If you already have Debian 11 installed on your server, you can skip this chapter.
Follow the steps from below to upgrade Debian 10 (Buster) to Debian 11 (Bullseye) if you either need to run the upgrade as part of your server’s maintenance, or if you have just rented a server with Debian 10 already installed, and the hosting company doesn’t offer servers with Debian 11 preinstalled.
First read the official upgrade documentation and take note, if what is described there is applicable to your situation.
As an additional precaution, open the default ssh port 22 in the firewall with UFW:
ufw allow 22
It’s important that you make sure you have a second way of accessing your server besides SSH, in case you loose SSH access to the server due to unexpected network issues that can happen during upgrade. Some hosting providers offer a web console that allows accessing the server without SSH, while other hosting companies, like OVH, offer a rescue mode boot up which you can use to log in to a different functional server and from there you can mount the hard drive of your disfunctional server, so that you can edit any files; then you can restart the server to apply the new changes.
6.1. Back up all the important data
Make a backup copy of all the important data stored on the server. For example, you should create compressed archives of the following directories:
/var/spool/asterisk. It’s also recommended to make a backup copy of the
/var/lib/apt/extended_states file and to save the output of the
dpkg --get-selections "*" command. Also, you will want to make backup copies of all the SQL databases of your websites and applications.
Save all the backups in a safe location, like on an external hard drive that only you can access, adding the date of the backup to the name of the folder in which you store them.
6.2. Check the sources.list file
We will install all available Buster updates before upgrading to Bullseye. On some systems, the package source is defined as “stable” in the
sources.list file instead of “buster” or “bullseye”. To avoid an accidential early upgrade to Bullseye, please check the
sources.list and ensure that it contains “buster” and not “stable” as source:
The content should be similar to this:
deb http://deb.debian.org/debian buster main deb-src http://deb.debian.org/debian buster main deb http://security.debian.org/debian-security buster/updates main deb-src http://security.debian.org/debian-security buster/updates main deb http://deb.debian.org/debian buster-updates main deb-src http://deb.debian.org/debian buster-updates main
Next, upgrade all Buster packages to prepare the system for the final upgrade to Bullseye.
Update the sources database:
Perform the first upgrade:
6.3. Check the state of installed packages to ensure that no packages are ‘on hold’ or with any error status
This test is important. You have to check the state of the installed packages to ensure that no packages are ‘on hold’ or with a status of ‘Half-Installed’ or ‘Failed-Config’ or with any error status. Your system and the
apt database must be in good standing before proceeding with the upgrade. If there are any ‘on hold’ or broken packages, you should fix these problems before the upgrade.
Check if any packages are ‘on hold’ with:
dpkg --get-selections | grep hold
Check if there are any packages with a status of ‘Half-Installed’, ‘Failed-Config’ or any error status, by running:
If both commands don’t return any packages, you can proceed with the upgrade.
/etc/apt/sources.list file again:
Replace its content with the following lines:
deb http://deb.debian.org/debian bullseye main deb-src http://deb.debian.org/debian bullseye main deb http://deb.debian.org/debian-security/ bullseye-security main deb-src http://deb.debian.org/debian-security/ bullseye-security main deb http://deb.debian.org/debian bullseye-updates main deb-src http://deb.debian.org/debian bullseye-updates main
Then run the following command to update the sources database:
It is recommended to do the upgrade in two steps, by first running “
apt-get upgrade” to install the base packages, then running “
apt full-upgrade” to do the actual distribution upgrade.
Then perform the distribution upgrade by running:
During the upgrade process you will be asked multiple times if you want to overwrite certain configuration files with the new versions, or to keep the current files. Each time, type
N and press Enter to keep the current configuration file, since you don’t want to loose the settings contained in that file.
A reboot will be required to finish the upgrade and load the new kernel:
To check which Debian version is currently installed on the system, take a look at the
The output should look like this:
PRETTY_NAME="Debian GNU/Linux 11 (bullseye)" NAME="Debian GNU/Linux" VERSION_ID="11" VERSION="11 (bullseye)" VERSION_CODENAME=bullseye ID=debian HOME_URL="https://www.debian.org/" SUPPORT_URL="https://www.debian.org/support" BUG_REPORT_URL="https://bugs.debian.org/"
Note: If, after upgrading Debian, Fail2ban gives an error when when you restart it, you will have to uninstall and then reinstall it. First copy the
/etc/fail2ban/jail.local file and the
/etc/fail2ban/filter.d directory to a safe location:
cp /etc/fail2ban/jail.local /root/Documents
cp -r /etc/fail2ban/filter.d /root/Documents
Then uninstall Fail2ban:
apt-get purge fail2ban
You also have to delete the entire Fail2ban directory before reinstalling:
rm -r /etc/fail2ban
apt-get install fail2ban
Don’t forget to configure Fail2ban after installation, using the configuration file and the filter files directory that you have saved earlier, like this:
cp /root/Documents/jail.local /etc/fail2ban
cp -r /root/Documents/filter.d/* /etc/fail2ban/filter.d
systemctl restart fail2ban
After you make sure that you can log in using SSH on your custom SSH port, you can close port
22 in the firewall with UFW:
ufw delete allow 22
6.7. Install and configure the latest version of PHP
Debian 11 comes with a new version of PHP, namely PHP 7.4. During the operating system upgrade, some PHP 7.4 packages will be installed, but to fully install PHP 7.4 and use it instead of PHP 7.3, which was the default version in Debian 10, you will have to install all the PHP 7.4 packages specified in the Install PHP chapter, and then configure PHP as described there. So, follow all the steps described in the Install PHP chapter.
After you have installed and configured PHP 7.4, you can uninstall all the
php7.3 packages with:
apt-get purge php7.3*