PFIL(9)

NAME
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

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);
    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);
    void
    pfil_add_hook(pfil_func_t, void *arg, int flags, struct pfil_head *);
    void
    pfil_remove_hook(pfil_func_t, void *arg, int flags, struct pfil_head *);
    int
    pfil_run_hooks(struct pfil_head *head, struct mbuf **mp, struct ifnet *, int dir, 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 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.