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 supressing host and service notifications during periods of planned outages
Ability to acknowlege 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
