code: def authenticate(self, request, username=None, password=None):
          
          if isinstance(username, basestring):
              username = username.encode('utf-8')
  
          if isinstance(password, basestring):
              password = password.encode('utf-8')
  
          server = self._get_server_from_settings()
          result = self._radius_auth(server, username, password)
  
          if result:
              return self.get_django_user(username, password)
  
          return None
docs: Check credentials against RADIUS server and return a User object or
          None.