Nagios is a network monitoring tool. I use it to track web servers, mail servers, and whatever else I have running on the LAN and on the Internet.
One common configuration issue is getting the Service Commands menu to work correctly. By default, it is visible in the UI, but disabled on the server backend. And on Debian, not all of the steps to enable it are particularly evident from the docs. Often, one will recieve the cryptic error Could not stat() command file pointing to /var/lib/nagios3/rw/nagios.cmd. This can be fixed without too much fuss.
Nagios Service Commands