docker on centos 7: are you trying to connect to a tls-enabled daemon without tls

After install docker on centos 7 and try to run a simple docker command such as "sudo docker images", it failes with this error message:

"Get http:///var/run/docker.sock/v1.18/images/json: dial unix /var/run/docker.sock: permission denied. Are you trying to connect to a TLS-enabled daemon without TLS?"

After some trial and error, it appears that centos actually does not launch docker service (not sure something to do with firewall in centos 7), anyway, running the "sudo service docker start" fix the problem.