I think the poster meant IP not port. Most proxy servers can be configured bind to a specific IP. So if your proxy server has an external IP and an internal I

I think the poster meant IP not port. Most proxy servers can be configured bind to a specific IP. So if your proxy server has an external IP and an internal IP, configure the proxy server to bind to the external IP. And then use IPTables to block any connections from the external IP to the internal network.