rfc2138.txt

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   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 should be 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
   what it should be, 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



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RFC 2138                         RADIUS                       April 1997


   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 should 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.  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.



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RFC 2138                         RADIUS                       April 1997


         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 that 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.






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RFC 2138                         RADIUS                       April 1997


3.  Packet Format

   Exactly one RADIUS packet is encapsulated in the UDP Data field [2],
   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.  There has been some
   confusion in the assignment of port numbers for this protocol.  The
   early deployment of RADIUS was done using the erroneously chosen 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




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RFC 2138                         RADIUS                       April 1997


   Codes 4 and 5 are covered in the RADIUS Accounting document [9], and
   are not further mentioned here.  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.

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
   should be treated as padding and should be ignored on reception.  If
   the packet is shorter than the Length field indicates, it should 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.








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RFC 2138                         RADIUS                       April 1997


      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.  A RADIUS
   server SHOULD 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 can add a Proxy-State Attribute, and
   when the proxy forwards a response, it removes the Proxy-State
   Attribute. Since Access-Accept and Access-Reject replies are
   authenticated on the entire packet contents, the stripping of the
   Proxy-State attribute would invalidate 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.



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RFC 2138                         RADIUS                       April 1997


Attributes

   Many Attributes may have multiple instances, in such a case the order
   of Attributes of the same Type SHOULD be preserved.  The order of
   Attributes of different Types is not required to be preserved.

   In the section below on "Attributes" where the text refers to which
   packets an attribute is allowed in, only packets with Codes 1, 2, 3
   and 11 and attributes defined in this document are covered in this
   document.  A summary table is provided at the end of the "Attributes"
   section.  To determine which Attributes are allowed in packets with
   codes 4 and 5 refer to the RADIUS Accounting document [9].

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).