What is Nagios?

Nagios is an Open Source IT system monitoring tool. It was designed to run on the Linux operating system and can monitor devices running Linux, Windows and Unix OSes.

Nagios software runs periodic checks on critical parameters of application, network and server resources. For example, Nagios can monitor memory use, disk use and microprocessor load, as well as the number of currently running processes and log files. Nagios also can monitor services such as Simple Mail Transfer Protocol (SMTP), Post Office Protocol 3, Hypertext Transfer Protocol (HTTP) and other common network protocols. Nagios initiates active checks, while passive checks come from external applications connected to the monitoring tool.

Originally released in 1999 as NetSaint, Nagios was developed by Ethan Galstad and subsequently refined by numerous contributors as an open source project. Nagios Enterprises, a company based around the Nagios Core technology, offers multiple products, such as Nagios XI, Log Server, Network Analyzer and Fusion.

How Nagios works

Users can choose to work in the command-line interface or select a web-based graphical user interface in some versions of Nagios and from third parties. Nagios' dashboard provides an overview of the critical parameters monitored on assets.

Based on the parameters and thresholds defined, Nagios can send out alerts if critical levels are reached. These notifications can be sent through email and text messages. An authorization system enables administrators to restrict access.

Nagios runs both agent-based and Agentless configurations. Independent agents are installed on any hardware or software system to collect data that is then reported back to the management server. Agentless monitoring uses existing protocols to emulate an agent. Both approaches can monitor file system use, OS metrics, service and process states. Examples of Nagios agents include Nagios Remote Data Processor (NRDP), Nagios Cross Platform Agent and NSClient++.

Nagios plugins

Nagios can also run remote scripts and plugins using the Nagios Remote Plugin Executor (NRPE) agent. NRPE enables remote monitoring of system metrics such as system load, memory and disk use. It consists of the check_nrpe plugin, which is stored on the local monitoring machine, and NRDP, which runs on the remote machine. Nagios uses a plugin to consolidate data from the NRPE agent before it goes to the management server for processing. NRPE can also communicate with Windows agents to monitor Windows machines.

Nagios supports plugins that are stand-alone add-ons and extensions so users can define targets and which target parameters to monitor. Nagios plugins process command-line arguments and communicate commands with Nagios Core.

There are around 50 plugins developed and maintained by Nagios, while there are over 3,000 from the community. These plugins are categorized into lists including hardware, software, cloud, OSes, security, log files and network connections. As an example, when used in conjunction with environmental-sensing systems, a Nagios plugin can share data on environmental variables, such as temperature, humidity or barometric pressure.

Nagios tools

Nagios has proven popular among small and large businesses, as well as internet service providers, educational institutions, government agencies, healthcare institutions, manufacturing companies and financial institutions.

Users can choose among free and paid options, depending on the needed services and support.

Nagios Core

The service that was originally known as Nagios is now referred to as Nagios Core. Core is freely available as an open source monitoring software for IT systems, networks and infrastructure. Core contains a wide array of infrastructure monitoring through allowing plugins to extend its monitoring capabilities. It is the base for paid Nagios monitoring systems.

Nagios Core has an optional web interface, which displays network status, notifications and log files. Core can notify its user when there are server or host issues. Additionally, Core can monitor network services such as SMTP, HTTP and Ping.

Nagios XI

Nagios XI is an extended interface of Nagios Core, intended as the enterprise-level version of the monitoring tool. XI acts as monitoring software, configuration manager and toolkit. While Nagios Core is free, XI must be purchased from Nagios Enterprises. Atop the same features as Core, XI adds preconfigured virtual machines (VMs), a web configuration user interface, performance graphing, a mobile application, dashboards, scheduled reporting and technical support through email.

Nagios XI monitors IT infrastructure components such as applications, OSes, networks and system metrics. Plugins are supported for these infrastructure components to expand on XI's monitoring capabilities.

Nagios commercial extensions

Nagios Log Server is a log monitoring and management tool that enables an organization to view, sort and configure logs from its IT infrastructure, including Windows event logs. Log Server can analyze, collect and store logged data based on custom and preassigned specifications. Administrators can set alerts to notify Log Server users when there is a potential threat or malfunction on a monitored asset. For example, an alert goes out to the Microsoft Exchange administrator when there are three failed login attempts to Exchange Server, meaning there could be an unwarranted person trying to guess the password to the system.

Nagios Network Analyzer tracks network traffic and bandwidth use. Network Analyzer can resolve network outages, abnormalities and security threats. Features include automated security alerts, customizable application monitoring, integration with Nagios IX and a bandwidth use calculator.

Nagios Fusion is an aggregation service for Nagios Core and Nagios XI servers that shows multiple systems in one view. Fusion condenses network management by centralizing features and data from Nagios XI and Core in one location, creating a granular view of a network infrastructure. With Fusion, administrators can specify which XI and Core servers are displayed and manage which users are allowed to view those servers. Additionally, Fusion users can log into any managed server and use cached or live data to configure charts and other graphics to appear on dashboards.

Nagios competitors

Nagios competitors include Zenoss, Zabbix, Microsoft System Center Operations Manager (SCOM) and SolarWinds, among other open source and commercial monitoring tools.


Zenoss is IT monitoring software for cloud, virtual and physical IT environments. Zenoss monitors servers, networks, VMs, databases and other hardware and software assets in an IT infrastructure. Similar to Nagios, Zenoss is available as an open source version called Zenoss Core or more extensive paid, supported options including Zenoss Service Dynamics and Zenoss as a Service. Service Dynamics is the on-site version of the software, while Zenoss as a Service is a software-as-a-service option. Similar to Nagios products, Zenoss products provide plugins, called ZenPacks, which extend monitoring capabilities.


Zabbix is an open source monitoring tool for Linux, Unix and Windows OSes that relies on agents to collect monitoring data. It can also use common protocols for agentless operation. The technology monitors physical and cloud assets, VMs, services and applications. Zabbix is evolving for cloud deployment, as well as on premises.

Microsoft SCOM

Microsoft SCOM enables users to configure, manage and monitor devices and applications via the same console. SCOM tracks server hardware, system services, OSes, hypervisors and applications. SCOM, like Nagios, relies on agents or agentless-based monitoring for its data collection, and supports plugins.


SolarWinds' Server & Application Monitor software works with applications, servers and databases. Server & Application Monitor includes performance monitoring, server management, alerts and reporting through agentless monitoring. Server & Application Monitor also supports other SolarWinds products.

This was last updated in July 2023

Continue Reading About Nagios

Dig Deeper on Containers and virtualization

Software Quality
App Architecture
Cloud Computing
Data Center