Check if your Core Keeper server is online
Core Keeper can be joined by game ID or queried directly on its port, which is why 'friends can join but the checker says offline' comes up so often with it.
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The ports Core Keeper uses
Joining by game ID runs through the publisher's relay and has nothing to do with your ports. This check uses a direct query on your server's UDP port, so it is testing a different path from the one your friends use. A server joined through the relay can be completely unreachable directly and still work fine for them.
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Still offline? Try this
Joining by game ID working fine means a failure on this page is normal, not something to fix. Forward the UDP port only if you want direct connections and a public listing on top.