Nagios is a powerful host and service monitoring program designed to run on most *NIX systems. It is written in C and designed to be fast and flexible. It has the ability to contact administrators via email, pager, or other user-defined methods.-Nagios is a powerful host and service monitoring program designed to run on most*NIX systems. It is written in C and designed to be fast and flexible. It has the ability to contact administrators via email, pager, or other user-defined methods.