Which URL does Apple request when connecting to a Wi-Fi network to look for redirects? I remember reading somewhere that when you connect to a Wi-Fi network, Apple tries accessing their own URL which is guaranteed not to follow any redirects. (E.g. http://apple.com/no-redirects). If it sees a redirect, it assumes you are on a Wi-Fi network that needs you to login, so it presents the login screen. Otherwise, if it was able to reach that URL with no redirects, it will just assume you are connected to the internet normally.

What is this URL, and where can I get more info about the mechanism Apple uses?
I found it. Apple uses http://www.apple.com/library/test/success.html

More information can be found at http://blog.erratasec.com/2010/09/apples-secret-wispr-request.html#.VLA9UsabLOY

The advantage of using a no redirect URL is that you can determine which state you are in:


Connected
Connected to Wi-Fi but Internet is inaccessible due to Wi-Fi router requiring user intervention.
Not connected


These are called captive portals, or, as Apple calls them, captive networks:


  Captive networks (often used in hotels, coffee shops, and other public places) may intercept your software’s HTTP requests and provide a login page instead of the expected data.