What it is

Nagios is a monitoring program for hosts, services, and network resources. It is written in C and released under the GNU General Public License, version 2.0. The project belongs to the Infrastructure & Operations / Monitoring & Observability ecosystem. Nagios 4.x can check network services, monitor host resources, expose status and history through optional CGI programs, and send notifications when problems occur or are resolved.

The concrete problem it solves is ongoing visibility into infrastructure health. Operators use it to watch whether services such as SMTP, POP3, HTTP, and PING are working, whether processor load or disk usage crosses thresholds, and whether a host is down or merely unreachable. It also supports parent host definitions, event handlers, and automatic log rotation so that monitoring results can be interpreted, acted on, and retained.

Key capabilities

• Nagios monitors network services, including SMTP, POP3, HTTP, and PING.
• Nagios monitors host resources, including processor load and disk usage.
• Nagios provides a plugin interface for user-developed service monitoring methods.
• Nagios supports parent host definitions to distinguish down hosts from unreachable hosts.
• Nagios can send notifications when problems occur and when they are resolved, via email, pager, or user-defined methods.
• Nagios allows event handlers for proactive problem resolution.
• Nagios includes automatic log file rotation and archiving, plus optional CGI web interface for current status, notification history, problem history, and logs.

Who uses it and how

• Operators use Nagios in self-managed infrastructure monitoring setups.
• Operators use plugin checks to monitor service availability and resource metrics such as processor load and disk usage.
• Network operators use parent host definitions to separate host failures from downstream reachability problems.
• On-call workflows use notifications for problem occurrence and problem resolution.
• Administrators use the optional web interface to review current network status, problem history, notification history, and log files.

Getting started

The README points to downloads from the Nagios website and to quickstart installation guides. Source code is hosted on GitHub, and documentation is provided through the Nagios homepage, user guide, and documentation library.

When to use it — and when not to

Nagios fits teams that want a self-hosted monitoring program in C and are willing to operate checks, notifications, web access, and log rotation themselves. It is less suitable for teams that want a fully managed monitoring service or do not want to install and maintain a C program, plugins, and a web interface. The provided metadata lists 206 open issues, and the README directs security issues to a dedicated contact, so operators should plan for configuration, upgrade, and issue triage work.