Recently I was called to adjust a firewall rule to allow access to 2 specific URL's only and nowhere else.
So I created a expression for the 2 specific URL and tested to see if it's working.
Guess what, it didn't! So further looking in to the issue I discovered that the specific developers had used more than a dozen domain to fetch images, scripts, ads, fonts, style sheets etc.
In order to test that I used the developer tools included in Firefox and Chrome, to access the tools either use the menu on each browser and locate them under tools or use Ctrl+Shift+I to open the console. Then select network and visit the page from an non restricted computer and disable any ad blockers that you might use.
Record all the domains and try to whitelist them in your rule.
It's a try and error procedure but helps especially if you are dealing with slow responding 3rd parties etc.
