How do I find out the browser's proxy settings?

The function you're looking for is WinHttpGetIEProxyConfigForCurrentUser(), which is documented at http://msdn.microsoft.com/en-us/library/aa384096(VS.85).aspx. This function is used by Firefox and Opera to get their proxy settings by default, although you can override them per-browser. Don't do that, though. The right thing to do (which is what everybody else does) is to just get the IE settings and assume that they're correct, since they almost always are.

Here's a sample of the relevant logic, which you should adapt for your needs:

if( WinHttpGetIEProxyConfigForCurrentUser( &ieProxyConfig ) ){    if( ieProxyConfig.fAutoDetect )    {        fAutoProxy = TRUE;    }    if( ieProxyConfig.lpszAutoConfigUrl != NULL )    {        fAutoProxy = TRUE;        autoProxyOptions.lpszAutoConfigUrl = ieProxyConfig.lpszAutoConfigUrl;    }}else{    // use autoproxy    fAutoProxy = TRUE;}if( fAutoProxy ){    if ( autoProxyOptions.lpszAutoConfigUrl != NULL )    {        autoProxyOptions.dwFlags = WINHTTP_AUTOPROXY_CONFIG_URL;    }    else    {        autoProxyOptions.dwFlags = WINHTTP_AUTOPROXY_AUTO_DETECT;        autoProxyOptions.dwAutoDetectFlags = WINHTTP_AUTO_DETECT_TYPE_DHCP | WINHTTP_AUTO_DETECT_TYPE_DNS_A;    }    // basic flags you almost always want    autoProxyOptions.fAutoLogonIfChallenged = TRUE;    // here we reset fAutoProxy in case an auto-proxy isn't actually    // configured for this url    fAutoProxy = WinHttpGetProxyForUrl( hiOpen, pwszUrl, &autoProxyOptions, &autoProxyInfo );}if ( fAutoProxy ){    // set proxy options for libcurl based on autoProxyInfo}else{    if( ieProxyConfig.lpszProxy != NULL )    {        // IE has an explicit proxy. set proxy options for libcurl here        // based on ieProxyConfig        //        // note that sometimes IE gives just a single or double colon        // for proxy or bypass list, which means "no proxy"    }    else    {        // there is no auto proxy and no manually configured proxy    }}

Here is a complete code sample how to call WinHttpGetIEProxyConfigForCurrentUser method from winhttp.dll library in C#

[TestClass]public class UnitTest1{    [StructLayout(LayoutKind.Sequential)]    public struct WinhttpCurrentUserIeProxyConfig    {        [MarshalAs(UnmanagedType.Bool)]        public bool AutoDetect;        [MarshalAs(UnmanagedType.LPWStr)]        public string AutoConfigUrl;        [MarshalAs(UnmanagedType.LPWStr)]        public string Proxy;        [MarshalAs(UnmanagedType.LPWStr)]        public string ProxyBypass;    }    [DllImport("winhttp.dll", SetLastError = true)]    static extern bool WinHttpGetIEProxyConfigForCurrentUser(ref WinhttpCurrentUserIeProxyConfig pProxyConfig);    [TestMethod]    public void TestMethod1()    {        var config = new WinhttpCurrentUserIeProxyConfig();        WinHttpGetIEProxyConfigForCurrentUser(ref config);        Console.WriteLine(config.Proxy);        Console.WriteLine(config.AutoConfigUrl);        Console.WriteLine(config.AutoDetect);        Console.WriteLine(config.ProxyBypass);    }}

There are registry keys for these values that you could get to directly of course. You could also do this in .NET without much hassle at all. I believe the WebClient object negotiates the proxy settings for you based on the current settings. This would look like this in C#:

using System.Net;string url = "http://www.example.com";WebClient client = new WebClient();byte[] fileBuffer = client.DownloadFile(url);

Or something close to that.