Question

I’m having trouble getting Proxy Automatic Configuration (PAC) in IE options to work as expected using .Net WebRequest.

According to this article:
Proxy Detection
Take the Burden Off Users with Automatic Configuration in .NET

The system proxy should be set by default with to each WebRequest.

That’s how the proxy.js pac file looks like:

function FindProxyForURL(url, host)
{
  return "PROXY ProxyServerName:3118; DIRECT;";
}

I also took a look at this post: How should I set the default proxy to use default credentials?

Which suggests to add this in the app.config:

<system.net>
  <defaultProxy useDefaultCredentials="true" />
</system.net>

Adding this did not help.

I created a small console application just to test this out.. here it is:

static void Main(string[] args)
{
    HttpWebRequest request = null;
    try
    {               
        String resolvedAddress = WebRequest.DefaultWebProxy.GetProxy(new Uri("http://www.google.com")).ToString();
        Console.WriteLine("Proxy for address is: " + resolvedAddress);

        Uri m_URLToTest = new Uri("http://www.google.com");
        request = WebRequest.Create(m_URLToTest) as HttpWebRequest;
        request.Method = "GET";
        request.KeepAlive = false;
        request.Timeout = 5000;
        request.Proxy = WebRequest.DefaultWebProxy;
        WebResponse response = request.GetResponse();
        StreamReader reader = new StreamReader(response.GetResponseStream());
        string message = reader.ReadToEnd();
    }
    catch (Exception ex)
    {
        Console.Write("Exception");
    }

}

The output:
Proxy for address is http://www.google.com

instead of Proxy for address is ProxyServerName:3118

It happens only when using auto configuration script…

Did I miss anything? Please help!

Answer 1

Found the solution!

It is really important that the mime type of the PAC file would be: [Content-type: application/x-ns-proxy-autoconfig]

Other mime types might not work.

Make sure using fiddler2 (with cache disabled) that the mime type is appropriate.
Some configurations might show Content-Type: text/plain which is bad.