The Apple operating systems detect the presence of a network that has captive portal enabled by attempting to request a web page from the Apple public website. This HTTP GET process retrieves a simple success.html file from the Apple web servers and the operating system uses the successful receipt of this file to assume that it is connected to an open network without the requirement for captive
If the success.html file is not received, the operating system conversely assumes that a captive portal is in place and presents the CNA automatically to prompt the user to perform a web authentication task. When the web authentication has completed successfully, the CNA window is closed automatically, which prevents the display of any subsequent welcome pages or redirecting of the user
to their configured home page. If the user chooses to cancel the CNA, the Wi-Fi connection to the open network is dropped automatically, which prevents any further interaction via the full browser or
The Apple Captive Network Assistant (CNA) feature in version 220.127.116.11.44 only, is an overlay that appears and prompts users automatically to login to the detected captive portal network without the need to explicitly open a web-browser. This type of login is useful on mobile devices where many of the common applications are not browser-based and these applications would otherwise fail to connect without the successful browser based authentication. Examples of these nonbrowser-based applications are email, social networking applications, corporate VPNs, and media streaming.
Hope this helps.