.\"	$NetBSD: pfil.9,v 1.16 2001/12/26 01:07:45 wiz Exp $
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.\" Copyright (c) 1996 Matthew R. Green
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.Dd November 10, 2000
.Dt PFIL 9
.Os
.Sh NAME
.Nm pfil ,
.Nm pfil_head_register ,
.Nm pfil_head_unregister ,
.Nm pfil_head_get ,
.Nm pfil_hook_get ,
.Nm pfil_add_hook ,
.Nm pfil_remove_hook ,
.Nm pfil_run_hooks
.Nd packet filter interface
.Sh SYNOPSIS
.Fd #include <sys/param.h>
.Fd #include <sys/mbuf.h>
.Fd #include <net/if.h>
.Fd #include <net/pfil.h>
.Ft int
.Fn pfil_head_register "struct pfil_head *head"
.Ft int
.Fn pfil_head_unregister "struct pfil_head *head"
.Ft struct packet_filter_hook *
.Fn pfil_hook_get "int dir" "struct pfil_head *head"
.Ft void
.Fn pfil_add_hook "int (*func)()" "void *arg" "int flags" "struct pfil_head *"
.Ft void
.Fn pfil_remove_hook "int (*func)()" "void *arg" "int flags" "struct pfil_head *"
.Ft int
.Fn (*func) "void *arg" "struct mbuf **mp" "struct ifnet *" "int dir"
.Ft int
.Fn pfil_run_hooks "struct pfil_head *head" "struct mbuf **mp" "struct ifnet *" "int dir"
.Sh DESCRIPTION
The
.Nm
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.
.Nm
Packet filtering points are registered with
.Fn pfil_head_register .
Filtering points are identified by a key (void *) and a data link type
(int) in the
.Em 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
.Xr 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
.Fn pfil_head_unregister
function.
.Pp
Packet filters register/unregister themselves with a filtering point
with the
.Fn pfil_add_hook
and
.Fn pfil_remove_hook
functions, respectively.  The head is looked up using the
.Fn 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.
.Pp
When a filter is invoked, the packet appears just as if it
.Dq 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 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.
.Pp
The
.Nm
interface is enabled in the kernel via the
.Sy PFIL_HOOKS
option.
.Sh SEE ALSO
.Xr bpf 4
.Sh HISTORY
The
.Nm
interface first appeared in
.Nx 1.3 .
The
.Nm
input and output lists were originally implemented as
.Fd <sys/queue.h>
.Dv LIST
structures;
however this was changed in
.Nx 1.4
to
.Dv 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.
.Pp
The
.Nm
interface was changed in 1.4T to accept a 3rd parameter to both
.Fn pfil_add_hook
and
.Fn pfil_remove_hook
, introducing the capability of per-protocol filtering.  This was done
primarily in order to support filtering of IPv6.
.Pp
In
.Nx 1.5K ,
the
.Nm
framework was changed to work with an arbitrary number of filtering points,
as well as be less IP-centric.
.Sh BUGS
The current
.Nm
implementation will need changes to suit a threaded kernel model.