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What port does a Core Keeper server use?
The answer depends on how the dedicated server is launched:
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| Networking mode | Port setting | Inbound forwarding | How players connect |
|---|---|---|---|
| Steam relay networking | Omit -port or leave SERVER_PORT empty in the documented Docker image |
No inbound game-port forwarding is required in the documented mode | Through Steam relay networking, not the server’s public IP |
| Direct connection | Set -port NUMBER or SERVER_PORT=NUMBER |
The selected port must be allowed by the host firewall; players outside the LAN generally also need router forwarding | To the server endpoint directly; -ip can select the local bind address |
That means “Core Keeper server port” is not automatically “27015.” A current server with no port argument has no fixed numeric listening port that you need to forward. If you want an IP-based connection, choose a port yourself, for example 7778, and configure the network for that choice.
Why older guides say 27015-27016
Many instructions still quote UDP ports 27015-27016 because an official Steam announcement from April 27, 2022 described those as the default ports for an experimental dedicated-server test. That announcement also instructed hosts to open and forward the ports for access from outside the local network. It documented that test branch, not a permanent numeric default for every later server package.
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The versioned Core Keeper server reference for version 1.2.1.5, verified September 7, 2026, gives the current rule: when -port is omitted, the server uses the Steam relay network and does not accept direct connections using an IP address. The same relay-versus-direct distinction also appeared in the older Linux server README for version 0.6.0.3. Relay behavior therefore predates the current release, while the historical 27015-27016 statement should be treated as version-specific.
The current reference does show -port 27015 as an example for connections on the same LAN. An example is not a universal default. You may use 27015 if it is free and convenient, but another available port works when it is supplied consistently to the server, firewall, router and connecting players.
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Choose relay or direct networking before changing anything
Use Steam relay mode when
- You want the simplest setup and players will join through Steam or a game ID.
- You do not want to expose or forward a game port on your router.
- Your network uses carrier-grade NAT, restrictive inbound rules or another arrangement that makes direct hosting difficult.
Leave -port out of the launch command. In this mode, trying to give players your public IP is the wrong connection method because the current reference says the server will not accept direct IP connections.
Use direct mode when
- You need clients to connect to a server IP and port, including a controlled LAN endpoint.
- You are hosting in an environment where Steam relay networking is unavailable or unsuitable.
- You can allow the selected port through the operating-system firewall and, for external players, the router.
Direct mode is an explicit configuration. Pick one port, set it with -port, restart the server, and distribute the resulting endpoint using the connection method supported by your server package.
How to change the dedicated-server port
The port is a startup argument, so change the command that launches the server rather than a saved world setting.
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- Find the launcher. Open the script or service that starts the dedicated server: commonly
Launch.baton Windows,launch.shon Linux, a custom service command, or a container entrypoint. - Add the argument. Append
-port NUMBERto the existing command. For example, use-port 7778. Keep the hyphen and place a space between the option and number. - Optionally set the bind address. Add
-ip ADDRESSonly when you need the server to bind to a particular local interface. The current reference says-iphas no effect unless-portis also set, and its default bind address is0.0.0.0. - Save and restart. Stop the running server cleanly, save the launcher, and start it again. A port argument read only at process startup will not change on a live process.
- Open the selected port. Permit the port in the host firewall. If players are outside your local network, create a router/NAT forward to the host and permit the same port there.
- Give players the correct endpoint. LAN players use the host’s reachable local address and selected port; external players use the public endpoint and selected port according to the direct-connection workflow for their client.
Use a port number that is not already occupied by another service. If your chosen number is unavailable, select another and change every matching configuration point together.
Windows batch-file example
If your existing Launch.bat contains a server executable followed by options, add the flag to that same line. The important part is the argument pair:
-port 7778Do not create a second unrelated command unless the launcher specifically requires it. Keep any existing world, account or save arguments intact, then restart the process.
Linux shell or service example
For a shell script, systemd service, supervisor command or other process manager, add the same pair to the executable’s argument list:
-port 7778 -ip 0.0.0.00.0.0.0 is the documented default bind address, so explicitly writing it is optional when that is the intended behavior. A more specific local address can be supplied with -ip, but it will not do anything unless a port is set too.
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Do not put the port in ServerConfig.json
The current server reference explicitly states that -port may only be set with command-line arguments. Adding or changing a port field in ServerConfig.json has no effect on the listening port. This is a common reason a server appears unchanged after an edit: the file may save successfully, but the launcher still starts without the required argument.
Use the startup script, service definition or container environment instead. After editing, restart the server and verify the command being executed actually contains the new value.
Firewall and router setup for direct connections
Host firewall
Allow the selected port for the server process in the operating system’s inbound firewall. Match the number you passed to -port. If the Docker example below is used, the protocol is published as UDP; follow the same protocol requirement in the host firewall rather than creating a rule for an unrelated protocol.
Test locally first. A client on the same LAN should use the host’s LAN address and selected port. A successful LAN test separates server and host-firewall errors from router or internet-path errors.
Router forwarding
For players outside the LAN, forward the selected external port to the host’s internal address. The 2022 official instructions required forwarding the listed UDP ports for outside-LAN access; in a current direct setup, apply that principle to the port you chose. Reserve the host’s local address in the router or otherwise keep it stable so the forwarding rule does not point at a different device later.
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Do not forward ports when you are intentionally using relay mode. Relay mode is designed to avoid inbound game-port forwarding in the documented configuration.
