Debian 11 has been released so it's time to upgrade. I only have postfix and a web server running on my server so it was pretty easy to make the switch from buster to bullseye. First edit /etc/apt/sources.list and replace the word buster with bullseye. That includes changing buster-updates to bullseye-updates. The only lines that are different are the URLs at the bottom. They need to have bullseye-security instead of buster-updates. After making the changes my sources.list looked like:

deb bullseye main
deb-src bullseye main
deb bullseye-updates main
deb-src bullseye-updates main
deb bullseye-security main
deb-src bullseye-security main

Then to do the upgrade just run

# apt update
# apt full-upgrade

and once it finishes successfully, reboot and you will be on Debian 11. You can then run

# apt autoremove
# apt update
# apt upgrade

again to remove the old kernel.

Almost everything was working fine after the upgrade from buster to bullseye, with the exception of PHP. When I visited a PHP page on my web server I was shown the source instead. This is because PHP was upgraded from version 7.3 to 7.4 and the configuration files weren't getting loaded. To fix it, make a symlink from conf-enabled to conf-available and restart apache:

# cd /etc/apache2/conf-enabled/
# ln -sf ../conf-available/php7.4-fpm.conf .
# /etc/init.d/apache2 restart

After that my PHP was working again. If you are not using php-fpm then it will probably be different.

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