The Tool Desk
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Quick diagnosis for a Factorio server that will not connect
Use the symptom that best matches what you see, then apply the checks in order. The fixes below are arranged from the most likely cause to the least common.
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|---|---|---|
| Factorio server not responding | Version/mod mismatch, blocked UDP, or the process never started | Compare versions, check mods, then inspect the server log and UDP firewall rules |
| Factorio couldn’t establish network communication with server | UDP port is unreachable or a router/firewall is dropping packets | Confirm the configured port, forward UDP on the router, and allow it through the host firewall |
| Unable to connect to dedicated server | Server is offline, loading a save incorrectly, or using incompatible content | Verify the process is running and that a save loaded successfully before testing from a client |
| Factorio server not showing in browser | Matchmaking credentials, verification settings, or blocked outbound traffic | Check server-settings.json, account credentials, and whether direct IP connection works |
| Factorio desync | Different or faulty mods, or a mod version changed on one side | Synchronize mods with the server and disable the suspected mod for a controlled test |
Factorio server not responding: check version parity first
The client and server must run the same Factorio version. A 2.0.x client cannot join a 2.1.x server, even if the server address and password are correct. This is the highest-value check because it takes seconds and prevents you from troubleshooting the wrong layer.
Check the client version
Read the version displayed on the client’s main menu. Record the complete number, including the major and minor parts, rather than relying on a label such as “latest.”
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Check the dedicated server version
Run the server executable with --version. Compare the output character-for-character with the client’s version. If they differ, update or roll back one installation so both use the same release line. Do not treat a January 2025 Steam community guide’s 2.0.77 reference as the current stable version; that date-specific information does not establish the version available in October 2026.
After an update, restart cleanly
Stop the old server process before replacing its files, then start the intended executable again. A service manager or startup script can otherwise keep launching an older binary while you inspect the newer directory.
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Every player and the server need identical mods and identical mod versions. A game can appear to connect and then disconnect with a desync when one side has different content. Mod parity is separate from base-game version parity: both must be correct.
Synchronize from the server browser
- Open the multiplayer server browser.
- Select the server entry.
- Choose Sync mods with server when the option is offered.
- Allow the download and restart the game if Factorio requests it.
- Join again only after the mod list and versions match.
Isolate a bad mod
If synchronization succeeds but the game still desyncs, record the mod named in the disconnect or desync details. Stop the server, remove or disable that mod on the server and clients, and run a short test on a copy of the save. Re-enable other mods one at a time only after the test is stable. This identifies a faulty or incompatible mod without risking the main save.
Headless servers that cannot download mods
The official Multiplayer instructions state that a headless server needs service-username and service-token in player-data.json when it must download mods. Without those credentials, the server may start without the required files or fail while preparing the save. Supply the credentials, restart, and watch the log for download or authentication errors.
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Factorio couldn’t establish network communication with server
Factorio server traffic uses UDP, not TCP. A TCP rule that appears open does not make the game reachable. The default game port is commonly cited as UDP 34197 by third-party hosting documentation; if your configuration specifies another port, use that value instead. The official command-line documentation exposes the port through the --port option but does not establish a universal default in the material available here.
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Confirm which port the process uses
- Inspect the startup command or service definition for
--port. - If no option is present, check the server’s effective configuration and the hosting panel rather than assuming a value.
- Use the same UDP port in the router rule, host firewall rule, and client address.
Forward UDP 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.
- Save the router configuration and restart the server process.
- Test from outside your home network, such as a phone on cellular data. A test from the same LAN can succeed even when internet forwarding is broken.
Allow the port through the host firewall
Create an inbound rule for UDP on the configured port and allow the Factorio executable. Antivirus suites can also block the process without a prominent warning. Temporarily disabling a security product can be a diagnostic test, but restore it and add a narrow allow rule instead of leaving protection off.
On Linux, restrictive ufw rules have been reported by VPS users as the cause of this error. Do not remove the firewall permanently: re-enable it and allow only the required UDP port and administration access.
Separate router, ISP, and server faults
Restart the router and repeat the external test. If a mobile-hotspot test works while the normal connection does not, the router or ISP path is implicated. If neither works, return to the server process, firewall, and port configuration. Hosted servers generally remove the home-router step, but their provider firewall still needs the UDP rule.
Unable to connect to dedicated server: verify startup and the save
A reachable port is not enough if the process failed before opening the game. Read the server log or console output before changing files. Startup errors, malformed configuration, missing saves, and mod failures are recorded there.
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Use the supported headless startup options
Headless mode starts from a save, has no in-game character, and pauses when nobody is connected. The official Multiplayer instructions document --start-server-load-latest for loading the newest save and --server-settings <path> for selecting a settings file. A typical command therefore has the form:
factorio --start-server-load-latest --server-settings /path/to/server-settings.jsonUse your actual executable and save location. If the command exits immediately, copy the first error from the console and correct that item before testing the network.
Confirm that a save loaded
Do not troubleshoot clients until the log shows that the server started and loaded a save. If no save exists, create one with the server’s supported save-generation workflow, then restart with the resulting file. A process that is running but has no loaded game will not behave like a joinable server.
