Known Issues

Some traffic seems to be missing?

Do you have any Traffic Filters enabled?  Check in the status bar, Filters tab, and Process Filter toolbar.

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 session 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

Fiddler2 now supports running on 64bit computers; you are unlikely to ever encounter an out-of-address-space problem on such computers.

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 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!