# Control and configure Docker with systemd Many Linux distributions use systemd to start the Docker daemon. This document shows a few examples of how to customise Docker's settings. ## Starting the Docker daemon Once Docker is installed, you will need to start the Docker daemon. $ sudo systemctl start docker # or on older distributions, you may need to use $ sudo service docker start If you want Docker to start at boot, you should also: $ sudo systemctl enable docker # or on older distributions, you may need to use $ sudo chkconfig docker on ## Custom Docker daemon options There are a number of ways to configure the daemon flags and environment variables for your Docker daemon. If the `docker.service` file is set to use an `EnvironmentFile` (often pointing to `/etc/sysconfig/docker`) then you can modify the referenced file. Check if the `docker.service` uses an `EnvironmentFile`: $ sudo systemctl show docker | grep EnvironmentFile EnvironmentFile=-/etc/sysconfig/docker (ignore_errors=yes) Alternatively, find out where the service file is located, and look for the property: $ sudo systemctl status docker | grep Loaded Loaded: loaded (/usr/lib/systemd/system/docker.service; enabled) $ sudo grep EnvironmentFile /usr/lib/systemd/system/docker.service EnvironmentFile=-/etc/sysconfig/docker You can customize the Docker daemon options using override files as explained in the [HTTP Proxy example](#http-proxy) below. The files located in `/usr/lib/systemd/system` or `/lib/systemd/system` contain the default options and should not be edited. ### Runtime directory and storage driver You may want to control the disk space used for Docker images, containers and volumes by moving it to a separate partition. In this example, we'll assume that your `docker.service` file looks something like: [Unit] Description=Docker Application Container Engine Documentation=https://docs.docker.com After=network.target docker.socket Requires=docker.socket [Service] Type=notify EnvironmentFile=-/etc/sysconfig/docker ExecStart=/usr/bin/docker daemon -H fd:// $OPTIONS LimitNOFILE=1048576 LimitNPROC=1048576 [Install] Also=docker.socket This will allow us to add extra flags to the `/etc/sysconfig/docker` file by setting `OPTIONS`: OPTIONS="--graph /mnt/docker-data --storage-driver btrfs" You can also set other environment variables in this file, for example, the `HTTP_PROXY` environment variables described below. ### HTTP proxy This example overrides the default `docker.service` file. If you are behind a HTTP proxy server, for example in corporate settings, you will need to add this configuration in the Docker systemd service file. First, create a systemd drop-in directory for the docker service: mkdir /etc/systemd/system/docker.service.d Now create a file called `/etc/systemd/system/docker.service.d/http-proxy.conf` that adds the `HTTP_PROXY` environment variable: [Service] Environment="HTTP_PROXY=http://proxy.example.com:80/" If you have internal Docker registries that you need to contact without proxying you can specify them via the `NO_PROXY` environment variable: Environment="HTTP_PROXY=http://proxy.example.com:80/" "NO_PROXY=localhost,127.0.0.0/8,docker-registry.somecorporation.com" Flush changes: $ sudo systemctl daemon-reload Verify that the configuration has been loaded: $ sudo systemctl show docker --property Environment Environment=HTTP_PROXY=http://proxy.example.com:80/ Restart Docker: $ sudo systemctl restart docker ## Manually creating the systemd unit files When installing the binary without a package, you may want to integrate Docker with systemd. For this, simply install the two unit files (service and socket) from [the github repository](https://github.com/docker/docker/tree/master/contrib/init/systemd) to `/etc/systemd/system`.