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Nagios

Nagios is a classic monitoring system built around host and service checks. It is still common in legacy environments and on teams that rely on plugin-based monitoring.

Core Model

  • Hosts represent systems being monitored.
  • Services represent checks on those systems.
  • Plugins return status codes and output text.
  • Notifications are triggered when a check changes state.

Check Types

  • Active checks are executed by Nagios.
  • Passive checks are submitted from an external source.

Typical Uses

  • HTTP, disk, process, and port checks.
  • Alerting on host availability.
  • Monitoring older infrastructure that predates modern observability stacks.

Example Service Definition

define service {
  use                 generic-service
  host_name           web-01
  service_description HTTP
  check_command       check_http
}

Practical Notes

  • Nagios works best when checks are small and reliable.
  • Plugin output should be human readable and actionable.
  • In newer environments, it is often replaced by Prometheus-based monitoring.