Known Issues

Some traffic seems to be missing?

See Missing Traffic.

I get a System.NET.WebException "The underlying connection was closed" when calling into WebServices.

When debugging a .Net application through Fiddler, you may see a System.Net.WebException, with message "The underlying connection was closed: A connection that was expected to be kept alive was closed by the server."

This is a bug in your application (it should handle this type of exception).

Note: This problem is very unlikely in Fiddler 2.2.8.5 and later, due to enhanced client connection reuse support.

Sometimes Fiddler throws an out-of-memory exception?

Sometimes, Fiddler may show a dialog containing the following text:

  Exception of type 'System.OutOfMemoryException' was thrown.
     at System.IO.MemoryStream.set_Capacity(Int32 value)
     at System.IO.MemoryStream.EnsureCapacity(Int32 value)
     at System.IO.MemoryStream.Write(Byte[] buffer, Int32 offset, Int32 count)
     at Fiddler.Session.Execute(Object objThreadstate)

Fiddler works by storing the entire request and response in memory.  If you are performing a huge download (hundreds of megabytes) it's possible that Fiddler cannot find a free memory block large enough to hold the entire contiguous response, and hence you'll run into this "out of memory" problem.  It's also possible that if you have thousands of sessions in the Fiddler session list, even a relatively small memory block will not be available to store a response a few megabytes in size. You can reduce the incidence of this problem by clearing the Web Sessions list (CTRL+X) or configuring it to automatically trim to the most recent two hundred sessions (Click the Filters tab, and click the "Keep only the most recent sessions" option at the bottom).

Developers can learn more about this here: http://blogs.msdn.com/ericlippert/archive/2009/06/08/out-of-memory-does-not-refer-to-physical-memory.aspx and here http://blogs.msdn.com/b/dotnet/archive/2011/10/04/large-object-heap-improvements-in-net-4-5.aspx.

Update: Fiddler2 now supports running on 64bit computers. If you're on a 64-bit machine, you'll never hit a problem.

If you're on a 32-bit machine, you can avoid out-of-memory errors when downloading huge files by adding the following code inside the OnPeekAtResponseHeaders function inside Rules > Customize Rules. The line in red will cause Fiddler not to keep a copy of the large file:

// This block enables streaming for files larger than 5mb
if (oSession.oResponse.headers.Exists("Content-Length"))
{
  var sLen = oSession.oResponse["Content-Length"];
  var iLen: Int32 = 0;
  if (!isNaN(sLen)){
    iLen = parseInt(sLen);
    if (iLen > 5120000) {
      oSession.bBufferResponse = false;
      oSession["ui-color"] = "yellow";
      oSession["log-drop-response-body"] = "save memory";
    }
  }
}

If you're using FiddlerCore or writing a Fiddler Extension, you can use code like this:

           Fiddler.FiddlerApplication.ResponseHeadersAvailable += delegate(Fiddler.Session oS)
           {
              // This block enables streaming for files larger than 5mb
                if (oS.oResponse.headers.Exists("Content-Length"))
                {
                    int iLen = 0;
                    if (int.TryParse(oS.oResponse["Content-Length"], out iLen))
                    {
                        // File larger than 5mb? Don't save its content
                        if (iLen > 5000000)
                        {
                            oS.bBufferResponse = false;
                            oS["log-drop-response-body"] = "save memory";
                        }
                    }
                }
            };

I get certificate errors or .NET security exceptions when debugging with Fiddler2.

Fiddler2 relies on a "man-in-the-middle" approach to HTTPS interception.  To your web browser, Fiddler2 claims to be the secure web server, and to the web server, Fiddler2 mimics the web browser.  In order to pretend to be the web server, Fiddler2 dynamically generates a HTTPS certificate chained to its own root certificate. 

The Fiddler root certificate is not trusted by your application (since Fiddler is not a Trusted Root Certification authority), and hence while Fiddler2 is intercepting your traffic, you'll see a HTTPS error message in your browser or receive a security exception in your .NET client application.  You can reconfigure Windows to trust Fiddler's bogus root to avoid error messages and enable logon to services like Passport and .NET Web Services. Note that you should never make this configuration change on a non-Test machine.

