interface. For example, when the user runs ``ip link set promisc
[on|off] dev <ifaceX>``. If the user sets this callback function to
NULL, but sets the ``port_id`` field to a value other than -1, a default
interface. For example, when the user runs ``ip link set promisc
[on|off] dev <ifaceX>``. If the user sets this callback function to
NULL, but sets the ``port_id`` field to a value other than -1, a default