NAME

       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

SYNOPSIS

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

       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);

       struct packet_filter_hook *
       pfil_hook_get(int dir, struct pfil_head *head);

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

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

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

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

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
       transformations.

       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 packet.

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)

HISTORY

       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.

BUGS

       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 statements.

       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 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 this.

Debian                                         September 29, 2004                                        PFIL(9)