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Core Keeper Connection Doctor

Nobody can join, or everyone was thrown out after a restart? Paste your server's Player.log, its ServerConfig.json, or both. This checks the failures Core Keeper servers actually hit, including the one that quietly changes your password every time the server starts. Everything stays in your browser.

Careful: a Core Keeper Player.log contains your server password in clear text, because the game writes the full join string into it. This page never uploads anything, but do not paste that log anywhere public.

Find it at $HOME/.config/unity3d/Pugstorm/Core Keeper/Player.log on Linux. The server's own stdout is mostly Unity memory settings and will not show the fault.

Sits in your server's data path, next to worlds/ and modloader/.

Want this on a real server? Supercraft hosts Core Keeper →

Would rather not babysit any of this? Supercraft runs Core Keeper dedicated servers with a stable password, mod.io mods and a clean shutdown on every restart, so the world is always saved.

Frequently asked questions

Why can my friends suddenly not join my Core Keeper server?

The usual cause is that no password was set. With an empty password the server creates a brand new random one every time it starts and writes it into GameID.txt. Players join using the string <ip>;<port>;;<password>, so the details everyone saved stop working after the next restart or update. Setting a password yourself makes it stable.

Why does my Core Keeper server crash right after starting?

If the log shows a failed dlopen of steamclient.so or “InitGameServer returned false”, the Steam game-server API could not initialise. Core Keeper looks for steamclient.so in $HOME/.steam/sdk64/ and the process dies shortly after Unity finishes loading. Linking that file from the server's own linux64 folder fixes it.

Why can nobody connect by IP address?

If the server is started without a port argument it uses the Steam relay network and refuses direct IP connections altogether. Starting it with an explicit port enables direct connections and cross-play. The game port and the port immediately above it both need to be open on UDP, because the second one is the query port.

Does a Core Keeper server save the world automatically?

No. Core Keeper writes the world inside its shutdown handler rather than on a timer, so a server that is force-killed loses everything since the last clean stop. A graceful stop shows a WorldSave line in the log and writes the world to <datapath>/worlds/<index>.world.gzip.

This tool: v1.3 · updated 18 Sep 2026