Debugging .NET Applications With Fiddler

If you want to debug a .NET application with Fiddler, you can force the application to proxy through Fiddler by adding this section to the application’s web.config or app.config:

<!-- The following section is to force use of Fiddler for all applications, including those running in service accounts -->  <system.net>
 <defaultProxy>
  <proxy autoDetect="false" bypassonlocal="false" proxyaddress="http://127.0.0.1:8888" usesystemdefault="false" />
 </defaultProxy>
</system.net>

Fiddler listens on port 8888 by default, but if you are on a shared system, you might run into a situation where that port is already in use by other users who are also using Fiddler.

In this case, you can easily change the port that your Fiddler is listening on under TOOLS | OPTIONS | CONNECTIONS tab:

fiddler_options

If you change the port, be sure to also change it in the proxyaddress URL in the configuration above.

For reference, here’s the relevant Fidder documentation page for this topic:

https://docs.telerik.com/fiddler/Configure-Fiddler/Tasks/ConfigureDotNETApp