🎮 Game Error Codes What it means & how to fix it

Core Keeper Can't Join Server? Here's the Fix

Core Keeper can't-join errors split by connection mode: in Direct-IP mode the server listens on the UDP port you pass as -port (commonly 27015) plus 27016, and both must be forwarded; in Relay / Game ID mode you need no port forwards at all. The most common mistake is joining with a Game ID when the server is actually listening on an IP.

Exact details for Core Keeper

Error codes

Friends can't join dedicated server Direct-IP ports closed

Cause: UDP 27015/27016 not forwarded.

Fix:
  1. Forward UDP 27015 (your -port value) and 27016 to the server's LAN IP, and open them in the firewall in both directions.
  2. Confirm players connect via the public IP + port, not a Game ID, when running Direct-IP mode.
  3. If port forwarding is blocked (CGNAT), switch to Relay/Game ID mode or use a tunnel service.
Joining by Game ID fails Wrong connection method

Cause: Server is on an IP but you're using a Game ID.

Fix:
  1. Match the join method to the server mode: IP:port for Direct-IP, Game ID only for Relay mode.
  2. Restart the server after switching modes so it re-registers.
Nobody can join after starting a dedicated server Build mismatch or firewall

Cause: Client and server on different versions, or firewall blocking.

Fix:
  1. Ensure every player's Core Keeper build exactly matches the server build.
  2. Add firewall exceptions for the server's UDP ports on both the machine and the router.

Fixes that work for most Core Keeper errors

  1. Direct-IP mode: forward UDP 27015 and 27016 to the server.
  2. Relay/Game ID mode: no port forward needed — use the Game ID.
  3. Never mix methods: IP:port for Direct-IP, Game ID for Relay.
  4. Keep client and server on the same build.
  5. Add firewall rules on both PC and router.
  6. Behind CGNAT? Use Relay mode or a tunnel.

FAQ

What ports does a Core Keeper dedicated server use?

In Direct-IP mode it uses the UDP port you set with -port (commonly 27015) plus 27016, and both must be forwarded and allowed through the firewall. Relay / Game ID mode needs no player-facing port forwarding.

Why can't my friends join my Core Keeper server?

Usually the Direct-IP ports (UDP 27015 and 27016) aren't forwarded, or players are trying to join with a Game ID when the server is listening on an IP. Match the join method to the server mode and open the ports.

Do I need to port forward for Core Keeper?

Only in Direct-IP mode. If you run the server in Relay / Game ID mode, players connect through a relay and no port forwarding is required — handy when you're behind CGNAT.

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