To handle this less centrally, you would need a distributed list of URLs to be used for captive portal checks, with servers handled by different entities,

To handle this less centrally, you would need a distributed list of URLs to be used for captive portal checks, with servers handled by different entities, and each device selecting one at random from the list. I dimly remember that the way Android does it is by requesting a list of randomly generated garbage domains (like http://aghepodlln/ http://aghepodlln/) and seeing if they get a response. If they do: captive portal.