Command Section
PFIL(9)                FreeBSD Kernel Developer's Manual               PFIL(9)

     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

     #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);)(void *arg, struct mbuf **mp, struct ifnet *, int dir, struct inpcb);
     pfil_head_register(struct pfil_head *head);

     pfil_head_unregister(struct pfil_head *head);

     struct pfil_head *
     pfil_head_get(int af, u_long dlt);

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

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

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

     pfil_rlock(struct pfil_head *, struct rm_priotracker *);

     pfil_runlock(struct pfil_head *, struct rm_priotracker *);

     pfil_wlock(struct pfil_head *);

     pfil_wunlock(struct pfil_head *);

     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

     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

     Currently, filtering points are implemented for the following link types:

        AF_INET   IPv4 packets.
        AF_INET6  IPv6 packets.
        AF_LINK   Link-layer packets.

     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!

     bpf(4), if_bridge(4), rmlock(9)

     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

     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.

     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

FreeBSD 11.1-RELEASE-p4         August 23, 2013        FreeBSD 11.1-RELEASE-p4
Command Section