacker to reply with a
         previously intercepted response.  Since it is expected that the
         same secret MAY be used to authenticate with servers in
         disparate geographic regions, the Request Authenticator field
         SHOULD exhibit global and temporal uniqueness.

         The Request Authenticator value in an Access-Request packet
         SHOULD also be unpredictable, lest an attacker trick a server
         into responding to a predicted future request, and then use the
         response to masquerade as that server to a future Access-
         Request.

         Although protocols such as RADIUS are incapable of protecting
         against theft of an authenticated session via realtime active
         wiretapping attacks, generation of unique unpredictable
         requests can protect against a wide range of active attacks
         against authentication.

         The NAS and RADIUS server share a secret.  That shared secret
         followed by the Request Authenticator is put through a one-way
         MD5 hash to create a 16 octet digest value which is xored with
         the password entered by the user, and the xored result placed

         in the User-Password attribute in the Access-Request packet.
         See the entry for User-Password in the section on Attributes
         for a more detailed description.

      Response Authenticator

         The value of the Authenticator field in Access-Accept, Access-
         Reject, and Access-Challenge packets is called the Response
         Authenticator, and contains a one-way MD5 hash calculated over
         a stream of octets consisting of: the RADIUS packet, beginning
         with the Code field, including the Identifier, the Length, the
         Request Authenticator field from the Access-Request packet, and
         the response Attributes, followed by the shared secret.  That
         is, ResponseAuth =
         MD5(Code+ID+Length+RequestAuth+Attributes+Secret) where +
         denotes concatenation.

   Administrative Note

      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.  This is to ensure a sufficiently large range for the
      secret to provide protection against exhaustive search attacks.
      The secret MUST NOT be empty (length 0) since this would allow
      packets to be trivially forged.

      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.

      When using a forwarding proxy, the proxy must be able to alter the
      packet as it passes through in each direction - when the proxy
      forwards the request, the proxy MAY add a Proxy-State Attribute,
      and when the proxy forwards a response, it MUST remove its Proxy-
      State Attribute if it added one.  Proxy-State is always added or
      removed after any other Proxy-States, but no other assumptions
      regarding its location within the list of attributes can be made.
      Since Access-Accept and Access-Reject replies are authenticated on
      the entire packet contents, the stripping of the Proxy-State
      attribute invalidates the signature in the packet - so the proxy
      has to re-sign it.

      Further details of RADIUS proxy implementation are outside the
      scope of this document.

4.  Packet Types

   The RADIUS Packet type is determined by the Code field in the first
   octet of the Packet.

4.1.  Access-Request

   Description

      Access-Request packets are sent to a RADIUS server, and convey
      information used to determine whether a user is allowed access to
      a specific NAS, and any special services requested for that user.
      An implementation wishing to authenticate a user MUST transmit a
      RADIUS packet with the Code field set to 1 (Access-Request).

      Upon receipt of an Access-Request from a valid client, an
      appropriate reply MUST be transmitted.

      An Access-Request SHOULD contain a User-Name attribute.  It MUST
      contain either a NAS-IP-Address attribute or a NAS-Identifier
      attribute (or both).

      An Access-Request MUST contain either a User-Password or a CHAP-
      Password or a 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
      User-Password or CHAP-Password.

      An Access-Request SHOULD contain a NAS-Port or NAS-Port-Type
      attribute or both unless the type of access being requested does
      not involve a port or the NAS does not distinguish among its
      ports.

      An Access-Request MAY contain additional attributes as a hint to
      the server, but the server is not required to honor the hint.

      When a User-Password is present, it is hidden using a method based
      on the RSA Message Digest Algorithm MD5 [3].

   A summary of the Access-Request packet 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                     Request Authenticator                     |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Attributes ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-

   Code

      1 for Access-Request.

   Identifier

      The Identifier field MUST be changed whenever the content of the
      Attributes field changes, and whenever a valid reply has been
      received for a previous request.  For retransmissions, the
      Identifier MUST remain unchanged.

   Request Authenticator

      The Request Authenticator value MUST be changed each time a new
      Identifier is used.

   Attributes

      The Attribute field is variable in length, and contains the list
      of Attributes that are required for the type of service, as well
      as any desired optional Attributes.

4.2.  Access-Accept

   Description

      Access-Accept packets are sent by the RADIUS server, and provide
      specific configuration information necessary to begin delivery of
      service to the user.  If all Attribute values received in an
      Access-Request are acceptable then the RADIUS implementation MUST
      transmit a packet with the Code field set to 2 (Access-Accept).

      On reception of an Access-Accept, the Identifier field is matched
      with a pending Access-Request.  The Response Authenticator field
      MUST contain the correct response for the pending Access-Request.
      Invalid packets are silently discarded.

   A summary of the Access-Accept packet 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                     Response Authenticator                    |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Attributes ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-

   Code

      2 for Access-Accept.

   Identifier

      The Identifier field is a copy of the Identifier field of the
      Access-Request which caused this Access-Accept.

   Response Authenticator

      The Response Authenticator value is calculated from the Access-
      Request value, as described earlier.

   Attributes

      The Attribute field is variable in length, and contains a list of
      zero or more Attributes.

4.3.  Access-Reject

   Description

      If any value of the received Attributes is not acceptable, then
      the RADIUS server MUST transmit a packet with the Code field set
      to 3 (Access-Reject).  It MAY include one or more Reply-Message
      Attributes with a text message which the NAS MAY display to the
      user.

   A summary of the Access-Reject packet 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                     Response Authenticator                    |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Attributes ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-

   Code

      3 for Access-Reject.

   Identifier

      The Identifier field is a copy of the Identifier field of the
      Access-Request which caused this Access-Reject.

   Response Authenticator

      The Response Authenticator value is calculated from the Access-
      Request value, as described earlier.

   Attributes

      The Attribute field is variable in length, and contains a list of
      zero or more Attributes.

4.4.  Access-Challenge

   Description

      If the RADIUS server desires to send the user a challenge
      requiring a response, then the RADIUS server MUST respond to the
      Access-Request by transmitting a packet with the Code field set to
      11 (Access-Challenge).

      The Attributes field MAY have one or more Reply-Message
      Attributes, and MAY have a single State Attribute, or none.
      Vendor-Specific, Idle-Timeout, Session-Timeout and Proxy-State
      attributes MAY also be included.  No other Attributes defined in
      this document are permitted in an Access-Challenge.

      On receipt of an Access-Challenge, the Identifier field is matched
      with a pending Access-Request.  Additionally, the Response
      Authenticator field MUST contain the correct response for the
      pending Access-Request.  Invalid packets are silently discarded.

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

      If the NAS supports challenge/response, receipt of a valid
      Access-Challenge indicates that a new Access-Request SHOULD be
      sent.  The NAS MAY display the text message, if any, to the user,
      and then prompt the user for a response.  It then sends its
      original Access-Request with a new request ID and Request
      Authenticator, with the User-Password Attribute replaced by the
      user's response (encrypted), and including the State Attribute
      from the Access-Challenge, if any.  Only 0 or 1 instances of the
      State Attribute can be present in an Access-Request.

      A NAS which supports PAP MAY forward the Reply-Message to the
      dialing client and accept a PAP response which it can use as
      though the user had entered the response.  If the NAS cannot do
      so, it MUST treat the Access-Challenge as though it had received
      an Access-Reject instead.

   A summary of the Access-Challenge packet 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                     Response Authenticator                    |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Attributes ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-

   Code

      11 for Access-Challenge.

   Identifier

      The Identifier field is a copy of the Identifier field of the
      Access-Request which caused this Access-Challenge.

   Response Authenticator

      The Response Authenticator value is calculated from the Access-
      Request value, as described earlier.

   Attributes

      The Attributes field is variable in length, and contains a list of
      zero or more Attributes.

5.  Attributes

   RADIUS Attributes carry the specific authentication, authorization,
   information and configuration details for the request and reply.

   The end of the list of Attributes is indicated by the Length of the
   RADIUS packet.

   Some Attributes MAY be included more than once.  The effect of this
   is Attribute specific, and is specified in each Attribute
   description.  A summary table is provided at the end of the
   "Attributes" section.

