modifying the error message window on browser

I need to modify the error message appear on the browser window after I block the web site using wfp. Like change the message “can’t reach this site” to my custom message. Is it possible to do this using wfp.

Assuming that you want this to work with many different browsers, to display a custom message, you need to accept the connection and proxy the request - returning some HTML that has your special error message. This is easy enough for standard HTTP, but SSL / TLS is more complex. It is possible because you are locally proxying the request, and can influence the certificate validation, but it is not easy