How to setup kernel fallback in CentOS6?

Updated: 28 September 2026 · Applies to: Ubuntu 24.04, Debian 12, AlmaLinux 9 and Rocky Linux 9

This article explains how to go back to an older kernel when a new one causes problems, and how to test kernels safely. CentOS 6 is obsolete; the steps are for Ubuntu 24.04, Debian 12, AlmaLinux 9 and Rocky Linux 9.

Boot an older kernel now

At the GRUB menu, choose the older kernel (on Ubuntu/Debian under Advanced options). To reach the menu on a VPS, open the VNC console in the VPS dashboard before restarting. Or, from a running system:

grubby --info=ALL | grep -E "^(index|kernel)"     # AlmaLinux / Rocky
grub2-reboot 1 && reboot                          # boot entry 1 once

Make the older kernel the default

grubby --set-default /boot/vmlinuz-5.14.0-570.el9.x86_64     # AlmaLinux / Rocky (use your version)
grubby --default-kernel

On Ubuntu/Debian set GRUB_DEFAULT=saved in /etc/default/grub, run update-grub, then grub-set-default with the menu entry name.

Remove the faulty kernel

dnf remove kernel-core-<version>          # AlmaLinux / Rocky
apt remove linux-image-<version>-generic   # Ubuntu

Never remove the kernel you are running (uname -r).

How it works

GRUB boots the saved default entry. You tell GRUB to try a new kernel once. If it works, you make it the default. If it hangs or panics, the next restart boots the old default again.

AlmaLinux / Rocky Linux 9

1. List the kernels and their index numbers: grubby --info=ALL | grep -E "^(index|kernel)"
2. Make sure the current, working kernel is the default: grubby --default-kernel
3. Let a kernel panic restart the server automatically after 10 seconds: grubby --update-kernel=ALL --args="panic=10"
4. Boot the new kernel once (index 0 in this example): grub2-reboot 0 && reboot
5. If the server comes back on the new kernel (uname -r), make it the default: grubby --set-default /boot/vmlinuz-<new-version>. If it did not boot, restart it (on a Ucartz VPS: Stop and Start in the VPS dashboard) and it starts the old kernel.

Ubuntu 24.04 / Debian 12

1. In /etc/default/grub set GRUB_DEFAULT=saved and add panic=10 to GRUB_CMDLINE_LINUX, then run update-grub.
2. Save the working kernel as default: grub-set-default "Advanced options for Ubuntu>Ubuntu, with Linux 6.8.0-45-generic" (use the names from grep -E "menuentry|submenu" /boot/grub/grub.cfg).
3. Try the new kernel once: grub-reboot "Advanced options for Ubuntu>Ubuntu, with Linux 6.8.0-50-generic" && reboot.