Comparisons that start from Nagios Core, the GPL-2.0 monitoring engine many teams still run, against the open-source monitoring platforms that grew out of it and document migrating from it.
Staying on Nagios Core or moving to Icinga 2, which began as a Nagios fork: compares the C engine with text configuration and CGI interface against Icinga's rule-based DSL, REST API, Director and built-in distributed monitoring with HA masters. Covers plugin reuse, how much configuration must be rewritten, licensing (GPL-2.0 versus GPL-3.0-or-later) and paid support from Nagios Enterprises or Icinga GmbH.
Nagios Core against Checkmk, a former Nagios fork whose free Community edition still runs the Nagios core: compares manual text configuration with Checkmk's rule-based setup and agents, what moves to the paid Checkmk Micro Core in Pro and Ultimate, the roughly 100-host scope Checkmk gives Community, and the effort to move an existing Nagios estate.
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