A number of Yosemite Macs are having problems with our organization’s Wi-Fi captive portal.
If you don’t first sign in via the window presented by Captive Network Assistant, the system appears to have zero network connectivity. You can’t ping anything, you can’t perform DNS lookups, etc. It’s as if the OS is actively blocking all attempted connectivity until it knows you have succeeded in authenticating.
Consequently, Browsers throw errors instead of presenting our captive portal page. This is a big problem, since in many cases the Captive Network Assistant app was previously disabled or deleted!
I found I could whitelist our network so that it lets applications try connecting even if the Captive Network Assistant has not yet been placated.
Just copy the command below, replacing NAMEOFNETWORK with the SSID of your captive portal network. A reboot may be necessary, but it may also be enough to simply switch Wi-Fi off and on again.
sudo /usr/libexec/PlistBuddy -c "Add ScrapingParameters:DisabledRealms:0 string @NAMEOFNETWORK" /Library/Preferences/SystemConfiguration/CaptiveNetworkSupport/Settings.plist