Nagios - Network Monitor

Nagios is a free, as in Open Source, network and service monitor designed to pro actively inform on network problems.

Some features from nagios.org;

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.

Nagios also offer an Enterprise option that includes professional support at nagios.com.

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