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

NAME
     pfil, pfil_head_register, pfil_head_unregister, pfil_head_get,
     pfil_add_hook, pfil_remove_hook, pfil_run_hooks, pfil_rlock,
     pfil_runlock, pfil_wlock, pfil_wunlock -- packet filter interface

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

     typedef int (*pfil_func_t)(void *arg, struct mbuf **mp, struct ifnet *, int dir, struct inpcb);

     int
     pfil_head_register(struct pfil_head *head);

     int
     pfil_head_unregister(struct pfil_head *head);

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

     void
     pfil_add_hook(pfil_func_t,	void *arg, int flags, struct pfil_head *);

     void
     pfil_remove_hook(pfil_func_t, void	*arg, int flags, struct	pfil_head *);

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

     void
     pfil_rlock(struct pfil_head *, struct rm_priotracker *);

     void
     pfil_runlock(struct pfil_head *, struct rm_priotracker *);

     void
     pfil_wlock(struct pfil_head *);

     void
     pfil_wunlock(struct pfil_head *);

DESCRIPTION
     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 trans-
     formations.

     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.  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 dif-
     ferent 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	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.

FILTERING POINTS
     Currently,	filtering points are implemented for the following link	types:

	AF_INET	  IPv4 packets.
	AF_INET6  IPv6 packets.
	AF_LINK	  Link-layer packets.

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

     The pfil_head_unregister()	function might sleep!

SEE ALSO
     bpf(4), if_bridge(4), rmlock(9)

HISTORY
     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
     order, 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.  pfil lock export was
     added in FreeBSD 10.0.

BUGS
     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
     this.

FreeBSD	10.1			August 23, 2013			  FreeBSD 10.1

NAME | SYNOPSIS | DESCRIPTION | FILTERING POINTS | RETURN VALUES | SEE ALSO | HISTORY | BUGS

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