Captive Network Assistant

A Captive Network Assistant (CNA) is the mini-browser your phone or laptop opens automatically when it detects that a WiFi network has a captive portal. It is the reason a venue's sign-in page "just appears" seconds after you join the WiFi, without you opening a browser. Apple coined the CNA name; Android and Windows have equivalents that behave the same way.

How captive portal detection works

Immediately after joining a network, the operating system silently requests a known URL and checks the response:

• Apple devices request a page on captive.apple.com.
• Android requests connectivitycheck.gstatic.com/generate_204 and expects an empty 204 response.
• Windows requests msftconnecttest.com/connecttest.txt.

If the expected response comes back, the internet is open and nothing happens. If the network intercepts the probe and redirects it, the OS concludes there is a captive portal and opens the CNA pointing at the splash page. Newer standards (RFC 8910 and RFC 8908) let networks advertise their portal explicitly instead of relying on interception, though probe-based detection remains the norm.

CNA quirks that affect splash pages

• It is a stripped-down browser: no address bar, no tabs, and limited cookie sharing with the device's main browser.
• It may close itself the moment the OS confirms connectivity, so post-login content should live on a redirect page, not in the CNA.
• App-store links, downloads and pop-ups generally do not work inside it.

Design implications

Because the CNA is unforgiving, splash pages should be a single lightweight screen with one action, with every asset served from a domain the walled garden allows. Heavy pages that work fine in Safari or Chrome can time out or render badly inside the assistant.