Fiddler Web Debugger

Fiddler is a web debugging tool which is owned, developed, and distributed by Eric Lawrence.

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  • Blog Post: Visual Studio 2012, IIS Express, and Fiddler

    From the mail bag... Q: When I use IE10 on Windows 8 to visit an IIS Express instance using the following URL http://ipv4.fiddler:2468/, I get an unexpected error message. HTTP/1.1 400 Bad Request Content-Type: text/html; charset=us-ascii Server: Microsoft-HTTPAPI/2.0 Date: Wed, 22 Aug 2012 19...
  • Blog Post: My book “Debugging with Fiddler”, is now available

    I’m delighted to announce that my new full-length book on Fiddler is now available. Weighing in at 330 pages, it covers every feature in Fiddler, and provides the most comprehensive available documentation of how to develop Fiddler extensions (including Inspectors, Transcoders, and IFiddlerExtensions...
  • Blog Post: .NET HTTPS connections timeout on SNI TLS Warning

    Recently, a handful of folks have emailed me complaining that some HTTPS sites cannot be reached by their .NET programs or from any program when Fiddler is configured to decrypt traffic. Notably, these users only have problems when running on Windows Vista or later. So, what’s going on? When...
  • Blog Post: New Tricks in Fiddler 2.3.8.2

    The new Fiddler Beta has a few useful tricks. First, you can copy a DataURI to your clipboard (from Fiddler itself, or anywhere else): and use the new Edit > Paste Files as Sessions command: … to instruct Fiddler to parse the DataURI and create a new Session for it....
  • Blog Post: News on Fiddler Surveys and Telemetry

    For this morning’s post, I wanted to give everyone a quick picture of some of the data I've collected about Fiddler usage, how I plan to respond, and how I will continue to learn more. I’ve recently had the chance to run some surveys and spend some time with developers who are using Fiddler...
  • Blog Post: Generating HTML5 AppCache Manifests with Fiddler

    HTML5 introduces the concept of an Application Cache, which allows a web developer to provide a manifest of pages that should be cached to permit offline use. You can see an instance of AppCache over on the IETestDrive site ; if you examine the markup, you can find that the HTML tag contains a manifest...
  • Blog Post: Fiddler and Windows 8 Metro-style applications

    Over on StackOverflow, a new Windows 8 user asked how to get Fiddler working with new Windows 8 Metro-style applications. These applications work somewhat differently than classic desktop applications, and require a bit of special configuration work to get Fiddler to work properly. Fear not, however...
  • Blog Post: Fiddler and Channel Binding Tokens Revisited

    Just under a year ago, I wrote a blog post about how the new “Extended Protection” feature (also known as Channel Binding Tokens or CBT) prevented seamless decryption of certain authenticated HTTPS traffic when Fiddler is running. The quick recap is that CBT binds a set of NTLM or Kerberos...
  • Blog Post: Swapping out JQuery with Fiddler

    This morning, someone asked me to look into a site-compatibility problem on a HTML5 demo site. When loading the site into IE9 and IE10, the F12 Developer Tools’ Script Debugger showed the following error: Now, obviously, IE does support getElementsByTagName, and I confirmed that the...
  • Blog Post: Debugging Windows Phone 7 device traffic with Fiddler

    Back in October, I showed how to debug Windows Phone emulator traffic with Fiddler . Since then, I’ve acquired the LG Quantum phone, and naturally, one of my first goals was to start looking at the traffic from mobile Internet Explorer and some of my WP7 applications. The process for capturing...
  • Blog Post: Fiddler and Channel-Binding-Tokens

    Note: Please see this post for an update. Some users of Fiddler who have HTTPS Decryption enabled have found that some of their internal HTTPS sites that used to work properly with Fiddler now endlessly prompt for credentials while Fiddler is running. Even typing the correct credentials into the authentication...
  • Blog Post: Fiddler and the Windows Phone 7 Emulator

    Note: This post is part of a larger set on using Fiddler with Windows Phone 7 . This post covers the Windows Phone Emulator only. Recently, some developers have asked me why Fiddler no longer works with the Windows Phone 7 emulator. Fiddler properly captured traffic from the CTP version of the emulator...
  • Blog Post: New Features in FiddlerCore 2.2.9.9

    Version 2.2.9.9 of the FiddlerCore .NET class library has been released . This new version includes a number of new features that make FiddlerCore more useful for a variety of traffic monitoring and capture scenarios. With the exception of the final improvement (Server Read Callback), all of these improvements...
  • Blog Post: Capturing (and changing) a Web Site with Fiddler

    Often, the IE team will encounter a problem in a complicated production website, and will need to "reduce the repro" in order to determine the root cause of a given bug. This is made more complicated by the fact that sometimes the live website will change under us and we may lose the repro before the...
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