title: Docker 1.12 certificate missing using daemon.json file
body: <p>I am running docker 1.12 on Ubuntu 16.04, I need to change the bip option, so I create a custom <code>/etc/docker/daemon.json</code> configuration file that looks like this:</p>
  
  <pre><code>root@simo:/etc/docker# cat daemon.json 
  {
      "authorization-plugins": [],
      "dns": ["8.8.8.8", "8.8.4.4"],
      "dns-opts": [],
      "dns-search": [],
      "exec-opts": [],
      "exec-root": "",
      "storage-driver": "",
      "storage-opts": [],
      "labels": [],
      "live-restore": true,
      "log-driver": "",
      "log-opts": {},
      "mtu": 0,
      "pidfile": "",
      "graph": "",
      "cluster-store": "",
      "cluster-store-opts": {},
      "cluster-advertise": "",
      "max-concurrent-downloads": 3,
      "max-concurrent-uploads": 5,
      "debug": true,
      "hosts": [],
      "log-level": "",
      "tls": false,
      "tlsverify": false,
      "tlscacert": "",
      "tlscert": "",
      "tlskey": "",
      "swarm-default-advertise-addr": "",
      "api-cors-header": "",
      "selinux-enabled": false,
      "userns-remap": "",
      "group": "",
      "cgroup-parent": "",
      "default-ulimits": {},
      "ipv6": false,
      "iptables": false,
      "ip-forward": false,
      "ip-masq": false,
      "userland-proxy": false,
      "ip": "0.0.0.0",
      "bridge": "",
      "bip": "172.17.42.1/16",
      "fixed-cidr": "",
      "fixed-cidr-v6": "",
      "default-gateway": "",
      "default-gateway-v6": "",
      "icc": false,
      "raw-logs": false,
      "registry-mirrors": [],
      "insecure-registries": [],
      "disable-legacy-registry": false,
      "default-runtime": "runc",
      "oom-score-adjust": -500
  }
  </code></pre>
  
  <p>Now, if I start the daemon it simply not work saying: 
  <code>
  FATA[0000] Could not load X509 key pair (cert: "", key: ""): open : no such file or directory
  </code></p>
  
  <p>Which is definitely correct, but doesn't let me use docker at all.</p>
  
  <p>If I remove the daemon.json file, however dockerd start correctly, so I am guessing that, either is possible to start dockerd without providing the certificates or there are certificates somewhere that I should use to run dockerd.</p>
  
  <p>How may I simply run dockerd using a daemon.json config file?</p>