pfil, pfil_head_register, pfil_head_unregister, pfil_head_get,
pfil_hook_get, pfil_add_hook, pfil_remove_hook, pfil_run_hooks - packet
pfil_head_register(struct pfil_head *head);
pfil_head_unregister(struct pfil_head *head);
struct packet_filter_hook *
pfil_hook_get(int dir, struct pfil_head *head);
pfil_add_hook(int (*func)(), void *arg, int flags, struct pfil_head *);
pfil_remove_hook(int (*func)(), void *arg, int flags,
struct pfil_head *);
(*func)(void *arg, struct mbuf **mp, struct ifnet *, int dir);
pfil_run_hooks(struct pfil_head *head, struct mbuf **mp, struct ifnet *,
The pfil framework allows for a specified function to be invoked for
every incoming or outgoing packet for a particular network I/O stream.
These hooks may be used to implement a firewall or perform packet transformations.
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. 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 filter may change which mbuf the mbuf
** argument references. The filter returns an 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
The pfil interface is enabled in the kernel via the PFIL_HOOKS option.
The pfil interface first appeared in NetBSD 1.3. The pfil input and output
lists were originally implemented as <sys/queue.h>
LIST structures; however this was changed in NetBSD 1.4 to TAILQ structures.
This change was to allow the input and output filters to be processed
in reverse order, to allow the same path to be taken, in or out of
The pfil interface was changed in 1.4T to accept a 3rd parameter to both
pfil_add_hook() and pfil_remove_hook() , introducing the capability of
per-protocol filtering. This was done primarily in order to support filtering
In NetBSD 1.5K, the pfil framework was changed to work with an arbitrary
number of filtering points, as well as be less IP-centric.
The current pfil implementation will need changes to suit a threaded kernel
BSD November 10, 2000 BSD
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