   If multiple Attributes with the same Type are present, the order of
   Attributes with the same Type MUST be preserved by any proxies.  The
   order of Attributes of different Types is not required to be
   preserved.  A RADIUS server or client MUST NOT have any dependencies
   on the order of attributes of different types.  A RADIUS server or
   client MUST NOT require attributes of the same type to be contiguous.

   Where an Attribute's description limits which kinds of packet it can
   be contained in, this applies only to the packet types defined in
   this document, namely Access-Request, Access-Accept, Access-Reject
   and Access-Challenge (Codes 1, 2, 3, and 11).  Other documents
   defining other packet types may also use Attributes described here.
   To determine which Attributes are allowed in Accounting-Request and
   Accounting-Response packets (Codes 4 and 5) refer to the RADIUS
   Accounting document [5].

   Likewise where packet types defined here state that only certain
   Attributes are permissible in them, future memos defining new
   Attributes should indicate which packet types the new Attributes may
   be present in.

   A summary of the 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  Value ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

      The Type field is one octet.  Up-to-date values of the RADIUS Type
      field are specified in the most recent "Assigned Numbers" RFC [6].
      Values 192-223 are reserved for experimental use, values 224-240
      are reserved for implementation-specific use, and values 241-255
      are reserved and should not be used.

      A RADIUS server MAY ignore Attributes with an unknown Type.

      A RADIUS client MAY ignore Attributes with an unknown Type.

      This specification concerns the following values:

          1      User-Name
          2      User-Password
          3      CHAP-Password
          4      NAS-IP-Address
          5      NAS-Port
          6      Service-Type
          7      Framed-Protocol
          8      Framed-IP-Address
          9      Framed-IP-Netmask
         10      Framed-Routing
         11      Filter-Id
         12      Framed-MTU
         13      Framed-Compression
         14      Login-IP-Host
         15      Login-Service
         16      Login-TCP-Port
         17      (unassigned)
         18      Reply-Message
         19      Callback-Number
         20      Callback-Id
         21      (unassigned)
         22      Framed-Route
         23      Framed-IPX-Network
         24      State
         25      Class
         26      Vendor-Specific
         27      Session-Timeout
         28      Idle-Timeout
         29      Termination-Action
         30      Called-Station-Id
         31      Calling-Station-Id
         32      NAS-Identifier
         33      Proxy-State
         34      Login-LAT-Service
         35      Login-LAT-Node
         36      Login-LAT-Group
         37      Framed-AppleTalk-Link
         38      Framed-AppleTalk-Network
         39      Framed-AppleTalk-Zone
         40-59   (reserved for accounting)
         60      CHAP-Challenge
         61      NAS-Port-Type
         62      Port-Limit
         63      Login-LAT-Port

   Length

      The Length field is one octet, and indicates the length of this
      Attribute including the Type, Length and Value fields.  If an
      Attribute is received in an Access-Request but with an invalid
      Length, an Access-Reject SHOULD be transmitted.  If an Attribute
      is received in an Access-Accept, Access-Reject or Access-Challenge
      packet with an invalid length, the packet MUST either be treated
      as an Access-Reject or else silently discarded.

   Value

      The Value field is zero or more octets and contains information
      specific to the Attribute.  The format and length of the Value
      field is determined by the Type and Length fields.

      Note that none of the types in RADIUS terminate with a NUL (hex
      00).  In particular, types "text" and "string" in RADIUS do not
      terminate with a NUL (hex 00).  The Attribute has a length field
      and does not use a terminator.  Text contains UTF-8 encoded 10646
      [7] characters and String contains 8-bit binary data.  Servers and
      servers and clients MUST be able to deal with embedded nulls.
      RADIUS implementers using C are cautioned not to use strcpy() when
      handling strings.

      The format of the value field is one of five data types.  Note
      that type "text" is a subset of type "string".

      text      1-253 octets containing UTF-8 encoded 10646 [7]
                characters.  Text of length zero (0) MUST NOT be sent;
                omit the entire attribute instead.

      string    1-253 octets containing binary data (values 0 through
                255 decimal, inclusive).  Strings of length zero (0)
                MUST NOT be sent; omit the entire attribute instead.

      address   32 bit value, most significant octet first.

      integer   32 bit unsigned value, most significant octet first.

      time      32 bit unsigned value, most significant octet first --
                seconds since 00:00:00 UTC, January 1, 1970.  The
                standard Attributes do not use this data type but it is
                presented here for possible use in future attributes.

5.1.  User-Name

   Description

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

      It MAY be sent in an Access-Accept packet, in which case the
      client SHOULD use the name returned in the Access-Accept packet in
      all Accounting-Request packets for this session.  If the Access-
      Accept includes Service-Type = Rlogin and the User-Name attribute,
      a NAS MAY use the returned User-Name when performing the Rlogin
      function.

   A summary of the User-Name 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

      1 for User-Name.

   Length

      >= 3

   String

      The String field is one or more octets.  The NAS may limit the
      maximum length of the User-Name but the ability to handle at least
      63 octets is recommended.

      The format of the username MAY be one of several forms:

      text      Consisting only of UTF-8 encoded 10646 [7] characters.

      network access identifier
                A Network Access Identifier as described in RFC 2486
                [8].

      distinguished name
                A name in ASN.1 form used in Public Key authentication
                systems.

5.2.  User-Password

   Description

      This Attribute indicates the password of the user to be
      authenticated, or the user's input following an Access-Challenge.
      It is only used in Access-Request packets.

      On transmission, the password is hidden.  The password is first
      padded at the end with nulls to a multiple of 16 octets.  A one-
      way MD5 hash is calculated over a stream of octets consisting of
      the shared secret followed by the Request Authenticator.  This
      value is XORed with the first 16 octet segment of the password and
      placed in the first 16 octets of the String field of the User-
      Password Attribute.

      If the password is longer than 16 characters, a second one-way MD5
      hash is calculated over a stream of octets consisting of the
      shared secret followed by the result of the first xor.  That hash
      is XORed with the second 16 octet segment of the password and
      placed in the second 16 octets of the String field of the User-
      Password Attribute.

      If necessary, this operation is repeated, with each xor result
      being used along with the shared secret to generate the next hash
      to xor the next segment of the password, to no more than 128
      characters.

      The method is taken from the book "Network Security" by Kaufman,
      Perlman and Speciner [9] pages 109-110.  A more precise
      explanation of the method follows:

      Call the shared secret S and the pseudo-random 128-bit Request
      Authenticator RA.  Break the password into 16-octet chunks p1, p2,
      etc.  with the last one padded at the end with nulls to a 16-octet
      boundary.  Call the ciphertext blocks c(1), c(2), etc.  We'll need
      intermediate values b1, b2, etc.

         b1 = MD5(S + RA)       c(1) = p1 xor b1
         b2 = MD5(S + c(1))     c(2) = p2 xor b2
                .                       .
                .                       .
                .                       .
         bi = MD5(S + c(i-1))   c(i) = pi xor bi

      The String will contain c(1)+c(2)+...+c(i) where + denotes
      concatenation.

      On receipt, the process is reversed to yield the original
      password.

   A summary of the User-Password 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

      2 for User-Password.

   Length

      At least 18 and no larger than 130.

   String

      The String field is between 16 and 128 octets long, inclusive.

5.3.  CHAP-Password

   Description

      This Attribute indicates the response value provided by a PPP
      Challenge-Handshake Authentication Protocol (CHAP) user in
      response to the challenge.  It is only used in Access-Request
      packets.

      The CHAP challenge value is found in the CHAP-Challenge Attribute
      (60) if present in the packet, otherwise in the Request
      Authenticator field.

   A summary of the CHAP-Password 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 4 5 6 7 8 9
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  CHAP Ident   |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

      3 for CHAP-Password.

   Length

      19

   CHAP Ident

      This field is one octet, and contains the CHAP Identifier from the
      user's CHAP Response.

   String

      The String field is 16 octets, and contains the CHAP Response from
      the user.

