Creating a Fiddler Extension

This article will quickly describe how you can create a Fiddler Extension. A Fiddler Extension can be loaded into the tool Fiddler by creating a Class library in .NET programmed in C#. If you use Fiddler 2, target .NET 2 or .NET 3.5. If you use Fiddler 4 (beta), you must actually target .NET 4 (this is not documented well on the Fiddler website at Fiddler 2).

First off, install Fiddler 4, if you have not installed it yet. You need at least Visual Studio 2005, but if you work with extensions for Fiddler 4, you must target .NET 4 (Visual Studio 2010 or 2012). Fiddler extensions are implemented using interfaces. Create a new solution in Visual Studio and select creating a Class Library. First off, add a reference to Fiddler.exe in your Fiddler installation folder. This exe file usually resides in the folder C:\Program Files (x86)\Fiddler2, the default location for installing Fiddler. After referencing the Fiddler executable, add also a reference to System.Windows.Forms, if your Fiddler extension will modify the GUI directly.

Here is a sample Fiddler extension that will remove a host by typing the command e.g. RemoveHost www.yourhosthere.com in the QuickExec command bar in Fiddler. Here www.yourhosthere.com is the hostname which is an example of a server to remove in the listing of Fiddler's capture of traffic. This Fiddler extension will remove all captures where we have a HTTP Header that contains a Host value of ww.yourhosthere.com We use the interface IHandleExecAction to add support for QuickExec here.

In addition, the Fiddler extension fakes the UserAgent by setting it to Violin. This extension is based on the tutorial at Fiddler website and the video of the Pluralsight course for Fiddler, which I have fully watched. The author, Robert Boedigheimer does an excellent job of explaining the possibilities of Fiddler tool at a basic, intermediate and advanced level. To add support for this we implement the interface IAutoTamper. Here is the source code of the extension I ended up writing in Visual Studio 2012. Remember to target .NET 4 Framework to support Fiddler 4!

using System;
using System.Collections.Generic;
using System.Text;
using System;
using System.Windows.Forms;
using Fiddler;


namespace TestFiddlerExtension
{
    public class Violin : Fiddler.IAutoTamper, IHandleExecAction    // Ensure class is public, or Fiddler won't see it!
    {
        string sUserAgent = "";

        public Violin()
        {
            /* NOTE: It's possible that Fiddler UI isn't fully loaded yet, so don't add any UI in the constructor.

               But it's also possible that AutoTamper* methods are called before OnLoad (below), so be
               sure any needed data structures are initialized to safe values here in this constructor */

            sUserAgent = "Violin";
        }

        public void OnLoad() { /* Load your UI here */ }
        public void OnBeforeUnload() { }

        public void AutoTamperRequestBefore(Session oSession)
        {
            oSession.oRequest["User-Agent"] = sUserAgent;
        }
        public void AutoTamperRequestAfter(Session oSession) { }
        public void AutoTamperResponseBefore(Session oSession) { }
        public void AutoTamperResponseAfter(Session oSession) { }
        public void OnBeforeReturningError(Session oSession) { }

        public bool OnExecAction(string sCommand)
        {
            string[] args = Fiddler.Utilities.Parameterize(sCommand);

            string command = args[0];

            if (command.ToLower() == "removehost")
            {

                if (args == null || args.Length != 2)
                {
                    FiddlerApplication.UI.SetStatusText("Specify host to remove");
                    return false;
                }

                string host = args[1];

                FiddlerApplication.UI.actSelectSessionsWithRequestHeaderValue("Host", host);
                FiddlerApplication.UI.actRemoveSelectedSessions();
            }

            return true; 

        }


    }
}

As you can see from the source code above, whenever we want to perform actions against the Fiddler GUI, we use the Fiddler.FiddlerApplication.UI object. This object has got methods for working with the Fiddler GUI. The class Fiddler.FiddlerUtilities has utility methods against fiddler. To work against Sessions (the individual rows of the Capture), we can see we can work against the Session by implementing IAutoTamper. The signature of the methods of these interfaces usually gives us the information we need, so our interface implementations can implement the behaviors of each specified interface. In addition to visiting the Project Properties of the Class Library and ensuring that .NET 4 framework is targeted, check that the Post build actions does the following:

copy "$(TargetPath)" "%userprofile%\My Documents\Fiddler2\Scripts\$(TargetFilename)"

The post action will compile the Fiddler Extension inside the class library and copy the DLL file to the users documents folder and subfolder Fiddler2\scripts folder. This is the users personal Fiddler Extensions. If you want to make your Fiddler extension available to all users, copy the compiled DLL file into the Scripts folder where Fiddler is installed, usually the folder:

C:\Program Files (x86)\Fiddler2

This article shows how you can create Fiddler extensions. This powerful HTTP Debugging proxy tools for analyzing network traffic is excellent and is able to capture traffic from diverse sources as WCF services bound to HTTPS or HTTP protocols, to acting as a reverse proxy for capturing traffic from mobile devices and regular capture of HTTP(S) traffic from a web browser.