https://github.com/thom311/libnl/blob/libnl3_2_25/lib/socket.c#L42.

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}')