Question

In Firefox internet connection is made through a proxy auto configuration file something.pac.

How do I know for a certain URL which proxy server is being used?

Answer 1

.pac file is just an ECMAscript – aka JavaScript. Check out the wikipedia article on the file format.

If you copy the PAC code you can process it to see what proxy is being used based on the target url. If you are feeling fancy, you can wrap the script into a web page (locally) to create a tool to evaluate locally.

Edit:

As an alternative to the method I started recommending, you might check out PACTester, available on Google Code. This will allow you to quickly test a range of options.

If you have .Net available and are interested in playing with C# then you can check out this article on MSDN which has code you can use in a similar fashion to the above.

To expand on the original method outlined above, there are a number of functions which may (and typically are) provided by the host browser. The basic function which must be implemented in a pac file is FindProxyForUrl(). This accepts two parameters: the url and the host (the host derived from the name of url). The “provided” functions include: isInNet(), localHostOrDomainIs(), isPlainHostName(), isResolvable(), etc.

If you are working in a Microsoft environment then you can check out this page on Technet which describes the .pac format with some useful examples.

Per the Microsoft documentation for isInNet():

The isInNet(host, pattern, mask) function returns TRUE if the host IP address matches the specified pattern. The mask indicates which part of the IP address to match (255=match, 0=ignore).

If you want to get technical, here is the Mozilla source code for the implementation of proxy auto-config related services. It specifies the JS code for isInNet() as:

200 function isInNet(ipaddr, pattern, maskstr) {
201     var test = /^(\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1,3})$/(ipaddr);
202     if (test == null) {
203         ipaddr = dnsResolve(ipaddr);
204         if (ipaddr == null)
205             return false;
206     } else if (test[1] > 255 || test[2] > 255 ||
207                test[3] > 255 || test[4] > 255) {
208         return false;    // not an IP address
209     }
210     var host = convert_addr(ipaddr);
211     var pat  = convert_addr(pattern);
212     var mask = convert_addr(maskstr);
213     return ((host & mask) == (pat & mask));
214     
215 }

Hope that helps!