5.4.  NAS-IP-Address

   Description

      This Attribute indicates the identifying IP Address of the NAS
      which is requesting authentication of the user, and SHOULD be
      unique to the NAS within the scope of the RADIUS server. NAS-IP-
      Address is only used in Access-Request packets.  Either NAS-IP-
      Address or NAS-Identifier MUST be present in an Access-Request
      packet.

      Note that NAS-IP-Address MUST NOT be used to select the shared
      secret used to authenticate the request.  The source IP address of
      the Access-Request packet MUST be used to select the shared
      secret.

   A summary of the NAS-IP-Address 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     |            Address
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            Address (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      4 for NAS-IP-Address.

   Length

      6

   Address

      The Address field is four octets.

5.5.  NAS-Port

   Description

      This Attribute indicates the physical port number of the NAS which
      is authenticating the user.  It is only used in Access-Request
      packets.  Note that this is using "port" in its sense of a
      physical connection on the NAS, not in the sense of a TCP or UDP
      port number.  Either NAS-Port or NAS-Port-Type (61) or both SHOULD
      be present in an Access-Request packet, if the NAS differentiates
      among its ports.

   A summary of the NAS-Port 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

      5 for NAS-Port.

   Length

      6

   Value

      The Value field is four octets.

5.6.  Service-Type

   Description

      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.

   A summary of the Service-Type 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

      6 for Service-Type.

   Length

      6

   Value

      The Value field is four octets.

       1      Login
       2      Framed
       3      Callback Login
       4      Callback Framed
       5      Outbound
       6      Administrative
       7      NAS Prompt
       8      Authenticate Only
       9      Callback NAS Prompt
      10      Call Check
      11      Callback Administrative

      The service types are defined as follows when used in an Access-
      Accept.  When used in an Access-Request, they MAY be considered to
      be a hint to the RADIUS server that the NAS has reason to believe
      the user would prefer the kind of service indicated, but the
      server is not required to honor the hint.

      Login               The user should be connected to a host.

      Framed              A Framed Protocol should be started for the
                          User, such as PPP or SLIP.

      Callback Login      The user should be disconnected and called
                          back, then connected to a host.

      Callback Framed     The user should be disconnected and called
                          back, then a Framed Protocol should be started
                          for the User, such as PPP or SLIP.

      Outbound            The user should be granted access to outgoing
                          devices.

      Administrative      The user should be granted access to the
                          administrative interface to the NAS from which
                          privileged commands can be executed.

      NAS Prompt          The user should be provided a command prompt
                          on the NAS from which non-privileged commands
                          can be executed.

      Authenticate Only   Only Authentication is requested, and no
                          authorization information needs to be returned
                          in the Access-Accept (typically used by proxy
                          servers rather than the NAS itself).

      Callback NAS Prompt The user should be disconnected and called
                          back, then provided a command prompt on the
                          NAS from which non-privileged commands can be
                          executed.

      Call Check          Used by the NAS in an Access-Request packet to
                          indicate that a call is being received and
                          that the RADIUS server should send back an
                          Access-Accept to answer the call, or an
                          Access-Reject to not accept the call,
                          typically based on the Called-Station-Id or
                          Calling-Station-Id attributes.  It is

                          recommended that such Access-Requests use the
                          value of Calling-Station-Id as the value of
                          the User-Name.

      Callback Administrative
                          The user should be disconnected and called
                          back, then granted access to the
                          administrative interface to the NAS from which
                          privileged commands can be executed.

5.7.  Framed-Protocol

   Description

      This Attribute indicates the framing to be used for framed access.
      It MAY be used in both Access-Request and Access-Accept packets.

   A summary of the Framed-Protocol 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

      7 for Framed-Protocol.

   Length

      6

   Value

      The Value field is four octets.

      1      PPP
      2      SLIP
      3      AppleTalk Remote Access Protocol (ARAP)
      4      Gandalf proprietary SingleLink/MultiLink protocol
      5      Xylogics proprietary IPX/SLIP
      6      X.75 Synchronous

5.8.  Framed-IP-Address

   Description

      This Attribute indicates the address to be configured for the
      user.  It MAY be used in Access-Accept packets.  It MAY be used in
      an Access-Request packet as a hint by the NAS to the server that
      it would prefer that address, but the server is not required to
      honor the hint.

   A summary of the Framed-IP-Address 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     |            Address
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            Address (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      8 for Framed-IP-Address.

   Length

      6

   Address

      The Address field is four octets.  The value 0xFFFFFFFF indicates
      that the NAS Should allow the user to select an address (e.g.
      Negotiated).  The value 0xFFFFFFFE indicates that the NAS should
      select an address for the user (e.g. Assigned from a pool of
      addresses kept by the NAS).  Other valid values indicate that the
      NAS should use that value as the user's IP address.

5.9.  Framed-IP-Netmask

   Description

      This Attribute indicates the IP netmask to be configured for the
      user when the user is a router to a network.  It MAY be used in
      Access-Accept packets.  It MAY be used in an Access-Request packet
      as a hint by the NAS to the server that it would prefer that
      netmask, but the server is not required to honor the hint.

   A summary of the Framed-IP-Netmask 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     |            Address
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            Address (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      9 for Framed-IP-Netmask.

   Length

      6

   Address

      The Address field is four octets specifying the IP netmask of the
      user.

5.10.  Framed-Routing

   Description

      This Attribute indicates the routing method for the user, when the
      user is a router to a network.  It is only used in Access-Accept
      packets.

   A summary of the Framed-Routing 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

      10 for Framed-Routing.

   Length

      6

   Value

      The Value field is four octets.

       0      None
       1      Send routing packets
       2      Listen for routing packets
       3      Send and Listen

5.11.  Filter-Id

   Description

      This Attribute indicates the name of the filter list for this
      user.  Zero or more Filter-Id attributes MAY be sent in an
      Access-Accept packet.

      Identifying a filter list by name allows the filter to be used on
      different NASes without regard to filter-list implementation
      details.

   A summary of the Filter-Id 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  Text ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

      11 for Filter-Id.

   Length

      >= 3

   Text

      The Text field is one or more octets, and its contents are
      implementation dependent.  It is intended to be human readable and
      MUST NOT affect operation of the protocol.  It is recommended that
      the message contain UTF-8 encoded 10646 [7] characters.

5.12.  Framed-MTU

   Description

      This Attribute indicates the Maximum Transmission Unit to be
      configured for the user, when it is not negotiated by some other
      means (such as PPP).  It MAY be used in Access-Accept packets.  It
      MAY be used in an Access-Request packet as a hint by the NAS to
      the server that it would prefer that value, but the server is not
      required to honor the hint.

   A summary of the Framed-MTU 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

      12 for Framed-MTU.

   Length

      6

   Value

      The Value field is four octets.  Despite the size of the field,
      values range from 64 to 65535.

5.13.  Framed-Compression

   Description

      This Attribute indicates a compression protocol to be used for the
      link.  It MAY be used in Access-Accept packets.  It MAY be used in
      an Access-Request packet as a hint to the server that the NAS
      would prefer to use that compression, but the server is not
      required to honor the hint.

      More than one compression protocol Attribute MAY be sent.  It is
      the responsibility of the NAS to apply the proper compression
      protocol to appropriate link traffic.

   A summary of the Framed-Compression 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

      13 for Framed-Compression.

   Length

      6

   Value

      The Value field is four octets.

       0      None
       1      VJ TCP/IP header compression [10]
       2      IPX header compression
       3      Stac-LZS compression

5.14.  Login-IP-Host

   Description

      This Attribute indicates the system with which to connect the user,
      when the Login-Service Attribute is included.  It MAY be used in
      Access-Accept packets.  It MAY be used in an Access-Request packet as
      a hint to the server that the NAS would prefer to use that host, but
      the server is not required to honor the hint.

   A summary of the Login-IP-Host 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     |            Address
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            Address (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      14 for Login-IP-Host.

   Length

      6

   Address

      The Address field is four octets.  The value 0xFFFFFFFF indicates
      that the NAS SHOULD allow the user to select an address.  The
      value 0 indicates that the NAS SHOULD select a host to connect the
      user to.  Other values indicate the address the NAS SHOULD connect
      the user to.

5.15.  Login-Service

   Description

      This Attribute indicates the service to use to connect the user to
      the login host.  It is only used in Access-Accept packets.

