How does windows check if I have a good internet connection?
Its NCSI (Network Connectivity Status Indicator) checks that:
dns.msftncsi.com resolves to 126.96.36.199 using DNS
http://www.msftncsi.com/ncsi.txt contains exactly: Microsoft NCSI
with no newlines or other non-printing characters.
Apple devices use
Which returns "Success" in some HTML.
Another old faithful is