CentOS / RHEL 7 : Change default kernel (boot with old kernel)

GRUB2 is the most common bootloader for RHEL 7 systems. A symlink to the GRUB2 config file should be present at /etc/grub2.cfg. The post describes changing the default kernel to a old kernel.

How GRUB2 selects which kernel to boot from

By default, the value for the directive GRUB_DEFAULT in the /etc/default/grub file is “saved”.

# cat /etc/default/grub
GRUB_TIMEOUT=5
GRUB_DEFAULT=saved
GRUB_DISABLE_SUBMENU=true
GRUB_TERMINAL_OUTPUT="console"
GRUB_CMDLINE_LINUX="nomodeset crashkernel=auto rd.lvm.lv=vg_os/lv_root rd.lvm.lv=vg_os/lv_swap rhgb quiet"
GRUB_DISABLE_RECOVERY="true"

This instructs GRUB 2 to load the kernel specified by the saved_entry directive in the GRUB 2 environment file, located at /boot/grub2/grubenv.

# cat /boot/grub2/grubenv 
# GRUB Environment Block
saved_entry=Red Hat Enterprise Linux Server (3.10.0-327.10.1.el7.x86_64) 7.2 (Maipo)

One can set another GRUB record to be the default, using the grub2-set-default command, which will update the GRUB 2 environment file. By default, the saved_entry value is set to the name of latest installed kernel of package type kernel. This is defined in /etc/sysconfig/kernel by the UPDATEDEFAULT and DEFAULTKERNEL directives.

# cat /etc/sysconfig/kernel
# UPDATEDEFAULT specifies if new-kernel-pkg should make
# new kernels the default
UPDATEDEFAULT=yes

# DEFAULTKERNEL specifies the default kernel package type
DEFAULTKERNEL=kernel

# MAKEDEBUG specifies if new-kernel-pkg should create non-default
# "debug" entries for new kernels.
MAKEDEBUG=yes

Change default kernel

To force a system to always use a particular menu entry, use the menu entry name as the key to the GRUB_DEFAULT directive in the /etc/default/grub file. The following command will print a list of the menu entries present in GRUB2’s configuration.

# awk -F\' /^menuentry/{print\$2} /etc/grub2.cfg
Red Hat Enterprise Linux Server (3.10.0-327.10.1.el7.x86_64) 7.2 (Maipo)                          ===> entry 0
Red Hat Enterprise Linux Server (3.10.0-229.el7.x86_64) 7.2 (Maipo)                               ===> entry 1
Red Hat Enterprise Linux Server (0-rescue-0cb6313ed65e4b36ba5daace11f3ad50) 7.2 (Maipo)           ===> entry 2

GRUB 2 supports using a numeric value as the key for the saved_entry directive to change the default order in which the kernel or operating systems are loaded. To specify which kernel should be loaded first, pass its number to the grub2-set-default command. The IDs are assigned in order the menu entries appear in the /etc/grub2.cfg file starting with 0. So the kernel 3.10.0-229.el7.x86_64 get an ID of 1.

# grub2-set-default 1

This will make 3.10.0-229.el7.x86_64 as defaul kernel which was the old kernel in the system.

Verify the new default kernel

Check the below file to see the kernel which will be loaded at next boot, crosscheck the numeric value with the menuentry in the /etc/default/grub file.

# cat /boot/grub2/grubenv |grep saved
saved_entry=1

Rebuild GRUB2

Changes to /etc/default/grub require rebuilding the grub.cfg file as follows:

# grub2-mkconfig -o /boot/grub2/grub.cfg

Reboot

Once you have verified everything and rebuilt the GRUB2 configuration file, you can go ahead an reboot the server for changes to take effect.

# shutdown -r now

CentOS / RHEL 5,6 : Change default kernel (boot with old kernel)