I didn't know either, so I did some Googling and found an old announcement[1] from 2009:
> A group of leading Nagios protagonists including members of the Nagios Community Advisory board and creators of multiple Nagios Addons have launched Icinga – a fork of Nagios, the prevalent open source monitoring system. This independent project [is based upon a] broader developer community. [...] Icinga takes all the great features of Nagios and combines it with the feature requests and patches of the user community.
It also looks like in 2014, Nagios centralized and appropriated a domain name and website used for hosting Nagios plugins, away from the community (its plugin developers)[2]:
> In the past, the domain "nagios-plugins.org" pointed to a server maintained by us, the Nagios Plugins Development Team. The domain itself had been transferred to Nagios Enterprises a few years ago, but we had an agreement that the project would continue to be independently run by the actual plugin maintainers.¹ Yesterday, the DNS records were modified to point to web space controlled by Nagios Enterprises instead. This change was done without prior notice.
> To make things worse, large parts of our web site were copied and are now served (with slight modifications²) by <http://nagios-plugins.org/>. Again, this was done without contacting us, and without our permission.
> This means we cannot use the name "Nagios Plugins" any longer.
> [Icinga developer]: "Six months before the fork, there was a bit of unrest among Nagios' extension developers [...] Community patches went unapplied for a long time[.]"
> [...]
> Two years ago, more or less when the split happened, [Nagios author] was having problems resolving [trademark] issues with a company called "Netways".
I'm still not sure what the effect is supposed to be tbh.
> A group of leading Nagios protagonists including members of the Nagios Community Advisory board and creators of multiple Nagios Addons have launched Icinga – a fork of Nagios, the prevalent open source monitoring system. This independent project [is based upon a] broader developer community. [...] Icinga takes all the great features of Nagios and combines it with the feature requests and patches of the user community.
It also looks like in 2014, Nagios centralized and appropriated a domain name and website used for hosting Nagios plugins, away from the community (its plugin developers)[2]:
> In the past, the domain "nagios-plugins.org" pointed to a server maintained by us, the Nagios Plugins Development Team. The domain itself had been transferred to Nagios Enterprises a few years ago, but we had an agreement that the project would continue to be independently run by the actual plugin maintainers.¹ Yesterday, the DNS records were modified to point to web space controlled by Nagios Enterprises instead. This change was done without prior notice.
> To make things worse, large parts of our web site were copied and are now served (with slight modifications²) by <http://nagios-plugins.org/>. Again, this was done without contacting us, and without our permission.
> This means we cannot use the name "Nagios Plugins" any longer.
There's some previous discussion of those controversies on HN here: https://news.ycombinator.com/item?id=9452013
From that article[3]:
> [Icinga developer]: "Six months before the fork, there was a bit of unrest among Nagios' extension developers [...] Community patches went unapplied for a long time[.]"
> [...]
> Two years ago, more or less when the split happened, [Nagios author] was having problems resolving [trademark] issues with a company called "Netways".
I'm still not sure what the effect is supposed to be tbh.
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1: https://icinga.com/blog/2009/05/06/announcing-icinga/
2: https://www.monitoring-plugins.org/archive/devel/2014-Januar...
3: https://web.archive.org/web/20160314090137/http://www.freeso...