THE article by Johannes Ullrich discusses the detection of captive portals encountered when connecting to public WiFi networks. It explains that captive portals are splashes screens requiring user acknowledgment or login, which modern browsers and operating systems aim to detect. Historically, it was easier to access these portals, but the widespread use of TLS has made detection more complicated.
The piece lists specific URLs used by various operating systems and browsers to check for internet connectivity and respond accordingly. For Windows, it references a Microsoft connectivity test, while for Apple, Android, Chrome, and Firefox, it provides equivalent check URLs, highlighting that these URLs facilitate the redirection necessary for users to access captive portals.