HTTP debugging with Fiddler

Microsoft Fiddler is an HTTP debugging proxy that displays raw web traffic as it travels between your PC and the network. Although there are browser plug-ins that do the same thing, such as ieHTTPHeaders and LiveHTTPHeaders, what really sets Fiddler apart is its ability to manipulate incoming or outgoing data, and its extensibility through .NET scripting.

These two MSDN articles show how to get the most out of Fiddler when developing and testing web applications:

• Fiddler PowerToy – Part 1: HTTP Debugging
• Fiddler PowerToy – Part 2: HTTP Performance

You can customise Fiddler’s behaviour using its event-based scripting subsystem, as shown in the FiddlerScript CookBook which gives plenty of examples. There is also a FiddlerScript Editor to make scripting even easier.