Captive portals are the common name for public or "as-a-service" Internet Connections which require payment, authentication, or terms of service/acceptable use policy (TOS/AUP) acceptance before allowing connectivity to a device.
Captive portals are typically seen in airports, hotels, coffee shops or really anywhere which gratis or paid wifi is offered. You may also see them in guest wifi networks in corporate or school environments.
A captive portal usually presents itself as a "gate" or pop-up in the browser wherein action is required by the end-user to provide credentials, payment, or accept the terms of service in order to reach the internet. Until the captive portal is cleared, the user is unable to browse any resource besides those within the subnet that the portal exists in.
Expected Behaviors and Scenarios
Most captive portals will redirect all browser requests (HTTP/HTTPS) to its local web portal. The local web portal is usually IP-based and not DNS-based. This means that there will be no behavioral issues caused when using the Umbrella roaming client on a computer which is connecting to a captive portal.
In the rare case where a captive portal uses DNS in some way to facilitate its service, the following behavior will occur before completing the requirements of the captive portal (payment, TOS/AUP acceptance, etc).
Third-party DNS Blocked
If the captive portal is blocking DNS requests destined for Umbrella, DNS connectivity will be blocked for approximately six seconds by the Umbrella roaming client. After six seconds, the Umbrella roaming client transitions to the Unprotected/Unencrypted state until it can again communicate with Umbrella.
Third-party DNS Redirected
If the captive portal is redirecting DNS requests destined for Umbrella, DNS connectivity will be blocked for approximately two to six seconds by the Umbrella roaming client. After this time, the Umbrella roaming client transitions to the Unprotected/Unencrypted state until it can again communicate with Umbrella.
Third-party DNS Allowed
If the captive portal is not manipulating or blocking DNS requests destined for Umbrella, the Umbrella roaming client will work as intended and may result in bypassing the login portion of the captive portal completely.