A docker cli client is installed inside this API. This docker client will contact the target docker server to get the information of that server. This is a clear-text TCP communication on port 2375. Which means the docker daemon should be exposed on port 2375 at the server.
Following steps can be followed to expose the the docker daemon on port 2375.
Edit/create the file /etc/docker/daemon.json
{
"metrics-addr" : "0.0.0.0:9323",
"experimental" : true,
"hosts": [ "unix:///var/run/docker.sock", "tcp://0.0.0.0:2375" ]
}
This will expose the metrics of the docker server on 9323 and you can connect to docker host locally by socket file and remotely by listening port 2375.
Please note: Anybody who can reach the docker server, will be able to access the docker daemon without any authentication as TLS is not activated here.
Now this will create a configuration conflict as we are specifying a different daemon address from the default. Docker listens on a socket by default. On Debian and Ubuntu systems using systemd, this means that a host flag -H is always used when starting dockerd. If you specify a hosts entry in the daemon.json, this causes a configuration conflict and Docker fails to start.
To work around this problem, create a new file /etc/systemd/system/docker.service.d/docker.conf with the following contents, to remove the -H argument that is used when starting the daemon by default.
[Service]
ExecStart=
ExecStart=/usr/bin/dockerd
Restart the docker and verify the listening port of docker on Ubuntu
service docker restart
systemctl daemon-reload
[ubuntu@localhost ~]# sudo netstat -ntlp | grep dockerd
tcp6 0 0 :::2375 :::* LISTEN 1382/dockerd
tcp6 0 0 :::9323 :::* LISTEN 1382/dockerd
More detail can be found here.