Docker is a container virtualization technology that has gained widespread popularity in recent times; it offers a more efficient way to deploy the application. With Docker, the applications reside inside the container on top of the Linux operating system.
Docker uses Kernel features such as cgroups and namespace to allow an independent container to run on single os instance.
In this post, you will learn how to install Docker on CentOS 7 / RHEL 7.
Choose any one of the methods to install Docker on CentOS 7.
Install from Docker (Official)
Docker is now available in two editions,
- Community Edition (CE)
- Enterprise Edition (EE)
Here, we will install Docker Comunity Edition (CE).
Uninstall older versions of Dockers, named docker or docker-engine along with associated dependencies.
yum -y remove docker-common docker container-selinux docker-selinux docker-engine
Do not worry about the contents inside /var/lib/docker/, all will be preserved.
yum -y install lvm2 device-mapper device-mapper-persistent-data device-mapper-event device-mapper-libs device-mapper-event-libs
The Docker Comunity package is now called docker-ce. Let’s add the CE repository for docker installation.
yum -y install wget wget https://download.docker.com/linux/centos/docker-ce.repo -O /etc/yum.repos.d/docker-ce.repo
Install the latest version of Docker CE using the following command.
yum -y install docker-ce
Eg: yum install docker-ce-19.03.0-3.el7
You can list the Docker versions available with yum list docker-ce –showduplicates | sort -r
Install Docker from Base Repository
Docker is available in the standard repository of CentOS, so we don’t have to search for the package.
For RHEL 7, you must have a valid Redhat subscription to enable Extras rpm’s repository on the server. Install it using the following command.
yum -y install docker
Now you have Docker installed onto your machine, start the Docker service in case if it is not started automatically after the installation
systemctl start docker systemctl enable docker
Verify Docker Installation
Once the service is started, verify your installation by running the following command.
docker run -it centos echo Hello-World
Let’s see what happens when we run the docker run command. Docker starts a container with centos base image since we are running this centos container for the first time, the output will look like below.
Unable to find image 'centos:latest' locally latest: Pulling from library/centos 8ba884070f61: Pull complete Digest: sha256:a799dd8a2ded4a83484bbae769d97655392b3f86533ceb7dd96bbac929809f3c Status: Downloaded newer image for centos:latest Hello-World
Docker looks for centos image locally, and it is not found, it starts downloading the centos image from Docker registry. Once the image has been downloaded, it will start the container and echo the command Hello-World in the console which you can see at the end of the output.
Allow Non-root access
As you can see in my command, for CentOS, I had to run Docker as a root user. To avoid this, you can follow below procedure to allow non-root users to run Docker containers.
Create a group called docker if it does not exist, run the following commands with root privileges.
Add a user that is to be a part of docker group. Replace raj with your own username.
Add a user to docker group.
usermod -aG docker raj
Now you can run a Docker with a non-root user.
FirwallD in CentOS 7 can conflict with Docker; it is recommended to disable the service.
systemctl stop firewalld systemctl disable firewalld
When firewalld is started or restarted it will remove the DOCKER chain from iptables, it prevents Docker from working properly.
If you still want to use systemd, firewalld is must be started before Docker service. In case if you start or restart firewalld after Docker, you will have to restart the Docker daemon.
Docker Basic Topics
Docker Advanced Topics
That’s All. Please share your feedback in the comments section.