Http Debugging with Fiddler

The information published in this post is now out-of-date and one or more links are invalid.

—IEBlog Editor, 21 August 2012

We’ve just published an MSDN article on a tool called Fiddler. As the article explains it is very useful for http debugging and was written by Eric Lawrence one of the Program Managers on the IE team.


Comments (31)

  1. Anonymous says:

    Thanks a lot for posting this, and thanks to the developer of the program. Just downloaded it and it looks great.

  2. Anonymous says:

    This is my favorite tool for debugging (lacking) websites. Thanks for the article.

    Do you know what the connection to /services/version.asmx is doing when you first start up fiddler?

    Here is hhsnap of fiddler at start up showing the connection.

    Howard Hoy

  3. Anonymous says:

    Version.asmx? Ah, that’s the beauty of a debugger– you can look at what’s being passed.

    Look at the TextView of the response and you’ll see that this page returns the values 0,9,9,8, representing the latest version of Fiddler available from the server.

    If you’d prefer not to auto-check the version, use the trivial registry script here:

  4. Anonymous says:

    Thanks.. nothing like using the same tool on itself. 🙂

  5. Anonymous says:

    Ahh cool a good article, long time coming, I have been using fiddler for quite some time now, nice little tool.

  6. Anonymous says:

    Can someone post the results of when you run fiddle through fiddler?

  7. Anonymous says:

    <<Can someone post the results of when you run fiddle through fiddler?>>

    Fiddler shows you everything that it receives and sends, so running two copies would merely show you exactly the same information twice. What in particular are you looking for?

  8. Anonymous says:

    Very useful tool! Many thanks.

    I guess it can be used on FF too, in the same way.

    Saves faffing around with inferior shareware tools like CommView.

    What qualifies something as a PowerToy? Why aren’t they ever updated? (I still use document tree and view partial source web accessories)

  9. Anonymous says:

    "PowerToy" is generally just something that we release as an optional addon with no promise of support.

    At present, I expect to continue updating Fiddler indefinitely, and I’m planning to write more articles on how to best use it. Please post if you have suggestions on articles you’d like to see.

    With regard to updates– I also still use Document Tree and View Partial Source. If ther are PowerToy updates you’d like to see, feel free to let us know and we’ll see what we can do.



  10. Anonymous says:

    Dave: number 2 (and possibly number 3) are actually COUNTERPRODUCTIVE for web developers. This stems directly from the zealotry of feature-bloat and the lack of adequate, controlled testing.

    Here is why.

    As I understand it, Firefox ‘cleans up’ certain HTML code before parsing it, and doesn’t store the original. You then go to ‘View Source’. It displays the CLEANED UP VERSION, not THE ORIGINAL DOWNLOADED FROM THE SERVER. This creates obfuscation instead of helping, when debugging and tracking obscure bugs related to certain improper tagging and the ilk, and is counterproductive.

    (I will concede that being able to Ctrl+Click to select a whole ‘chunk’ of a page at a time, followed by ‘View Selection Source’, is actually very useful… Is this the first known example of Firefox ‘innovation’ or was this copied from Opera too, like everything else?)

    To stay ontopic, Fiddler is far more extensible than Live HTTP headers, which isn’t even scriptable. Eric, you have created a novel debugging tool, let’s see how fast the more advanced ideas are stol -errr – imitated… by the ‘innovators’ :o)

  11. Anonymous says:

    Vis-a-vis #1: LiveHTTPHeaders is a tool to view HTTPHeaders. Fiddler is a HTTP Debugger; viewing headers is only a small part of its functionality. You can use Fiddler to flag/rewrite or reroute traffic on the fly, either manually or via the scripting engine.

    For tasks #2,3, try the IE5 Web Developer Accessories.

  12. Anonymous says:

    Hm, the Link is dead. I get just a redirect to a piece of Junk-HTML:

    Does somebody the real address?

  13. Anonymous says:

    Which link is dead? This one:


    It appears to work from here. You might try using the link on the homepage?

  14. Anonymous says:

    Oy. We really do try, I promise.

    Can you send me the details of what broke (e.g. what UA you were sending) and I’ll pass it along to the folks who own the scripts?


  15. Anonymous says:

    Erm this might be a stupid question, but why oh why does MSDN have UA-dependent behavior?

    Even with the best of intentions, bugs and/or general ineptitude in this regard are inevitably going to lead to exposure to anti-competitive accusations!

  16. Anonymous says:

    Interesting tool. Seems to work just fine with Firefox except one annoying bug: when Fiddler isn’t running, Firefox can’t connect to the proxy while IE can. Any ideas?

  17. Anonymous says:

    IE/WinINET exposes an api to "announce" that a new proxy server should be used (in this case, Fiddler). FireFox exposes no such API.

