I'm running a squid Proxy behind a firewall in my network. The Proxy server does not have any Global Ip address and since i did not have any one point authentication for my network I'm running it without any Authentication (ie anyone in my network can access the proxy server.

Lately i am getting mail from my ISP that i'm running an open proxy and that i should close it. Since i', not using any direct ip address and the server is not directly accesible from outside (internet) i felt that it cannot be an open proxy. Can someone tell me if my asumtion is right and how do i explain to my isp regarding the complaint that they have sent me. Thanks.

use a site like Sheilds Up! to scan your source internet address for open ports. if you have a router forwarding external ports to the inside go over these and review them carefully. did tehy say what port number is open for this?