Some blocked sites ( by url filtering ) not redirecting to the page " Web page Blocked " ,Instead it shows " The web page not available " .
But some are redirecting to the page " Web page Blocked " as expected
This happens when users are trying to access blocked site over HTTPS. With ssl connection firewall cannot inject response page (blocked page), and since the connection is blocked by the firewall, end users get "Web page not available" option.
You can either enable ssl decryption or just configure response page as mentioned in above document. In either case, you will need to install root certificate in all machine in order to not get the browser error. Hope this helps. Thank you.
If you already use SSL decryption or the the method with the url proxy, you should keep in mind that it won't work if the user surf through another proxy: Block Pages are Not Displayed When Using SSL and a Web-Proxy
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