rfc2865.txt

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      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





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RFC 2865                         RADIUS                        June 2000


   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.




















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RFC 2865                         RADIUS                        June 2000


   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.







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RFC 2865                         RADIUS                        June 2000


   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





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RFC 2865                         RADIUS                        June 2000


         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.






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RFC 2865                         RADIUS                        June 2000


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].












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