What is CAPPORT?

Definition

CAPPORT is an IETF standard that lets a network tell a device, in a structured way, that it is behind a captive portal, where the login page is, and what the session state is. It replaces the guesswork of connectivity probes with an explicit API, giving operating systems accurate portal prompts and session information.

The problem it solves

For years, devices discovered captive portals by accident. The operating system would request a known URL, expect a specific response, and infer from anything unexpected that a portal was in the way. It works often enough to have become universal, but it is fragile: HTTPS breaks interception, DNS-over-HTTPS bypasses the network resolver, and the device has no way to learn how much time or data is left in a session. CAPPORT defines a proper channel instead. The network advertises an API endpoint during address configuration, and the device queries it directly.

What the API returns

The endpoint responds with a small JSON document describing whether the device is currently captive, the URL of the portal page to open if it is, and optionally when the session expires or how much data remains. That last part is the meaningful upgrade for guests: instead of a session silently dying and traffic mysteriously stopping, the device can surface remaining time or data in the system interface. It also means the portal prompt appears reliably and closes when authorisation actually completes.

Where it stands today

Support is arriving gradually. Major mobile platforms have implemented parts of the specification and several network vendors expose the API, but the installed base of hardware and firmware in real venues moves slowly. In practice a portal in 2026 should support CAPPORT where the hardware allows while continuing to work correctly for devices that only do the old probe-based detection. It is an additive improvement to the guest experience, not a replacement for a portal that already behaves well.

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