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 pfil_head *
pfil_head_get(int af, u_long dlt);
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,
        struct inpcb *);
int
pfil_run_hooks(struct pfil_head  *head,  struct  mbuf  **mp,
struct ifnet *,
        int dir, struct inpcb *);

DESCRIPTION

The pfil framework allows for a specified function to be in

voked 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. Filtering points may be unregistered

with the
pfil_head_unregister() function.

Packet filters register/unregister themselves with a filter

ing point with
the pfil_add_hook() and pfil_remove_hook() functions, re

spectively. 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 net

work 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.

RETURN VALUES

If successful, pfil_head_get() returns the pfil_head struc

ture 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), bridge(4)

HISTORY

The pfil interface first appeared in NetBSD 1.3. The pfil

input and output lists were originally implemented as #include

<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 fil

ters 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 pa

rameter to both
pfil_add_hook() and pfil_remove_hook(), introducing the ca

pability 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 ar

bitrary number
of filtering points, as well as be less IP-centric.

Fine-grained locking was added in FreeBSD 5.2.

BUGS

The pfil_hook_get() function is only safe for internal use.

FreeBSD implements only hooks for AF_INET and AF_INET6.

Packets diverted
through these hooks have data in host byte order contrary to

the above
statements.

The bridge(4) diverts inbound AF_INET traffic, but contrary

to the above
statements, the data is provided in host byte order.

When a pfil_head is being modified, no traffic is diverted

(to avoid
deadlock). This means that traffic may be dropped uncondi

tionally for a
short period of time. pfil_run_hooks() will return ENOBUFS

to indicate
this.

BSD September 29, 2004

BSD