Configure and run Docker on various distributions

After successfully installing Docker, the docker daemon runs with its default configuration.

In a production environment, system administrators typically configure the docker daemon to start and stop according to an organization’s requirements. In most cases, the system administrator configures a process manager such as SysVinit, Upstart, or systemd to manage the docker daemon’s start and stop.

Running the docker daemon directly

The Docker daemon can be run directly using the dockerd command. By default it listens on the Unix socket unix:///var/run/docker.sock

$ dockerd

INFO[0000] +job init_networkdriver()
INFO[0000] +job serveapi(unix:///var/run/docker.sock)
INFO[0000] Listening for HTTP on unix (/var/run/docker.sock)
...
...

Configuring the docker daemon directly

If you’re running the Docker daemon directly by running dockerd instead of using a process manager, you can append the configuration options to the docker run command directly. Other options can be passed to the Docker daemon to configure it.

Some of the daemon’s options are:

Flag	Description
-D, --debug=false	Enable or disable debug mode. By default, this is false.
-H,--host=[]	Daemon socket(s) to connect to.
--tls=false	Enable or disable TLS. By default, this is false.

Here is an example of running the Docker daemon with configuration options:

$ dockerd -D --tls=true --tlscert=/var/docker/server.pem --tlskey=/var/docker/serverkey.pem -H tcp://192.168.59.3:2376

These options :

• Enable -D (debug) mode
• Set tls to true with the server certificate and key specified using --tlscert and --tlskey respectively
• Listen for connections on tcp://192.168.59.3:2376

The command line reference has the complete list of daemon flags with explanations.

Daemon debugging

As noted above, setting the log level of the daemon to “debug” or enabling debug mode with -D allows the administrator or operator to gain much more knowledge about the runtime activity of the daemon. If faced with a non-responsive daemon, the administrator can force a full stack trace of all threads to be added to the daemon log by sending the SIGUSR1 signal to the Docker daemon. A common way to send this signal is using the kill command on Linux systems. For example, kill -USR1 <daemon-pid> sends the SIGUSR1 signal to the daemon process, causing the stack dump to be added to the daemon log.

Note: The log level setting of the daemon must be at least “info” level and above for the stack trace to be saved to the logfile. By default the daemon’s log level is set to “info”.

The daemon will continue operating after handling the SIGUSR1 signal and dumping the stack traces to the log. The stack traces can be used to determine the state of all goroutines and threads within the daemon.

Ubuntu

Ubuntu 16.04 and newer use systemd as the init system. Ubuntu 14.04 uses upstart. Use the appropriate command below to check whether Docker is running:

Init system	Command
systemd	sudo systemctl is-active
upstart	sudo status docker

You can also use Docker itself to check whether Docker is running:

$ docker info

Running Docker

You can start/stop/restart the docker daemon using

$ sudo start docker

$ sudo stop docker

$ sudo restart docker

Configuring Docker

The instructions below depict configuring Docker on a system that uses upstart as the process manager. As of Ubuntu 15.04, Ubuntu uses systemd as its process manager. For Ubuntu 15.04 and higher, refer to control and configure Docker with systemd.

You configure the docker daemon in the /etc/default/docker file on your system. You do this by specifying values in a DOCKER_OPTS variable.

To configure Docker options:

1. Log into your host as a user with sudo or root privileges.

2. If you don’t have one, create the /etc/default/docker file on your host. Depending on how you installed Docker, you may already have this file.

3. Open the file with your favorite editor.

   $ sudo vi /etc/default/docker
   

4. Add a DOCKER_OPTS variable with the following options. These options are appended to the docker daemon’s run command.

   DOCKER_OPTS="-D --tls=true --tlscert=/var/docker/server.pem --tlskey=/var/docker/serverkey.pem -H tcp://192.168.59.3:2376"
   

   These options :

   • Enable -D (debug) mode
   • Set tls to true with the server certificate and key specified using --tlscert and --tlskey respectively
   • Listen for connections on tcp://192.168.59.3:2376

   The command line reference has the complete list of daemon flags with explanations.

5. Save and close the file.

6. Restart the docker daemon.

   $ sudo restart docker
   

7. Verify that the docker daemon is running as specified with the ps command.

    $ ps aux | grep docker | grep -v grep
   

Logs

By default logs for Upstart jobs are located in /var/log/upstart and the logs for docker daemon can be located at /var/log/upstart/docker.log

$ tail -f /var/log/upstart/docker.log

INFO[0000] Loading containers: done.
INFO[0000] Docker daemon commit=1b09a95-unsupported graphdriver=aufs version=1.11.0-dev
INFO[0000] +job acceptconnections()
INFO[0000] -job acceptconnections() = OK (0)
INFO[0000] Daemon has completed initialization

CentOS / Red Hat Enterprise Linux / Fedora

As of 7.x, CentOS and RHEL use systemd as the process manager. As of 21, Fedora uses systemd as its process manager.

