There's no standard protocol in widespread use. Typically operating systems attempt to fetch a known resource when connecting to a new Wi-Fi network, and if it redirects, then they treat the destination as the captive portal sign-in page and display it with special chrome / modally. The URL each platform attempts to load varies by vendor and version.

Do you have any links to resources or know what name(s) people use when talking about this strategy? I'd be interested to know which URL Debian attempts to fetch, but I'm not sure how I'd even begin to search for the answer.

"Captive portal detection"

And here's your answer: http://network-test.debian.org/nm

Thanks!

cf. http://blog.erratasec.com/2010/09/apples-secret-wispr-reques...

My understanding is that this is just the OS trying to fetch a known page over HTTP and, if redirected, assuming it got a login portal and presenting it to the user.

I assume this can continue "working" (with your noted caveat that it doesn't even always work today) forever, as the process that checks doesn't need to understand HSTS or to ever try HTTPS, regardless of what actual web browsers start doing.

Basically, they try to phone home and if it redirects, they assume it's a captive portal login page. It's a sort of controversial practice, because it means that the OS vendor now always knows where and when you're accessing the internet.

On Nougat it seems to work flawlessly most of the time. I wonder what the protocol is. Maybe a request to the access point's DNS server to see what IP is returned and then connect to that?