217 lines
6.1 KiB
Groff
217 lines
6.1 KiB
Groff
.\" $NetBSD: pfil.9,v 1.22 2003/07/01 13:04:06 wiz Exp $
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.\"
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.\" Copyright (c) 1996 Matthew R. Green
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. The name of the author may not be used to endorse or promote products
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.\" derived from this software without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
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.\" BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
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.\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
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.\" AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
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.\" OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd September 29, 2004
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.Dt PFIL 9
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.Os
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.Sh NAME
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.Nm pfil ,
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.Nm pfil_head_register ,
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.Nm pfil_head_unregister ,
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.Nm pfil_head_get ,
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.Nm pfil_hook_get ,
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.Nm pfil_add_hook ,
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.Nm pfil_remove_hook ,
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.Nm pfil_run_hooks
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.Nd packet filter interface
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.Sh SYNOPSIS
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.In sys/param.h
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.In sys/mbuf.h
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.In net/if.h
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.In net/pfil.h
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.Ft int
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.Fn pfil_head_register "struct pfil_head *head"
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.Ft int
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.Fn pfil_head_unregister "struct pfil_head *head"
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.Ft "struct pfil_head *"
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.Fn pfil_head_get "int af" "u_long dlt"
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.Ft "struct packet_filter_hook *"
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.Fn pfil_hook_get "int dir" "struct pfil_head *head"
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.Ft void
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.Fn pfil_add_hook "int (*func)()" "void *arg" "int flags" "struct pfil_head *"
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.Ft void
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.Fn pfil_remove_hook "int (*func)()" "void *arg" "int flags" "struct pfil_head *"
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.Ft int
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.Fn (*func) "void *arg" "struct mbuf **mp" "struct ifnet *" "int dir" "struct inpcb *"
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.Ft int
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.Fn pfil_run_hooks "struct pfil_head *head" "struct mbuf **mp" "struct ifnet *" "int dir" "struct inpcb *"
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.Sh DESCRIPTION
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The
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.Nm
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framework allows for a specified function to be invoked for every
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incoming or outgoing packet for a particular network I/O stream.
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These hooks may be used to implement a firewall or perform packet
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transformations.
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.Pp
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Packet filtering points are registered with
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.Fn pfil_head_register .
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Filtering points are identified by a key
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.Pq Vt "void *"
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and a data link type
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.Pq Vt int
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in the
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.Vt pfil_head
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structure.
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Packet filters use the key and data link type to look up the filtering
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point with which they register themselves.
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The key is unique to the filtering point.
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The data link type is a
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.Xr bpf 4
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DLT constant indicating what kind of header is present on the packet
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at the filtering point.
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Filtering points may be unregistered with the
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.Fn pfil_head_unregister
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function.
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.Pp
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Packet filters register/unregister themselves with a filtering point
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with the
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.Fn pfil_add_hook
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and
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.Fn pfil_remove_hook
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functions, respectively.
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The head is looked up using the
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.Fn pfil_head_get
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function, which takes the key and data link type that the packet filter
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expects.
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Filters may provide an argument to be passed to the filter when
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invoked on a packet.
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.Pp
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When a filter is invoked, the packet appears just as if it
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.Dq came off the wire .
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That is, all protocol fields are in network byte order.
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The filter is called with its specified argument, the pointer to the
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pointer to the
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.Vt mbuf
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containing the packet, the pointer to the network
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interface that the packet is traversing, and the direction
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.Dv ( PFIL_IN
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or
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.Dv PFIL_OUT )
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that the packet is traveling.
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The filter may change which mbuf the
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.Vt "mbuf\ **"
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argument references.
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The filter returns an error (errno) if the packet processing is to stop, or 0
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if the processing is to continue.
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If the packet processing is to stop, it is the responsibility of the
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filter to free the packet.
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.Sh RETURN VALUES
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If successful,
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.Fn pfil_head_get
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returns the
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.Vt pfil_head
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structure for the given key/dlt.
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The
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.Fn pfil_add_hook
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and
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.Fn pfil_remove_hook
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functions
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return 0 if successful.
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If called with flag
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.Dv PFIL_WAITOK ,
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.Fn pfil_remove_hook
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is expected to always succeed.
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.Pp
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The
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.Fn pfil_head_unregister
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function
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might sleep!
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.Sh SEE ALSO
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.Xr bpf 4 ,
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.Xr if_bridge 4
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.Sh HISTORY
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The
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.Nm
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interface first appeared in
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.Nx 1.3 .
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The
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.Nm
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input and output lists were originally implemented as
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.In sys/queue.h
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.Dv LIST
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structures;
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however this was changed in
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.Nx 1.4
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to
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.Dv TAILQ
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structures.
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This change was to allow the input and output filters to be processed in
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reverse order, to allow the same path to be taken, in or out of the kernel.
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.Pp
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The
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.Nm
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interface was changed in 1.4T to accept a 3rd parameter to both
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.Fn pfil_add_hook
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and
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.Fn pfil_remove_hook ,
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introducing the capability of per-protocol filtering.
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This was done primarily in order to support filtering of IPv6.
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.Pp
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In 1.5K, the
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.Nm
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framework was changed to work with an arbitrary number of filtering points,
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as well as be less IP-centric.
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.Pp
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Fine-grained locking was added in
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.Fx 5.2 .
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.Sh BUGS
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The
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.Fn pfil_hook_get
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function
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is only safe for internal use.
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.Pp
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.Fx
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implements only hooks for
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.Dv AF_INET
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and
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.Dv AF_INET6 .
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Packets diverted through these hooks have data in
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host byte order contrary to the above statements.
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.Pp
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The
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.Xr if_bridge 4
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diverts
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.Dv AF_INET
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and
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.Dv AF_INET6
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traffic according to its sysctl settings, but contrary to the above
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statements, the data is provided in host byte order.
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.Pp
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When a
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.Vt pfil_head
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is being modified, no traffic is diverted
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(to avoid deadlock).
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This means that traffic may be dropped unconditionally for a short period
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of time.
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.Fn pfil_run_hooks
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will return
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.Er ENOBUFS
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to indicate this.
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