   A summary of the Login-Service 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

      15 for Login-Service.

   Length

      6

   Value

      The Value field is four octets.

       0   Telnet
       1   Rlogin
       2   TCP Clear
       3   PortMaster (proprietary)
       4   LAT
       5   X25-PAD
       6   X25-T3POS
       8   TCP Clear Quiet (suppresses any NAS-generated connect string)

5.16.  Login-TCP-Port

   Description

      This Attribute indicates the TCP port with which the user is to be
      connected, when the Login-Service Attribute is also present.  It
      is only used in Access-Accept packets.

   A summary of the Login-TCP-Port 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

      16 for Login-TCP-Port.

   Length

      6

   Value

      The Value field is four octets.  Despite the size of the field,
      values range from 0 to 65535.

5.17.  (unassigned)

   Description

      ATTRIBUTE TYPE 17 HAS NOT BEEN ASSIGNED.

5.18.  Reply-Message

   Description

      This Attribute indicates text which MAY be displayed to the user.

      When used in an Access-Accept, it is the success message.

      When used in an Access-Reject, it is the failure message.  It MAY
      indicate a dialog message to prompt the user before another
      Access-Request attempt.

      When used in an Access-Challenge, it MAY indicate a dialog message
      to prompt the user for a response.

      Multiple Reply-Message's MAY be included and if any are displayed,
      they MUST be displayed in the same order as they appear in the
      packet.

   A summary of the Reply-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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  Text ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

      18 for Reply-Message.

   Length

      >= 3

   Text

      The Text field is one or more octets, and its contents are
      implementation dependent.  It is intended to be human readable,
      and MUST NOT affect operation of the protocol.  It is recommended
      that the message contain UTF-8 encoded 10646 [7] characters.

5.19.  Callback-Number

   Description

      This Attribute indicates a dialing string to be used for callback.
      It MAY be used in Access-Accept packets.  It MAY be used in an
      Access-Request packet as a hint to the server that a Callback
      service is desired, but the server is not required to honor the
      hint.

   A summary of the Callback-Number 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

      19 for Callback-Number.

   Length

      >= 3

   String

      The String field is one or more octets.  The actual format of the
      information is site or application specific, and a robust
      implementation SHOULD support the field as undistinguished octets.

      The codification of the range of allowed usage of this field is
      outside the scope of this specification.

5.20.  Callback-Id

   Description

      This Attribute indicates the name of a place to be called, to be
      interpreted by the NAS.  It MAY be used in Access-Accept packets.

   A summary of the Callback-Id 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

      20 for Callback-Id.

   Length

      >= 3

   String

      The String field is one or more octets.  The actual format of the
      information is site or application specific, and a robust
      implementation SHOULD support the field as undistinguished octets.

      The codification of the range of allowed usage of this field is
      outside the scope of this specification.

5.21.  (unassigned)

   Description

      ATTRIBUTE TYPE 21 HAS NOT BEEN ASSIGNED.

5.22.  Framed-Route

   Description

      This Attribute provides routing information to be configured for
      the user on the NAS.  It is used in the Access-Accept packet and
      can appear multiple times.

   A summary of the Framed-Route 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     |  Text ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

      22 for Framed-Route.

   Length

      >= 3

   Text

      The Text field is one or more octets, and its contents are
      implementation dependent.  It is intended to be human readable and
      MUST NOT affect operation of the protocol.  It is recommended that
      the message contain UTF-8 encoded 10646 [7] characters.

      For IP routes, it SHOULD contain a destination prefix in dotted
      quad form optionally followed by a slash and a decimal length
      specifier stating how many high order bits of the prefix to use.
      That is followed by a space, a gateway address in dotted quad
      form, a space, and one or more metrics separated by spaces.  For
      example, "192.168.1.0/24 192.168.1.1 1 2 -1 3 400". The length
      specifier may be omitted, in which case it defaults to 8 bits for
      class A prefixes, 16 bits for class B prefixes, and 24 bits for
      class C prefixes.  For example, "192.168.1.0 192.168.1.1 1".

      Whenever the gateway address is specified as "0.0.0.0" the IP
      address of the user SHOULD be used as the gateway address.

5.23.  Framed-IPX-Network

   Description

      This Attribute indicates the IPX Network number to be configured
      for the user.  It is used in Access-Accept packets.

   A summary of the Framed-IPX-Network 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

      23 for Framed-IPX-Network.

   Length

      6

   Value

      The Value field is four octets.  The value 0xFFFFFFFE indicates
      that the NAS should select an IPX network for the user (e.g.
      assigned from a pool of one or more IPX networks kept by the NAS).
      Other values should be used as the IPX network for the link to the
      user.

5.24.  State

   Description

      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.

      In either usage, the client MUST NOT interpret the attribute
      locally.  A packet must have only zero or one State Attribute.
      Usage of the State Attribute is implementation dependent.

   A summary of the State 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

      24 for State.

   Length

      >= 3

   String

      The String field is one or more octets.  The actual format of the
      information is site or application specific, and a robust
      implementation SHOULD support the field as undistinguished octets.

      The codification of the range of allowed usage of this field is
      outside the scope of this specification.

5.25.  Class

   Description

      This Attribute is available to be sent by the server to the client
      in an Access-Accept and SHOULD be sent unmodified by the client to
      the accounting server as part of the Accounting-Request packet if
      accounting is supported.  The client MUST NOT interpret the
      attribute locally.

   A summary of the Class 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

      25 for Class.

   Length

      >= 3

   String

      The String field is one or more octets.  The actual format of the
      information is site or application specific, and a robust
      implementation SHOULD support the field as undistinguished octets.

      The codification of the range of allowed usage of this field is
      outside the scope of this specification.

5.26.  Vendor-Specific

   Description

      This Attribute is available to allow vendors to support their own
      extended Attributes not suitable for general usage.  It MUST not
      affect the operation of the RADIUS protocol.

      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.

   A summary of the Vendor-Specific 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       |            Vendor-Id
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        Vendor-Id (cont)           |  String...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

      26 for Vendor-Specific.

   Length

      >= 7

   Vendor-Id

      The high-order octet is 0 and the low-order 3 octets are the SMI
      Network Management Private Enterprise Code of the Vendor in
      network byte order, as defined in the "Assigned Numbers" RFC [6].

   String

      The String field is one or more octets.  The actual format of the
      information is site or application specific, and a robust
      implementation SHOULD support the field as undistinguished octets.

      The codification of the range of allowed usage of this field is
      outside the scope of this specification.

      It SHOULD be encoded as a sequence of vendor type / vendor length
      / value fields, as follows.  The Attribute-Specific field is
      dependent on the vendor's definition of that attribute.  An
      example encoding of the Vendor-Specific attribute using this
      method follows:

       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       |            Vendor-Id
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           Vendor-Id (cont)           | Vendor type   | Vendor length |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Attribute-Specific...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

      Multiple subattributes MAY be encoded within a single Vendor-
      Specific attribute, although they do not have to be.

5.27.  Session-Timeout

   Description

      This Attribute sets the maximum number of seconds of service to be
      provided 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.

   A summary of the Session-Timeout 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

      27 for Session-Timeout.

   Length

      6

   Value

      The field is 4 octets, containing a 32-bit unsigned integer with
      the maximum number of seconds this user should be allowed to
      remain connected by the NAS.

5.28.  Idle-Timeout

   Description

      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.

   A summary of the Idle-Timeout 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

      28 for Idle-Timeout.

   Length

      6

   Value

      The field is 4 octets, containing a 32-bit unsigned integer with
      the maximum number of consecutive seconds of idle time this user
      should be permitted before being disconnected by the NAS.

5.29.  Termination-Action

   Description

      This Attribute indicates what action the NAS should take when the
      specified service is completed.  It is only used in Access-Accept
      packets.

   A summary of the Termination-Action 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

      29 for Termination-Action.

   Length

      6

   Value

      The Value field is four octets.

       0      Default
       1      RADIUS-Request

      If the Value is set to RADIUS-Request, upon termination of the
      specified service the NAS MAY send a new Access-Request to the
      RADIUS server, including the State attribute if any.

