What Is This?
Nagios® is a system and network monitoring application. It watches hosts and services that you specify, alerting you when things go bad and when they get better.
Nagios was originally designed to run under Linux, although it should work under most other unices as well.
Some of the many features of Nagios® include:
The only requirement of running Nagios is a machine running Linux (or UNIX variant) and a C compiler. You will probably also want to have TCP/IP configured, as most service checks will be performed over the network.
You are not required to use the CGIs included with Nagios. However, if you do decide to use them, you will need to have the following software installed...
Nagios® is licensed under the terms of the GNU General Public License Version 2 as published by the Free Software Foundation. This gives you legal permission to copy, distribute and/or modify Nagios under certain conditions. Read the 'LICENSE' file in the Nagios distribution or read the online version of the license for more details.
Nagios® is provided AS IS with NO WARRANTY OF ANY KIND, INCLUDING THE WARRANTY OF DESIGN, MERCHANTABILITY, AND FITNESS FOR A PARTICULAR PURPOSE.
Several people have contributed to Nagios by either reporting bugs, suggesting improvements, writing plugins, etc. A list of some of the many contributors to the development of Nagios can be found at http://www.nagios.org.
Downloading The Latest Version
You can check for new versions of Nagios at http://www.nagios.org.
Other Monitoring Utilities
In case you weren't aware, there are other network monitoring utilities available besides Nagios. I think Nagios is a pretty good contender, but I'm obviously biased. Here are links to a few other free monitoring utilities. There are many others that are not listed here - search the net (or Freshmeat) and you'll find them.