# Install Docker for Kata Containers on CentOS

> **Note:**
>
> - This guide assumes you have
>   [already installed the Kata Containers packages](../centos-installation-guide.md).
>
> - If you do not want to copy or type all these instructions by hand, you can use the
>   [`kata-manager`](https://github.com/kata-containers/tests/blob/master/cmd/kata-manager/kata-manager.sh)
>   script to install the packaged system including your chosen container
>   manager. Alternatively, you can generate a runnable shell script from
>   individual documents using the
>   [`kata-doc-to-script`](https://github.com/kata-containers/tests/blob/master/.ci/kata-doc-to-script.sh) script.

1. Install the latest version of Docker with the following commands:

   > **Note:** This step is only required if Docker is not installed on the system.

   ```bash
   $ sudo yum-config-manager --add-repo https://download.docker.com/linux/centos/docker-ce.repo
   $ sudo yum -y install docker-ce
   ```

   For more information on installing Docker please refer to the
   [Docker Guide](https://docs.docker.com/engine/installation/linux/centos).

2. Configure Docker to use Kata Containers by default with one of the following methods:

    1. systemd

        ```bash
        $ sudo mkdir -p /etc/systemd/system/docker.service.d/
        $ cat <<EOF | sudo tee /etc/systemd/system/docker.service.d/kata-containers.conf
        [Service]
        ExecStart=
        ExecStart=/usr/bin/dockerd -D --add-runtime kata-runtime=/usr/bin/kata-runtime --default-runtime=kata-runtime
        EOF
        ```

    2. Docker `daemon.json`

        Add the following definitions to `/etc/docker/daemon.json`:

        ```json
        {
          "default-runtime": "kata-runtime",
          "runtimes": {
            "kata-runtime": {
              "path": "/usr/bin/kata-runtime"
            }
          }
        }
        ```

3. Restart the Docker systemd service with the following commands:

    ```bash
    $ sudo systemctl daemon-reload
    $ sudo systemctl restart docker
    ```

4. Run Kata Containers

    You are now ready to run Kata Containers:

    ```bash
    $ sudo docker run busybox uname -a
    ```

    The previous command shows details of the kernel version running inside the
    container, which is different from the host kernel version.