[Osiris]

Source How to install Nagios on Ubuntu server

Are you one of those special geeks that think there is never TMI (too much information)? If that describes you then Nagios is the monitor for you. Once installed, Nagios will keep you busy with more information about your system than you ever thought possible. But it’s not just a matter of running apt-get install nagios. No, there’s more to the installation than that.
In this article you will see how to install Nagios on a working Ubuntu Server installation. The release I used was 9.04, but you should be able to use 8.04 just as easily.