I love Fiddler, it is a great aide in developing web applications, web tests and in general figuring out what is happening between your browser and server.
That said, it can be rather frustrating to get Fiddler working under certain circumstances. Over the years I have collected the following answers to the most common annoyances. Most of the following information I found on blogs, forum posts, and newsgroups. I have collected it here for convenience.
Two of the most common errors and fixes when trying to use Fiddler for a proxy debugger.
1. [Fiddler] Connection to localhost. failed:
[Fiddler] Connection to localhost. failed.
Exception Text: No connection could be made because the target machine actively refused it
Go to Fiddler Options and uncheck 'enable IPv6'
The alternative would be to configure your local IIS to listen to IPv6 traffic but this won't help if you are using the built in dev server that comes with Visual Studio
2. If you are not seeing any traffic on your local machine:
IE7 and the .NET Framework are hardcoded not to send requests for Localhost through any proxies, and as a proxy, Fiddler cannot intercept such traffic. There are several fixes to this problem.
The quickest fix is to substitute your machine name for localhost in the url so instead of http://localhost:1841/MyPage.aspx use http://mymachinename:1841/MyPage.aspx.
Another fix is to insert a '.' (period) right after the localhost in the url. e.g. http://localhost.:1841/MyPage.aspx
You can also edit your local hosts file to alias 127.0.0.1 to something like 'local'
A great deal of information on specific configuration issues in a variety of client systems is available on the Fiddler web site on the Configuring Clients page.
Enjoy!
Robert Porter