There is no fixed numeric default port when you run the current Core Keeper dedicated server without a -port argument. In that configuration, the server uses Steam relay networking (SDR), so players connect through Steam rather than directly to your public IP. For a LAN or internet connection by IP address, add a UDP port such as -port 7778, then allow that port through the host firewall and, for players outside your network, forward it on the router.
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The short answer: which port does Core Keeper use?
The answer depends on how the dedicated server is launched:
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|---|---|---|---|---|
| Steam relay (SDR) | Leave -port out (or leave Docker SERVER_PORT empty) |
None is assigned for direct listening | Steam relay networking; not the server’s public IP | Not required in the documented relay setup |
| Direct connection | Add -port NUMBER (or set SERVER_PORT) |
Chosen by you; the current reference shows 27015 as a same-LAN example |
The server’s IP address and selected UDP port | Required on the host firewall; normally also on the router for internet players |
Therefore, saying “the Core Keeper port is 27015” is incomplete. 27015 is an example in the current server reference, not a universal default. The current reference for Core Keeper 1.2.1.5, verified September 7, 2026, states that omitting -port makes the server use the Steam relay network and prevents direct IP connections.
Why older guides list UDP 27015-27016
An official Steam announcement from April 27, 2022, covered an experimental dedicated-server test. It said: “The ports used by the server are, by default, UDP ports 27015-27016,” and instructed administrators to open and forward those ports for access from outside the local network.
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That statement describes the 2022 test branch. It should not be treated as the current server’s automatic setting. The modern versioned reference uses a different model: no -port means relay networking, while adding -port explicitly opts into direct connections. An older Linux README for version 0.6.0.3 also documented the relay-versus-direct distinction, so relay mode is not merely a recent workaround.
If you are maintaining an old installation or following a guide written for the experimental test, check the version and startup command before opening 27015-27016. On a current installation, open only the port you deliberately assign.
How relay and direct connections differ
Steam relay mode
Start the server without -port. Core Keeper then uses Steam Datagram Relay networking. Players can use the server’s Steam-based join method, such as a game ID, without connecting to a listening game port on your public address. Because there is no direct game-port listener in this documented mode, you normally do not create an inbound router rule for Core Keeper.
This explains a common symptom: a world works when joined through a game ID, but an IP-address connection fails. The server is operating in relay mode by design; it was never configured to accept direct IP connections.
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Direct mode
Add -port NUMBER to the command that starts the server. Core Keeper then listens for direct traffic on the selected UDP port. The port must be permitted by the operating-system firewall. Players on another network generally also need a router port-forwarding rule to the host’s private address.
Direct mode is useful when you specifically need IP/LAN connectivity or when your hosting environment requires a published UDP endpoint. It also makes the network path your responsibility: the selected port, bind address, firewall rule and router rule must agree.
Change the port on a dedicated server
The port is a startup argument. Change the script, service definition or container command that launches the server, then restart it.
- Find the real startup command. It may be
Launch.baton Windows,launch.shon Linux, a system service, a custom shell command or a container entrypoint. - Add the argument. Append
-port NUMBER, replacingNUMBERwith the UDP port you want, for example-port 7778. - Optionally set the bind address. Add
-ip ADDRESSonly when you need to bind to a particular local interface. The current reference gives0.0.0.0as the default bind address.-iphas no effect unless-portis also set. - Save and restart. Stop the running server completely and start it from the modified command. A running process will not adopt a changed argument until it is restarted.
- Open the same UDP port. Add a host-firewall rule and, for external players, forward that UDP port from the router to the server’s private IP address.
- Give players the direct endpoint. Use the reachable address and the port selected in the startup command, following the connection method provided by your server version.
Windows example
Open the existing Launch.bat file in a text editor. Do not replace the command’s other options; append the port argument to it. A simplified command has this shape:
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CoreKeeperServer.exe -port 7778If the file already contains world, ID or save-location arguments, leave them intact and add -port 7778 at the end. Run the batch file again after saving it.
Linux example
Edit the command in launch.sh, a service unit or your own launcher. Append the argument to the command that starts the server:
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./CoreKeeperServer -port 7778The executable name can differ between packages, so preserve the command supplied by your installation and add only the argument. If you run the server through a service manager, edit the service’s start command and reload/restart that service using its normal procedure.
Do not edit ServerConfig.json for this setting
The current server reference explicitly says that -port may only be set with command-line arguments. Setting a port value in ServerConfig.json has no effect. If the server continues to use relay networking after you edited that file, that behavior is expected; put the argument in the launch command instead.
