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.\" Copyright (c) 1996 Matthew R. Green
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.Dd August 4, 1996
.Dt PFIL 9
.Os
.Sh NAME
.Nm pfil ,
.Nm pfil_hook_get ,
.Nm pfil_add_hook ,
.Nm pfil_remove_hook
.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 struct packet_filter_hook *
.Fn pfil_hook_get "int" "struct pfil_head *"
.Ft void
.Fn pfil_add_hook "int (*func)()" "int flags" "struct pfil_head *"
.Ft void
.Fn pfil_remove_hook "int (*func)()" "int flags" "struct pfil_head *"
.\"(void *, int, struct ifnet *, int, struct mbuf **)
.Sh DESCRIPTION
The
.Nm
interface allows a function to be called on every incoming or outgoing
packets.  The hooks for these are embedded in the
.Fn ip_input
and
.Fn ip_output
routines.  The
.Fn pfil_hook_get
function returns the first member of a particular hook, either the in or out
list.  The
.Fn pfil_add_hook
function takes a function of the form below as it's first argument, and the
flags for which lists to add the function to.  The possible values for these
flags are some combination of PFIL_IN and PFIL_OUT.  The
.Fn pfil_remove_hook
removes a hook from the specified lists.
.Pp
The
.Va func
argument is a function with the following prototype.
.Pp
.Fn func "void *data" "int hlen" "struct ifnet *net" "int dir" "struct mbuf **m"
.Pp
The
.Va data
describes the packet.  Currently, this may only be a pointer to a ip structure.  The
.Va net
and
.Va m
arguments describe the network interface and the mbuf holding data for this
packet.  The
.Va dir
is the direction; 0 for incoming packets and 1 for outgoing packets.  if the function
returns non-zero, this signals an error and no further processing of this packet is
performed.  The function should set errno to indicate the nature of the error.
It is the hook's responsibiliy to free the chain if the packet is being dropped.
.Pp
The
.Nm
interface is enabled in the kernel via the
.Sy PFIL_HOOKS
option.
.Sh RETURN VALUES
If successful
.Fn pfil_hook_get
returns the first member of the packet filter list,
.Fn pfil_add_hook
and
.Fn pfil_remove_hook
are expected to always succeed.
.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.
.Sh BUGS
The current
.Nm
implementation will need changes to suit a threaded kernel model.
.Sh SEE ALSO
.Xr bpf 4