What about setting up some honeypot open-with-no-IP wifi hotspots near google HQ and near places where googlers are known to congregate (or in residential areas with high density of googlers working from home). Once it's their problem, they'll fix it pronto.

I lack a solution but would like to commiserate. We had some config errors on one of our VLANs, where DNS want reachable. The phones helpfully would just say no internet access and then bounce to 4g without any other sorry of error. What I would give to be able to just SSH into all these consumer services and just grep through the logs.

See https://github.com/leoleozhu/android-captive-portal and https://success.tanaza.com/s/article/How-Automatic-Detection...

If you could return valid internal DNS entries for these domains, and serve up a 204 to the right requests, you should be able to stay online.

You need to ensure you don't do it for every possible domain - I believe Chrome also tests random non-existent domains too, to ensure it gets back the expected error.

I have a "captive" or "promiscuous" dns server (it responds with its own IP for all requests) for my esp projects so this info is exactly what I need. Thanks a lot!

In my experience, I had no network until I answered "no internet connection, disconnect?" dialog that pops up in that case. Maybe you could also say "don't ask again". Then it was working fine.

What is not working fine though, is that I cannot access the internet through the mobile network at the same time. But this seems to really depend on the phone: it works on iPhones and on my previous Android phone, but not on a Pixel 5. I would like to download updates for the esp32 firmware through the mobile network, but this prevents it.

Chromecasts get initially configured by connecting your phone to a wifi network hosted by the device, so I wonder how Google makes that work.