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

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

     #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);

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

     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);

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

     pfil_add_hook_flags(pfil_func_flags_t, void *arg, int flags, struct pfil_head *);

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

     pfil_remove_hook_flags(pfil_func_flags_t, void *arg, int flags, struct pfil_head *);

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

     pfil_rlock(struct pfil_head *, struct rm_priotracker *);

     pfil_runlock(struct pfil_head *, struct rm_priotracker *);

     pfil_wlock(struct pfil_head *);

     pfil_wunlock(struct pfil_head *);

     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.  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 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 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 responsi-
     bility 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	de-

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

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

     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), rmlock(9)

     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.  pfil lock export was
     added in FreeBSD 10.0.

     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

FreeBSD	13.0			March 10, 2018			  FreeBSD 13.0


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