Unless you can configure you router to forward some incoming traffic to the computer where A4Proxy is installed, A4Proxy will not be able to check proxies in the Loopback mode. In order to check anonymity of proxies reliably, A4Proxy uses a special technique. It makes the proxy connect to your own computer, as if it was a web server, and analyzes the incoming proxy requests (the proxy connects to A4Proxy software). In other words, a connection loop is formed: your computer -> proxy server -> your computer. In order for this method to work, there must be no obstacles for the incoming connections to your computer. E.g., if you run a firewall, it will block all incoming connections and as a result A4Proxy will not get any response from proxies and all of them will be marked as non-functioning. The same will happen if there is a router between your computer and the Internet.
Note that during the checking requests come to your computer from the “outside”, opposite to ordinary browsing, when requests go from your computer to the destination servers. Incoming requests are never required for ordinary surfing and therefore they are often blocked by firewalls and sometimes by providers, and routers have to be specifically configured to forward them into the network.
A4Proxy uses the port selected in the Check Port option on the Proxy Options tab for ordinary proxy checking (you will see incoming requests on port 80 if you haven’t changed that port); also, A4Proxy tests proxies on support for Secure HTTP and FTP protocols using ports 443 and 21 correspondingly. These additional tests are run if you check the option “Reset Data” on the Proxy Options tab AND the option “Check for HTTPS/FTP support on reset data” on the Proxy Options tab. It is incoming traffic on these ports that you will need to forward into your network to the computer where A4Proxy is installed. In addition to that, your will need to enter the IP address of your Internet connection in the option called “Use Fixed IP address”, Options menu, Special Options dialog in A4Proxy.
However, in the near future we are going to release a new version, which will be able to check proxies from behind firewalls and routers without any problems, as it will be using a different testing method in such cases (the program is currently under testing). Updates will be free for all our customers so you will not have any problems using A4Proxy.