mitmproxy/doc-src/transparent/linux.html

On Linux, mitmproxy integrates with the iptables redirection mechanism to
achieve transparent mode.

<ol class="tlist">

    <li> <a href="@!urlTo('ssl.html')!@">Install the mitmproxy
    certificates on the test device</a>. </li>

    <li> Enable IP forwarding:

    <pre class="terminal">sysctl -w net.ipv4.ip_forward=1</pre>

    You may also want to consider enabling this permanently in
    <b>/etc/sysctl.conf</b>.

    </li>

    <li> If your target machine is on the same physical network and you configured it to use a custom gateway,
    disable ICMP redirects:

    <pre class="terminal">echo 0 | sudo tee /proc/sys/net/ipv4/conf/*/send_redirects</pre>

    You may also want to consider enabling this permanently in
    <b>/etc/sysctl.conf</b> as demonstrated <a href="http://unix.stackexchange.com/a/58081">here</a>.

    </li>

    <li> Create an iptables ruleset that redirects the desired traffic to the
    mitmproxy port. Details will differ according to your setup, but the
    ruleset should look something like this:

<pre class="terminal">iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -j REDIRECT --to-port 8080
iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 443 -j REDIRECT --to-port 8080</pre>
    
    </li>

    <li> Fire up mitmproxy. You probably want a command like this:

        <pre class="terminal">mitmproxy -T --host</pre>

        The <b>-T</b> flag turns on transparent mode, and the <b>--host</b>
        argument tells mitmproxy to use the value of the Host header for URL
        display.

    </li>

    <li> Finally, configure your test device to use the host on which mitmproxy is
    running as the default gateway.</li>

</ol>


For a detailed walkthrough, have a look at the <a href="@!urlTo('tutorials/transparent-dhcp.html')!@"><i>Transparently proxify virtual machines</i></a> tutorial.