Check if your Left 4 Dead 2 server is online
Left 4 Dead 2 replies to the usual Source query, but because of its lobby system a server that answers is not necessarily one people can join.
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The ports Left 4 Dead 2 uses
Queries go over UDP on the game port. The catch is particular to this game: the server can be set to accept players only through Steam lobbies, in which case a direct connection is turned away even though the port is open and this check passes.
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Still offline? Try this
Passing here while direct connects fail points at the lobby-only option, not the router. Switching it off lets people connect directly; you give up the matchmaking integration in exchange.