pfil(9) - NetBSD Manual Pages

PFIL(9)                      NetBSD Kernel Manual                      PFIL(9)


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 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);

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


DESCRIPTION

     The pfil framework allows for a specified function to be invoked for ev-
     ery incoming or outgoing packet for a particular network I/O stream.
     These hooks may be used to implement a firewall or perform packet trans-
     formations.  pfil 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 errno if the packet pro-
     cessing 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.

     The pfil interface is enabled in the kernel via the PFIL_HOOKS option.


SEE ALSO

     bpf(4)


HISTORY

     The pfil interface first appeared in NetBSD 1.3.  The pfil input and out-
     put lists were originally implemented as <sys/queue.h>
     LIST structures; however this was changed in NetBSD 1.4 to TAILQ struc-
     tures.  This change was to allow the input and output filters to be pro-
     cessed 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 fil-
     tering of IPv6.

     In NetBSD 1.5K, the pfil framework was changed to work with an arbitrary
     number of filtering points, as well as be less IP-centric.


BUGS

     The current pfil implementation will need changes to suit a threaded ker-
     nel model.

NetBSD 1.6                     November 10, 2000                             2