The captive portal technique forces an HTTP client on a network to see a special web page (usually for authentication purposes) before surfing the Internet normally. This is done by intercepting all HTTP traffic, regardless of address, until the user is allowed to exit the portal. You will see captive portals in use at most Wi-Fi hotspots. It can be used to control wired access (e.g. apartment houses, business centers, open Ethernet jacks) as well.
=Software Captive Portals=
Examples of captive portal software packages running on PC hardware are:
[http://www.wifidog.org/ WifiDog] (embedded Linux - OpenWRT, Linux)
Gateway / Centralized central server solution
[http://patronsoft.com/firstspot FirstSpot] (Windows)
[http://m0n0.ch/wall/ m0n0wall] (embedded FreeBSD)
[http://www.nocat.net NoCatAuth] (Linux)
[http://www.opensplash.org/ OpenSplash] (FreeBSD)
[http://www.shmoo.com/~bmc/software/wicap/announce.html wicap] (OpenBSD)
[http://www.chillispot.org// chillispot] (Linux)
[http://www.publicip.net/ Public IP] (Linux)
[http://www.pfsense.org/ PfSense] (FreeBSD)
[http://www.iea-software.com/airmarshal AirMarshal] (Linux)
[http://wiki.personaltelco.net/index.cgi/PortalSoftware Other wiki list of captive portals]
=Hardware Captive Portals=
Examples of captive portal hardware include:
Cisco [http://www.cisco.com/en/US/products/sw/netmgtsw/ps533/products_user_guide_book09186a0080126bfe.html BBSM-Hotspot]
Cisco [http://www.cisco.com/en/US/tech/tk888/tk890/tsd_technology_support_protocol_home.html Site Selection Gateway (SSG)] / [http://www.cisco.com/en/US/products/hw/routers/ps314/products_data_sheet09186a0080091f30.html Subscriber Edge Services (SESM)]
Nomadix [http://www.nomadix.com/products Gateway]
Aptilo [http://www.aptilo.com/solutions_public_access.htm Access Gateway]
Captive portals are gaining increasing use on free open wireless networks where instead of authenticating users, they often display a message from the provider along with the terms of use. Although the legal standing is still unclear, at least in USA, common thinking is that forcing users to click through a page that displays terms of use and explicitly releases the provider from any liability mitigates potential problems.
= See also =
HTTP proxy