





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.




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RFC 2865                         RADIUS                        June 2000


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



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      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).







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RFC 2865                         RADIUS                        June 2000


   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



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   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].




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   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





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   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





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   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.



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   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.



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   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.






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RFC 2865                         RADIUS                        June 2000


   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.




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   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 direc                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   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