Most Factorio connection failures come from four checks: the client and server must run the same game version, every player must have the same mod set, the server’s UDP port must be reachable, and the server must finish starting without an error. Check those in that order, then use the server log to confirm what failed. The steps below cover “factorio server not responding,” “couldn’t establish network communication with server,” dedicated-server connection failures, missing browser listings, authentication errors, and desyncs.
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Start with the symptom and the fastest test
Record the exact message and whether the failure affects everyone or only one player. A server that appears online but rejects one player usually has a version, mod, password, or account-verification problem. A server that nobody can reach usually has a startup, firewall, router, or port problem.
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- Only one player fails: compare that player’s Factorio version, mods, login state, and password entry with a player who can join.
- Everyone fails from outside the host network: test the server locally, then inspect UDP forwarding and firewall rules.
- The process exits or never creates a usable game: read the startup log before changing network settings.
- The server is absent from the browser: verify listing credentials and network reachability separately; a server can be reachable by direct address without appearing in the public list.
Do not change several variables at once. Apply one fix, restart the server if required, and test again so the successful change is identifiable.
“Factorio server not responding”: verify version parity first
Client and server builds must match. A 2.0.x client cannot join a 2.1.x server, and a patch-level mismatch can also prevent a connection. The stable release changes over time, so do not use an old guide’s version number as a target.
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Check the client version
The client’s main menu shows the installed version. Ask every player to compare that number with the host’s version, including the major and minor portions.
Check the dedicated server version
Run the server executable with the version option:
factorio --versionUse the equivalent path to the executable if it is not on your shell’s PATH. Update either side until the versions are identical, then restart the server and try again. If you use an automatic updater, confirm that it did not update the client while leaving a manually installed server behind.
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Every joining client needs the same mods, at the same versions, as the server. A player can often select Sync mods with server in the multiplayer browser to obtain the server’s set. If synchronization fails, download or copy the required mod versions through the normal Factorio mod workflow, then reconnect.
Recognize a mod mismatch
- The connection reaches the server but fails during joining or loading.
- The game reports a mod mismatch or produces a desync after a map loads.
- Only players with a different mod collection are rejected.
Disable one suspected mod on a test copy of the server and connect again. Do not remove a mod from a live save without a backup: removing content can change entities or prevent the save from loading. If the server starts with no mods but fails with the full set, add mods back in small groups until the offending mod or version is isolated.
Fix “unable to connect to dedicated server” by testing the UDP path
Factorio multiplayer traffic uses UDP, not TCP. The default game port is commonly cited as UDP 34197 by third-party hosting documentation, while the official command-line documentation confirms that the port is configurable with --port without establishing that default value. Use the actual port configured on your server rather than assuming a default.
Confirm the port the server is using
Look at the launch command or service definition for an option such as:
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Open the host firewall for UDP
Create an inbound rule for the server’s UDP port. For a Linux host using UFW, the pattern is:
sudo ufw allow <PORT>/udpUse your operating system’s firewall interface or rule syntax on other systems. Antivirus or endpoint-security software can silently block the executable; temporarily test with that filter disabled only long enough to confirm the cause, then create a narrow allow rule and re-enable protection.
Forward the port on a home router
- Give the server computer a stable local address, preferably with a DHCP reservation.
- Create a port-forward rule for the Factorio UDP port to that local address.
- Ensure the router rule is UDP-only unless your network documentation requires another setting.
- Restart the Factorio server and test from a different network, such as a phone hotspot.
A local test can succeed even when internet players cannot connect, because local traffic may bypass the router’s inbound rules. Conversely, a hotspot test that works when the home connection fails points toward the router or ISP rather than the Factorio installation. Some routers handle long-lived UDP sessions poorly; restarting the router can clear a stale mapping, but it is not a substitute for a correct forward and firewall rule.
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Why a Factorio server is not showing in the browser
Browser visibility and direct connectivity are separate. First try a direct connection using the server address and port. If direct connection works, focus on listing and authentication settings; if it fails too, return to the UDP checks.
Set listing credentials
For a public listing, the server settings need a Factorio account username and token. The token is obtained from the account profile at factorio.com. Enter the values in the server settings used at startup, not in an unused example file. Keep the token private and replace it if it has been exposed.
Check the verification requirement
The official Matchmaking API describes require_user_verification as true when a server requires users to be logged in to join. Players must therefore link and sign in to a Factorio account before joining a server with that requirement enabled. If guests should be allowed, review that setting deliberately rather than treating an authentication error as a network failure.
Allow time for listing and avoid false negatives
Restart after changing credentials, wait for the server to complete startup, and search using the exact server name or region filters. A private server, a server that cannot reach the matchmaking service, or one blocked by an outbound firewall may not appear even though its process is running.
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“server-settings.json authentication error”: validate the files and account
The settings file controls password, visibility, credentials, and verification behavior. The official Factorio multiplayer guidance recommends creating it by copying data/server-settings.example.json from the Factorio application directory and editing the copy. Do not edit the example in place if an update may overwrite it.
Use the file you think you are using
- Copy
data/server-settings.example.jsonto a separate server-settings file. - Edit that copy with valid JSON syntax and the intended values.
