Nagios

Key takeaways

Nagios belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Nagios to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Nagios from memory before moving on to harder problems.

Reference excerpt

Nagios () is an open-source network and infrastructure monitoring system. It monitors hosts, services, and network devices, sending alerts when components fail and again when they recover. Originally written by Ethan Galstad in 1999 as NetSaint, it was renamed Nagios in 2002 after a trademark dispute. The name is a recursive acronym: "Nagios Ain't Gonna Insist On Sainthood." Nagios uses a plugin-based architecture: the core scheduler runs check plugins at configured intervals, processes the results, and triggers notifications. Hundreds of community-developed plugins exist for monitoring specific services and hardware. Nagios is written in C and runs on Linux and other Unix systems. It is licensed under the GPLv2. Galstad founded Nagios Enterprises in 2007 to provide commercial support and the proprietary Nagios XI product. The project's plugin-architecture model influenced several derivative projects, including Icinga (2009), Shinken (2009), Naemon (2013), and Checkmk.

History Galstad released NetSaint 0.0.1 on March 14, 1999. The initial release supported host and service monitoring via plugins, with notifications by email and pager. NetSaint was registered on SourceForge about two months before the project that became OpenNMS. In 2002, Galstad renamed the project to Nagios after the owners of a similarly named trademark raised a legal challenge. Although an agreement was reached, Galstad chose to change the name to avoid future disputes. "Agios" (hagios) transliterates the Greek word for "saint," linking the new name to the old. Nagios 2.0 (February 2006) added a redesigned web interface and the Event Broker (NEB) module API, which allowed third-party software to access Nagios's internal data structures. Nagios 3.0 (March 2008) introduced adaptive monitoring and improved performance for large installations. Nagios 4.0 (September 2013) added a redesigned CGI interface, worker processes for parallel check execution, and reduced CPU usage on large deployments.

Icinga fork In May 2009, a group of community developers forked Nagios as Icinga, citing frustration with slow development, lack of community involvement in decision-making, and what they considered insufficient response to submitted patches. Icinga initially maintained compatibility with Nagios plugins and configuration files. Icinga 2 (2014) was a ground-up rewrite with a new configuration language and distributed monitoring support.

Plugins controversy On January 16, 2014, Nagios Enterprises redirected the nagios-plugins.org domain to a server it controlled without notifying the community plugin team. The displaced maintainers continued their work under the name "Monitoring Plugins" at monitoring-plugins.org.

Architecture

Plugin model Nagios Core does not perform monitoring checks directly. Instead, it executes external programs (plugins) that test a specific service or resource and return a status code: 0 (OK), 1 (WARNING), 2 (CRITICAL), or 3 (UNKNOWN). This design allows administrators to write custom plugins in any language. The project maintains an official plugin set, and hundreds of community plugins are available from the Monitoring Plugins project and Nagios Exchange.

Agents NRPE The Nagios Remote Plugin Executor runs plugins on remote Linux/Unix systems and returns results to the Nagios server via the check_nrpe plugin. NRPE 4.0.1 was deprecated in January 2020. NRDP The Nagios Remote Data Processor uses HTTP and XML for flexible passive check submission, replacing the older NSCA protocol. NCPA The Nagios Cross Platform Agent runs on Windows, Linux, and macOS, providing both active and passive check capabilities through a REST API. NSClient++ A Windows-focused agent that reports system metrics (CPU, memory, disk, processes) to Nagios via the check_nt plugin.

Web interface Nagios Core provides a CGI-based web interface for viewing host and service status, alert history, and availability reports. The interface displays a tactical overview, status maps, and alert histograms.

Nagios XI Nagios XI is a proprietary monitoring platform built on Nagios Core. It provides a PHP-based web interface, a configuration wizard system, customizable dashboards, capacity planning reports, and integration with RRDtool and Highcharts for graphing. The backend uses MySQL for configuration and event data (via the NDOUtils module). Nagios XI runs on CentOS and RHEL.

Worked examples

Example 1 — a first encounter with Nagios

Start with the simplest possible case. Write down what Nagios claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Nagios before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Nagios ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Nagios

In research: Nagios appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Nagios in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Nagios is common in secondary-school and first-year university syllabi. It links to neighbouring topics 1999 software, Free network management software, Internet Protocol based network software, so understanding it makes those chapters shorter.
In everyday life: Look for Nagios outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Nagios in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Nagios means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Nagios out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Nagios in simple terms?

Nagios () is an open-source network and infrastructure monitoring system. It monitors hosts, services, and network devices, sending alerts when components fail and again when they recover.

Why does Nagios matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Nagios?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Nagios.