5.30.  Called-Station-Id

   Description

      This Attribute allows the NAS to send in the Access-Request packet
      the phone number that the user called, using Dialed Number
      Identification (DNIS) or similar technology.  Note that this may
      be different from the phone number the call comes in on.  It is
      only used in Access-Request packets.

   A summary of the Called-Station-Id 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

      30 for Called-Station-Id.

   Length

      >= 3

   String

      The String field is one or more octets, containing the phone
      number that the user's call came in on.

      The actual format of the information is site or application
      specific.  UTF-8 encoded 10646 [7] characters are recommended, but
      a robust implementation SHOULD support the field as
      undistinguished octets.

      The codification of the range of allowed usage of this field is
      outside the scope of this specification.

5.31.  Calling-Station-Id

   Description

      This Attribute allows the NAS to send in the Access-Request packet
      the phone number that the call came from, using Automatic Number
      Identification (ANI) or similar technology.  It is only used in
      Access-Request packets.

   A summary of the Calling-Station-Id 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

      31 for Calling-Station-Id.

   Length

      >= 3

   String

      The String field is one or more octets, containing the phone
      number that the user placed the call from.

      The actual format of the information is site or application
      specific.  UTF-8 encoded 10646 [7] characters are recommended, but
      a robust implementation SHOULD support the field as
      undistinguished octets.

      The codification of the range of allowed usage of this field is
      outside the scope of this specification.

5.32.  NAS-Identifier

   Description

      This Attribute contains a string identifying the NAS originating
      the Access-Request.  It is only used in Access-Request packets.
      Either NAS-IP-Address or NAS-Identifier MUST be present in an
      Access-Request packet.

      Note that NAS-Identifier MUST NOT be used to select the shared
      secret used to authenticate the request.  The source IP address of
      the Access-Request packet MUST be used to select the shared
      secret.

   A summary of the NAS-Identifier 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

      32 for NAS-Identifier.

   Length

      >= 3

   String

      The String field is one or more octets, and should be unique to
      the NAS within the scope of the RADIUS server.  For example, a
      fully qualified domain name would be suitable as a NAS-Identifier.

      The actual format of the information is site or application
      specific, and a robust implementation SHOULD support the field as
      undistinguished octets.

      The codification of the range of allowed usage of this field is
      outside the scope of this specification.

5.33.  Proxy-State

   Description

      This Attribute is available to be sent by a proxy server to
      another server when forwarding an Access-Request and MUST be
      returned unmodified in the Access-Accept, Access-Reject or
      Access-Challenge.  When the proxy server receives the response to
      its request, it MUST remove its own Proxy-State (the last Proxy-
      State in the packet) before forwarding the response to the NAS.

      If a Proxy-State Attribute is added to a packet when forwarding
      the packet, the Proxy-State Attribute MUST be added after any
      existing Proxy-State attributes.

      The content of any Proxy-State other than the one added by the
      current server should be treated as opaque octets and MUST NOT
      affect operation of the protocol.

      Usage of the Proxy-State Attribute is implementation dependent.  A
      description of its function is outside the scope of this
      specification.

   A summary of the Proxy-State 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

      33 for Proxy-State.

   Length

      >= 3

   String

      The String field is one or more octets.  The actual format of the
      information is site or application specific, and a robust
      implementation SHOULD support the field as undistinguished octets.

      The codification of the range of allowed usage of this field is
      outside the scope of this specification.

5.34.  Login-LAT-Service

   Description

      This Attribute indicates the system with which the user is to be
      connected by LAT.  It MAY be used in Access-Accept packets, but
      only when LAT is specified as the Login-Service.  It MAY be used
      in an Access-Request packet as a hint to the server, but the
      server is not required to honor the hint.

      Administrators use the service attribute when dealing with
      clustered systems, such as a VAX or Alpha cluster. In such an
      environment several different time sharing hosts share the same
      resources (disks, printers, etc.), and administrators often
      configure each to offer access (service) to each of the shared
      resources. In this case, each host in the cluster advertises its
      services through LAT broadcasts.

      Sophisticated users often know which service providers (machines)
      are faster and tend to use a node name when initiating a LAT
      connection.  Alternately, some administrators want particular
      users to use certain machines as a primitive form of load
      balancing (although LAT knows how to do load balancing itself).

   A summary of the Login-LAT-Service 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

      34 for Login-LAT-Service.

   Length

      >= 3

   String

      The String field is one or more octets, and contains the identity
      of the LAT service to use.  The LAT Architecture allows this
      string to contain $ (dollar), - (hyphen), . (period), _
      (underscore), numerics, upper and lower case alphabetics, and the
      ISO Latin-1 character set extension [11].  All LAT string
      comparisons are case insensitive.

5.35.  Login-LAT-Node

   Description

      This Attribute indicates the Node with which the user is to be
      automatically connected by LAT.  It MAY be used in Access-Accept
      packets, but only when LAT is specified as the Login-Service.  It
      MAY be used in an Access-Request packet as a hint to the server,
      but the server is not required to honor the hint.

   A summary of the Login-LAT-Node 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

      35 for Login-LAT-Node.

   Length

      >= 3

   String

      The String field is one or more octets, and contains the identity
      of the LAT Node to connect the user to.  The LAT Architecture
      allows this string to contain $ (dollar), - (hyphen), . (period),
      _ (underscore), numerics, upper and lower case alphabetics, and
      the ISO Latin-1 character set extension.  All LAT string
      comparisons are case insensitive.

5.36.  Login-LAT-Group

   Description

      This Attribute contains a string identifying the LAT group codes
      which this user is authorized to use.  It MAY be used in Access-
      Accept packets, but only when LAT is specified as the Login-
      Service.  It MAY be used in an Access-Request packet as a hint to
      the server, but the server is not required to honor the hint.

      LAT supports 256 different group codes, which LAT uses as a form
      of access rights.  LAT encodes the group codes as a 256 bit
      bitmap.

      Administrators can assign one or more of the group code bits at
      the LAT service provider; it will only accept LAT connections that
      have these group codes set in the bit map. The administrators
      assign a bitmap of authorized group codes to each user; LAT gets
      these from the operating system, and uses these in its requests to
      the service providers.

   A summary of the Login-LAT-Group 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

      36 for Login-LAT-Group.

   Length

      34

   String

      The String field is a 32 octet bit map, most significant octet
      first.  A robust implementation SHOULD support the field as
      undistinguished octets.

      The codification of the range of allowed usage of this field is
      outside the scope of this specification.

5.37.  Framed-AppleTalk-Link

   Description

      This Attribute indicates the AppleTalk network number which should
      be used for the serial link to the user, which is another
      AppleTalk router.  It is only used in Access-Accept packets.  It
      is never used when the user is not another router.

   A summary of the Framed-AppleTalk-Link 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

      37 for Framed-AppleTalk-Link.

   Length

      6

   Value

      The Value field is four octets.  Despite the size of the field,
      values range from 0 to 65535.  The special value of 0 indicates
      that this is an unnumbered serial link.  A value of 1-65535 means
      that the serial line between the NAS and the user should be
      assigned that value as an AppleTalk network number.

5.38.  Framed-AppleTalk-Network

   Description

      This Attribute indicates the AppleTalk Network number which the
      NAS should probe to allocate an AppleTalk node for the user.  It
      is only used in Access-Accept packets.  It is never used when the
      user is another router.  Multiple instances of this Attribute
      indicate that the NAS may probe using any of the network numbers
      specified.

   A summary of the Framed-AppleTalk-Network 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

      38 for Framed-AppleTalk-Network.

   Length

      6

   Value

      The Value field is four octets.  Despite the size of the field,
      values range from 0 to 65535.  The special value 0 indicates that
      the NAS should assign a network for the user, using its default
      cable range.  A value between 1 and 65535 (inclusive) indicates
      the AppleTalk Network the NAS should probe to find an address for
      the user.

5.39.  Framed-AppleTalk-Zone

   Description

      This Attribute indicates the AppleTalk Default Zone to be used for
      this user.  It is only used in Access-Accept packets.  Multiple
      instances of this attribute in the same packet are not allowed.

