Install Docker CentOS 7: Step-by-Step Guide

Docker is a powerful platform for building, shipping, and running applications in containers. This guide provides a comprehensive, step-by-step process for installing Docker Community Edition (CE) on CentOS 7. It covers prerequisites, different installation methods (using the repository is recommended), and post-installation setup to ensure a smooth and secure experience.

Prerequisites:

• CentOS 7: This guide assumes you have a clean or existing CentOS 7 system. The instructions may work on other RHEL-based distributions, but are optimized for CentOS 7.
• Root or Sudo Access: You’ll need root privileges or a user account with sudo access to perform the installation.
• Internet Connection: An active internet connection is required to download the necessary packages.
• 64-bit Architecture: Docker requires a 64-bit version of CentOS 7.

Installation Methods:

There are several ways to install Docker on CentOS 7. The recommended and most common method is using the official Docker repository. We’ll cover that method in detail, then briefly mention the alternative methods.

1. Installing Docker CE using the Repository (Recommended):

This is the preferred method as it ensures you get the latest stable release and makes future updates easier.

Step 1: Update System Packages

Before installing anything, it’s crucial to update your system’s package index:

bash
sudo yum update -y

The -y flag automatically answers “yes” to any prompts, streamlining the process.

Step 2: Install Required Packages

Install the necessary utilities for managing repositories:

bash
sudo yum install -y yum-utils device-mapper-persistent-data lvm2

• yum-utils: Provides the yum-config-manager utility, which is essential for adding and managing repositories.
• device-mapper-persistent-data and lvm2: These packages are dependencies for the recommended overlay2 storage driver. Docker uses a storage driver to manage the layers of container images. overlay2 is generally preferred over the older devicemapper driver on CentOS 7.

Step 3: Add the Docker Repository

Add the official Docker CE repository to your system:

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

This command adds the repository information to your system’s yum configuration.

Step 4: (Optional) Enable the edge or test repositories

If you want to install pre-release versions (e.g., for testing or development), you can enable the edge or test repositories:

“`bash

For the edge (monthly) releases:

sudo yum-config-manager –enable docker-ce-edge

For the test (nightly) releases:

sudo yum-config-manager –enable docker-ce-test
“`

Important: Only enable these repositories if you understand the risks associated with using pre-release software. For production environments, stick to the stable repository (enabled by default). You usually would not enable both edge and test. Choose one or neither.

Step 5: Install Docker CE

Now, install the Docker CE package:

bash
sudo yum install docker-ce docker-ce-cli containerd.io -y

This command installs:

• docker-ce: The Docker Engine.
• docker-ce-cli: The Docker command-line interface (CLI).
• containerd.io: A container runtime that Docker uses.

Step 6: Start the Docker Service

Start the Docker daemon:

bash
sudo systemctl start docker

Step 7: Enable Docker on Boot

Ensure Docker starts automatically on system boot:

bash
sudo systemctl enable docker

Step 8: Verify the Installation

Verify that Docker is installed and running correctly by running the hello-world image:

bash
sudo docker run hello-world

This command downloads a test image and runs it in a container. If successful, you’ll see output confirming that Docker is working, including a message like:

Hello from Docker!
This message shows that your installation appears to be working correctly.
...

2. Alternative Installation Methods (Less Recommended):

• Installing from a Package (.rpm): You can download the .rpm package directly from the Docker website and install it using yum localinstall. However, this method doesn’t automatically handle dependencies or updates, making it less convenient.
  bash
# Download the appropriate .rpm package
# Example (replace with the actual URL and filename):
# wget https://download.docker.com/linux/centos/7/x86_64/stable/Packages/docker-ce-<VERSION>.el7.x86_64.rpm
# sudo yum localinstall docker-ce-<VERSION>.el7.x86_64.rpm -y

• Using a Convenience Script: Docker provides a convenience script (get-docker.sh) that automates the installation process. While convenient, it’s generally recommended to understand the steps involved and use the repository method for better control and maintainability. Use this method with caution and only if you trust the source of the script.
  bash
# curl -fsSL https://get.docker.com -o get-docker.sh
# sudo sh get-docker.sh
  This script detects your OS and installs Docker. Inspect the script before running it to understand what it does.

Post-Installation Setup:

Step 9: (Recommended) Manage Docker as a Non-Root User

Running Docker commands with sudo all the time can be cumbersome and potentially risky. It’s best practice to add your user to the docker group, allowing you to run Docker commands without sudo.

bash
sudo usermod -aG docker $USER

• usermod -aG docker: Adds the user to the docker group.
• $USER: This is an environment variable that automatically represents your current username.

Important: After running this command, you need to log out and log back in (or open a new terminal session) for the group membership change to take effect. Alternatively, you can use the following command to apply the group changes without logging out (but logging out is still recommended):

bash
newgrp docker

After logging back in (or using newgrp), verify that you can run Docker commands without sudo:

bash
docker run hello-world

Step 10: (Optional) Configure Docker to Use a Different Storage Driver

By default, CentOS 7 often uses the devicemapper storage driver with loop-lvm mode, which is not recommended for production. The overlay2 driver is generally preferred. The installation steps above should have configured overlay2 automatically, but it’s good to verify.

Check the current storage driver:

bash
docker info | grep "Storage Driver"

If the output shows Storage Driver: overlay2, you’re all set. If it shows devicemapper, and you see a warning about loop-lvm in the docker info output, you should switch to overlay2.

To switch to overlay2:

1. Stop Docker: sudo systemctl stop docker
2. Backup (Optional but Recommended): If you have existing containers and images, back them up before making changes to the storage driver.

3. Edit /etc/docker/daemon.json: Create the file if it doesn’t exist. Add the following content:

   json
{
"storage-driver": "overlay2"
}

4. Start Docker: sudo systemctl start docker

5. Verify: docker info | grep "Storage Driver" (should now show overlay2).

Step 11: (Optional) Configure Docker Daemon Options

You can further configure the Docker daemon by editing /etc/docker/daemon.json. This file allows you to set various options, such as:

• log-driver: Configure the logging driver (e.g., json-file, syslog, journald).
• log-opts: Configure logging options (e.g., max-size, max-file).
• dns: Specify custom DNS servers for containers.
• insecure-registries: Allow connections to insecure Docker registries (use with caution).
• bip: Specify a custom IP address and netmask for the Docker bridge network

For example, to limit the size and number of log files for containers using the json-file driver:

json
{
"log-driver": "json-file",
"log-opts": {
"max-size": "10m",
"max-file": "3"
}
}

Important: After making any changes to /etc/docker/daemon.json, restart the Docker daemon:

bash
sudo systemctl restart docker

Troubleshooting:

• Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?: This error usually means the Docker daemon isn’t running. Make sure you’ve started it with sudo systemctl start docker. Also, ensure you’ve added your user to the docker group and logged out/back in if you’re trying to run Docker commands without sudo.
• Permission denied while trying to connect to the Docker daemon socket…`: Similar to the above, this often indicates a permissions issue. Double-check your group membership and ensure the Docker daemon is running.
• Errors related to storage driver: If you encounter issues with the storage driver, consult the Docker documentation for specific troubleshooting steps for your chosen driver.
• yum errors: If you have problems with yum (e.g., repository errors), ensure your internet connection is working, and check the repository URLs for any typos. You might need to clean the yum cache (sudo yum clean all).

This detailed guide covers the essential steps for installing and configuring Docker CE on CentOS 7. By following these instructions carefully, you should have a working Docker environment ready for your containerization needs. Remember to consult the official Docker documentation for more advanced configurations and troubleshooting.