docs: def guess_cls(self):
          """Guess the packet class that must be used on the interface"""
  
          # Get the data link type
          try:
              ret = fcntl.ioctl(self.ins, BIOCGDLT, struct.pack('I', 0))
              ret = struct.unpack('I', ret)[0]
          except IOError:
              cls = conf.default_l2
              warning("BIOCGDLT failed: unable to guess type. Using %s !",
                      cls.name)
              return cls
  
          # Retrieve the corresponding class
          try:
              return conf.l2types[ret]
          except KeyError:
              cls = conf.default_l2
              warning("Unable to guess type (type %i). Using %s", ret, cls.name)