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I am connected to my own router, and I know for a fact that I do not have a captive portal installed. Firefox displays a banner at the top of the page saying that "you must log in to this network before you can access the Internet.", with a button that says "Open Network Login Page" to its right.
The link forwards to http://detectportal.firefox.com/success.txt. Notably only one machine is affected. Just sort of wondering what the possible reasons are for getting suddenly seeing this.

I also know that someone hasn't set up a fake router SSID- because I can see that I'm connected to the router via the router's web-config utility

  • I also know that someone hasn't set up a fake router SSID- because I can see that I'm connected to the router via the router's web-config utility. – John Doe Jul 3 '17 at 19:15
  • Not sure... maybe your router has a special configuration or a captive portal? Did you tried on other browser? from other pc or mobile device? is the same behavior? – OscarAkaElvis Jul 3 '17 at 20:30
  • Maybe your router is altering the success.txt page in some way, so Firefox doesn't recognise the clean response - so assumes a captive portal? – Douglas Leeder Jul 3 '17 at 20:46
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    I see the banner in the corporate network I am working in because of a proxy that meddles with HTTP traffic. From Firefox 54 onwards you can disable the captive portal check by setting network.captive-portal-service.enabled to false in about:config. – herzbube Sep 4 '17 at 7:30
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The captive portal detection works by requesting http://detectportal.firefox.com/success.txt (or something similar) and seeing if the expected contents are returned. Possibly something on your router, or computer is altering the page, so Firefox is assuming that there is a captive portal in the way?

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