pfil

PFIL(9)                   BSD Kernel Developer's Manual                  PFIL(9)

NAME
     pfil, pfil_head_register, pfil_head_unregister, pfil_head_get,
     pfil_add_hook, pfil_add_hook_flags, pfil_remove_hook,
     pfil_remove_hook_flags, 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);

     typedef int (*pfil_func_flags_t)(void *arg, struct mbuf **mp, struct ifnet *, int dir, int flags, 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);

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

     int
     pfil_add_hook_flags(pfil_func_flags_t, void *arg, int flags, struct pfil_head *);

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

     int
     pfil_remove_hook_flags(pfil_func_flags_t, void *arg, int flags, struct pfil_head *);

     int
     pfil_run_hooks(struct pfil_head *head, struct mbuf **mp, struct ifnet *, int dir, int flags, 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 flags argument will indicate if an outgoing
     packet is simply being forwarded with the value PFIL_FWD.  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.

BSD                              March 10, 2018                              BSD