Change default kernel on Boot - CentOS

Recently a friend of mine had his server compromised. This was because no updates were done on his server for the past 2 years. When he did, among other things he needed to update his default kernel so that when the server was rebooted it knew which kernel to load.

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June 9, 2020
Accepted Answer

Before we begin, I would like to mention that the tutorial is intended for CentOS 6,7,8/RHEL/OEL 7 machines only and it won’t work on Ubuntu.

Imagine the following scenario. You haven’t updated your server in a looong long time and you now wish to update every package on it. This is what we’ll follow on this tutorial with the inclusion of changing the default kernel as well.

Now, firstly, we need to SSH to our server:

ssh root@XXX.XXX.XXX.XXX

where XXX.XXX.XXX.XXX is your actual server’s IP address.

Once in, we need to start updating the services. This can be easily achievable with the command yum ;

yum update

This will find all packages on your server and update everyone that needs updating, including the kernel. It might take some time but you’ll have the most recent package based on your repositories afterwards.

Having said that, this doesn’t mean your server has actually updated it’s default kernel. This in most cases needs to be done manually by yourself.

The information for the kernels is located in your grub config /boot/grub2/grub.cfg which is set as a symlink to /etc/grub2.cfg. To actually check the installed versions run the following command

awk -F\' '$1=="menuentry " {print $2}' /etc/grub2.cfg

It will show something in the mists of

Linux NameOfServer 4.18.0-117.3.1.el8_1.x86_64
Linux NameOfServer 4.18.0-147.3.1.el8_1.x86_64
Linux NameOfServer 4.18.0-147.8.1.el8_1.x86_64

Now, if we take a closer look, we’ll see all of the mentioned lines are almost similar however the last one is the latest. We’ll want to set that as our default kernel. To do, we run the command

grub2-set-default 2

Yes, it’s the line number 3 but denoted as entry 2.

Now that you’ve executed the command, you should be good to go on rebooting the server. Once it boots it should be with the latest kernel installed on your machine.

Regards, KDSys