I remember someday from a few years ago, because i just got off the phone with a customer calling me way too early! (meaning i still was in my pyjamas)

C:"Hey NNP, why si that software not available (He refers to fail2ban on his server)
Me: "It's there" (shows him terminal output)
C: " But i cannot invoke it, there is no fail2ban command! you're lieing"
Me: "well, try that sudoers command i gave you (basically it just tails all the possible log files in /var/log ) , do you see that last part with fail2ban on it?
C: "Yeah, but there is only a file descriptor! nothing is showing! It doesnt do anything.
Me: "That's actually good, it means that fail2ban does not detect any anomalies so it does not need to log it"
C:" How can you be sure!?"
Me: "Shut up and trust me, i am ROOT"

(Fail2ban is a software service that checks log files like your webserver or SSH to detect floods or brute force attempts, you set it up by defining some "jails" that monitor the things you wish to watch out for. A sane SSH jail is to listen to incoming connection attempts and after 5 or 10 attempts you block that user's IP address on firewall level. It uses IPtables. Can be used for several other web services like webservers to detect and act upon flooding attempts. It uses the logfiles of those services to analyze them and to take the appropriate action. One those jails are defined and the service is up, you should see as little log as possible for fail2ban.)