Nagios is a host and service monitor designed to inform you of network problems before your clients, end-users or managers do. It has been designed to run under the Linux operating system, but works fine under most *NIX variants as well. The monitoring daemon runs intermittent checks on hosts and services you specify using external "plugins" which return status information to Nagios. When problems are encountered, the daemon can send notifications out to administrative contacts in a variety of different ways (email, instant message, SMS, etc.). Current status information, historical logs, and reports can all be accessed via a web browser.

Features
Nagios has a lot of features, making it a very powerful monitoring tool. Some of the major features are listed below:

• Monitoring of network services (SMTP, POP3, HTTP, NNTP, PING, etc.)
• Monitoring of host resources (processor load, disk and memory usage, running processes, log files, etc.)
• Monitoring of environmental factors such as temperature
• Simple plugin design that allows users to easily develop their own host and service checks
• Ability to define network host hierarchy, allowing detection of and distinction between hosts that are down and those that are unreachable
• Contact notifications when service or host problems occur and get resolved (via email, pager, or other user-defined method)
• Optional escalation of host and service notifications to different contact groups
• Ability to define event handlers to be run during service or host events for proactive problem resolution
• Support for implementing redundant and distributed monitoring servers
• External command interface that allows on-the-fly modifications to be made to the monitoring and notification behavior through the use of event handlers, the web interface, and third-party applications
• Retention of host and service status across program restarts
• Scheduled downtime for suppressing host and service notifications during periods of planned outages
• Ability to acknowledge problems via the web interface
• Web interface for viewing current network status, notification and problem history, log file, etc.
• Simple authorization scheme that allows you restrict what users can see and do from the web interface

License
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.