Every site has a lot of weekly / daily attempts to abuse your forms to submit their bad content, create users, login... Here we will try to ban some of those IPs directly in the server firewall, so they can't even reach Drupal.

At first, this method might not be applicable for you. If you have a professional hosting / PaaS, your provider could (should ?) already take care of this. At least globally, maybe not specifically for Drupal attacks. For this method, you must have be able to log on your server with a user that has admin rights. This post is especially written for Debian Buster, but it should be more or less the same for other Debian versions. For other distributions, you might have to tweak some things.

Then, as you will see in the conclusion, I tend to think that this method is not sustainable in the long term. This really has to be taken as an experiment.