Redirect via DNS to captive portal, how?

Question

WiFi

Friends, read a lot, searched a lot. I noticed that every week they ask questions on various captive portals. But the question is a little different. There are a bunch of services that allow you to run "social wi-fi". Almost everywhere, a router flashing is required ... But there are a few who ask to register their DNS servers in the router settings and Voilà!
Question. What free stuff can work the same way? Most solutions require a physical connection of the router to the port... Thank you very much!