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Do anyone tried to play on the Internet?
I had little-to-no luck, me and two friends of mine tried but it never happened we all could join one game.

For example, I create one of the friend cannot join (but sees the server game). The other friend creates and I cannot enter, etc..

What can we try? We all use the Linux version, in SP it seems working fine.
I'm not sure of the port forwarding on Linux, but you may have to set a specific port on your routers to allow incoming connections.