docs: def init_default_cb():
      """https://github.com/thom311/libnl/blob/libnl3_2_25/lib/socket.c#L42."""
      global default_cb
      nlcb = os.environ.get('NLCB', '').lower()
      if not nlcb:
          return
  
      if nlcb == 'default':
          default_cb = NL_CB_DEFAULT
      elif nlcb == 'verbose':
          default_cb = NL_CB_VERBOSE
      elif nlcb == 'debug':
          default_cb = NL_CB_DEBUG
      else:
          _LOGGER.warning('Unknown value for NLCB, valid values: {default | verbose | debug}')