PFIL(9)

NAME
pfil, pfil_head_register, pfil_head_unregister, pfil_link, pfil_run_hooks - packet filter interface

SYNOPSIS
#include <sys/param.h>
#include <sys/mbuf.h>
#include <net/pfil.h>
pfil_head_t
pfil_head_register(struct pfil_head_args *args);
void
pfil_head_unregister(struct pfil_head_t *head);
pfil_hook_t
pfil_add_hook(struct pfil_hook_args *);
void
pfil_remove_hook(pfil_hook_t);
int
pfil_link(struct pfil_link_args *args);
int
pfil_run_hooks(phil_head_t *, pfil_packet_t, struct ifnet *, int, struct inpcb *);

DESCRIPTION
The pfil framework allows for a specified function or a list of functions 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, for historical reasons named heads, are registered with pfil_head_register(). The function is supplied with special versioned struct pfil_head_args structure that specifies type and features of the head as well as human readable name. If the filtering point to be ever destroyed, the subsystem that created it must unregister it with call to pfil_head_unregister().

Packet filtering systems may register arbitrary number of filters, for historical reasons named hooks. To register a new hook pfil_add_hook() with special versioned struct pfil_hook_args structure is called. The structure specifies type and features of the hook, pointer to the actual filtering function and user readable name of the filtering module and ruleset name. Later hooks can be removed with pfil_remove_hook() functions.

To connect existing hook to an existing head function pfil_link() shall be used. The function is supplied with versioned struct pfil_link_args structure that specifies either literal names of hook and head or pointers to them. Typically pfil_link() is called by filtering modules to autoregister their default ruleset and default filtering points. It also serves on the kernel side of ioctl(2) when user changes pfil configuration with help of pfilctl(8) utility.

For every packet traveling through a head the latter shall invoke pfil_run_hooks(). The function can accept either struct mbuf * pointer or a void * pointer and length. In case if a hooked filtering module cannot understand void * pointer pfil will provide it with a fake one. All calls to pfil_run_hooks() are performed in network epoch(9).

HEADS (filtering points)
By default kernel creates the following heads:

inet
    IPv4 packets.

inet6
    IPv6 packets.

ethernet
    Link-layer packets.

Default rulesets are automatically linked to these heads to preserve historical behaviour.

SEE ALSO
ipfilter(4), ipfw(4), pf(4), pfilctl(8)

HISTORY
The pfil interface first appeared in NetBSD 1.3. The pfil interface was imported into FreeBSD 5.2. In FreeBSD 13.0 the interface was significantly rewritten.