How Automatic Detection of Captive Portal works

How do Wi-Fi client devices (smartphones, tablets, laptops) can detect the Captive Portal and automatically open the Splash Page?

 

Answer

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.

 

Differences between Client devices

All client devices use the above described strategy to find out if they are behind a captive portal, but the URL might vary depending on the specific model of smartphone, tablet, laptop and depending on the specific OS version. In the following you can find the list of domains that are contacted by each model in order to detect the captive portal.
 

Android Captive Portal Detection

  • clients3.google.com

 

Apple iPhone, iPad with iOS 6 Captive Portal Detection

  • gsp1.apple.com
  • *.akamaitechnologies.com
  • www.apple.com
  • apple.com

 

Apple iPhone, iPad with iOS 7, 8, 9 and recent versions of OS X

  • www.appleiphonecell.com

  • *.apple.com

  • www.itools.info

  • www.ibook.info

  • www.airport.us

  • www.thinkdifferent.us

  • *.apple.com.edgekey.net

  • *.akamaiedge.net

  • *.akamaitechnologies.com

 

Windows

  • ipv6.msftncsi.com
  • ipv6.msftncsi.com.edgesuite.net
  • www.msftncsi.com
  • www.msftncsi.com.edgesuite.net
  • teredo.ipv6.microsoft.com
  • teredo.ipv6.microsoft.com.nsatc.net

 

 

 

NOTE:
Consider that you can disable the Automatic Detection of Captive Portal on specific client devices simply adding all the listed domains for that specific device to the Walled Garden, as described in the article:
How to enable/disable the Automatic Detection of Captive Portal and automatic opening of the Splash Page

 

 

Fonte: https://success.tanaza.com/s/article/How-Automatic-Detection-of-Captive-Portal-works

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