Captive portal detection, popup implementation?

Based on hostapd, I m building a captive portal. - My Linux Machine provides a Wifi access. - iPad's and Android clients-tablets connect this Wifi.

Generally, any client OS check if a url is reachable, if not : client OS states it is captive, and displays a popup browser window. Popup is used for login, presentation or else.

Id like to display such a popup, to present my machine's service. But I dont get it. I ve avoided the net forward though. All connexions are redirected in the machine localhost website.

Why dont I get such a popup ? How to get it ? How/Where should I implement it on my localhost ?