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Core Keeper does not have a single numeric default port in the current dedicated-server package. When you omit -port, the server uses Steam relay networking, so clients connect through Steam rather than to a port you forward on your router. For a direct LAN or internet connection, add a launch argument such as -port 7778, then allow that port through the host firewall and, for users outside your LAN, forward it on the router.
The often-quoted UDP ports 27015-27016 came from an April 27, 2022 experimental dedicated-server announcement. They are historical instructions, not proof that every current server listens on that pair.
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What port does a current Core Keeper server use?
In the current Core Keeper dedicated-server reference (version 1.2.1.5, verified September 7, 2026), -port is optional. If you leave it out, the server uses the Steam relay network and does not accept direct connections by IP address. There is therefore no current numeric “default port” to enter into a port-forwarding rule in relay mode.
A numeric port exists only when you explicitly select direct networking. For example:
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CoreKeeperServer.exe -port 7778The number 7778 is an example, not a reserved Core Keeper value. The current reference also shows 27015 as an example for same-LAN connections. Choose an unused port, use the same number in your firewall and router rules, and give direct-connection clients the address and port your setup requires.
At a glance
| Question | Current answer |
|---|---|
| Is there a fixed default port? | No. Omitting -port selects Steam relay networking instead of a fixed direct-listening port. |
| Is Core Keeper server port 27015? | 27015 is a documented same-LAN example. It is not a universal current default. |
Can I change the port in ServerConfig.json? |
No. The current reference says -port may be set only as a command-line argument. |
| Do I need port forwarding? | Not for the documented relay mode. Direct connections from outside your LAN generally require forwarding the selected port. |
Why guides mention UDP 27015–27016
An official Steam announcement dated April 27, 2022, described an experimental dedicated-server test. It stated that the server used UDP ports 27015-27016 by default and that those ports had to be opened and forwarded for connections from outside the local network.
That announcement describes the 2022 testing branch. It should not be copied as the current default without checking the server version and launch method. The current versioned reference uses a different rule: omit -port and the server uses Steam relay networking; set -port when you want direct IP connectivity.
An older Linux README for version 0.6.0.3 also documented the relay-versus-direct distinction. Relay behavior is therefore not merely a recent workaround, while the exposed 27015–27016 pair belongs specifically to the historical test instructions.
Relay mode and direct mode
| Mode | How it is selected | Inbound firewall/router work | How clients connect |
|---|---|---|---|
| Steam relay (SDR) | Omit -port; in the documented Docker image, leave SERVER_PORT empty. |
No inbound game-port forwarding in the documented configuration. | Through Steam relay networking, commonly using the game’s ID or Steam-based invitation flow rather than the host’s public IP. |
| Direct connection | Set -port NUMBER or SERVER_PORT=NUMBER. |
The selected port must be allowed by the host firewall. Internet users generally also need a router/NAT forward. | To the server endpoint directly. -ip can limit the local bind address when -port is set. |
The practical symptom is easy to recognize: a server can work when joined by a game ID but fail when someone tries to enter an IP address. That usually means it is operating in relay mode, where direct IP connections are intentionally disabled.
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How to change the dedicated-server port
Make the change in the command that starts the server, not in the world or server configuration file. You need permission to edit the launch script or service, and the chosen port must not already be occupied by another program.
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- Find the startup command. On Windows this is commonly
Launch.bat; on Linux it may belaunch.sh, a systemd service, a shell script, or a custom container entrypoint. - Add the port argument. Append
-port NUMBER, replacingNUMBERwith your chosen value. For example:CoreKeeperServer.exe -port 7778 - Optionally set the bind address. Add
-ip ADDRESSonly if the server must listen on a particular local interface, such as a specific LAN address. The current reference says-iphas no effect unless-portis also set, and its default bind address is0.0.0.0. - Save the command and restart the server. The argument is read at startup, so an already-running process will continue using its previous networking mode until it is stopped and launched again.
- Open the same port where needed. Allow the selected port in the host firewall. For players outside your LAN, create a router port-forward rule to the server computer’s private IP address.
- Give clients the correct endpoint. LAN players use the host’s private address and selected port when the chosen connection method asks for it. Internet players use the public address and forwarded port, subject to your ISP and router setup.
Windows batch-file example
A minimal batch-file command can look like this:
CoreKeeperServer.exe -port 7778If you need a specific interface, the form is:
CoreKeeperServer.exe -port 7778 -ip 192.168.1.50Use the computer’s actual private address in place of 192.168.1.50. If that address changes later, a fixed DHCP reservation or static address prevents the router rule from pointing at the wrong machine.
Linux or service command example
Put the arguments in the command used by the service or shell script:
./CoreKeeperServer -port 7778For a service manager, edit the service’s startup line, reload the service definition according to your distribution’s procedure, and restart the service. The important detail is that -port must be present in the actual process command line; adding it to an unrelated environment file or world setting will not change the listening port.
