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It's usually possible to use another provider, as long as it uses the same protocol. But there may be different flavors, e.g. with differing default values and what-not, so YMMV. I have used the OpenLDAP provider to authenticate over AD before.

Okay... i tried a few different things but all i get is redirected back to rancher with this error: Logging in failed: Your account may not be authorized to log in.

If possible, I would recommend SSH:ing into one of the upstream cluster's nodes and try to run some CLI tool to try and authenticate some request manually. That way to check that the IDP is reachable from Rancher (no firewall messing things up) and that the account you use do have proper access.