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Http Debugging with Fiddler

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The information published in this post is now out-of-date and one or more links are invalid.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

Thanks
-Dave

Comments

  • Anonymous
    January 01, 2003
    Thanks a lot for posting this, and thanks to the developer of the program. Just downloaded it and it looks great.
  • Anonymous
    January 01, 2003
    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.

    http://www.doyousnap.com/portal/albums/7/121.aspx

    Howard Hoy

  • Anonymous
    January 01, 2003
    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: http://www.fiddlertool.com/fiddler/version.asp



  • Anonymous
    January 01, 2003
    Thanks.. nothing like using the same tool on itself. :-)
  • Anonymous
    January 01, 2003
    Ahh cool a good article, long time coming, I have been using fiddler for quite some time now, nice little tool.
  • Anonymous
    January 01, 2003
    Can someone post the results of when you run fiddle through fiddler?
  • Anonymous
    January 01, 2003
    <<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?
  • Anonymous
    January 01, 2003
    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)
  • Anonymous
    January 01, 2003
    "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.

    Thanks!

    -E
  • Anonymous
    January 01, 2003
    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)
  • Anonymous
    January 01, 2003
    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.
  • Anonymous
    January 01, 2003
    The comment has been removed
  • Anonymous
    January 01, 2003
    Which link is dead? This one: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnwebgen/html/IE_IntroFiddler.asp

    ?

    It appears to work from here. You might try using the link on the msdn.microsoft.com/ie homepage?
  • Anonymous
    January 01, 2003
    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?

    Thanks!
  • Anonymous
    January 01, 2003
    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!
  • Anonymous
    January 01, 2003
    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?
  • Anonymous
    January 01, 2003
    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.

    -E
  • Anonymous
    January 01, 2003
    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.
  • Anonymous
    January 01, 2003
    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?
  • Anonymous
    January 01, 2003
    The comment has been removed
  • Anonymous
    January 01, 2003
    I can't get to http://www.fiddlertool.com with any browser.... is it dead?
  • Anonymous
    January 01, 2003
    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.
  • Anonymous
    January 01, 2003
    Eric,

    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.
  • Anonymous
    January 01, 2003
    The comment has been removed
  • Anonymous
    January 01, 2003
    The Cave &raquo; I Love Fiddler
  • Anonymous
    January 01, 2003
    Eric,

    I'm trying to use fiddler AND dotNet fitnesse.org'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.
  • Anonymous
    January 01, 2003
    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 http://www.fiddlertool.com/bbs/

    Thanks,

    Eric
  • Anonymous
    January 01, 2003
    Hi,

    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 : http://www.debugbar.com

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

    Best Regards.
  • Anonymous
    January 01, 2003
    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!
  • Anonymous
    January 01, 2003
    The comment has been removed