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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 #include <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.