   A summary of the Framed-AppleTalk-Zone 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 4
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

      39 for Framed-AppleTalk-Zone.

   Length

      >= 3

   String

      The name of the Default AppleTalk Zone to be used for this user.
      A robust implementation SHOULD support the field as
      undistinguished octets.

      The codification of the range of allowed usage of this field is
      outside the scope of this specification.

5.40.  CHAP-Challenge

   Description

      This Attribute contains the CHAP Challenge sent by the NAS to a
      PPP Challenge-Handshake Authentication Protocol (CHAP) user.  It
      is only used in Access-Request packets.

      If the CHAP challenge value is 16 octets long it MAY be placed in
      the Request Authenticator field instead of using this attribute.

   A summary of the CHAP-Challenge 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

      60 for CHAP-Challenge.

   Length

      >= 7

   String

      The String field contains the CHAP Challenge.

5.41.  NAS-Port-Type

   Description

      This Attribute indicates the type of the physical port of the NAS
      which is authenticating the user.  It can be used instead of or in
      addition to the NAS-Port (5) attribute.  It is only used in
      Access-Request packets.  Either NAS-Port (5) or NAS-Port-Type or
      both SHOULD be present in an Access-Request packet, if the NAS
      differentiates among its ports.

   A summary of the NAS-Port-Type 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

      61 for NAS-Port-Type.

   Length

      6

   Value

      The Value field is four octets.  "Virtual" refers to a connection
      to the NAS via some transport protocol, instead of through a
      physical port.  For example, if a user telnetted into a NAS to

      authenticate himself as an Outbound-User, the Access-Request might
      include NAS-Port-Type = Virtual as a hint to the RADIUS server
      that the user was not on a physical port.

      0       Async
      1       Sync
      2       ISDN Sync
      3       ISDN Async V.120
      4       ISDN Async V.110
      5       Virtual
      6       PIAFS
      7       HDLC Clear Channel
      8       X.25
      9       X.75
      10      G.3 Fax
      11      SDSL - Symmetric DSL
      12      ADSL-CAP - Asymmetric DSL, Carrierless Amplitude Phase
              Modulation
      13      ADSL-DMT - Asymmetric DSL, Discrete Multi-Tone
      14      IDSL - ISDN Digital Subscriber Line
      15      Ethernet
      16      xDSL - Digital Subscriber Line of unknown type
      17      Cable
      18      Wireless - Other
      19      Wireless - IEEE 802.11

      PIAFS is a form of wireless ISDN commonly used in Japan, and
      stands for PHS (Personal Handyphone System) Internet Access Forum
      Standard (PIAFS).

5.42.  Port-Limit

   Description

      This Attribute sets the maximum number of ports to be provided to
      the user by the NAS.  This Attribute MAY be sent by the server to
      the client in an Access-Accept packet.  It is intended for use in
      conjunction with Multilink PPP [12] or similar uses.  It MAY also
      be sent by the NAS to the server as a hint that that many ports
      are desired for use, but the server is not required to honor the
      hint.

   A summary of the Port-Limit 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

      62 for Port-Limit.

   Length

      6

   Value

      The field is 4 octets, containing a 32-bit unsigned integer with
      the maximum number of ports this user should be allowed to connect
      to on the NAS.

5.43.  Login-LAT-Port

   Description

      This Attribute indicates the Port with which the user is to be
      connected by LAT.  It MAY be used in Access-Accept packets, but
      only when LAT is specified as the Login-Service.  It MAY be used
      in an Access-Request packet as a hint to the server, but the
      server is not required to honor the hint.

   A summary of the Login-LAT-Port 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

      63 for Login-LAT-Port.

   Length

      >= 3

   String

      The String field is one or more octets, and contains the identity
      of the LAT port to use.  The LAT Architecture allows this string
      to contain $ (dollar), - (hyphen), . (period), _ (underscore),
      numerics, upper and lower case alphabetics, and the ISO Latin-1
      character set extension.  All LAT string comparisons are case
      insensitive.

5.44.  Table of Attributes

   The following table provides a guide to which attributes may be found
   in which kinds of packets, and in what quantity.

   Request   Accept   Reject   Challenge   #    Attribute
   0-1       0-1      0        0            1   User-Name
   0-1       0        0        0            2   User-Password [Note 1]
   0-1       0        0        0            3   CHAP-Password [Note 1]
   0-1       0        0        0            4   NAS-IP-Address [Note 2]
   0-1       0        0        0            5   NAS-Port
   0-1       0-1      0        0            6   Service-Type
   0-1       0-1      0        0            7   Framed-Protocol
   0-1       0-1      0        0            8   Framed-IP-Address
   0-1       0-1      0        0            9   Framed-IP-Netmask
   0         0-1      0        0           10   Framed-Routing
   0         0+       0        0           11   Filter-Id
   0-1       0-1      0        0           12   Framed-MTU
   0+        0+       0        0           13   Framed-Compression
   0+        0+       0        0           14   Login-IP-Host
   0         0-1      0        0           15   Login-Service
   0         0-1      0        0           16   Login-TCP-Port
   0         0+       0+       0+          18   Reply-Message
   0-1       0-1      0        0           19   Callback-Number
   0         0-1      0        0           20   Callback-Id
   0         0+       0        0           22   Framed-Route
   0         0-1      0        0           23   Framed-IPX-Network
   0-1       0-1      0        0-1         24   State [Note 1]
   0         0+       0        0           25   Class
   0+        0+       0        0+          26   Vendor-Specific
   0         0-1      0        0-1         27   Session-Timeout
   0         0-1      0        0-1         28   Idle-Timeout
   0         0-1      0        0           29   Termination-Action
   0-1       0        0        0           30   Called-Station-Id
   0-1       0        0        0           31   Calling-Station-Id
   0-1       0        0        0           32   NAS-Identifier [Note 2]
   0+        0+       0+       0+          33   Proxy-State
   0-1       0-1      0        0           34   Login-LAT-Service
   0-1       0-1      0        0           35   Login-LAT-Node

   0-1       0-1      0        0           36   Login-LAT-Group
   0         0-1      0        0           37   Framed-AppleTalk-Link
   0         0+       0        0           38   Framed-AppleTalk-Network
   0         0-1      0        0           39   Framed-AppleTalk-Zone
   0-1       0        0        0           60   CHAP-Challenge
   0-1       0        0        0           61   NAS-Port-Type
   0-1       0-1      0        0           62   Port-Limit
   0-1       0-1      0        0           63   Login-LAT-Port
   Request   Accept   Reject   Challenge   #    Attribute

   [Note 1] 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 User-Password or
   CHAP-Password.

   [Note 2] An Access-Request MUST contain either a NAS-IP-Address or a
   NAS-Identifier (or both).

   The following table defines the meaning of the above table entries.

0     This attribute MUST NOT be present in packet.
0+    Zero or more instances of this attribute MAY be present in packet.
0-1   Zero or one instance of this attribute MAY be present in packet.
1     Exactly one instance of this attribute MUST be present in packet.

6.  IANA Considerations

   This section provides guidance to the Internet Assigned Numbers
   Authority (IANA) regarding registration of values related to the
   RADIUS protocol, in accordance with BCP 26 [13].

   There are three name spaces in RADIUS that require registration:
   Packet Type Codes, Attribute Types, and Attribute Values (for certain
   Attributes).

   RADIUS is not intended as a general-purpose Network Access Server
   (NAS) management protocol, and allocations should not be made for
   purposes unrelated to Authentication, Authorization or Accounting.

6.1.  Definition of Terms

   The following terms are used here with the meanings defined in
   BCP 26: "name space", "assigned value", "registration".

   The following policies are used here with the meanings defined in
   BCP 26: "Private Use", "First Come First Served", "Expert Review",
   "Specification Required", "IETF Consensus", "Standards Action".

6.2.  Recommended Registration Policies

   For registration requests where a Designated Expert should be
   consulted, the IESG Area Director for Operations should appoint the
   Designated Expert.

   For registration requests requiring Expert Review, the ietf-radius
   mailing list should be consulted.

