I run Debian 10 Buster on all of my Proxmox VMs. This week, Debian 11 Bullseye was released, so it’s time to upgrade. You can check with cat /etc/os-release what OS version you’re running at the moment. For further details on the upgrade process, check out the official Debian upgrade guide.

I strongly recommend executing all of these steps in a tmux session. The SSH connection might get interrupted during the upgrade and with tmux you can easily reconnect to the terminal. Furthermore, I also strongly recommend making a backup of your system before attempting the upgrade.

First, upgrade all packages to the lastest version available in Buster:

sudo apt update
sudo apt full-upgrade

Now the repository has to be changed from Buster to Bullseye. In the repository configuration file /etc/apt/sources.list, replace buster with bullseye. The security suite is now named bullseye-security instead of buster/updates and has to be renamed as well. I also noticed that all the sources were defined with HTTP connections, so let’s convert those to HTTPS while we’re at it. It’s a good idea to either keep a backup of the old /etc/apt/sources.list file or comment out the existing lines instead of modifying them. That way you can revert in case anything goes wrong.

In summary,

deb http://deb.debian.org/debian/ buster main
deb http://deb.debian.org/debian/ buster-updates main
deb http://security.debian.org/debian-security buster/updates main


deb https://deb.debian.org/debian/ bullseye main
deb https://deb.debian.org/debian/ bullseye-updates main
deb https://deb.debian.org/debian-security bullseye-security main contrib

Check with ls -l /etc/apt/sources.list.d/ whether there are any custom source configuration files that need to change from buster to bullseye. If so, change them as well.

Now it’s time to perform the upgrade and remove unused dependencies:

sudo apt clean
sudo apt update
sudo apt full-upgrade
sudo apt autoremove

Finally, reboot the machine with sudo shutdown -r now. You’ll be running Debian 11 Bullseye.