rfc2058.txt

radius服务器

   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.







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RFC 2058                         RADIUS                     January 1997


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.

        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



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RFC 2058                         RADIUS                     January 1997


        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.

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.

   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 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-








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RFC 2058                         RADIUS                     January 1997


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 will be 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.








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RFC 2058                         RADIUS                     January 1997


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



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RFC 2058                         RADIUS                     January 1997


   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.

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).

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




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RFC 2058                         RADIUS                     January 1997


     An Access-Request MUST contain a User-Name attribute.  It SHOULD
     contain either a NAS-IP-Address attribute or NAS-Identifier
     attribute (or both, although that is not recommended).  It MUST
     contain either a User-Password attribute or CHAP-Password
     attribute.  It 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 [1].

   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.