Manual Pages  — PFIL

Packet filtering points are registered with pfil_head_register(). Filtering points are identified by a key (void *) and a data link type (int) in the pfil_head structure. Packet filters use the key and data link type to look up the filtering point with which they register themselves. The key is unique to the filtering point. The data link type is a bpf(4) DLT constant indicating what kind of header is present on the packet at the filtering point. Each filtering point uses common per-VNET rmlock by default. This can be changed by specifying PFIL_FLAG_PRIVATE_LOCK as flags field in the pfil_head structure. Note that specifying private lock can break filters sharing the same ruleset and/or state between different data link types. Filtering points may be unregistered with the pfil_head_unregister() function.

Packet filters register/unregister themselves with a filtering point with the pfil_add_hook() and pfil_remove_hook() functions, respectively. The head is looked up using the pfil_head_get() function, which takes the key and data link type that the packet filter expects. Filters may provide an argument to be passed to the filter when invoked on a packet.

When a filter is invoked, the packet appears just as if it "came off the wire". That is, all protocol fields are in network byte order. The filter is called with its specified argument, the pointer to the pointer to the mbuf containing the packet, the pointer to the network interface that the packet is traversing, and the direction ( PFIL_IN or PFIL_OUT) that the packet is traveling. The flags argument will indicate if an outgoing packet is simply being forwarded with the value PFIL_FWD. The filter may change which mbuf the mbuf ** argument references. The filter returns an error (errno) if the packet processing is to stop, or 0 if the processing is to continue. If the packet processing is to stop, it is the responsibility of the filter to free the packet.

Every filter hook is called with pfil read lock held. All heads uses the same lock within the same VNET instance. Packet filter can use this lock instead of own locking model to improve performance. Since pfil uses rmlock(9) pfil_rlock() and pfil_runlock() require struct rm_priotracker to be passed as argument. Filter can acquire and release writer lock via pfil_wlock() and pfil_wunlock() functions. See rmlock(9) for more details.