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<table class="head">
<tr>
<td class="head-ltitle">TAP(4)</td>
<td class="head-vol">Device Drivers Manual</td>
<td class="head-rtitle">TAP(4)</td>
</tr>
</table>
<div class="manual-text">
<section class="Sh">
<h1 class="Sh" id="NAME"><a class="permalink" href="#NAME">NAME</a></h1>
<p class="Pp"><code class="Nm">tap</code>, <code class="Nm">vmnet</code>
— <span class="Nd">Ethernet tunnel software network
interface</span></p>
</section>
<section class="Sh">
<h1 class="Sh" id="SYNOPSIS"><a class="permalink" href="#SYNOPSIS">SYNOPSIS</a></h1>
<p class="Pp"><code class="Cd">device tuntap</code></p>
</section>
<section class="Sh">
<h1 class="Sh" id="DESCRIPTION"><a class="permalink" href="#DESCRIPTION">DESCRIPTION</a></h1>
<p class="Pp">The <code class="Nm">tap</code> interface is a software loopback
mechanism that can be loosely described as the network interface analog of
the <a class="Xr">pty(4)</a>, that is, <code class="Nm">tap</code> does for
network interfaces what the <a class="Xr">pty(4)</a> driver does for
terminals.</p>
<p class="Pp">The <code class="Nm">tap</code> driver, like the
<a class="Xr">pty(4)</a> driver, provides two interfaces: an interface like
the usual facility it is simulating (an Ethernet network interface in the
case of <code class="Nm">tap</code>, or a terminal for
<a class="Xr">pty(4)</a>), and a character-special device
“control” interface. A client program transfers Ethernet
frames to or from the <code class="Nm">tap</code> “control”
interface. The <a class="Xr">tun(4)</a> interface provides similar
functionality at the network layer: a client will transfer IP (by default)
packets to or from a <a class="Xr">tun(4)</a> “control”
interface.</p>
<p class="Pp">The network interfaces are named
“<code class="Li">tap0</code>”,
“<code class="Li">tap1</code>”, etc., one for each control
device that has been opened. These Ethernet network interfaces persist until
<span class="Pa">if_tuntap.ko</span> module is unloaded, or until removed
with "ifconfig destroy" (see below).</p>
<p class="Pp"><code class="Nm">tap</code> devices are created using interface
cloning. This is done using the “ifconfig tap<b class="Sy">N</b>
<span class="No">create</span>” command. This is the preferred method
of creating <code class="Nm">tap</code> devices. The same method allows
removal of interfaces. For this, use the “ifconfig
tap<b class="Sy">N</b> <span class="No">destroy</span>” command.</p>
<p class="Pp">If the <a class="Xr">sysctl(8)</a> variable
<var class="Va">net.link.tap.devfs_cloning</var> is non-zero, the
<code class="Nm">tap</code> interface permits opens on the special control
device <span class="Pa">/dev/tap</span>. When this device is opened,
<code class="Nm">tap</code> will return a handle for the lowest unused
<code class="Nm">tap</code> device (use <a class="Xr">devname(3)</a> to
determine which).</p>
<p class="Pp"></p>
<div class="Bf Em">Disabling the legacy devfs cloning functionality may break
existing applications which use <code class="Nm">tap</code>, such as VMware
and <a class="Xr">ssh(1)</a>. It therefore defaults to being enabled until
further notice.</div>
<p class="Pp">Control devices (once successfully opened) persist until
<span class="Pa">if_tuntap.ko</span> is unloaded or the interface is
destroyed.</p>
<p class="Pp">Each interface supports the usual Ethernet network interface
<a class="Xr">ioctl(2)</a>s and thus can be used with
<a class="Xr">ifconfig(8)</a> like any other Ethernet interface. When the
system chooses to transmit an Ethernet frame on the network interface, the
frame can be read from the control device (it appears as
“input” there); writing an Ethernet frame to the control
device generates an input frame on the network interface, as if the
(non-existent) hardware had just received it.</p>
<p class="Pp" id="read">The Ethernet tunnel device, normally
<span class="Pa">/dev/tap</span><b class="Sy">N</b>, is exclusive-open (it