After successfully installing Docker for CentOS/Red Hat Enterprise Linux/Fedora, you can check the running status in this way:

$ sudo systemctl status docker

Running Docker

You can start/stop/restart the docker daemon using

$ sudo systemctl start docker

$ sudo systemctl stop docker

$ sudo systemctl restart docker

If you want Docker to start at boot, you should also:

$ sudo systemctl enable docker

Configuring Docker

For CentOS 7.x and RHEL 7.x you can control and configure Docker with systemd.

Previously, for CentOS 6.x and RHEL 6.x you would configure the docker daemon in the /etc/sysconfig/docker file on your system. You would do this by specifying values in a other_args variable. For a short time in CentOS 7.x and RHEL 7.x you would specify values in a OPTIONS variable. This is no longer recommended in favor of using systemd directly.

For this section, we will use CentOS 7.x as an example to configure the docker daemon.

To configure Docker options:

1. Log into your host as a user with sudo or root privileges.

2. Create the /etc/systemd/system/docker.service.d directory.

   $ sudo mkdir /etc/systemd/system/docker.service.d
   

3. Create a /etc/systemd/system/docker.service.d/docker.conf file.

4. Open the file with your favorite editor.

   $ sudo vi /etc/systemd/system/docker.service.d/docker.conf
   

5. Override the ExecStart configuration from your docker.service file to customize the docker daemon. To modify the ExecStart configuration you have to specify an empty configuration followed by a new one as follows:

   [Service]
   ExecStart=
   ExecStart=/usr/bin/dockerd -H fd:// -D --tls=true --tlscert=/var/docker/server.pem --tlskey=/var/docker/serverkey.pem -H tcp://192.168.59.3:2376
   

   These options :

   • Enable -D (debug) mode
   • Set tls to true with the server certificate and key specified using --tlscert and --tlskey respectively
   • Listen for connections on tcp://192.168.59.3:2376

   The command line reference has the complete list of daemon flags with explanations.

6. Save and close the file.

7. Flush changes.

   $ sudo systemctl daemon-reload
   

8. Restart the docker daemon.

   $ sudo systemctl restart docker
   

9. Verify that the docker daemon is running as specified with the ps command.

   $ ps aux | grep docker | grep -v grep
   

Logs

systemd has its own logging system called the journal. The logs for the docker daemon can be viewed using journalctl -u docker

$ sudo journalctl -u docker
May 06 00:22:05 localhost.localdomain systemd[1]: Starting Docker Application Container Engine...
May 06 00:22:05 localhost.localdomain docker[2495]: time="2015-05-06T00:22:05Z" level="info" msg="+job serveapi(unix:///var/run/docker.sock)"
May 06 00:22:05 localhost.localdomain docker[2495]: time="2015-05-06T00:22:05Z" level="info" msg="Listening for HTTP on unix (/var/run/docker.sock)"
May 06 00:22:06 localhost.localdomain docker[2495]: time="2015-05-06T00:22:06Z" level="info" msg="+job init_networkdriver()"
May 06 00:22:06 localhost.localdomain docker[2495]: time="2015-05-06T00:22:06Z" level="info" msg="-job init_networkdriver() = OK (0)"
May 06 00:22:06 localhost.localdomain docker[2495]: time="2015-05-06T00:22:06Z" level="info" msg="Loading containers: start."
May 06 00:22:06 localhost.localdomain docker[2495]: time="2015-05-06T00:22:06Z" level="info" msg="Loading containers: done."
May 06 00:22:06 localhost.localdomain docker[2495]: time="2015-05-06T00:22:06Z" level="info" msg="Docker daemon commit=1b09a95-unsupported graphdriver=aufs version=1.11.0-dev"
May 06 00:22:06 localhost.localdomain docker[2495]: time="2015-05-06T00:22:06Z" level="info" msg="+job acceptconnections()"
May 06 00:22:06 localhost.localdomain docker[2495]: time="2015-05-06T00:22:06Z" level="info" msg="-job acceptconnections() = OK (0)"

Note: Using and configuring journal is an advanced topic and is beyond the scope of this article.