Generate the rescue kernel boot entry in CentOS Stream 9

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Generate the rescue kernel

Regenerating the vmlinuz and initramfs for the rescue kernel of currently installed kernel under CentOS Stream 9 is really simple. There is a package dracut-config-rescue, which delivers a bash script (/usr/lib/kernel/install.d/51-dracut-rescue.install) to help generate a rescue kernel.

STEP 1) Move the old rescue kernel in a backup directory.

Remove the current rescue kernel from the /boot.

[root@srv ~]# ls -altr /boot/|grep rescue
-rwxr-xr-x.  1 root root 10030216 Apr 12  2021 vmlinuz-0-rescue-b2a198ecbfdd451cb905f76f825af01e
-rw-------.  1 root root 77700560 Apr 12  2021 initramfs-0-rescue-b2a198ecbfdd451cb905f76f825af01e.img
[root@srv ~]# mkdir /tmp/old-rescue
[root@srv ~]# mv /boot/*-rescue-* /tmp/old-rescue/
[root@srv ~]# ls -altr /tmp/old-rescue/
total 85684
-rwxr-xr-x. 1 root root 10030216 Apr 12  2021 vmlinuz-0-rescue-b2a198ecbfdd451cb905f76f825af01e
-rw-------. 1 root root 77700560 Apr 12  2021 initramfs-0-rescue-b2a198ecbfdd451cb905f76f825af01e.img
drwxrwxrwt. 9 root root     4096 Oct  5 10:00 ..
drwxr-xr-x. 2 root root     4096 Oct  5 10:01 .
[root@srv ~]# mv /boot/loader/entries/b2a198ecbfdd451cb905f76f825af01e-0-rescue.conf /tmp/old-rescue/

STEP 2) Regenerate the rescue kernel and the Grub boot entry.

Regenerate the with the /usr/lib/kernel/install.d/51-dracut-rescue.install the rescue kernel and the Grub entry by executing the following command:

[root@srv ~]# /usr/lib/kernel/install.d/51-dracut-rescue.install add $(uname -r) /boot /boot/vmlinuz-$(uname -r)

The command does not output anything on successful generation, but there are 3 new files with rescue in the name:

[root@srv ~]# find /boot/ -name '*rescue*'

Here are the valid arguments to generate the rescue kernel:

  1. add – the command what to do the script.
  2. kernel version – the kernel version, for which the script to generate the rescue kernel.
  3. boot directory – the boot directory, where the rescue kernel will be saved.
  4. kernel image – the kernel image against the script will produce the rescue kernel.

Bonus) Additional information.

It is interesting to mention, now, the script /usr/lib/kernel/install.d/51-dracut-rescue.install seems unfinished, because it does not include “USAGE” output and “remove” command is not implemented! The usage part is even stranger, because when the script is executed with wrong or without arguments it throws error for missing “usage command” (in fact, “usage” bash function):

[root@srv ~]# /usr/lib/kernel/install.d/51-dracut-rescue.install
/usr/lib/kernel/install.d/51-dracut-rescue.install: line 129: usage: command not found

The remove command is just not implemented and it exits the script with 0, which WILL NOT remove a kernel rescue entry.

[root@srv ~]# grep remove -A 4 /usr/lib/kernel/install.d/51-dracut-rescue.install
        exit 0


This is the situation for the latest version at present:

[root@srv ~]# dnf info dracut-config-rescue
Last metadata expiration check: 1:35:30 ago on Wed 05 Oct 2022 09:06:59 AM UTC.
Installed Packages
Name         : dracut-config-rescue
Version      : 057
Release      : 13.git20220816.el9
Architecture : x86_64
Size         : 3.5 k
Source       : dracut-057-13.git20220816.el9.src.rpm
Repository   : @System
From repo    : baseos
Summary      : dracut configuration to turn on rescue image generation
URL          :
License      : GPLv2+ and LGPLv2+ and GPLv2
Description  : This package provides the configuration to turn on the rescue initramfs
             : generation with dracut.

More topics on CentOS Stream 9 here.

removing the default kernel in CentOS 8 – remove elrepo kernel

Removing the default kernel aka the loaded kernel in CentOS 8 maybe challenging because the package is protected and cannot be removed by the yum or dnf.
Here is the case: an elrepo kernel-ml loaded and the dnf prints it cannot remove the package, because it is protected:

[root@srv ~]# dnf remove kernel-ml kernel-ml-core kernel-ml-modules
 Problem: The operation would result in removing the following protected packages: kernel-ml-core
(try to add '--skip-broken' to skip uninstallable packages or '--nobest' to use not only best candidate packages)
[root@srv ~]# uname -a
Linux srv.localhost 5.10.4-1.el8.elrepo.x86_64 #1 SMP Tue Dec 29 11:04:23 EST 2020 x86_64 x86_64 x86_64 GNU/Linux
[root@srv ~]# grubby --default-kernel

The system is booted up with the kernel we are trying to remove, which is impossible.

The solution is to set a new default kernel and load it. Then dnf will be able to remove the first kernel.

For CentOS 7, just use the yum instead of dnf command.
Using grubby is really easy and straightforward:

STEP 1) List all installed and available to boot kernels

[root@srv ~]# grubby --info=ALL |grep ^kernel

STEP 2) Select the kernel to load the next time

[root@srv ~]# grubby --set-default "/boot/vmlinuz-4.18.0-259.el8.x86_64"
The default is /boot/loader/entries/45e12f0814fd4947b99cbdcb88950361-4.18.0-259.el8.x86_64.conf with index 1 and kernel /boot/vmlinuz-4.18.0-259.el8.x86_64

Keep on reading!