RADIUS attribute from rfc2865

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User-Name

This Attribute indicates the name of the user to be authenticated.
      It MUST be sent in Access-Request packets if available.

      It MAY be sent in an Access-Accept packet, in which case the
      client SHOULD use the name returned in the Access-Accept packet in
      all Accounting-Request packets for this session.  If the Access-
      Accept includes Service-Type = Rlogin and the User-Name attribute,
      a NAS MAY use the returned User-Name when performing the Rlogin
      function.

   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.

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

      text      Consisting only of UTF-8 encoded 10646  characters.

      network access identifier
                A Network Access Identifier as described in.

      distinguished name
                A name in ASN.1 form used in Public Key authentication
                systems.

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