Linux binary errors
A forum report describes a Linux “203” launch error resolved with chmod +x on the binary, but that report is not an official Factorio diagnosis. Check executable permissions and architecture, then obtain a fresh headless download from the official Factorio site if the file may be incomplete or corrupted. Preserve the existing saves and settings before replacing binaries.
Factorio server not showing in browser
Browser listing depends on matchmaking and account settings in addition to local networking. A server can be joinable by direct address yet absent from the public list.
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Check verification requirements
In the official Matchmaking API, require_user_verification is true when a server requires joining users to be logged in. Players must then have a linked Factorio account. If you do not intend to require account login, review that setting in server-settings.json; if you do require it, make sure every player completes the account link before testing.
Set public-listing credentials
For a public listing, the server username and token must be present in the server settings. The token is obtained from the owner’s factorio.com profile. A missing, expired, or mistyped token can prevent registration even when direct connections work. Keep the token private and do not paste it into a public issue or chat.
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Clear out junk files and repair common Windows errorsFree Scan →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Use direct connection as a control test
Try the server’s address and port directly. If direct connection works, focus on matchmaking credentials, verification, and outbound firewall rules. If direct connection fails too, return to version, mods, process startup, and UDP reachability rather than changing listing settings.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.server-settings.json authentication error and require_user_verification
Authentication failures are configuration and account problems, not evidence that the game binary is damaged. Edit the copy of the example settings file, not the packaged template: the official setup instructions say to copy data/server-settings.example.json from the Factorio application directory and edit the copy.
Validate the settings file
- Ensure the file is valid JSON: matching braces, quoted property names, and no trailing comma.
- Start the server with the exact file path passed to
--server-settings. - Check the log for the first parse or authentication error after restarting.
- Confirm that usernames, tokens, and verification values belong to the intended Factorio account.
Understand the login requirement
When require_user_verification is enabled, a player who is not logged in or whose account is not linked will be refused. Resolve the account link on the client, then reconnect. Changing the setting while the server is running has no effect until the server reloads its configuration.
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Do not confuse a password with account verification
A game password controls access to the server; verification controls whether joining users must authenticate with Factorio. Diagnose the two independently. Enter the configured password exactly as supplied by the administrator. A case-sensitivity warning appeared in third-party search material but was not verified against an official source, so treat any claim about special password casing as unconfirmed.
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Game password and rejected joins
First establish that the server is reachable and that versions and mods match. Only then investigate the password. Read the value currently loaded from server-settings.json, remove accidental leading or trailing spaces, and have the player re-enter it manually rather than pasting a value that may include a newline. If the administrator changed the file, restart the server so the new value is active.
Self-hosted versus rented Factorio servers
The troubleshooting order is the same, but ownership of each layer differs.
| Layer | Home self-hosting | Rented hosting |
|---|---|---|
| Factorio version and mods | You install, update, and preserve matching files | The provider may offer installers or panels, but you still must keep clients synchronized |
| UDP port | You configure router forwarding and the computer’s firewall | The provider normally supplies the public port; you must enable it in the provider firewall or panel |
| Files and logs | Full access to executable, saves, settings, and logs | Access depends on the panel, file manager, or support policy |
| Internet path | Your router and ISP can introduce UDP timeouts | The provider’s network and firewall are the relevant external path |
If a hosted instance fails, ask the provider to confirm that the allocated UDP port is open and that the selected image has the intended Factorio version. Do not assume a provider’s TCP test represents game connectivity.
A repeatable troubleshooting procedure
- Record the exact symptom. Note whether the server is absent from the browser, refuses a direct connection, disconnects during loading, or desyncs after joining.
- Read the server log. Capture the first error after startup; later messages may only be consequences.
- Compare versions. Check the client menu and run the server with
--version. - Compare mods. Use Sync mods with server, then test without a suspect mod if a desync remains.
- Confirm process and save. Start with the intended executable and settings path, and verify that a save loaded.
- Verify UDP reachability. Match the configured port in the server, router, host firewall, and client address.
- Test from another network. A mobile hotspot distinguishes local-LAN success from internet failure.
- Check authentication. Review
require_user_verification, linked accounts, server username, and token. - Retest after one change. Change one variable at a time and keep the successful setting, so you know which fix mattered.
What not to change blindly
- Do not forward every port or disable the firewall permanently; Factorio needs the configured UDP port.
- Do not replace the server binary before backing up saves, mods, and settings.
- Do not assume a browser-listing problem means the game is offline; direct connection can prove otherwise.
- Do not copy a historical version number into an update script without checking the release you intend to run.
- Do not edit the example JSON in place if the server expects a separate settings file.
When to escalate
Contact a host or community support channel with a minimal, useful report: Factorio client and server versions, operating system, whether the server is self-hosted or rented, the configured UDP port, the exact client error, the first relevant log error, and whether direct connection from a different network worked. Remove passwords, account tokens, and private IP details before sharing. This information lets others distinguish compatibility, routing, authentication, and startup faults without guessing.
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