Intercepting port 80 traffic is exactly what a transparent proxy does. http://en.wikipedia.org/wiki/Proxy_server#Transparent_proxy http://en.wikipedia.org/wik

Intercepting port 80 traffic is exactly what a transparent proxy does. http://en.wikipedia.org/wiki/Proxy_server#Transparent_proxy http://en.wikipedia.org/wiki/Proxy_server#Transparent_proxy They call it transparent because the client does not need to support using a proxy server or even be aware that it is happening. Transparent proxies are common at corporations that filter web browsing. It is harder to circumvent than DNS blocking. I suppose that it is no longer a transparent proxy once it starts modifying the requests or responses. But even transparent proxies generally serve an error message in some cases, like when a domain name doesn't exist or a server does not respond on port 80. So they are rarely, if ever, fully transparent.