Upgrading Debian 10 ("Buster") to Debian 11 ("Bullseye")

Category: Debian
Created: 2021-08-20

Upgrading a Debian installation to the next major version number works quite smoothly. Depending on the speed of the server and the duration of the reboot, an upgrade is usually completed in under 15 minutes.

Always upgrade Debian Linux from one version to the next higher, do not skip versions!

To upgrade a server from Debian 9 to Debian 11, you must first upgrade from 9 to 10 and then from 10 to 11. Reboot the server after each upgrade!

Follow the steps below to upgrade:

update /etc/apt/sources.list (replace buster with bullseye)

The name of the archive for security updates has also changed - from <codename>/updates to bullseye-security. Therefore change the corresponding line so that it looks something like this (adapt the mirror server if necessary):

deb https://deb.debian.org/debian-security bullseye-security main contrib
if necessary, update the file /etc/apt/sources.list.d/liveconfig.list for the LiveConfig PHP repository in the same way
run apt update
run apt upgrade
run apt full-upgrade. While running, you will be asked whether some existing configuration files should be replaced - in most cases it is better to keep the existing configuration (answer with no).
run apt-get autoremove to remove all packages that are no longer required
reboot the server (that’s important to switch the kernel and all kernel modules)
then log in to LiveConfig as admin and go to Server Management → E-Mail. Save the configuration for Postfix and for Dovecot again (this will update the configuration files and adapt them for Debian Buster).

You can also find detailed information on upgrading from Debian 10 in the official documentation.

Possible problems

fail2ban possibly doesn’t send e-mails when the mail tool from the package bsd-mailx is used (see Release Notes)

Currently we’re not aware of any more problems. If you have any issues, please let us know so we can update this article accordingly.

New in Debian 11

Some things have changed with Debian Buster (see Release Notes). Some important changes are:

XFS: the mount options barrier/nobarrier has been removed. Check your /etc/fstab - partitions using these options will fail to mount.
security archive layout has changed (see instructions above - edit /etc/apt/sources.list and change buster/updates to bullseye-security)