- Start the server with an explicit path:
factorio --server-settings <path-to-server-settings.json>Check the launch log for the resolved settings path. A common mistake is editing one file while a service or script starts the server with another.
Resolve account and token errors
- Confirm the username belongs to the Factorio account used to obtain the token.
- Regenerate or recopy the token if it was revoked, truncated, or surrounded by accidental whitespace.
- Make sure the host can make outbound connections to Factorio’s matchmaking service.
- Decide whether
require_user_verificationshould be enabled, and tell players that account login is mandatory when it is.
If the server is intended to run without public matchmaking, remove unnecessary listing credentials and test a direct connection instead of debugging a listing that you do not need.
Game-password failures
Enter the password exactly as it is stored in server-settings.json. Verify spelling, punctuation, spaces, and which settings file the server loaded. A case-sensitivity claim appeared in an unverified search summary, so treat case behavior as unconfirmed rather than relying on it; test the exact value and, if necessary, set a temporary simple password to isolate the issue. Change that temporary password after testing.
Read the server log before changing the installation
The server log is the first diagnostic tool. It records configuration parsing errors, port-binding failures, authentication problems, mod download errors, save loading failures, and shutdown causes.
What to look for
- JSON or settings errors: correct the named file, key, comma, quote, or value.
- Address already in use: another Factorio process or service is using the configured port; stop it or choose a different port and forward that port.
- Mod download or authentication errors: verify account credentials, outbound access, and the mod’s availability.
- Save or prototype errors: restore a known-good save or remove the newly added mod from a backup copy.
- Clean startup followed by no players: the process may be healthy; continue with client, listing, and UDP tests.
Do not infer success merely because a process remains visible. A usable headless server should finish loading a save and continue listening on its configured port.
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Headless startup, saves, and mod downloads
Headless mode starts from a save, has no in-game character, and pauses when nobody is connected. It is therefore normal for simulation activity to stop while the server is empty; that is not the same as a crashed server.
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The official command-line options include:
factorio --start-server-load-latest --server-settings <path-to-server-settings.json>Use a specific save option instead when you need a named file. Confirm in the log that the save loaded successfully before asking players to connect.
Permit headless mod downloads
The official multiplayer documentation states that a headless server needs service-username and service-token in player-data.json when it must download mods. These credentials are separate from the settings file’s public-listing values. If a server fails only while obtaining mods, check those fields, outbound connectivity, and available disk space.
Linux launch and binary problems
A reported “203” execution error
A Linux forum report attributed an error code “203” to a binary without execute permission and suggested:
chmod +x <factorio-binary>This is a community report, not a verified official fix. Check ownership, permissions, architecture, and the executable path first; apply the permission change only to a binary you trust and obtained from the official distribution.
Suspected corrupted download
If the binary is truncated, the process exits immediately, or the log shows unreadable files, compare the download size and checksum when available and download a fresh copy manually from the official Factorio site. Preserve the save and configuration directories before replacing the program files.
Self-hosted machine versus rented host
The troubleshooting boundary depends on who controls the network:
| Area | Home or self-hosted machine | Rented hosted server |
|---|---|---|
| Router forwarding | You must reserve a local address and forward the UDP port. | The provider normally supplies the public endpoint and port mapping. |
| Firewall | You configure the operating-system firewall and any security suite. | The provider controls its network firewall; you may still need a panel rule. |
| Files and logs | Full access to binaries, saves, settings, and logs. | Access depends on the provider’s panel, file manager, or support policy. |
| Best first test | Connect locally, then test from a hotspot. | Check the panel’s allocated port, service status, and exposed logs. |
Hosting documentation is often written for that provider’s panel and may not apply to a self-hosted installation. Translate every instruction into the equivalent control you actually have.
Factorio desync: isolate the mod or state difference
A desync means a client and server no longer calculate the same game state. Mod mismatches are a common cause, so establish exact version and mod parity before investigating anything more exotic.
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- Have every player synchronize mods from the server.
- Record the first moment the desync occurs and what action preceded it.
- Test with a backup save and the suspected mod disabled.
- If the desync disappears, restore mods one at a time or in small groups.
- Keep the working mod versions pinned for all players and the server.
Do not overwrite the only copy of a save while testing. A reproducible desync with identical versions and mods should be documented with the relevant log details before seeking help.
Quick Recap
A practical decision tree
- Can the server process stay running and load a save? If no, fix the log-reported startup, file, save, or mod error.
- Do client and server versions match exactly? If no, update or roll back one side.
- Do all clients have identical mods and versions? If no, synchronize them and retest.
- Can a player on the host’s local network connect? If no, inspect the server port, host firewall, and launch settings.
- Can a player on another network connect? If no, inspect router forwarding, ISP behavior, and external firewall rules.
- Does direct connection work but the browser not list the server? Check listing username, token, visibility, outbound access, and verification requirements.
- Does only one player fail? Compare that player’s login, mods, version, and password with a working player.
What not to assume
- Do not assume a version number from an old guide is still the current stable release.
- Do not open a TCP rule when the relevant Factorio traffic is UDP.
- Do not edit the example settings file and assume the running service uses it.
- Do not treat a missing browser listing as proof that the server port is closed.
- Do not remove mods from a production save without a backup.
- Do not leave the firewall disabled after a test; create the narrow UDP rule that fixes the problem.
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