cannot be opened if it is already open) and is restricted to the super-user,
unless the <a class="Xr">sysctl(8)</a> variable
<var class="Va">net.link.tap.user_open</var> is non-zero. If the
<a class="Xr">sysctl(8)</a> variable
<var class="Va">net.link.tap.up_on_open</var> is non-zero, the tunnel device
will be marked “up” when the control device is opened. A
<a class="permalink" href="#read"><code class="Fn">read</code></a>() call
will return an error (<code class="Er">EHOSTDOWN</code>) if the interface is
not “ready”. Once the interface is ready,
<code class="Fn">read</code>() will return an Ethernet frame if one is
available; if not, it will either block until one is or return
<code class="Er">EWOULDBLOCK</code>, depending on whether non-blocking I/O
has been enabled. If the frame is longer than is allowed for in the buffer
passed to <code class="Fn">read</code>(), the extra data will be silently
dropped.</p>
<p class="Pp" id="write">A <a class="Xr">write(2)</a> call passes an Ethernet
frame in to be “received” on the pseudo-interface. Each
<a class="permalink" href="#write"><code class="Fn">write</code></a>() call
supplies exactly one frame; the frame length is taken from the amount of
data provided to <code class="Fn">write</code>(). Writes will not block; if
the frame cannot be accepted for a transient reason (e.g., no buffer space
available), it is silently dropped; if the reason is not transient (e.g.,
frame too large), an error is returned. The following
<a class="Xr">ioctl(2)</a> calls are supported (defined in
<code class="In"><<a class="In">net/if_tap.h</a>></code>):</p>
<dl class="Bl-tag">
<dt id="TAPSIFINFO"><a class="permalink" href="#TAPSIFINFO"><code class="Dv">TAPSIFINFO</code></a></dt>
<dd>Set network interface information (line speed and MTU). The type must be
the same as returned by <code class="Dv">TAPGIFINFO</code> or set to
<code class="Dv">IFT_ETHER</code> else the <a class="Xr">ioctl(2)</a> call
will fail. The argument should be a pointer to a <var class="Va">struct
tapinfo</var>.</dd>
<dt id="TAPGIFINFO"><a class="permalink" href="#TAPGIFINFO"><code class="Dv">TAPGIFINFO</code></a></dt>
<dd>Retrieve network interface information (line speed, MTU and type). The
argument should be a pointer to a <var class="Va">struct
tapinfo</var>.</dd>
<dt id="TAPSDEBUG"><a class="permalink" href="#TAPSDEBUG"><code class="Dv">TAPSDEBUG</code></a></dt>
<dd>The argument should be a pointer to an <var class="Va">int</var>; this
sets the internal debugging variable to that value. What, if anything,
this variable controls is not documented here; see the source code.</dd>
<dt id="TAPGDEBUG"><a class="permalink" href="#TAPGDEBUG"><code class="Dv">TAPGDEBUG</code></a></dt>
<dd>The argument should be a pointer to an <var class="Va">int</var>; this
stores the internal debugging variable's value into it.</dd>
<dt id="TAPGIFNAME"><a class="permalink" href="#TAPGIFNAME"><code class="Dv">TAPGIFNAME</code></a></dt>
<dd>Retrieve network interface name. The argument should be a pointer to a
<var class="Va">struct ifreq</var>. The interface name will be returned in
the <var class="Va">ifr_name</var> field.</dd>
<dt id="TAPSTRANSIENT"><a class="permalink" href="#TAPSTRANSIENT"><code class="Dv">TAPSTRANSIENT</code></a></dt>
<dd>The argument should be a pointer to an <var class="Va">int</var>; this
sets the transient flag on the <code class="Nm">tap</code> device. A
transient <code class="Nm">tap</code> will be destroyed upon last
close.</dd>
<dt id="TAPGTRANSIENT"><a class="permalink" href="#TAPGTRANSIENT"><code class="Dv">TAPGTRANSIENT</code></a></dt>
<dd>The argument should be a pointer to an <var class="Va">int</var>; this
stores the current state (enabled or disabled) of the transient flag into
it.</dd>
<dt id="FIONBIO"><a class="permalink" href="#FIONBIO"><code class="Dv">FIONBIO</code></a></dt>
<dd>Turn non-blocking I/O for reads off or on, according as the argument
