# Nagios — The Industry Standard in IT Infrastructure Monitoring

> The original open-source monitoring system for hosts, services, and network devices with a 25-year track record and thousands of community plugins.

## Install

Save as a script file and run:

# Nagios — The Industry Standard in IT Infrastructure Monitoring

## Quick Use
```bash
# Install on Ubuntu
sudo apt install nagios4 nagios-plugins
# Start the service
sudo systemctl start nagios4
# Access web interface at http://localhost/nagios4
# Default: nagiosadmin / (password set during install)
```

## Introduction
Nagios Core is the original open-source monitoring engine that has been watching servers, switches, and services since 1999. Its plugin architecture has spawned an ecosystem of thousands of community-maintained checks, and its alert pipeline has set the standard that newer monitoring tools still follow.

## What Nagios Does
- Monitors host and service availability via active and passive checks
- Sends alerts through email, SMS, and third-party notification handlers
- Tracks performance data for capacity planning and trend analysis
- Detects network outages and distinguishes root cause from symptoms
- Provides a web interface for status dashboards and acknowledgements

## Architecture Overview
Nagios Core runs as a single daemon that schedules and executes check plugins at defined intervals. Plugins are external programs that return a status code (OK/WARNING/CRITICAL/UNKNOWN) and optional performance data. The core processes results through a notification pipeline with contact groups, escalation chains, and time period filters. State is persisted to flat files and rendered through CGI-based web pages.

## Self-Hosting & Configuration
- Install via package manager or compile from source
- Define hosts, services, and contacts in object definition files
- Place check plugins in /usr/lib/nagios/plugins/
- Configure notification commands for your alerting channels
- Use NRPE or check_by_ssh for remote host monitoring

## Key Features
- Plugin architecture with 5000+ community-maintained checks available
- Parent-child host relationships for intelligent root cause detection
- Flexible notification system with escalation and dependencies
- Event handlers for automatic remediation actions
- External command interface for integration with automation tools

## Comparison with Similar Tools
- **Icinga** — Modern fork with better API and clustering; Nagios is simpler but less scalable
- **Zabbix** — All-in-one with built-in graphing; Nagios relies on add-ons for visualization
- **Prometheus** — Pull-based metrics; Nagios is check-based with active polling
- **Sensu** — Cloud-native redesign of the monitoring concept; Nagios is traditional but proven

## FAQ
**Q: How is Nagios Core different from Nagios XI?**
A: Core is the free open-source engine. XI is the commercial product with a modern web UI, wizards, and enterprise support.

**Q: Can I monitor cloud resources with Nagios?**
A: Yes. Community plugins exist for AWS CloudWatch, Azure Monitor, and GCP metrics. Use check_http for endpoint monitoring.

**Q: How do I monitor remote Linux servers?**
A: Install NRPE (Nagios Remote Plugin Executor) on remote hosts. It runs local checks and returns results to the Nagios server.

**Q: Is Nagios still relevant in 2025?**
A: Yes for organizations with existing deployments and custom plugins. For greenfield projects, newer tools may offer better developer experience.

## Sources
- https://github.com/NagiosEnterprises/nagioscore
- https://www.nagios.org/documentation/

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Author: Script Depot