Note:

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 any CentOS-specific packages or replacing them with the Oracle Linux equivalent. As the README.md states in the Oracle centos2ol repository on GitHub, 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

Note: When using the free lab environment, see Oracle Linux Lab Basics for connection and other usage instructions.

  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 currently disabled, enabling it at a later date could render the system 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 or you are using a hands-on lab. (Do not attempt to remove the kernel using yum in the Oracle-provided lab environment. This system has only a single kernel installed. Removing the kernel entry of the only working kernel or the incorrect kernel may cause the system to fail to boot and require booting into single-user mode to fix the broken server.)

  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 or you are using a hands-on lab. (Do not attempt to remove the kernel using yum in the Oracle provided lab environment. This system has only a single kernel installed. Removing the kernel entry of the only working kernel or the incorrect kernel may cause the system to fail to boot and require booting into single-user mode to fix the broken server.)

  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.

Note: For the hands-on lab, there are several CentOS and Oracle repositories and a single non-CentOS repository named zfs. The zfs repository needs to be disabled in this section.

  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
    

    Note: For the hands-on lab, the /var/cache is under the / mount point, which has 29G available. This system has the 5G free recommended by the centos2ol project.

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.

Note: For the hands-on lab, although there are no cron jobs configured in this environment, the recommendation is to review the different files and their contents.

  1. List root’s cron jobs.

    sudo crontab -l
    
  2. List user’s cron jobs.

    sudo crontab -u oracle -l
    
  3. List daily, hourly, weekly, monthly cron jobs.

    sudo ls -al /etc/cron*
    

    Then check the individual files using less or 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, change the current working directory into the centos2ol directory created.

    See the usage options for the script, pass 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 the 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.

More Learning Resources

Explore other labs on docs.oracle.com/learn or access more free learning content on the Oracle Learning YouTube channel. Additionally, visit education.oracle.com/learning-explorer to become an Oracle Learning Explorer.

For product documentation, visit Oracle Help Center.

More Learning Resources

Explore other labs on docs.oracle.com/learn or access more free learning content on the Oracle Learning YouTube channel. Additionally, visit education.oracle.com/learning-explorer to become an Oracle Learning Explorer.

For product documentation, visit Oracle Help Center.