Hello :)

After a fresh install of Centreon, I could install some Monitoring Connectors (I think these are the default/free ones). But I also see many others greyed out — I assume those require a license?

You need a licence to install the other Monitoring Connectors, you can request a free IT100 trial here which is valid for an unlimited period for monitoring up to 100 hosts.

I understand that Monitoring Connectors (or Plugin Packs?) are templates that help configure services, but I’m not sure how they relate to the actual plugin scripts (like those in /usr/lib/centreon/plugins/).

Plugin Packs is the former name for Monitoring Connectors. They refer to the same thing. They contain templates that set default value in the macros contained in the commands. These commands execute the plugins in the /usr/lib/centreon/plugins/ folder (but not only this scripts).

I also don’t really get the relationship with the GitHub repo (https://github.com/centreon/centreon-plugins). Can I just clone that repo and use it without a license? if I clone it will i be able to use all plugins and won’t need to install connectors/packs etc?

The Github repo can be cloned and used for monitoring but this is more suitable for develoment purpose that for production. You can either use the github repo or the packaged plugins on an unlicensed centreon by installing the plugins from our packages repositories (added by default on Centreon servers). You will then have to manually create your commands and your templates and/or hosts.

If I do clone the GitHub repo, should I clone it on the Central server, on each poller, or both?

That's not how the repo is used, so I wouldn't advise you to do it that way. Instead, install the plugin packages from our repos, whether on Central or Pollers.

If I do that, will that be enough to monitor hosts without requiring internet access later (since my pollers will soon be in an internal network without internet)?

To monitor hosts without internal access, there are offline versions of licences, such as the free IT100 licences (here is the documentation explaining the different licences).

I’ve read some docs saying to use dnf install centreon-plugin-Cloud-Aws-Ec2-Api or similar — but that seems to imply a license too? Or does that work without one?

This command installs a packaged plugin which is free and open source. What is licensed is the centreon-pack-* packages (which we call Monitoring Connectors).

I also noticed that even after installing Monitoring Connectors from the web UI, nothing shows up in /usr/lib/centreon/plugins/, which adds to my confusion. Does installing a connector not install the actual plugin scripts?

In the web interface, check that you have selected automatic installation of plugins in the top right-hand corner of Configuration > Monitoring Connectors Manager.
Then, once you export your configuration, if at least one host or service requires the plugin, it will be installed automatically.
This requires the central/poller to have access to internet or to a local mirror of our repository.

What’s actually needed to get working plugin checks running,

To make the plugin checks work, you need to have the plugin package and then either deploy the services using the monitoring connectors if you have a licence, or create the commands and services associated with the plugin yourself.

Whether I can work fully offline if I prep everything in advance,

You can work offline, but you will not automatically receive monitoring connect updates during the monthly releases (you will have to download them by hand, see this doc page).

And how much of this is blocked by not having a Centreon commercial license.

You can use the IT100 licence free of charge for an unlimited period, but after 100 hosts you need to switch to a commercial licence.

If i get the free 100 IT license and install some packs, will i be able to use them later even if i have more than 100 hosts? knowing i’ll go offline, or what exaclty, please help, for example i need the jenkins, F5 bigIP,paloalto and vmware vcenter ones, do i just install with the dnf command those scripts without the packs from the UI or?

You can no longer use the monitoring connectors when you exceed 100 hosts (whether it's an offline or online licence) but under these conditions it would be legitimate to consider investing more in a tool that serves you so much ;)

Best regards

​@ldubrunfaut and if i want to just use the github plugins without the need for a license? and set up everything manually? can i do that? its not me its the company that wont let me get a license so i need to figure it out myself(im just an intern) so far i git cloned the repos and all on the centreon central and the pollers(im currenlty in online envirenemnt but ill switch to offline after i make sure evertyhing works) im just not sure if i can just use those github repos just like that call plugins direclty with the scripts or is there some step im missing because i keep getting a lot of weird errors

You can just do 

dnf install centreon-plugin-\*

and then you don’t have anything to install all is free and without licence. But you need to configurate all services and host manually.