How to bypass the firewall on a proxy server?

Question

Computer networks

All traffic on our network goes through port 3128 of the main proxy. Proxy settings are set locally, through group policies. It is possible to change, but without this proxy traffic does not go further than the internal network.
This proxy has a firewall with a white list of sites that are allowed to access. When you try to access other sites, it displays the "site is prohibited" page, but, what is remarkable, these sites ping normally, and when tracing it is clear that the packets normally reach the desired servers.
Actually the question: plz explain how this protection works, that site tracing goes to the right server, but web pages are blocked, and how can this be bypassed? Is it possible that this is not a complete block, but only certain protocols?
PS The options for searching for a proxy among the allowed IPs have already been considered, I scanned the ports, I did not find a single proxy.

Answer 1

DNS Tunneling