Most Factorio connection failures come from one of four mismatches: the game or mods differ, the wrong join method or address is being used, UDP port 34197 (or the server’s changed port) is not reachable, or NAT/firewall rules block the host. Check those in that order, then use the server-type steps below.
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Start with the symptom that matches your situation
Use this quick triage before changing settings. It prevents you from troubleshooting the router when the actual problem is a different mod set or an incorrectly started server.
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|---|---|---|
| Nobody can join | Server process, game setup, or host network | Confirm the server is running, versions and mods match, then verify the configured UDP port, router forwarding, and host firewall. |
| LAN players join, Internet players cannot | Public reachability | Check the host’s public IP, UDP forwarding, firewall policy, and whether a Steam or virtual-LAN path works. |
| Only one player cannot join | That player’s client or local network | Compare the exact game version and mods, Steam status, local firewall/antivirus permissions, and the address and port entered. |
| The server appears in a list but joining fails | Discovery succeeded; the connection did not | Treat listing and reachability as separate tests. NAT traversal or UDP acceptance can still fail after a listing is visible. |
| Only a dedicated server fails | Startup or bind configuration | Start it with a save, inspect its --port and --bind settings, and test with --mp-connect. |
1. Make every game instance identical
Factorio requires every participant and the server to run exactly the same game version and the same mods. This is a connection requirement, not a performance tweak. A player can be unable to join even when the server is online and its address is correct if one installation has a different release, a different mod version, an extra mod, or a missing mod.
Check the game version
- On the server host, note the version shown by the Factorio launcher or main menu.
- Have each player check the same location on their own installation.
- Update or switch the outlier so all installations display the identical version before trying again.
Do not assume that an automatic update affected every installation at the same time. A dedicated server may be using a separate installation from the host’s desktop copy, so check that executable too.
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Check the complete mod set
- Compare the enabled-mod list on the server with each player’s list.
- Match both membership and versions; “the same mods” means no extras, omissions, or different releases.
- Apply the same changes to the server and all clients, then restart the game or server before reconnecting.
If only one friend fails while everyone else joins, begin here. It is the fastest way to separate a client mismatch from an Internet routing problem.
2. Pick the least fragile connection method
Factorio supports Steam invites, the public server browser, Play on LAN, and direct IP:port joining. Choose the method that matches the network rather than forcing direct addressing when a simpler route is available.
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| Steam invite | Everyone is using Steam and the host can invite friends | Steam running and friends available through the overlay or invite flow | It does not remove version/mod mismatches or a host that is not actually serving a game. |
| Public server browser | The server is configured to advertise publicly | A running server and a reachable Internet connection | A visible listing does not prove that the final UDP connection will succeed. |
| Play on LAN | All players are on the same local network | Each device connected to the same LAN | Players outside that LAN generally cannot use the private address. |
| Direct IP:port | You know the server address and its configured port | The host’s public address for Internet play, or a private LAN address for local play | Internet hosts need UDP reachability, forwarding, and a firewall rule. |
| Virtual LAN | The host cannot configure router forwarding | A virtual-LAN tool installed and joined by every player | It adds another software network layer and must be configured consistently. |
Factorio’s client-server mode relays multiplayer traffic through the server. That allows LAN and outside players to participate in the same game, although the relay path can add an extra network hop.
3. Joining over a local network
When everyone is physically or virtually on the same LAN, use Play on LAN or the host’s private IP rather than the public Internet address. The local path avoids most router NAT complications.
LAN checklist
- Confirm every device is connected to the same home, office, or virtual-LAN network.
- Start the multiplayer game on the host and leave it running while the client searches.
- Use Play on LAN first. If it does not appear, use direct IP:port with the host’s private address and configured UDP port.
- Check that the host firewall permits Factorio’s UDP traffic on the active network profile.
- Do not use a guest Wi-Fi network that isolates wireless clients from one another.
If LAN players can join but a player on the Internet cannot, stop changing LAN settings. The remaining fault is almost certainly public reachability, NAT, port forwarding, or an Internet-facing firewall rule.
4. Hosting for Internet players: expose the correct UDP port
Factorio uses UDP only. The default server port is UDP 34197, although the server can be configured to use another port. The number in the router rule, server configuration, and join address must be the same.
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Find the port the server actually uses
- Check the server launch configuration or command line for
--port. - If no custom port is specified, use 34197.
- Tell clients to enter the host’s public IP followed by that port, for example
PUBLIC_IP:34197. Replace the example with the actual address and port; do not enter the router’s private address for an Internet player.
Create the router forwarding rule
- Give the hosting computer a stable private address, either with a reserved DHCP lease or a static configuration.
- Open the router’s port-forwarding or virtual-server page.
- Forward the server’s UDP port (34197 by default) to that computer’s private IP address. Select UDP, not only TCP.
- Save the rule and restart the server if the router requires it.
Forwarding the wrong protocol, forwarding to an old private IP, or forwarding a different port than the server listens on produces the same symptom as a closed port: clients time out or cannot establish network communication.
Allow the traffic through the host
Create an inbound firewall exception for Factorio or for the configured UDP port on the host computer. Check both the operating-system firewall and any security suite that filters network traffic. If an antivirus or firewall product blocks UDP, the router can be configured correctly and the connection will still fail.
