Gateway Pop Up Window Login Page not working

Hi there

So i run a proxy that uses DHCP to send the IP address of the proxy server to be the default gateway for all devices that join the network. When the user joins the network and opens a browser, they will be presented with a login page to continue to be able to browse.

However on the IMAC, a pop up window containing the same login page appears when the computer starts up (this doesnt appear to be a normal browser pop up window). None of the login/download certificate links work because Apple have injected some Apple urls into the page. This same login page works correctly if you skip the pop up box and head straight to the browser to do it.

My clients would like a workaround however, and i was wondering if anyone had come accross something like this, and if so, is there a way to remove the pop up box completely?

Cheers!