[2.0.76]Infinite Map Download

[civalion]

Greetings,

I created a Factorio server on my homeserver as Docker Container (image: factoriotools/factorio).
No mods (except Space Age), default difficulty/gameplay.

The Problem:
A friend gets stuck on the map download step. Other friends can join without issue.
The log files are pretty much identical in both cases (verbose logging disabled).

When using a mobile hotspot, therefore skipping his router, he was able to connect. So, the problem must be with his router (Speedport; ISP brand router). Unfortunately this device has a firewall, which can not be configured/disabled. Port forwarding also changed nothing. From what I've found so far this could be because the router firewall possibly interprets the map download as UDP Package Spam or DOS attack. I tried playing around with the package size, since that's easily done in the factorio server config, but so far no luck.

When hosting a game through steam, on the computer I play factorio on (choosing "Host new game" in the menu), connection is no problem, so the issue may also be with the server, but I have no clue what factorio is doing differently when running headless and with UI. At least the way it's currently setup.

Kind Regards

[Loewchen]

You can reduce the maximum map transfer speed to an extremely low value and see if the issue disappears.

[civalion]

I reduced the upload speed in the factorio server settings, but no change.

I find this very weird. When I host factorio on one computer, everything works fine, while on the other, it doesn't, resulting in the described problem.
At the same time, my friend realized his computer is connecting to a wifi repeater, instead of directly to the router. When he connects directly to the router, by unplugging the repeater, everything works fine again.

Now, it's not an option for me to switch host computer, and for him it's not an option to _not_ use the repeater.

The question is, why is factorio behaving so problematic and only in this specific case? And so reliable! No other application hast this issue (yet).
Maybe it's also worth of note, the "issue"-host is running Linux Mint and the "no issue"-host is running Win11.

[Techjar]

Ah, yeah that indicates the issue is with NAT traversal. Some WiFi repeaters create what is called a double NAT, which depending on the configuration can prevent UDP hole punching (the technique Factorio uses to establish connections) from working. As for hosting on different computers changing the behavior, I would hazard a guess that the problematic one has overly strict local firewall settings.

As for the reason why removing only one of the problematic sides fixes the issue, that's because as long as one side can punch a hole through the NAT the other one can just connect and everything works.