Windows 10 – Fix WiFi Only Working with Low MTU

google-wifiinternet connectionnetworkingwindows 10wsl2

Got a mystifying issue with my WiFi. It suddenly stopped working and after much trial and error I got it work by lowering the MTU to 316.

netsh interface ipv4 set subinterface "Wi-Fi" mtu=316

Anything higher than that just hangs and end up with a ERR_CONNECTION_CLOSED error.

How the issue started

  • I was uploading a big file to the cloud from WSL and then the internet suddenly stopped working permanently. Not only in WSL but also in Windows.
  • Then I connected to the guest WiFi in the same router (it's a Google Wifi) and it worked fine for weeks. But then one day I uploaded another big file from WSL the same way as before and the guest WiFi stopped working permanently too. The first time I though it was a coincidence, but twice…

What I have tried

  • Tried using a WiFi USB adapter, and it has the same issue. So this rules out hardware/driver problems.
  • It works fine (mtu=1500) if I connect my phone to the Google Wifi and create a WiFi hotspot, and connect the computer to that WiFi hotspot (basically using the phone to forward the connection), so the issue only happens when I connect directly to that WiFi network.
  • There are other computers and devices in the same WiFi network and they work fine.
  • Tried factory reset the router, changing the password of the WiFi, changing the ssid of the WiFi, changing the name of the computer, using a random MAC address (just in case the AI in Google Wifi was doing something funny with this computer for some reason).
  • Tried using a VPN.
  • Tried shutdown WSL, and also uninstalled it completely, because why not.
  • Tried everything on this page, including "Network Reset". Also tried deleting WiFi network profiles netsh wlan delete profile name=*.

I ran out of ideas of what to try, besides reinstalling Windows from scratch. Also I can't really 100% rule out if the problem is in Windows or in the Google Wifi router, though I'm leaning towards a Windows issue.

Any suggestions?

Best Answer

Finally solved the issue.

It was caused by a MSI motherboard bloatware called "cFosSpeed". It affected all network adapters, not only the one in the motherboard.

My theory is that somehow it associated a bad configuration to the Google Wifi gateway IP so that only that specific network got broken, explaining the weird symptoms in the question above.

Anyway, removing it solved the problem. Also just stopping the service didn't work, I had to completely purge it.

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