title: Cannot connect to the Docker daemon (port 2375)
body: <p>I have installed Docker on Ubuntu 16.04 server, using the manual on this page: <a href="https://docs.docker.com/cs-engine/1.13/" rel="noreferrer">https://docs.docker.com/cs-engine/1.13/</a>, so, using these steps:</p>
  
  <pre><code>curl -fsSL 'https://sks-keyservers.net/pks/lookup?op=get&amp;search=0xee6d536cf7dc86e2d7d56f59a178ac6c6238f52e' | sudo apt-key add -
  sudo add-apt-repository    "deb https://packages.docker.com/1.13/apt/repo/ \
  ubuntu-$(lsb_release -cs) \
  main"
  
  sudo apt-get update
  sudo apt-get -y install docker-engine
  </code></pre>
  
  <p>I have installed it on two servers and I need them to see each other, I needed to let Docker daemon listen on port 2375 (probably doesn't matter, but using this manual: <a href="https://github.com/yeasy/cello/blob/master/docs/deployment.md" rel="noreferrer">https://github.com/yeasy/cello/blob/master/docs/deployment.md</a>)</p>
  
  <p>So I created the conf file:</p>
  
  <pre><code>sudo mkdir -p /etc/systemd/system/docker.service.d
  sudo vim /etc/systemd/system/docker.service.d/override.conf
  </code></pre>
  
  <p>Added this to the override.conf:</p>
  
  <pre><code>[Service]
  DOCKER_OPTS="$DOCKER_OPTS -H tcp://0.0.0.0:2375 -H unix:///var/run/docker.sock --api-cors-header='*' --default-ulimit=nofile=8192:16384 --default-ulimit=nproc=8192:16384"
  EnvironmentFile=-/etc/default/docker
  ExecStart=
  ExecStart=/usr/bin/dockerd -H fd:// $DOCKER_OPTS
  </code></pre>
  
  <p>Then:</p>
  
  <pre><code>$ sudo systemctl daemon-reload
  $ sudo systemctl restart docker.service
  </code></pre>
  
  <p>Tested the connection between servers like this:</p>
  
  <pre><code>$ docker -H 10.101.35.61:2375 version
  </code></pre>
  
  <p>The response:</p>
  
  <pre><code>Client:
   Version:      1.13.1-cs4
   API version:  1.27
   Go version:   go1.7.5
   Git commit:   e46aec0
   Built:        Mon May 22 18:46:40 2017
   OS/Arch:      linux/amd64
  Cannot connect to the Docker daemon at tcp://10.101.35.61:2375. Is the docker daemon running?
  </code></pre>
  
  <p>Tried restarting the server, same problem. Tried to run with sudo. Tried adding the user to group docker:</p>
  
  <pre><code>sudo usermod -aG docker $USER
  </code></pre>
  
  <p>Didn't help. I have disabled firewall on both servers. When I check the ports open on the server with <code>sudo lsof -i</code>, I can't see anything listening to port 2375 - I am guessing Docker should be listening to it?</p>