Why editing ServerConfig.json does not work
The current server reference explicitly says that -port may be set only with command-line arguments. Setting a port value in ServerConfig.json has no effect. This explains a common failed setup: the file appears to contain the desired number, but the server still behaves as relay-only or continues listening on the old direct port.
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After editing the launch command, verify the process was really restarted with the new argument. If a control panel, shortcut, scheduled task, or container starts the server instead of the file you edited, change the command at that actual launch point.
Opening the port on the host and router
Host firewall
For direct mode, create an inbound rule for the selected game port and the protocol used by the server. The historical official instructions identified the Core Keeper server ports as UDP, and the documented Docker example publishes the game port as UDP. If your operating system asks for a protocol, make the rule match the server package’s current documentation rather than opening every protocol unnecessarily.
- Restrict the rule to the dedicated-server executable when the firewall supports application rules.
- Use the exact selected port instead of opening a broad range.
- Keep the rule on the network profiles where the server is actually reachable.
- After changing the port, remove or disable obsolete rules so they do not create confusion.
Router or NAT forwarding
Router forwarding is needed when the connecting player is outside the host’s local network and the server is in a private address space. Create a rule that maps the chosen external port and protocol to the same port on the server computer’s private IP. The 2022 instructions explicitly required forwarding the listed UDP ports for outside-LAN access; the same principle applies when you choose a different direct port today.
Reserve the server computer’s private IP in DHCP. Without a reservation, the computer may receive a new address after a reboot, leaving the forward aimed at the wrong device. Also check whether the ISP uses carrier-grade NAT: if the router’s WAN address is not the same address visible from the internet, ordinary port forwarding may not be sufficient.
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Test locally first. A successful LAN connection proves that the process, bind address, and host firewall are broadly correct. It does not prove that the router forward or ISP path works. Test internet access from a different network, such as a phone hotspot, rather than from inside the same LAN; some routers do not support hairpin NAT and will make a working server appear unreachable from its own public address.
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Docker configuration
The escapingnetwork/core-keeper-dedicated Docker image uses the SERVER_PORT environment variable for direct mode. The variable has no default. Setting it switches the container to direct-connection behavior; leaving it empty keeps the documented relay behavior.
The example Compose mapping publishes the same UDP port on both sides:
environment:
SERVER_PORT: "7778"
ports:
- "7778:7778/udp"If you select another number, replace both occurrences in the mapping and the environment value. Publishing a host port that differs from SERVER_PORT can make the container listen internally on one number while the router or client reaches another.
SERVER_IP is used only when a port is set. When no direct port is configured, the documented container behavior binds to 0.0.0.0. As with a non-container installation, the Docker host’s firewall and, for internet players, the router still have to permit the selected UDP port.
Troubleshooting by symptom
The game ID works, but an IP address does not
This is expected when -port is omitted. The server is using Steam relay networking and will not accept direct IP connections. Either continue using the Steam-based join method or add -port NUMBER, restart the server, publish or forward that port as appropriate, and provide clients with the direct endpoint.
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LAN players connect, but internet players cannot
Check the host firewall first, then verify that the router forwards the selected port to the server’s current private IP. Confirm that the external test is coming from a different network and that the router’s WAN address is publicly reachable. Carrier-grade NAT, double NAT, or an upstream modem in router mode can prevent inbound connections even when the local rule looks correct.
No one can connect after changing the port
- Confirm the startup command contains
-portand that the process was restarted. - Check that the firewall rule uses the same number and protocol.
- For Docker, compare
SERVER_PORTwith the publishedhost:container/udpmapping. - Make sure another application is not already using the selected port.
- If
-ipis present, verify that the address belongs to an active interface on the host. - Ensure clients are using the new port, not a bookmark or script containing the old one.
The server starts, but direct connections still fail
Look at the launch output or service status to confirm the argument was accepted. A frequent mistake is editing ServerConfig.json while leaving the command unchanged; that file cannot set the port in the current package. Another is setting -ip without -port; the reference says the IP argument has no effect in that combination.
Port 27015 is already in use
There is no requirement to use 27015. Select another unused port, such as 7778, and change the launch argument, firewall rule, router forward, Docker mapping, and client instructions together. The historical 27015–27016 pair should not force your modern installation into a conflict.
Choosing a port safely
Pick a stable, unused port that is easy to document for your group. Avoid selecting a port already assigned to another service on the same host, and do not expose unrelated administrative services just because a number is available. The game port should be the only new inbound rule required for the dedicated server.
Keep the server computer’s private address stable, record the selected number beside the startup command, and remove stale forwards when migrating from an older setup. If you later return to relay mode, remove -port, clear SERVER_PORT in Docker, stop forwarding the game port, and restart the server.
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For current Core Keeper dedicated servers, “default port” is a networking-mode question. No -port means Steam relay networking and no fixed numeric port to forward. Adding -port NUMBER enables direct connections; that number becomes the port you must allow on the host, publish in Docker if applicable, and forward for players outside the LAN. The 27015–27016 figures are historical 2022 test instructions, while 27015 in current documentation is an example rather than a universal default.
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