After changing a rule, test from a player who is not on the host’s LAN. Testing the public address from inside the same network can be affected by the router’s hairpin-NAT behavior and is not a reliable substitute for an outside test.
5. Dedicated or headless server checks
A headless server runs without Factorio’s graphical interface. A process can exist without actually loading a save or listening on the address you expect, so verify startup arguments before debugging the router.
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Launch the dedicated process with the --start-server SAVE option, replacing SAVE with the save file path or name used by your installation. Starting without a save can leave you with a running process that is not serving the intended game.
Verify port and bind settings
- Inspect the command line or configuration for
--port. If absent, the default is UDP 34197. - Inspect
--bind. A bind setting that restricts the process to the wrong interface can make it unreachable from LAN or Internet clients. - Confirm the router forwards the same port to the machine running the headless process.
- Review the server log for startup errors, save-loading failures, or messages indicating that the port is already in use.
Test with --mp-connect
Use Factorio’s --mp-connect ADDRESS option to test a client connection to a specific address. Supply the server’s private address for a LAN test or its public address and port for an external test. A successful LAN test followed by an external failure points away from the Factorio process and toward forwarding, NAT, or the host firewall.
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6. Understand NAT traversal and virtual-LAN workarounds
A server behind NAT needs a publicly reachable address for ordinary Internet clients unless everyone is on the same LAN. If you cannot configure the router, a virtual-LAN tool such as Hamachi can place all participating computers on a shared virtual network; each player then joins using the host’s virtual-LAN address.
Why a listed server can still reject a connection
Factorio’s matching-server process can help peers discover one another and attempts NAT punching with keepalives. The developers reported an expected success rate of about 70% under the conditions they tested in 2016. That historical figure is not a current universal reliability guarantee. A server can therefore appear in a list while the final UDP path still fails because the particular NAT devices do not permit the required packets.
When to choose a virtual LAN
- The host cannot access the router or the Internet provider uses restrictive NAT.
- Port forwarding is unavailable, unreliable, or blocked by policy.
- All players can install and join the same virtual network.
Use a virtual-LAN path consistently: every player must be connected to the same virtual network, and the host firewall must allow the virtual adapter’s traffic. Do not combine a private LAN address, public address, and virtual address at random; choose the address belonging to the method you are testing.
7. Diagnose the exact failure message
“Couldn’t establish network communication with server”
- Recheck exact version and mod parity.
- Confirm the server is still running and has loaded the save.
- Verify the address and port, especially if the server uses a non-default
--port. - For Internet play, confirm UDP forwarding and the host firewall.
Connection timeout
A timeout usually means packets are not reaching the server or replies are not returning. Check public-versus-private addressing, router forwarding, double NAT, and firewall rules. If LAN works, focus on the public boundary rather than reinstalling the game.
Server not showing up
Check that the server is advertising through the method you selected and that it is not bound to the wrong interface. Use direct IP:port as an independent test. If direct joining works but the listing does not, the game is reachable and the remaining issue is discovery or advertisement.
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Only one friend is rejected
Have that player compare the displayed game version, enabled mods and mod versions, Steam connectivity, local firewall permissions, and the exact address and port. If other clients can join the same server, changing the router is unlikely to help that one player.
8. A clean order for retesting
- Stop the server and close Factorio on every client.
- Align the game version and complete mod set.
- Start the server with the intended save and record its port and bind settings.
- Test from a client on the same LAN using Play on LAN or the private IP.
- If LAN succeeds, test from outside the LAN using the public IP and forwarded UDP port.
- If forwarding is impossible, connect everyone through a virtual-LAN method and test the virtual address.
- After each change, test one variable at a time so the successful fix is identifiable.
This sequence separates game compatibility, server startup, local reachability, and public reachability. It also prevents a server-listing result from being mistaken for proof that the UDP session itself works.
Quick Recap
Common mistakes that look like network failures
- Using TCP forwarding: Factorio uses UDP only, so a TCP-only rule does not expose the game.
- Forwarding 34197 when the server changed its port: The router and join address must follow the actual
--portvalue. - Forwarding to a changing private IP: A DHCP change can silently send traffic to the wrong computer.
- Entering a private address from the Internet: Internet players need the host’s public address unless they are on the same LAN or virtual LAN.
- Assuming discovery equals connectivity: NAT punching and UDP acceptance can fail after a server is listed.
- Ignoring the dedicated process: A headless executable must be started with a save and checked for the expected bind and port.
- Changing several layers at once: Align the game, test LAN, then test the Internet path so each result narrows the cause.
Final checklist
- Every player and the server show exactly the same Factorio version.
- Every player and the server have exactly the same mods and mod versions.
- The selected join method matches the network: Steam, browser, LAN, direct IP:port, or virtual LAN.
- The server is running the intended save.
- The configured port is known; UDP 34197 is the default.
- The router forwards that UDP port to the correct, stable private IP.
- The host firewall and security software permit the traffic.
- A dedicated server’s
--binddoes not exclude the interface clients use. - External tests use the public address; LAN tests use the private address.
- A listed server is tested separately for actual connection success.
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