   Packet Type Codes have a range from 1 to 254, of which 1-5,11-13 have
   been allocated.  Because a new Packet Type has considerable impact on
   interoperability, a new Packet Type Code requires Standards Action,
   and should be allocated starting at 14.

   Attribute Types have a range from 1 to 255, and are the scarcest
   resource in RADIUS, thus must be allocated with care.  Attributes
   1-53,55,60-88,90-91 have been allocated, with 17 and 21 available for
   re-use.  Attributes 17, 21, 54, 56-59, 89, 92-191 may be allocated
   following Expert Review, with Specification Required.  Release of
   blocks of Attribute Types (more than 3 at a time for a given purpose)
   should require IETF Consensus.  It is recommended that attributes 17
   and 21 be used only after all others are exhausted.

   Note that RADIUS defines a mechanism for Vendor-Specific extensions
   (Attribute 26) and the use of that should be encouraged instead of
   allocation of global attribute types, for functions specific only to
   one vendor's implementation of RADIUS, where no interoperability is
   deemed useful.

   As stated in the "Attributes" section above:

      "[Attribute Type] Values 192-223 are reserved for experimental
      use, values 224-240 are reserved for implementation-specific use,
      and values 241-255 are reserved and should not be used."

   Therefore Attribute values 192-240 are considered Private Use, and
   values 241-255 require Standards Action.

   Certain attributes (for example, NAS-Port-Type) in RADIUS define a
   list of values to correspond with various meanings.  There can be 4
   billion (2^32) values for each attribute. Adding additional values to
   the list can be done on a First Come, First Served basis by the IANA.

7.  Examples

   A few examples are presented to illustrate the flow of packets and
   use of typical attributes.  These examples are not intended to be
   exhaustive, many others are possible.  Hexadecimal dumps of the
   example packets are given in network byte order, using the shared
   secret "xyzzy5461".

7.1.  User Telnet to Specified Host

   The NAS at 192.168.1.16 sends an Access-Request UDP packet to the
   RADIUS Server for a user named nemo logging in on port 3 with
   password "arctangent".

   The Request Authenticator is a 16 octet random number generated by
   the NAS.

   The User-Password is 16 octets of password padded at end with nulls,
   XORed with MD5(shared secret|Request Authenticator).

      01 00 00 38 0f 40 3f 94 73 97 80 57 bd 83 d5 cb
      98 f4 22 7a 01 06 6e 65 6d 6f 02 12 0d be 70 8d
      93 d4 13 ce 31 96 e4 3f 78 2a 0a ee 04 06 c0 a8
      01 10 05 06 00 00 00 03

       1 Code = Access-Request (1)
       1 ID = 0
       2 Length = 56
      16 Request Authenticator

      Attributes:
       6  User-Name = "nemo"
      18  User-Password
       6  NAS-IP-Address = 192.168.1.16
       6  NAS-Port = 3

   The RADIUS server authenticates nemo, and sends an Access-Accept UDP
   packet to the NAS telling it to telnet nemo to host 192.168.1.3.

   The Response Authenticator is a 16-octet MD5 checksum of the code
   (2), id (0), Length (38), the Request Authenticator from above, the
   attributes in this reply, and the shared secret.

      02 00 00 26 86 fe 22 0e 76 24 ba 2a 10 05 f6 bf
      9b 55 e0 b2 06 06 00 00 00 01 0f 06 00 00 00 00
      0e 06 c0 a8 01 03

       1 Code = Access-Accept (2)
       1 ID = 0 (same as in Access-Request)
       2 Length = 38
      16 Response Authenticator

      Attributes:
       6  Service-Type (6) = Login (1)
       6  Login-Service (15) = Telnet (0)
       6  Login-IP-Host (14) = 192.168.1.3

7.2.  Framed User Authenticating with CHAP

   The NAS at 192.168.1.16 sends an Access-Request UDP packet to the
   RADIUS Server for a user named flopsy logging in on port 20 with PPP,
   authenticating using CHAP.  The NAS sends along the Service-Type and
   Framed-Protocol attributes as a hint to the RADIUS server that this
   user is looking for PPP, although the NAS is not required to do so.

   The Request Authenticator is a 16 octet random number generated by
   the NAS, and is also used as the CHAP Challenge.

   The CHAP-Password consists of a 1 octet CHAP ID, in this case 22,
   followed by the 16 octet CHAP response.

      01 01 00 47 2a ee 86 f0 8d 0d 55 96 9c a5 97 8e
      0d 33 67 a2 01 08 66 6c 6f 70 73 79 03 13 16 e9
      75 57 c3 16 18 58 95 f2 93 ff 63 44 07 72 75 04
      06 c0 a8 01 10 05 06 00 00 00 14 06 06 00 00 00
      02 07 06 00 00 00 01

       1 Code = 1     (Access-Request)
       1 ID = 1
       2 Length = 71
      16 Request Authenticator

      Attributes:
       8  User-Name (1) = "flopsy"
      19  CHAP-Password (3)
       6  NAS-IP-Address (4) = 192.168.1.16
       6  NAS-Port (5) = 20
       6  Service-Type (6) = Framed (2)
       6  Framed-Protocol (7) = PPP (1)

   The RADIUS server authenticates flopsy, and sends an Access-Accept
   UDP packet to the NAS telling it to start PPP service and assign an
   address for the user out of its dynamic address pool.

   The Response Authenticator is a 16-octet MD5 checksum of the code
   (2), id (1), Length (56), the Request Authenticator from above, the
   attributes in this reply, and the shared secret.

      02 01 00 38 15 ef bc 7d ab 26 cf a3 dc 34 d9 c0
      3c 86 01 a4 06 06 00 00 00 02 07 06 00 00 00 01
      08 06 ff ff ff fe 0a 06 00 00 00 02 0d 06 00 00
      00 01 0c 06 00 00 05 dc

       1 Code = Access-Accept (2)
       1 ID = 1 (same as in Access-Request)
       2 Length = 56
      16 Response Authenticator

      Attributes:
       6  Service-Type (6) = Framed (2)
       6  Framed-Protocol (7) = PPP (1)
       6  Framed-IP-Address (8) = 255.255.255.254
       6  Framed-Routing (10) = None (0)
       6  Framed-Compression (13) = VJ TCP/IP Header Compression (1)
       6  Framed-MTU (12) = 1500

7.3.  User with Challenge-Response card

   The NAS at 192.168.1.16 sends an Access-Request UDP packet to the
   RADIUS Server for a user named mopsy logging in on port 7.  The user
   enters the dummy password "challenge" in this example.  The challenge
   and response generated by the smart card for this example are
   "32769430" and "99101462".

   The Request Authenticator is a 16 octet random number generated by
   the NAS.

   The User-Password is 16 octets of password, in this case "challenge",
   padded at the end with nulls, XORed with MD5(shared secret|Request
   Authenticator).

      01 02 00 39 f3 a4 7a 1f 6a 6d 76 71 0b 94 7a b9
      30 41 a0 39 01 07 6d 6f 70 73 79 02 12 33 65 75
      73 77 82 89 b5 70 88 5e 15 08 48 25 c5 04 06 c0
      a8 01 10 05 06 00 00 00 07

       1 Code = Access-Request (1)
       1 ID = 2
       2 Length = 57
      16 Request Authenticator

      Attributes:
       7 User-Name (1) = "mopsy"
      18 User-Password (2)
       6  NAS-IP-Address (4) = 192.168.1.16
       6  NAS-Port (5) = 7

   The RADIUS server decides to challenge mopsy, sending back a
   challenge string and looking for a response.  The RADIUS server
   therefore and sends an Access-Challenge UDP packet to the NAS.

   The Response Authenticator is a 16-octet MD5 checksum of the code
   (11), id (2), length (78), the Request Authenticator from above, the
   attributes in this reply, and the shared secret.

   The Reply-Message is "Challenge 32769430.  Enter response at prompt."

   The State is a magic cookie to be returned along with user's
   response; in this example 8 octets of data (33 32 37 36 39 34 33 30
   in hex).

