Using an Internet cafe in Bayern made me discover a new software: « Patronsoft FirstSpot(tm) »: access point: « a Windows-based Wi-Fi Hotspot Management software designed to track and secure your WLAN visitor-based networks or hotspots in a centralized way. Based on captive portal technology, FirstSpot allows users to login only utilizing a web browser ».
Of course, there is bound to be « open source linux-based solutions »: