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.