I've wondered this too. Doesn't DHCP support extensible options? Couldn't you just send an option that said, "hey, crappy portal welcome site at {URL}"? It'd be fixed to a particular webpage (so, you can't use another protocol), but that's the state of things today.
It seems like there are only two things captive-portals are "for", though: accepting EULAs, and presenting a login form for the proxy. Both of these could be better if built into browser/OS chrome.
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I've wondered this too. Doesn't DHCP support extensible options? Couldn't you just send an option that said, "hey, crappy portal welcome site at {URL}"? It'd be fixed to a particular webpage (so, you can't use another protocol), but that's the state of things today.
It seems like there are only two things captive-portals are "for", though: accepting EULAs, and presenting a login form for the proxy. Both of these could be better if built into browser/OS chrome.