Add pfil(9) subroutines and manpage from NetBSD.

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+.\"	$FreeBSD$
+.\"
+.\" Copyright (c) 1996 Matthew R. Green
+.\" All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\" 3. The name of the author may not be used to endorse or promote products
+.\"    derived from this software without specific prior written permission.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+.\" BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+.\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+.\" AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+.\" OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+.\" SUCH DAMAGE.
+.\"
+.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