      0b 02 00 4e 36 f3 c8 76 4a e8 c7 11 57 40 3c 0c
      71 ff 9c 45 12 30 43 68 61 6c 6c 65 6e 67 65 20
      33 32 37 36 39 34 33 30 2e 20 20 45 6e 74 65 72
      20 72 65 73 70 6f 6e 73 65 20 61 74 20 70 72 6f
      6d 70 74 2e 18 0a 33 32 37 36 39 34 33 30

       1 Code = Access-Challenge (11)
       1 ID = 2 (same as in Access-Request)
       2 Length = 78
      16 Response Authenticator

      Attributes:
      48  Reply-Message (18)
      10  State (24)

   The user enters his response, and the NAS send a new Access-Request
   with that response, and includes the State Attribute.

   The Request Authenticator is a new 16 octet random number.

   The User-Password is 16 octets of the user's response, in this case
   "99101462", padded at the end with nulls, XORed with MD5(shared
   secret|Request Authenticator).

   The state is the magic cookie from the Access-Challenge packet,
   unchanged.

      01 03 00 43 b1 22 55 6d 42 8a 13 d0 d6 25 38 07
      c4 57 ec f0 01 07 6d 6f 70 73 79 02 12 69 2c 1f
      20 5f c0 81 b9 19 b9 51 95 f5 61 a5 81 04 06 c0
      a8 01 10 05 06 00 00 00 07 18 10 33 32 37 36 39
      34 33 30

       1 Code = Access-Request (1)
       1 ID = 3 (Note that this changes.)
       2 Length = 67
      16 Request Authenticator

      Attributes:
       7  User-Name = "mopsy"
      18  User-Password
       6  NAS-IP-Address (4) = 192.168.1.16
       6  NAS-Port (5) = 7
      10  State (24)

   The Response was incorrect (for the sake of example), so the RADIUS
   server tells the NAS to reject the login attempt.

   The Response Authenticator is a 16 octet MD5 checksum of the code
   (3), id (3), length(20), the Request Authenticator from above, the
   attributes in this reply (in this case, none), and the shared secret.

      03 03 00 14 a4 2f 4f ca 45 91 6c 4e 09 c8 34 0f
      9e 74 6a a0

       1 Code = Access-Reject (3)
       1 ID = 3 (same as in Access-Request)
       2 Length = 20
      16 Response Authenticator

      Attributes:
         (none, although a Reply-Message could be sent)

8.  Security Considerations

   Security issues are the primary topic of this document.

   In practice, within or associated with each RADIUS server, there is a
   database which associates "user" names with authentication
   information ("secrets").  It is not anticipated that a particular
   named user would be authenticated by multiple methods.  This would
   make the user vulnerable to attacks which negotiate the least secure
   method from among a set.  Instead, for each named user there should
   be an indication of exactly one method used to authenticate that user
   name.  If a user needs to make use of different authentication
   methods under different circumstances, then distinct user names
   SHOULD be employed, each of which identifies exactly one
   authentication method.

   Passwords and other secrets should be stored at the respective ends
   such that access to them is as limited as possible.  Ideally, the
   secrets should only be accessible to the process requiring access in
   order to perform the authentication.

   The secrets should be distributed with a mechanism that limits the
   number of entities that handle (and thus gain knowledge of) the
   secret.  Ideally, no unauthorized person should ever gain knowledge
   of the secrets.  It is possible to achieve this with SNMP Security
   Protocols [14], but such a mechanism is outside the scope of this
   specification.

   Other distribution methods are currently undergoing research and
   experimentation.  The SNMP Security document [14] also has an
   excellent overview of threats to network protocols.

   The User-Password hiding mechanism described in Section 5.2 has not
   been subjected to significant amounts of cryptanalysis in the
   published literature.  Some in the IETF community are concerned that
   this method might not provide sufficient confidentiality protection
   [15] to passwords transmitted using RADIUS.  Users should evaluate
   their threat environment and consider whether additional security
   mechanisms should be employed.

9.  Change Log

   The following changes have been made from RFC 2138:

   Strings should use UTF-8 instead of US-ASCII and should be handled as
   8-bit data.

   Integers and dates are now defined as 32 bit unsigned values.

   Updated list of attributes that can be included in Access-Challenge
   to be consistent with the table of attributes.

   User-Name mentions Network Access Identifiers.

   User-Name may now be sent in Access-Accept for use with accounting
   and Rlogin.

   Values added for Service-Type, Login-Service, Framed-Protocol,
   Framed-Compression, and NAS-Port-Type.

   NAS-Port can now use all 32 bits.

   Examples now include hexadecimal displays of the packets.

   Source UDP port must be used in conjunction with the Request
   Identifier when identifying duplicates.

   Multiple subattributes may be allowed in a Vendor-Specific attribute.

   An Access-Request is now required to contain either a NAS-IP-Address
   or NAS-Identifier (or may contain both).

   Added notes under "Operations" with more information on proxy,
   retransmissions, and keep-alives.

   If multiple Attributes with the same Type are present, the order of
   Attributes with the same Type MUST be preserved by any proxies.

   Clarified Proxy-State.

   Clarified that Attributes must not depend on position within the
   packet, as long as Attributes of the same type are kept in order.

   Added IANA Considerations section.

   Updated section on "Proxy" under "Operations".

   Framed-MTU can now be sent in Access-Request as a hint.

   Updated Security Considerations.

   Text strings identified as a subset of string, to clarify use of
   UTF-8.

10.  References

   [1]   Rigney, C., Rubens, A., Simpson, W. and S. Willens, "Remote
         Authentication Dial In User Service (RADIUS)", RFC 2138, April
         1997.

   [2]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March, 1997.

   [3]   Rivest, R. and S. Dusse, "The MD5 Message-Digest Algorithm",
         RFC 1321, April 1992.

   [4]   Postel, J., "User Datagram Protocol", STD 6, RFC 768, August
         1980.

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

   [6]   Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC
         1700, October 1994.

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

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

   [9]   Kaufman, C., Perlman, R., and Speciner, M., "Network Security:
         Private Communications in a Public World", Prentice Hall, March
         1995, ISBN 0-13-061466-1.

   [10]  Jacobson, V., "Compressing TCP/IP headers for low-speed serial
         links", RFC 1144, February 1990.

   [11]  ISO 8859. International Standard -- Information Processing --
         8-bit Single-Byte Coded Graphic Character Sets -- Part 1: Latin
         Alphabet No. 1, ISO 8859-1:1987.

   [12]  Sklower, K., Lloyd, B., McGregor, G., Carr, D. and T.
         Coradetti, "The PPP Multilink Protocol (MP)", RFC 1990, August
         1996.

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

   [14]  Galvin, J., McCloghrie, K. and J. Davin, "SNMP Security
         Protocols", RFC 1352, July 1992.

   [15]  Dobbertin, H., "The Status of MD5 After a Recent Attack",
         CryptoBytes Vol.2 No.2, Summer 1996.

11.  Acknowledgements

   RADIUS was originally developed by Steve Willens of Livingston
   Enterprises for their PortMaster series of Network Access Servers.

12.  Chair's Address

   The working group can be contacted via the current chair:

   Carl Rigney
   Livingston Enterprises
   4464 Willow Road
   Pleasanton, California  94588

   Phone: +1 925 737 2100
   EMail: cdr@telemancy.com

13.  Authors' Addresses

   Questions about this memo can also be directed to:

   Carl Rigney
   Livingston Enterprises
   4464 Willow Road
   Pleasanton, California  94588

   Phone: +1 925 737 2100
   EMail: cdr@telemancy.com

   Allan C. Rubens
   Merit Network, Inc.
   4251 Plymouth Road
   Ann Arbor, Michigan  48105-2785

   EMail: acr@merit.edu

   William Allen Simpson
   Daydreamer
   Computer Systems Consulting Services
   1384 Fontaine
   Madison Heights, Michigan  48071

   EMail: wsimpson@greendragon.com

   Steve Willens
   Livingston Enterprises
   4464 Willow Road
   Pleasanton, California  94588

   EMail: steve@livingston.com

14.  Full Copyright Statement

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or 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
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Acknowledgement

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

 

User Contributions:

imesjon
Report this comment as inappropriate
Aug 30, 2005 @ 4:16 pm
Does RADIUS, like KERBEROS, utilize NTP to synchronize the workstation's date and time with the server.

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