I don't see any traffic in Fiddler.
See the Configuring clients topic.
I don't see traffic sent to localhost or 127.0.0.1.
Some traffic seems to be missing?
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 126.96.36.199 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:
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
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).
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 using FiddlerCore or writing a Fiddler Extension, you can use code like this:
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
This can happen if you have the Microsoft Word 97 viewer
installed. That tool sets the registry key
NT\CurrentVersion\Fonts\Tahoma (TrueType) to tahoma.FOT.
Fiddler crashes on startup complaining about the "Configuration System"
configuration section system.serviceModel.
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:
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)
Using REGEDIT, add a new STRING under HKCU\Software\Microsoft\Fiddler2 named ExclusivePort with value False
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