I work for a very small company that is on limited budget, but would like to do there best at preventing access to proxy servers via the web to circumvent our web policies or blocked sites. The current layout for my infrastructure is a PIX 506E at the Firewall (Cisco PIX Firewall Version 6.3(1)) and I have implemented a real on the cheap solution for web filtering by running an Windows 2003 server with DNS/DHCP and IIS on it. I route all DNS through the Windows server where I created Forward Lookup Zones for the sites that I want to block to point back to my IIS server where they receive a warning page (my local index.html) if there is no zone then it passing along to our ISPs DNS for accurate resolution.
Is there a way to better protect the company from users going to the numerous amounts of proxy sites to circumvent the in-house DNS?
Please let me know your thoughts on this one.