When forwarding still does not work
- Confirm the server was restarted after adding
-port. - Check that the firewall rule uses the same port and protocol as the server.
- Verify the router forwards to the correct internal host address.
- Check whether the internet connection uses carrier-grade NAT or another upstream router that prevents inbound connections.
- Make sure the client is using the direct endpoint rather than a Steam-relay game-ID path.
Docker: set SERVER_PORT and publish the same UDP port
The escapingnetwork/core-keeper-dedicated Docker image exposes the same relay-versus-direct choice through environment variables. SERVER_PORT has no default. Leaving it unset keeps the documented relay behavior; setting it switches the container to direct-connection behavior.
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services:
core-keeper:
image: escapingnetwork/core-keeper-dedicated
environment:
SERVER_PORT: "7778"
ports:
- "7778:7778/udp"The exact Compose layout can include additional settings, but the essential relationship is SERVER_PORT=7778 and 7778:7778/udp. If those numbers differ, the published host port will not match the port the server is configured to use.
SERVER_IP is used only when a port is set. When no port is configured, the image binds to 0.0.0.0 according to its documented behavior. For direct hosting, set SERVER_IP only when you need a particular interface; otherwise leave the default bind behavior in place.
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Docker checklist
- Set
SERVER_PORTto the chosen number for direct mode. - Publish the identical number with the
/udpsuffix. - Allow that UDP port through the host firewall.
- Forward it on the router for external clients.
- Recreate or restart the container after changing environment variables.
How clients should connect
Relay connection
With no -port argument, use the Steam-based connection method supplied by the game and server workflow, such as a game ID where applicable. Do not substitute the host’s public IP: the current reference says relay mode will not accept direct IP connections.
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When -port is set, clients on the same network should use the host’s reachable LAN address and the selected port. A value such as 27015 can be used as a LAN example, but there is no requirement that every installation use it.
Direct internet connection
External clients need the public endpoint and selected port, and the host must have the firewall and router rules described above. If LAN clients work but internet clients fail, focus on NAT, forwarding and upstream-network restrictions rather than changing the world or save configuration.
Troubleshooting by symptom
| Symptom | Likely cause | What to check |
|---|---|---|
| Game ID works, IP address does not | The server is still in relay mode | Confirm the launch command includes -port NUMBER; relay mode does not accept direct IP connections |
| Changing ServerConfig.json changed nothing | The port is not read from that file | Move the setting to the launcher, service command or container environment and restart |
| LAN clients cannot join | Wrong bind address, host firewall block or wrong port | Check -ip, the firewall rule, the LAN address and the exact number after -port |
| LAN works but outside players cannot join | Missing or incorrect router forwarding, upstream NAT or external firewall block | Forward the selected port to the correct host, permit it on the host firewall and check for carrier-grade NAT or another upstream router |
| Docker clients cannot connect directly | SERVER_PORT and published port do not match, or the protocol mapping is missing |
Use the same number on both sides of host:container/udp, recreate the container and open the host’s UDP port |
-ip appears ignored |
No port was supplied | Set -port as well; the current reference says -ip has no effect by itself |
| A new port appears unavailable | Another service or an existing server process is using it | Stop duplicate instances, choose an unused number and update every matching firewall, router and client setting |
A reliable change-and-test sequence
- Record whether the server currently uses relay mode or a direct port.
- Choose an unused port, such as
7778; do not assume 27015 is mandatory. - Add
-port 7778to the actual startup command, or setSERVER_PORT=7778for the documented Docker image. - Add
-ip ADDRESSonly if binding to a specific interface is necessary. - Restart the server or recreate the container.
- Allow the selected port using the required protocol in the host firewall.
- Test from a client on the same LAN using the LAN endpoint.
- If internet access is required, forward the same port to the host and test from outside the LAN.
- If the test fails, compare each layer in order: launcher, server bind, host firewall, router forwarding, upstream NAT, then client endpoint.
This order avoids changing several variables at once. A successful LAN test proves that the server process and local path are working before you troubleshoot the router or internet connection.
What to remember about 27015
27015is a valid example shown for same-LAN direct connections in the current reference, not a guaranteed current default.- The official April 27, 2022 experimental announcement described UDP
27015-27016as that test’s default pair. - Current installations that omit
-portuse Steam relay networking instead of listening for direct IP connections on a fixed documented number. - If you select 27015 or any other number, configure that exact value in the launch argument, firewall, router, Docker mapping and client endpoint.
Security and maintenance considerations
Direct hosting exposes a reachable service endpoint, so open only the selected game port and keep the forwarding target limited to the intended host. If you do not need direct IP access, leaving the port unset avoids creating an inbound game-port rule in the documented relay configuration.
Document the chosen port beside the launcher or deployment files. Future administrators should be able to see whether the server is intentionally in relay mode or direct mode without guessing from an old router rule. When upgrading the dedicated-server package, recheck its versioned reference: the 2022 numeric-default guidance demonstrates why port behavior should be verified against the release you are actually running.
Bottom line
A current Core Keeper dedicated server has no single numeric default port when -port is omitted; it uses Steam relay networking. To host through a direct IP or LAN endpoint, add -port NUMBER to the startup command, optionally add -ip ADDRESS, restart, and open or forward the selected port. Editing ServerConfig.json will not change it. The old 27015-27016 advice describes an April 2022 experimental test, while 27015 today is only an example you may choose.
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