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PFIL(9)			 BSD Kernel Developer's	Manual		       PFIL(9)

     pfil, pfil_head_register, pfil_head_unregister, pfil_head_get,
     pfil_hook_get, pfil_add_hook, pfil_remove_hook, pfil_run_hooks -- packet
     filter interface

     #include <sys/param.h>
     #include <sys/mbuf.h>
     #include <net/if.h>
     #include <net/pfil.h>

     pfil_head_register(struct pfil_head *head);

     pfil_head_unregister(struct pfil_head *head);

     struct pfil_head *
     pfil_head_get(int af, u_long dlt);

     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,
	 struct	inpcb *);

     pfil_run_hooks(struct pfil_head *head, struct mbuf	**mp, struct ifnet *,
	 int dir, struct inpcb *);

     The pfil framework	allows for a specified function	to be invoked for ev-
     ery incoming or outgoing packet for a particular network I/O stream.
     These hooks may be	used to	implement a firewall or	perform	packet trans-

     Packet filtering points are registered with pfil_head_register().	Fil-
     tering 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 fil-
     ter 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 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.

     If	successful, pfil_head_get() returns the	pfil_head structure for	the
     given key/dlt.  The pfil_add_hook() and pfil_remove_hook()	functions re-
     turn 0 if successful.  If called with flag	PFIL_WAITOK,
     pfil_remove_hook()	is expected to always succeed.

     The pfil_head_unregister()	function might sleep!

     bpf(4), if_bridge(4)

     The pfil interface	first appeared in NetBSD 1.3.  The pfil	input and out-
     put 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 or-
     der, to allow the same path to be taken, in or out	of the kernel.

     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 fil-
     tering of IPv6.

     In	1.5K, the pfil framework was changed to	work with an arbitrary number
     of	filtering points, as well as be	less IP-centric.

     Fine-grained locking was added in FreeBSD 5.2.

     The pfil_hook_get() function is only safe for internal use.

     FreeBSD implements	only hooks for AF_INET and AF_INET6.  Packets diverted
     through these hooks have data in host byte	order contrary to the above

     The if_bridge(4) diverts AF_INET and AF_INET6 traffic according to	its
     sysctl settings, but contrary to the above	statements, the	data is	pro-
     vided in host byte	order.

     When a pfil_head is being modified, no traffic is diverted	(to avoid
     deadlock).	 This means that traffic may be	dropped	unconditionally	for a
     short period of time.  pfil_run_hooks() will return ENOBUFS to indicate

BSD			      September	29, 2004			   BSD


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