Unable to activate Fail2Ban jails in Plesk: Backend ‘systemd’ failed to initialize due to No module named systemd

Symptoms

• When enabling Fail2Ban jails, the operation fails with:

  Unable to switch on the selected jails: f2bmng failed: fail2ban [7690]: ERROR NOK: ("Failed to initialize any backend for Jail 'plesk-roundcube'",)
  ERROR:main:Command '['/usr/bin/fail2ban-client', 'reload', '--if-exists', 'plesk-roundcube']' returned non-zero exit status 255
  ERROR:main:Failed to reload following jails due to errors in configuration: plesk-roundcube.

• The following error message is logged in /var/log/fail2ban.log:

  Backend 'systemd' failed to initialize due to No module named systemd

Cause

systemd is set as default backend in jail.local file:

grep -r 'backend' /etc/fail2ban/jail.local
backend = systemd

Note: file could be different, e.g. /etc/fail2ban/jail.d/local.conf

Resolution

1. Connect to the Plesk server via SSH.

2. Open the file /etc/fail2ban/jail.local in a text editor. In this example, we are using the vi editor:

   # vi /etc/fail2ban/jail.local

3. Remove or comment the following line, or change it to backend = auto:

   backend = systemd

4. Save the changes and close the file.

5. Restart the Fail2Ban service:

   # systemctl restart fail2ban.service