what does fd:// mean exactly in dockerd -H fd:// : <p><a href="https://docs.docker.com/engine/reference/commandline/dockerd/#daemon-socket-option" rel="noreferrer">Docker daemon documentation</a> suggests the following <code>hosts</code> option for most setups:</p>

<pre><code>dockerd -H fd://
</code></pre>

<p>I guess <code>fd</code> stands for file descriptor. I don't understand how <code>fd</code> is used for socket communication. </p>

<p>I understand the following options:</p>

<pre><code>-H unix:///var/run/docker.sock -H tcp://192.168.59.106 -H tcp://10.10.10.2
</code></pre>

<p>These are unix domain sockets and tcp sockets. I know how to call docker daemon using these sockets:</p>

<pre><code>docker -H tcp://0.0.0.0:2375 ps
</code></pre>

<p>But if I started docker daemon using <code>-H fd://</code>, the following call gives error:</p>

<pre><code>$ docker -H fd:// ps
error during connect: Get http:///v1.26/containers/json: http: no Host in request URL
</code></pre>

<p>So what is the meaning of <code>fd://</code>? Is there any use for it?</p>
