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<title>Building TUN based virtual networks with socat</title>
<link rel="stylesheet" type="text/css" href="dest-unreach.css">
</head>

<body>

<h1>Building TUN based virtual networks with socat</h1>

<h2>Introduction</h2>
<p>
Some operating systems allow the generation of virtual network interfaces that
do not connect to a wire but to a process that simulates the network. Often
these devices are called TUN or TAP.
</p>
<p>
socat provides an address type that creates a TUN device on Linux; the other
socat address can be any type; it transfers the "wire" data as desired.
</p>
<p>
This document shows how a simple virtual network can be created between
two hosts that may be far (many network hops) apart. On both hosts a socat
instance is started that connects to the other host using TCP and creates a TUN
device. See <a href="socat-openssltunnel.html">socat-openssltunnel.html</a> for
a guide on securing the connection using SSL.
</p>
<p>
The following IP addresses are used in the example; replace them in the
following commands with the requirements of your situation:</p>
<table border="1">
<tr><th>host</th><th>address</th><th>mask</th></tr>
<tr><td>physical "server" address</td><td>1.2.3.4</td><td>n/a</td></tr>
<tr><td>physical "client" address</td><td>n/a</td><td>n/a</td></tr>
<tr><td>TUN on "server"</td><td>192.168.255.1</td><td>255.255.255.0</td></tr>
<tr><td>TUN on "client"</td><td>192.168.255.2</td><td>255.255.255.0</td></tr>
</table>
<p>The TCP connection uses port 11443.</p>

<p>On "default" Linux installations, creating TUN/TAP devices might require
root privilege.</p>

<!-- discussion -->
<h2>Generate TUN devices with socat</h2>
<p>In this section two instances of socat are used to generate TUN devices on
different hosts and connect the "wire" sides, providing a simple virtual
network. 
</p>
<p>
We distinguish server and client only with respect to the connection between
the two socat instances; the TUN interfaces both have the same quality.
</p>

<h3>TUN Server</h3>

<span class="frame"><span class="shell">socat -d -d TCP-LISTEN:11443,reuseaddr TUN:192.168.255.1/24,up</span></span>
<p>After starting this command, socat will wait for a connection and then
create a TUN pseudo network device with address 192.168.255.1; the bit number
specifies the mask of the network that is pretended to be connected on this
interface.</p>

<h3>TUN Client</h3>
<span class="frame"><span class="shell">socat TCP:1.2.3.4:11443 TUN:192.168.255.2/24,up</span></span>
<p>This command should establish a connection to the server and create the TUN
device on the client.</p>

<h3>Seeing it work</h3>

<p>
After successful connection both TUN interfaces should be active and transfer
date between each other using the TCP connection. Try this by pinging
192.168.255.1 from the client and 192.168.255.2 from the server.
</p>

<h3>TCP/IP version 6</h3>

<p>IPv6 as transport should work just like any TCP/IPv6 connection.</p>

<p>Creation of an IPv6 virtual interface is not directly possible, but you can
generate an IPv4 interface as described above, and add IPv6 addresses using
the <tt>ifconfig</tt> command.

<h2>Troubleshooting</h2>

<h3>Test TUN integration</h3>
<p>
If you get error messages like this:</p>
<table border="1" bgcolor="#e08080"><tr><td><tt>... E unknown device/address "tun"</tt></td></tr></table>
<p>your socat executable probably does not provide TUN/TAP support. Potential
reasons: you are not on Linux or are using an older version of socat.
</p>

<h3>Missing kernel support</h3>
<p>An error message like:</p>
<table border="1" bgcolor="#e08080"><tr><td><tt>... E open("/dev/net/tun", 02, 0666): No such file or directory</tt></td></tr></table>
<p>indicates that your kernel either needs to load the tun module or does not
  have TUN/TAP support compiled in. Try to load the module:</p>
<span class="frame"><span class="shell">modprobe tun</span></span>
<p>and check
 for /dev/net/tun. If that does not succeed you need to
 rebuild your kernel with the appropriate configuration (probably under
<b>Device driver / Network device support / Network device / Universal TUN/TAP</b>).
</p>

<h3>TUN cloning device permissions</h3>
<p>An error message like:</p>
<table border="1" bgcolor="#e08080"><tr><td><tt>... E open("/dev/net/tun", 02, 0666): Permission denied</tt></td></tr></table>
<p>indicates that you do not have permission to read or write the TUN cloning
device. Check its permission and ownership.</p>

<h3>Interface down</h3>
<p>If no error occurs but the pings do not work check if the network devices
have been created:</p>
<span class="frame"><span class="shell">ifconfig tun0</span></span>
<p>The output should look like:</p>
<pre>
tun0      Link encap:UNSPEC  HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00  
          inet addr:192.168.255.1  P-t-P:192.168.255.1  Mask:255.255.255.0
          UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:500 
          RX bytes:0 (0.0 b)  TX bytes:0 (0.0 b)
</pre>
<p>Check the "UP" keyword; you forget the "up" option in the socat command if
 it is missing.<p>
<p>Check if the correct IP address and network mask are displayed.</p>

<h3>Routing</h3>
<p></p>
<span class="frame"><span class="shell">netstat -an |fgrep 192.168.255</span></span>
<p>The output should look like:</p>
<pre>
192.168.255.0   0.0.0.0         255.255.255.0   U         0 0          0 tun0
</pre>

<h3>Other problems</h3>
<p>Another reason for failure might be iptables.</p>
<p>Run socat with options <tt>-d -d -d</tt>, this will show every data transfer
between the two processes. Each ping probe should cause a forth and a back
transfer.<p>

<h2>History</h2>
<p>
Linux TUN/TAP support was added to socat in version 1.6.0.</p>

<p>This document was last modified in February 2010.</p>

<h2>More info about socat TUN/TAP support</h2>

<h3>Links regarding this tutorial</h3>
<a href="socat.html#ADDRESS_TUN">socat address tun</a><br>

<h3>socat options for TUN/TAP addresses</h3>
<a href="socat.html#GROUP_TUN">TUN/TAP options</a><br>

<h2>References</h2>
<a href="http://www.dest-unreach.org/socat">socat home page</a><br>
<a href="socat.html">socat man page</a><br>
<a href="http://openvpn.net/">OpenVPN home page</a><br>
<a href="http://en.wikipedia.org/wiki/TUN/TAP">TUN/TAP on Wikipedia</a><br>

<p>
<small>Copyright: Gerhard Rieger 2007-2010</small><br>
<small>License: <a href="http://www.fsf.org/licensing/licenses/fdl.html">GNU Free Documentation License (FDL)</a></small>
</p>

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