Nagios is an open-source IT infrastructure monitoring tool that helps organizations track the health and performance of their systems, networks, and applications in real time. It continuously checks hosts, services, and network protocols, alerting administrators when issues such as downtime, slow performance, or failures occur. Nagios supports both active and passive checks and can be extended using community-contributed plugins. Alerts can be sent via email, SMS, or integrations with ticketing systems. Nagios is widely used in DevOps environments.
Nagios is owned and maintained by Nagios Enterprises, LLC, a company based in Saint Paul, Minnesota, USA.
Nagios Core (the free, open-source version) is developed and maintained by both the Nagios Enterprises team and the wider open-source community.
Nagios XI and some other commercial editions (such as Nagios Log Server, Network Analyzer, and Fusion) are developed, supported, and sold by Nagios Enterprises, LLC. The company is responsible for ongoing product development, enterprise features, security updates, support, and official documentation.
Information on Nagios Enterprises can be found at Nagios Enterprises. While the open-source community information resides on Nagios | Open Source Monitoring and Network Management.
Nagios Core, the free open-source edition is distributed under the GNU General Public License, version 2 (GPLv2). This means it is free to download, use, modify, and distribute, provided distributed derivative works also remain under GPLv2 (see: Frequently Asked Questions about the GNU Licenses - GNU Project - Free Software Foundation).
Nagios XI and the other commercial products available from Nagios Enterprises are distributed under commercial, proprietary licenses.
Nagios has both free and paid offerings. Here’s how it breaks down:
Nagios Core (Free / Open-Source)
Nagios Core is completely open source and free to use. It provides basic monitoring capabilities, plugin support, community resources, etc. There is no licensing fee for the software itself, however any released products built upon it must be distributed under a similar license.
Nagios XI (Paid / Commercial)
Nagios XI is the commercial product. It’s not free (except for limited usage). While there is a “free” version of XI, but it's highly restricted: it includes 7 Nodes or 100 Services, whichever limit is hit first, and does not include support. Full-featured plans / SKUs of Nagios XI are based on the number of Nodes (or Hosts) monitored. Pricing varies depending on scale (50 Nodes up to unlimited).
By default, Nagios is designed to run on Linux/Unix-based systems. Nagios can be run on Windows using virtualization.
The Nagios Cross-Platform Agent (NCPA) is an open-source monitoring tool that delivers monitoring for Windows, Linux, and Mac OS X.
Nagios is primarily designed to run on Linux, most hypervisors are Linux-based. As such Nagios can often be run within hypervisor kernels. Whilst Nagios can be run within a hypervisor kernel, this configuration is usually unsupported by hypervisor vendors. For enterprises looking for fully supported configurations alternative monitoring tools are usually adopted.
The product that eventually became Nagios was initially developed in 1999 and it retains traditional monitoring tool paradigms. Some regard Nagios as a somewhat legacy product compared to today’s modern AIOps-powered monitoring platforms, however Nagios retains a large user base and Nagios Enterprises have a web page to counteract or explain some of the most common gripes cited around the tool, see: Why Nagios Is Still Relevant in 2025.
