Networking – Sign-In to Network protocol


When I connect to a hotel network that does not require a WPA key, on my PC, the first browser access will be trapped and directed to the login page. This is understandable, and I can guess the network equipment provisions access by MAC address.

On my Android (5.0) phone, a Sign-in to Network screen opens and it shows that same web page as what I see on the PC. What is the protocol that makes this happen?

Best Answer

Direct quote from How Automatic Detection of Captive Portal works (follow link for full text)

Basic strategy behind Captive Portal detection
The Automatic Detection of Captive Portal mechanism is based on a simple verification, done by the Operational System (OS) of the client device (smartphone, tablet, laptop).
It simply tries to reach a specific URL and verify that such URL returns a well-known result.

  • If a Captive Portal is not in place, the result will match the expected one and the OS will know that there is full access to internet.
  • If the URL returns a result other than the expected one, then the OS will detect that there is a Captive Portal in place and that it's needed to proceed with authentication in order to get full access to internet: in this case the OS will open the Splash Page automatically.

Further Readings
Windows 7 Network Awareness: How Windows knows it has an internet connection
Captive portals - MSDN

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