title: what does fd:// mean exactly in dockerd -H fd://
body: <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>