Downgrade the Linux kernel on an Azure Ubuntu VM

Sometimes it might be that you need to downgrade the Linux kernel on an Azure Ubuntu VM.

Especially when you use Azure Site Recovery for BCDR. As the ASR agent has a slight delay on supporting the latest Linux kernel versions.

-> https://docs.microsoft.com/en-us/azure/site-recovery/azure-to-azure-support-matrix#supported-ubuntu-kernel-versions-for-azure-virtual-machines

Ubuntu was one of the first Linux distributions offering an Azure optimized kernel. Since then the Azure optimized kernel is the default one for Ubuntu VMs on Azure installed by the official Azure Marketplace images.

First step is to identify the necessary packages that you need to install. This is done by executing the following command.

> sudo apt search linux-azure | grep 5.4.0-1025

Afterwards you know which packages needs to be installed.

> sudo apt install linux-image-5.3.0-1009-azure \
linux-tools-5.3.0-1009-azure \
linux-cloud-tools-5.3.0-1009-azure \
linux-headers-5.3.0-1009-azure \
linux-modules-5.3.0-1009-azure \
linux-modules-extra-5.3.0-1009-azure

In the next step the actual kernel gets removed.

As I did my demo with Ubuntu 18.04 on Azure, I experienced that it is required adding the unsigned kernel image package to the removal list. Otherwise apt installs it during the removal process of the actual kernel.

> sudo apt remove linux-headers-5.4.0-1025-azure \
linux-image-5.4.0-1025-azure \
linux-image-unsigned-5.4.0-1025-azure

During the process you confirm with <No> that you do not want to abort the removal process.

As the last step you initiate a reboot with 

The welcome screen should now state the target kernel version.