<var class="Va">int</var>'s value is or is not zero (Writes are always
nonblocking).</dd>
<dt id="FIOASYNC"><a class="permalink" href="#FIOASYNC"><code class="Dv">FIOASYNC</code></a></dt>
<dd>Turn asynchronous I/O for reads (i.e., generation of
<code class="Dv">SIGIO</code> when data is available to be read) off or
on, according as the argument <var class="Va">int</var>'s value is or is
not zero.</dd>
<dt id="FIONREAD"><a class="permalink" href="#FIONREAD"><code class="Dv">FIONREAD</code></a></dt>
<dd>If any frames are queued to be read, store the size of the first one into
the argument <var class="Va">int</var>; otherwise, store zero.</dd>
<dt id="TIOCSPGRP"><a class="permalink" href="#TIOCSPGRP"><code class="Dv">TIOCSPGRP</code></a></dt>
<dd>Set the process group to receive <code class="Dv">SIGIO</code> signals,
when asynchronous I/O is enabled, to the argument
<var class="Va">int</var> value.</dd>
<dt id="TIOCGPGRP"><a class="permalink" href="#TIOCGPGRP"><code class="Dv">TIOCGPGRP</code></a></dt>
<dd>Retrieve the process group value for <code class="Dv">SIGIO</code> signals
into the argument <var class="Va">int</var> value.</dd>
<dt id="SIOCGIFADDR"><a class="permalink" href="#SIOCGIFADDR"><code class="Dv">SIOCGIFADDR</code></a></dt>
<dd>Retrieve the Media Access Control (<code class="Dv">MAC</code>) address of
the “remote” side. This command is used by the VMware port
and expected to be executed on descriptor, associated with control device
(usually <span class="Pa">/dev/vmnet</span><b class="Sy">N</b> or
<span class="Pa">/dev/tap</span><b class="Sy">N</b>). The
<var class="Va">buffer</var>, which is passed as the argument, is expected
to have enough space to store the <code class="Dv">MAC</code> address. At
the open time both “local” and “remote”
<code class="Dv">MAC</code> addresses are the same, so this command could
be used to retrieve the “local” <code class="Dv">MAC</code>
address.</dd>
<dt id="SIOCSIFADDR"><a class="permalink" href="#SIOCSIFADDR"><code class="Dv">SIOCSIFADDR</code></a></dt>
<dd>Set the Media Access Control (<code class="Dv">MAC</code>) address of the
“remote” side. This command is used by VMware port and
expected to be executed on a descriptor, associated with control device
(usually <span class="Pa">/dev/vmnet</span><b class="Sy">N</b>).</dd>
</dl>
<p class="Pp">The control device also supports <a class="Xr">select(2)</a> for
read; selecting for write is pointless, and always succeeds, since writes
are always non-blocking.</p>
<p class="Pp">On the last close of the data device, the interface is brought
down (as if with “ifconfig tap<b class="Sy">N</b>
<span class="No">down</span>”) and has all of its configured
addresses deleted unless the device is a <i class="Em">VMnet</i> device, or
has <code class="Dv">IFF_LINK0</code> flag set. All queued frames are thrown
away. If the interface is up when the data device is not open, output frames
are thrown away rather than letting them pile up.</p>
<p class="Pp">The <code class="Nm">tap</code> device can also be used with the
VMware port as a replacement for the old <i class="Em">VMnet</i> device
driver. <i class="Em">VMnet</i> devices do not <a class="Xr">ifconfig(8)</a>
themselves down when the control device is closed. Everything else is the
same.</p>
<p class="Pp">In addition to the above mentioned <a class="Xr">ioctl(2)</a>
calls, there is an additional one for the VMware port.</p>
<dl class="Bl-tag">
<dt id="VMIO_SIOCSIFFLAGS"><a class="permalink" href="#VMIO_SIOCSIFFLAGS"><code class="Dv">VMIO_SIOCSIFFLAGS</code></a></dt>
<dd>VMware <code class="Dv">SIOCSIFFLAGS</code>.</dd>
</dl>
</section>
<section class="Sh">
<h1 class="Sh" id="SEE_ALSO"><a class="permalink" href="#SEE_ALSO">SEE
ALSO</a></h1>
<p class="Pp"><a class="Xr">inet(4)</a>, <a class="Xr">intro(4)</a>,
<a class="Xr">tun(4)</a></p>
</section>
</div>
<table class="foot">
<tr>
<td class="foot-date">January 13, 2020</td>
<td class="foot-os">FreeBSD 15.0</td>
</tr>
</table>
|
