Captive Portal

A captive portal is a 'landing' web page, presented by a Layer 3 brand or Layer 2 Operator and shown to users before they gain broader access to URL or http-based Internet services. Often used to present a landing or log-in page, the portal intercepts observed packets until such time as the user is authorized to launch browser sessions. After being redirected to a web page which may require authentication, payment, acceptance of EULA/acceptable use policies or other valid credentials that the host and user agree to, the user is granted conditional Internet access. Captive portals are used for a broad range of mobile and pedestrian broadband services, including cable and commercially provided Wi-Fi, home hotspots and others, and can also be used to provide access to enterprise and residential wired networks (such as apartment houses, hotel rooms, business centers).

The login page itself is presented to the client, and is stored either locally at the gateway, or at the web server hosting that page (requiring access to an approved access list, or "white-list", an essential feature of the secure enterprise). Depending on the feature set of the gateway, multiple web servers can be white-listed (say for iframes or links within the login page). In addition to white-listing the URLs of web hosts, some gateways can white-list TCP ports. The MAC address of attached clients can also be used to bypass the login process for specified devices.

This technique has occasionally been referred to as UAM (Universal Access Method) in implementations and standards forums.

Captive portals are mainly used in wireless open networks, where the users are shown a welcome message informing them of the conditions of access (allowed ports, liability, etc.). Administrators tend to do this so that their own users take responsibility for their actions and to avoid any major problems. It is discussed whether this delegation of responsibility is legally valid.

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Wikipedia