A técnica é conhecida como a criação de um "Portal Cativo" . O conceito descrito neste site é simples:
Captive portals allow you to leverage a common browser as a secure authentication device. They also have the potential to allow you to do everything securely via SSL and IPSec and setup per user quality of service rules, and still maintain an open network.
No que diz respeito à implementação, depende do produto de gerenciamento de rede que está sendo usado. Normalmente, ele é tratado por um appliance de firewall. Por exemplo, este site tem instruções sobre como usar as configurações do Captive Portal em NG Firewall :
Captive Portal allows administrators to require network users to log in or accept a network usage policy before accessing the internet. Captive Portal can authenticate users against Untangle's built-in Local Directory, Active Directory (if Directory Connector is installed), or RADIUS. It can be used to set up policies (for Policy Manager) by username (or group name if using Active Directory) rather than IP. While Captive Portal is running, captured machines will be forced to authenticate (or just press OK) on the Captive Portal page before they are able to access the internet.
Idem com a documentação do pfSense :
The Captive Portal function in pfSense allows you to secure a network by requiring a username and password (or just a click through), entered on a portal page.