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 every
     incoming or outgoing packet for a particular network I/O stream.  These
     hooks may be used to implement a firewall or perform packet

     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 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

     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!

     bpf(4), bridge(4)

     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 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 filtering
     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 bridge(4) diverts inbound AF_INET traffic, but contrary to the above
     statements, the data is provided 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