Nagios
Nagios is an open source IT infrastructure monitoring tool that checks the status of hosts and services and alerts administrators when something goes down or crosses a defined threshold.
Definition
Nagios is an open source IT infrastructure monitoring tool that checks the status of hosts and services and alerts administrators when something goes down or crosses a defined threshold.
Overview
Nagios is one of the oldest and most widely deployed open source monitoring tools, originally released in the late 1990s under the name NetSaint before being renamed. It established a monitoring model that many later tools — including its own forks — still follow: a central Nagios server periodically runs checks (via plugins) against hosts and services, and raises alerts when a check returns a warning or critical state. Nagios's plugin architecture is its defining feature. Checks are simple executables that return a status code and output text, which means anything scriptable — disk space, HTTP response codes, database connectivity, custom application health checks — can be monitored without waiting on built-in support. This flexibility made Nagios a default choice for Linux system administrators managing on-premises and early cloud infrastructure. Compared to modern observability platforms built around metrics time series and distributed tracing, Nagios is fundamentally a status-checking and alerting tool rather than a deep telemetry platform. It remains in wide use for straightforward up/down and threshold monitoring, and it directly inspired forks and alternatives such as Icinga and agent-based tools like Checkmk.
Key Features
- Plugin-based architecture allowing any scriptable check to be monitored
- Active checks run on a schedule against hosts and services
- Passive check support for external systems to submit status updates
- Escalation and notification rules for alerting via email, SMS, or scripts
- Web interface for viewing host/service status and historical availability
- Large community-maintained plugin ecosystem covering common infrastructure checks
- Configuration via text files, enabling version-controlled monitoring definitions