rfc2138.txt

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      is received in an Access-Accept, Access-Reject or Access-Challenge
      packet with an invalid length, the packet MUST either be treated
      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 a "string" in RADIUS does not require termination by an
      ASCII NUL because the Attribute already has a length field.














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RFC 2138                         RADIUS                       April 1997


      The format of the value field is one of four data types.

      string    0-253 octets

      address   32 bit value, most significant octet first.

      integer   32 bit value, most significant octet first.

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


5.1.  User-Name

   Description

      This Attribute indicates the name of the user to be authenticated.
      It is only used in Access-Request packets.

   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.








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RFC 2138                         RADIUS                       April 1997


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

      monolithic Consisting only of alphanumeric characters.  This
                 simple form might be used to locally manage a NAS.

      simple    Consisting only of printable ASCII characters.

      name@fqdn SMTP address.  The Fully Qualified Domain Name (with or
                without trailing dot) indicates the realm in which the
                name part applies.

      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 [4] pages 109-110.  A more precise
      explanation of the method follows:





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RFC 2138                         RADIUS                       April 1997


      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.



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RFC 2138                         RADIUS                       April 1997


      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.  It is only used
      in Access-Request packets.  Either NAS-IP-Address or NAS-
      Identifier SHOULD be present in an Access-Request packet.












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RFC 2138                         RADIUS                       April 1997


   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.














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RFC 2138                         RADIUS                       April 1997


    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.  Despite the size of the field,
      values range from 0 to 65535.

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.





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RFC 2138                         RADIUS                       April 1997