Control and configure Docker with systemd

Estimated reading time: 5 minutes

Many Linux distributions use systemd to start the Docker daemon. This document shows a few examples of how to customize 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.

The recommended way is to use a systemd drop-in file (as described in the systemd.unit documentation). These are local files named <something>.conf in the /etc/systemd/system/docker.service.d directory.

However, if you had previously used a package which had an EnvironmentFile (often pointing to /etc/sysconfig/docker) then for backwards compatibility, you drop a file with a .conf extension into the /etc/systemd/system/docker.service.d directory including the following:

[Service]
EnvironmentFile=-/etc/sysconfig/docker
EnvironmentFile=-/etc/sysconfig/docker-storage
EnvironmentFile=-/etc/sysconfig/docker-network
ExecStart=
ExecStart=/usr/bin/dockerd $OPTIONS \
          $DOCKER_STORAGE_OPTIONS \
          $DOCKER_NETWORK_OPTIONS \
          $BLOCK_REGISTRY \
          $INSECURE_REGISTRY

To check if the docker.service uses an EnvironmentFile:

$ systemctl show docker | grep EnvironmentFile

EnvironmentFile=-/etc/sysconfig/docker (ignore_errors=yes)

Alternatively, find out where the service file is located:

$ systemctl show --property=FragmentPath docker

FragmentPath=/usr/lib/systemd/system/docker.service

$ 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 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

[Service]
Type=notify
# the default is not to use systemd for cgroups because the delegate issues still
# exists and systemd currently does not support the cgroup feature set required
# for containers run by docker
ExecStart=/usr/bin/dockerd
ExecReload=/bin/kill -s HUP $MAINPID
# Having non-zero Limit*s causes performance problems due to accounting overhead
# in the kernel. We recommend using cgroups to do container-local accounting.
LimitNOFILE=infinity
LimitNPROC=infinity
LimitCORE=infinity
# Uncomment TasksMax if your systemd version supports it.
# Only systemd 226 and above support this version.
#TasksMax=infinity
TimeoutStartSec=0
# set delegate yes so that systemd does not reset the cgroups of docker containers
Delegate=yes
# kill only the docker process, not all processes in the cgroup
KillMode=process

[Install]
WantedBy=multi-user.target

This will allow us to add extra flags via a drop-in file (mentioned above) by placing a file containing the following in the /etc/systemd/system/docker.service.d directory:

[Service]
ExecStart=
ExecStart=/usr/bin/dockerd --graph=/mnt/docker-data --storage-driver=overlay

You can also set other environment variables in this file, for example, the HTTP_PROXY environment variables described below.

To modify the ExecStart configuration, specify an empty configuration followed by a new configuration as follows:

[Service]
ExecStart=
ExecStart=/usr/bin/dockerd --bip=172.17.42.1/16

If you fail to specify an empty configuration, Docker reports an error such as:

docker.service has more than one ExecStart= setting, which is only allowed for Type=oneshot services. Refusing.

HTTP proxy

This example overrides the default docker.service file.

If you are behind an HTTP proxy server, for example in corporate settings, you will need to add this configuration in the Docker systemd service file.

Create a systemd drop-in directory for the docker service:
```
$ mkdir -p /etc/systemd/system/docker.service.d
```
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,127.0.0.1,docker-registry.somecorporation.com"
```
Flush changes:
```
$ sudo systemctl daemon-reload
```

Verify that the configuration has been loaded:

$ systemctl show --property=Environment docker
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 to /etc/systemd/system.

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