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
       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.  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  different  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  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.  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.

Debian                                           August 23, 2013                                         PFIL(9)