    To make life more convenient when using Fiddler + non-WinINET clients, Fiddler writes a BrowserPAC.js file to its scripts folder when it starts and closes. This file is a "proxy autoconfiguration" file in the legacy Netscape PAC format. By pointing FireFox at this file, FireFox will use Fiddler when Fiddler is running, and not try to use Fiddler when Fiddler is not running.

    The problem you’ve hit is that FireFox (doubtless for performance reasons) only appears to load this file once– at FireFox boot time.

    While you can dig into the advanced FireFox UI and click the "Load script" button on the Proxy configuration dialog, you are better off simply restarting FireFox.

    Or, you can not use the PAC file and instead use one of the assorted FireFox plugins to put a "use what proxy" button directly on the FireFox chrome.


  18. Anonymous says:

    Regarding UA-dependent site design: Often to our peril, we try to do the best thing for our customer. When a particular version of a browser misbehaves, we often try to work around the problem so our site is visible in this browser. The danger is that the bug later gets fixed and our site isn’t updated to expect the fix.

    It’s a tough situation (where we’ve made the wrong call from time to time) but it’s always helpful if when reporting an issue, details about how to reproduce the problem are also reported, so we can fix the problem.

  19. Anonymous says:

    Jep. That’s a redirect.

    Oh wait … it’s has a broken browser sniffer. Switching my UA helps.

    Is there really no one at microsoft who knows how to set up a working website?

  20. Anonymous says:

    Firefox has similar tools:

    1. An extension "Live HTTP Headers" for HTTP debugging, it’s a much smaller download than the tool mentioned above and does most of the things Fiddler does. I find it very useful and works on all the platforms Firefox works on.

    2. Built in ‘View Selection Source’ in the context menu when you select some text and open the context menu.

    3. DOM Inspector (an optional component) that shows you the document tree of HTML, XML and XUL documents.

  21. Anonymous says:

    I can’t get to with any browser…. is it dead?

  22. Anonymous says:

    The site had some downtime on the 3rd, but it was up again within a few hours. Sorry about that.

    Right now Fiddler is hosted on my personal domain (owing to its unofficial origins). It will be moving to a Microsoft-hosted site in the near future.

  23. Anonymous says:


    Is there any chance that Fiddler might be open-sourced?

    Part of why I ask is that I am trying to familiarize myself with .Net programming the same way I’ve learned other languages (i.e., by learning from examples of working code), and the examples in much of the MSDN documentation are less than stellar in illuminating how to make things dance. Fiddler looks like it would be a treasure trove of insight into how to work both ends of HTTP sessions, not to mention handling the content thereof.

  24. Anonymous says:

    We’re looking into providing the Fiddler source on GotDotNet as a sample project.

    One downside to using this code for learning purposes is that the code is written by me, not a professional developer. Thus, it’s sloppy and not particularly well architected. I wrote this application to learn C#. Hence, there’s much that could be better.

    Another challenge is that the code uses two components for which I purchased a development license. I cannot redistribute those components for developers, so certain parts of the functionality would be limited in any shared source effort.

    One very exciting development is the new .NET Framework 2.0. It includes several awesome new classes that will eliminate the need to write some of the trickiest parts of Fiddler, including the Compression part (which is currently supported via the excellent Xceed Compression library).

    If I had to guess how it will all turn out: When .NET 2.0 ships, I’ll clean up the Fiddler code, eliminate the components, utilize the new 2.0 classes, and ship version 1.1 as Shared Source on GotDotNet. (Please do not read this as a guarantee, as I can’t make one yet)

    In the meantime, if you have any particular questions about how to do this or that, simply shoot me an email and I’ll try to help you out. Also, I can talk about those topics in a future column.



  25. Anonymous says:

    The Cave &raquo; I Love Fiddler

  26. Anonymous says:


    I’m trying to use fiddler AND dotNet’s fitness framework. I’m writing my test fixtures with it and wondering if you have any suggestions regarding the use of fiddler + fitnesse.

    The problems I’m getting are mostly related to NTLM, otherwise, the fiddler is an excellent tool! Thanks a lot.

  27. Anonymous says:

    Can you provide a URL to the tool you’re referring to? I wasn’t abel to turn anything up in a quick search.

    Feel free to post to the Fiddler discussion forum at



  28. Anonymous says:


    I just downloaded and tested Fiddler and it is a very usefull http tool.

    I developed another tool similar to fiddle with other features (dom inspector, javascript console, etc) that can also be usefull for developers.

    It is delivered as a toolbar and horizontal bar for IE. It’s the DebugBar :

    Do not hesitate to test it adn send feedback, we are working on new versions and new features.

    Best Regards.

  29. Anonymous says:

    Great tool. I have been using a tool called Paros , which works in a similar manner. This tool is more powerful (if less user friendly… which is always the tradeoff with powerful tools).

    Nice job!

  30. Anonymous says:

    Are you suggesting that Fiddler is less user friendly? If so, please do drop me a line and let me know where I can improve it.

    Thanks, Eric

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