Captive portal test
http://clients3.google.com/generate_204
http://www.gstatic.com/generate_204
http://172.217.6.238/generate_204
http://captiveportal.kuketz.de
https://e.foundation/net_204/
https://elementary.io/generate_204
http://httpstat.us/204
http://captive.apple.com/hotspot-detect.html
http://www.apple.com/library/test/success.html
http://attwifi.apple.com/library/test/success.html
http://detectportal.firefox.com/canonical.html (returns http 200, html meta redirect)
http://detectportal.firefox.com/ (returns http 200, text "success")