I don't understand how this (and other similar sites mentioned) work. What is going on with the network that makes visiting a non-SSL site make it work? Anyone care to offer an explanation or have a link to one?
If google.com were a non-HTTPS website then the router could simply then serve up any arbitrary content it wanted. i.e., it's own login/access form.
However, since google is an HTTPS website, the browser asks for an HTTPS certificate from this fake google.com and the router is unable to deliver a "real" certificate—this then breaks the "login flow" since instead of seeing a login form you see a "danger, do not continue, bad certificate" page.
I remember in public school, IT had a website blocker that was effectively was a captive portal that would just route you to a "BLOCKED" web page and only could handle blocking non-HTTPS domains. Many sites were already going HTTPS by senior year so it was trivial to go to https://facebook.com and have full access. Another possibility was using a cgi-proxy. The site blocker was a blacklist so I hosted my own cgi-proxy when the one commonly used got blacklisted.
https://en.wikipedia.org/wiki/Captive_portal#Require_Web_Bro...
When you connect to wifi, many (most?) OSes and/or browsers issue requests to http servers run by apple/firefox/microsoft. If the response is a portal, a popup is shown to allow the user to login (typically logging in would authorise the IP and/or mac address in a database and allow traffic to pass).
However some (poorly implemented) hotspots have specific holes to allow these pages to load without the portal (whitelisting the domains), then when you try to visit http://othersite.com, you will be redirected. Other portals may timeout after X minutes or X MB of use and require reauthentication, which the OS/browser may not detect.
As more and more sites use https, while the connection can be stopped (by stopping traffic going to port 443), a correctly behaving browser should ignore any traffic coming from a redirected https connection, as it wouldn't have a valid certificate - it's the whole point.
Many sites also implement headers which tell a browser "you will always connect to me on http", and the browser will refuse to downgrade to http://othersite.com, even if port 443 (https) is blocked but port 80 is open.
By visiting neverssl.com or similar sites, you can in those situations trigger the captive portal login page.