See the Decrypting HTTPS traffic with Fiddler2 for information on resolving this issue.

Fiddler's "Automatic Authentication" feature doesn't work when server and client are on the same machine?

If IIS and the client are on the same machine, then a feature called "Loopback protection" is causing the authentication request to fail because your computer recognizes that it is authenticating to itself, and it is unexpected (due to the proxy).

You'll need to set DisableLoopbackCheck=1 as described here: http://support.microsoft.com/kb/926642

Fiddler crashes on startup complaining about the Tahoma font

---------------------------
Sorry, you may have found a bug...
---------------------------
Fiddler has encountered an unexpected problem.

Font 'Tahoma' does not support style 'Regular'.
Source: System.Drawing
at System.Drawing.Font.CreateNativeFont()

This can happen if you have the Microsoft Word 97 viewer installed. That tool sets the registry key HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Fonts\Tahoma (TrueType) to tahoma.FOT.
To fix the issue, change the following registry key from:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Fonts
"Tahoma (TrueType)"="tahoma.FOT"

to

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Fonts
"Tahoma (TrueType)"="TAHOMA.TTF"

If that does not help, you may be missing the Tahoma font on your computer (it should be in c:\windows\fonts), or the .NET Framework installation is corrupt. If you see the Font file, then try reinstalling the .NET Framework and all updates from WindowsUpdate.

Fiddler crashes on startup complaining about the "Configuration System"

---------------------------
Sorry, you may have found a bug...
---------------------------
Fiddler has encountered an unexpected problem. If you believe this is a bug in Fiddler, please copy this message by hitting CTRL+C, and submit a bug report using the Help | Send Feedback menu.
Configuration system failed to initialize
Source: System.Configuration
at System.Configuration.ConfigurationManager.PrepareConfigSystem()
at System.Configuration.ConfigurationManager.GetSection(String sectionName)

System.Configuration.ConfigurationErrorsException: Unrecognized configuration section system.serviceModel. (c:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Config\machine.config line 134)
or

System.Configuration.ConfigurationErrorsException: Unrecognized configuration section runtime. (C:\Program Files (x86)\Fiddler2\Fiddler.exe.Config line 2)

This error message indicates that one of the .NET Framework's configuration files is corrupt. The most common fix for this is to visit WindowsUpdate and install all available .NET Framework updates. If that doesn't work, try re-installing the .NET Framework. If that doesn't work, try editing the file specified in the error message to correct whatever the error message is complaining about.

Fiddler Crashes on Startup with an unhelpful message box

If you see this message box when starting Fiddler:
 
   

...it generally means that your .NET Framework installation is corrupt.  If you uninstall and reinstall the .NET 2.0 Framework, the problem is usually resolved.

Obscure problems...

1. If you're seeing incomplete HTTP Responses, ensure "Use HTTP1.1 through proxy servers" is checked on IE's Tools | Internet Option | Advanced tab.  (Or in your browser of choice).
2. When connecting to http://localhost on a WindowsXP version of IIS, you may see many HTTP/403 errors.  This is caused by WindowsXP's 10 connection limit.  To reduce the incidence of this problem, ensure that "Reuse Connections to Servers" is checked in the Tools | Fiddler Options | Connections dialog.
3. Microsoft ISA Firewall client may cause Fiddler to detach.  Learn more.
4. When starting Fiddler under nonadmin account (ordinary User) you may see an error message:
   
   Unable to bind to port [Localhost: 8888]. This is usually due to another running copy of Fiddler.
   (An attempt was made to access a socket in a way forbidden by its access permissions)
   
   Fix:
   Close Fiddler.
   Using REGEDIT, add a new STRING under HKCU\Software\Microsoft\Fiddler2 named ExclusivePort with value False

Other problems?

Got a problem not listed above?  Use the "Contact" link to send me mail.  Thanks!