Packet Filters
In principle, the pseudo-devices involved with packet filtering are as follows:
Packet Filter interface
The pfil interface is purely in the stack and supports packet-filtering hooks. Packet filters can register hooks that are
called when packet processing is taking place; in essence, pfil is a list of callbacks for certain events. In addition to
being able to register a filter for incoming and outgoing packets, pfil provides support for interface attach/detach and
address change notifications.
Berkeley Packet Filter
The Berkeley Packet Filter (BPF) provides link-layer access to data available on the network through interfaces attached to
the system.