199 lines
6.2 KiB
Groff
199 lines
6.2 KiB
Groff
.\" $NetBSD: tap.4,v 1.4 2005/01/10 22:35:01 wiz Exp $
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.\"
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.\" Copyright (c) 2004, 2005 The NetBSD Foundation.
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.\" All rights reserved.
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.\"
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.\" This code is derived from software contributed to the NetBSD Foundation
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.\" by Quentin Garnier.
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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. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the NetBSD
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.\" Foundation, Inc. and its contributors.
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.\" 4. Neither the name of The NetBSD Foundation nor the names of its
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.\" contributors may be used to endorse or promote products derived
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.\" from this software without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
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.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
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.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
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.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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.\" POSSIBILITY OF SUCH DAMAGE.
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.\"
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.Dd January 10, 2005
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.Dt TAP 4
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.Os
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.Sh NAME
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.Nm tap
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.Nd virtual Ethernet device
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.Sh SYNOPSIS
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.Cd pseudo-device tap
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.Sh DESCRIPTION
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The
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.Nm
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driver allows the creation and use of virtual Ethernet devices.
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Those interfaces appear just as any real Ethernet NIC to the kernel,
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but can also be accessed by userland through a character device node in order
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to read frames being sent by the system or to inject frames.
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.Pp
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In that respect it is very similar to what
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.Xr tun 4
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provides, but the added Ethernet layer allows easy integration with machine
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emulators or virtual Ethernet networks through the use of
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.Xr bridge 4
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with tunneling.
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.Ss INTERFACE CREATION
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Interfaces may be created in two different ways:
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using the
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.Xr ifconfig 8
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.Cm create
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command with a specified device number,
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or its
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.Xr ioctl 2
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equivalent,
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.Dv SIOCIFCREATE ,
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or using the special cloning device
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.Pa /dev/tap .
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.Pp
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The former works the same as any other cloning network interface:
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the administrator can create and destroy interfaces at any time,
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notably at boot time.
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This is the easiest way of combining
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.Nm
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and
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.Xr bridge 4 .
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Later, userland will actually access the interfaces through the specific
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device nodes
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.Pa /dev/tapN .
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.Pp
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The latter is aimed at applications that need a virtual Ethernet device for
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the duration of their execution.
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A new interface is created at the opening of
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.Pa /dev/tap ,
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and is later destroyed when the last process using the file descriptor closes
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it.
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.Ss CHARACTER DEVICES
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Whether the
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.Nm
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devices are accessed through the special cloning device
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.Pa /dev/tap
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or through the specific devices
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.Pa /dev/tapN ,
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the possible actions to control the matching interface are the same.
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.Pp
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When using
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.Pa /dev/tap
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though, as the interface is created on-the-fly, its name is not known
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immediately by the application.
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Therefore the
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.Dv TAPGIFNAME
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ioctl is provided.
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It should be the first action an application using the special cloning device
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will do.
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It takes a pointer to a
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.Ft struct ifreq
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as an argument.
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.Pp
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Ethernet frames sent out by the kernel on a
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.Nm
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interface can be obtained by the controlling application with
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.Xr read 2 .
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It can also inject frames in the kernel with
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.Xr write 2 .
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There is absolutely no validation of the content of the injected frame,
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it can be any data, of any length.
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.Pp
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One call of
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.Xr write 2
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will inject a single frame in the kernel, as one call of
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.Xr read 2
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will retrieve a single frame from the queue, to the extent of the provided
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buffer.
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If the buffer is not large enough, the frame will be truncated.
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.Pp
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.Nm
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character devices support the
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.Dv FIONREAD
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ioctl which returns the size of the next available frame,
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or 0 if there is no available frame in the queue.
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.Pp
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They also support non-blocking I/O through the
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.Dv FIONBIO
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ioctl.
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In that mode,
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.Er EWOULDBLOCK
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is returned by
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.Xr read 2
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when no data is available.
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.Pp
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Asynchronous I/O is supported through the
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.Dv FIOASYNC ,
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.Dv FIOSETOWN ,
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and
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.Dv FIOGETOWN
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ioctls.
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The first will enable
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.Dv SIGIO
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generation, while the two other configure the process group that
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will receive the signal when data is ready.
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.Pp
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Synchronisation may also be achieved through the use of
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.Xr select 2 ,
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.Xr poll 2 ,
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or
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.Xr kevent 2 .
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.Ss ETHERNET ADDRESS
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When a
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.Nm
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device is created, it is assigned an Ethernet address
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of the form f0:0b:a4:xx:xx:xx.
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This address can later be changed in two ways:
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through a sysctl node, or an ioctl call.
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.Pp
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The sysctl node is net.link.tap.\*[Lt]iface\*[Gt].
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Any string of six colon-separated hexadecimal numbers will be accepted.
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Reading that node will provide a string representation of the current
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Ethernet address.
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.Pp
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The address can also be changed with the
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.Dv SIOCSIFPHYADDR
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ioctl, which is used the same way as with
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.Xr gif 4 .
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The difference is in the family of the address which is passed inside the
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.Ft struct ifreqalias
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argument, which should be set to
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.Dv AF_LINK .
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This ioctl call should be made on a socket, as it is not available on
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the ioctl handler of the character device interface.
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.Sh FILES
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.Bl -tag -compact -width /dev/tap[0-9]*
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.It Pa /dev/tap
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cloning device
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.It Pa /dev/tap[0-9]*
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individual character device nodes
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.El
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.Sh SEE ALSO
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.Xr bridge 4 ,
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.Xr gif 4 ,
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.Xr tun 4 ,
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.Xr ifconfig 8
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.Sh HISTORY
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The
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.Nm
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driver first appeared in
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.Nx 3.0 .
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