Switch from CentOS 8 to Oracle Linux 8

Introduction

The following tutorial provides step-by-step procedures to automatically switch a CentOS 8 instance to Oracle Linux 8 by removing or replacing any CentOS-specific packages with the Oracle Linux equivalent. The README.md in the Oracle centos2ol repository on GitHub states that the script used in this tutorial is a work in progress and not designed to handle all possible configurations.

Please ensure you have a complete backup of the system before starting this process if the script cannot successfully convert the system.

Be sure to review the centos2ol project README.md file for the latest details.

Objectives

In this lab, you’ll:

Prerequisite

Check the CentOS Version

  1. Open a terminal and connect to your instance.

  2. Get the version of CentOS.

    sudo cat /etc/redhat-release

    sudo cat /etc/os-release

    Both of these commands show the instance is running CentOS 8.

Check for Non-Standard Kernels

  1. Get a list of installed kernels using yum.

    sudo yum list installed kernel

    The listing shows all the kernels installed using yum. We strongly recommend removing all non-standard kernels; for example, any kernel installed and not provided by the base or updates repo. Includes removing any centosplus kernels.

  2. Get a list of other installed kernels using grubby.

    sudo grubby --info=ALL | grep ^kernel

    kernel list using grubby

    The list shows all the kernels configured for use. Again, we strongly recommend removing all non-standard kernels.

Remove Non-Standard Kernels with Yum

Because of the GRUB2 BootHole vulnerability, the Oracle Linux Secure Boot shim can only boot kernels signed by Oracle, and we can only replace the default CentOS kernels. While this may not impact a system if SecureBoot is disabled, enabling it later could render it unbootable. For that reason, we strongly recommend removing all non-standard kernels; for example, any installed kernel not provided by either the base or updates repo, including the CentOSPlus kernels.

Note: Skip this section if your system does not have non-standard kernels based on the description above.

  1. Remove any non-standard kernels.

    sudo yum remove <KERNEL>

    Where <KERNEL> is is the complete package name provided by rpm -q kernel.

Remove Non-Standard Kernels with Grubby

grubby is a command-line tool for updating and displaying information about the grub boot loader’s configuration files. This tool allows an administrator to disable kernels installed outside of yum.

Note: Skip this section if your system does not have non-standard kernels based on the description in the previous section.

  1. Get the index assigned to each of the installed kernel’s boot entries.

    sudo grubby --info=ALL | grep -E "^kernel|^index"

    index list using grubby

  2. Remove any non-standard kernel’s boot entries.

    grubby --remove-kernel=<MENU_INDEX>

    Where <MENU_INDEX> is the index value returned from the previous command’s output.

Check DNF Configuration

  1. Get a list of enabled repositories.

    sudo dnf repolist

    list of enabled repositories

  2. Update all installed packages.

    sudo dnf -y upgrade

Remove or Disable Non-CentOS Repositories

Disable all non-CentOS repositories. This step helps avoid package conflicts with third-party repositories during the switch. You can re-enable the repos after the switch.

  1. Disable non-CentOS repositories.

    sudo dnf config-manager --set-disabled <REPOSITORY_NAME>

    Replace <REPOSITORY_NAME> with the repository name of any non-CentOS repository enabled on your system.

Check Free Space in /var/cache

  1. Get a disk space usage report.

    df -h

Check for Automatic Updates in Cron

There are several ways an administrator can enable automatic updates. This section checks for cron jobs that execute dnf directly or using a script. Disable these jobs, if found, to avoid them running during the switching process.

  1. List root’s cron jobs.

    sudo crontab -l

  2. List the user’s cron jobs.

    sudo crontab -u <username> -l

    Where <username> is the name of your user account on the system.

  3. List daily, hourly, weekly, and monthly cron jobs.

    sudo ls -al /etc/cron*

    Then, check the individual files using less or the editor of choice.

  4. List contents of /etc/crontab.

    sudo less /etc/crontab

Disable DNF Automatic Updates

Another way to automatically apply updates is with dnf-automatic.

  1. Check for the dnf-automatic package.

    sudo dnf list installed dnf-automatic

    dnf-automatic installed

    If the output shows the dnf-automatic package not installed on your system, skip to the next section.

  2. Check the state of the dnf-automatic systemd timer.

    sudo systemctl is-enabled dnf-automatic.timer
sudo systemctl is-active dnf-automatic.timer

  3. Disable the dnf-automatic systemd timer if it is active and running.

    sudo systemctl stop dnf-automatic.timer
sudo systemctl disable dnf-automatic.timer

Download and Run centos2ol Script

  1. Download the centos2ol.sh script from GitHub.

    The simplest way to get the script is to use curl:

    curl -O https://raw.githubusercontent.com/oracle/centos2ol/main/centos2ol.sh

    If you have Git installed, use the clone option to pull the repository from GitHub.

    git clone https://github.com/oracle/centos2ol.git

  2. Run the centos2ol.sh script.

    If you used git clone, change the current working directory to the centos2ol directory.

    See the usage options for the script by passing the option -h.

    sudo bash centos2ol.sh -h

    Now, run the script and wait for it to complete.

    sudo bash centos2ol.sh

    As part of the process, the default kernel is switched to the latest release of Oracle’s Unbreakable Enterprise Kernel (UEK) to enable extensive performance and scalability improvements to the process scheduler, memory management, file systems, and networking stack. We also replace the existing CentOS kernel with the equivalent Red Hat Compatible Kernel (RHCK), which specific hardware or application may require if it has imposed strict kernel version restrictions.

  3. Reboot the system.

    sudo reboot

Confirm System Switch to Oracle Linux

  1. Using the terminal, again connect to your instance.

  2. Check the distribution version and kernel details.

    cat /etc/os-release
cat /etc/redhat-release
uname -r

Contribute to the centos2ol GitHub Project

  1. Get support.

    Open a GitHub issue for non-security-related bug reports, questions, or requests for enhancements.

For More Information

Oracle Linux Documentation
Oracle Linux Training
Oracle Linux Training Station

More Learning Resources

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