captive portal
termnetworking
The page a network forces a new client to see before granting access, implemented by intercepting the first requests.
It works by lying: the network answers DNS and HTTP for names it does not own so the browser lands somewhere else. That is indistinguishable from an attack, which is precisely why it breaks more every year - certificate validation, encrypted DNS and pinned applications all refuse the interception on purpose. Operating systems now probe a known address to detect the situation and open a restricted window, which is the industry admitting the technique is a hack it has agreed to accommodate.
Also known as: hotspot login