The real solution is for the captive portal is to signal that there is a captive portal. Unfortunately that has turned into an arms race: some operating system

The real solution is for the captive portal is to signal that there is a captive portal. Unfortunately that has turned into an arms race: some operating systems switch to a different browser when they detect a captive portal. So captive portals try to avoid detection, etc. Basically what you want is package that tries to detect a captive portal and when it detects one alerts the user and offer to start a browser that uses the DHCP-supplied DNS resolvers to interact with the portal.