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Docker: set SERVER_PORT and publish the matching UDP port
For the escapingnetwork/core-keeper-dedicated image, SERVER_PORT has no default. Setting it switches the container to direct-connection behavior. The host-published port and container port must match in the documented Compose pattern:
services:
core-keeper:
image: escapingnetwork/core-keeper-dedicated
environment:
SERVER_PORT: "7778"
SERVER_IP: "0.0.0.0"
ports:
- "7778:7778/udp"Use the port number you chose consistently in both places. The left side of the mapping is the host port; the right side is the container’s UDP port. If you change one side without the other, traffic can reach the host but not the game process.
SERVER_IP is used only when a port is set. Without a port, the documented behavior binds to 0.0.0.0 and keeps relay mode. In a container deployment, also check the host firewall and any cloud or router firewall in front of the Docker host.
Firewall and router setup for direct connections
Host firewall
Create an inbound rule for the exact UDP port in your launch command, such as UDP 7778. A rule for TCP alone does not satisfy a UDP game listener. Restrict the rule to the server application or trusted network ranges when your operating system allows that without blocking intended players.
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Router forwarding
For players outside your home or office network, create a UDP port-forwarding rule from the router’s public interface to the server’s private IPv4 address. The internal destination must be the machine running Core Keeper, and the internal port must match the server’s selected port.
Give the server host a stable private address (for example, a DHCP reservation) before creating the rule. Otherwise the machine may receive a different address later and the forward will point to the wrong device.
LAN-only connections
Players on the same LAN can usually connect to the server’s private address when direct mode is enabled. The current reference uses -port 27015 as a same-LAN example; you may choose another unused UDP port. You still need a host-firewall rule, even when no router forwarding is involved.
Choosing a port safely
Core Keeper does not require you to use 27015 for a current direct server. Pick an unused UDP port that your operating system, router and hosting provider permit, and record it in one place so the launch command, firewall rule, router rule and player instructions all match.
- Use one deliberate port rather than opening a broad UDP range.
- Do not assume a historical pair such as 27015-27016 is required by the current package.
- Check for conflicts with another game server or service on the same host.
- If a hosting provider supplies an assigned port, use that value in
-portand publish/forward the same value. - Changing the port does not replace authentication or access controls; it only changes where direct traffic is accepted.
Troubleshooting by symptom
| Symptom | Likely cause | What to check |
|---|---|---|
| Game ID joining works, IP joining fails | The server is in relay mode because -port is absent |
Add -port NUMBER, restart, then configure firewall and forwarding for direct access |
| Changing ServerConfig.json changed nothing | The port is command-line-only | Put -port NUMBER in the actual startup command |
| LAN clients cannot connect after choosing a port | Host firewall, wrong private IP, or bind-address issue | Confirm the process was restarted, allow the selected UDP port, and verify -ip is not bound to an unreachable interface |
| Internet clients cannot connect but LAN clients can | Missing or incorrect router forwarding, or an upstream firewall | Forward the same UDP port to the server’s stable private IP and check any cloud/ISP firewall |
| Docker clients cannot connect | SERVER_PORT and the published UDP mapping do not match |
Set the same number in SERVER_PORT and HOST:CONTAINER/udp, then recreate/restart the container |
Server appears to ignore -ip |
-ip was supplied without -port |
Set both arguments; the reference says -ip has no effect by itself |
| Players use an old 27015-27016 guide and still fail | The guide targets the 2022 experimental test | Check the installed server version and configure the current launch mode explicitly |
A reliable configuration checklist
- Decide whether players will use Steam relay networking or a direct IP endpoint.
- For relay mode, remove
-portand leave DockerSERVER_PORTempty. - For direct mode, add one explicit
-port NUMBERargument. - Use
-ip ADDRESSonly alongside-port, when a specific local interface is required. - Restart the server after every launch-command change.
- Allow the selected UDP port through the host firewall.
- For outside-LAN players, forward that UDP port to the server host’s stable private address.
- In Docker, publish the identical UDP port and set
SERVER_PORTto that number. - Tell players which connection method the server is configured to accept; a relay game ID and a direct IP endpoint are not interchangeable.
What to remember about Core Keeper ports
On the current dedicated-server package, omission of -port is a meaningful choice: it selects Steam relay networking rather than a hidden numeric default. A direct server has the port you assign, such as 7778, and that UDP port must be reachable through every network layer between the player and the host. The often-quoted UDP 27015-27016 pair belongs to the April 2022 experimental announcement, while 27015 in the current documentation is an example for direct/LAN use. Configure the startup argument—not ServerConfig.json—and keep firewall, router and Docker mappings consistent with it.
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