Fiddler is a free proxy that logs HTTP(S) traffic on the host computer. It offers a rich user interface that supports inspection of requests and responses, setting breakpoints, and tampering with incoming or outgoing data. It also supports numerous transformations and previewers, e.g., unchunking and decompression via GZIP, DEFLATE ,or BZIP2, and displaying images directly within the preview pane.
Fiddler is freeware available for download in a stable v2.2.0.7 and a recently updated beta v2.x that provides an enhanced UI and bug fixes. There is also a deprecated v1.3 for developers still using the.NET Framework 1.1.
The most primitive use of Fiddler is to view the contents of single request and response, though this basic ability proves useful when developing rich web functionality with AJAX -- debugging problems or just trying to get a full picture of the AJAX traffic.
Fiddler also offers views into the traffic over many requests, with simple but useful timeline and visualization tools.
Beyond inspection and analysis, Fiddler also offers the ability to set breakpoints and tamper with data in requests and responses, especially useful for security and input sanitation testing.
Out of the box, Fiddler offers useful functionality, but it also has an event-based scripting subsystem allowing extensive customization, including a syntax-aware script editor. And for .NET developers, the most differentiating aspect of Fiddler is that it can be extended using any .NET language. There is good set of videos to help get started using Fiddler.
Fiddler will auto-configure the debugging proxy for IE6 and IE7, but the proxy is compatible with Firefox, Opera and virtually any application using the HTTP(S) protocol. For example, Firefox allows manually specifying a proxy through Menu > Preferences > Network > Settings > Manual Proxy Configuration: 127.0.0.1 Port 8888.
While Fiddler is unique in its tight integration with .NET, there are many other well-known tools that offer similar functionality:
- Tamper Data Firefox Addon - allows manipulation of HTTP headers and post params
- FireBug Firefox Addon - provides traffic analysis and live manipulation of DOM elements, CSS, and JavaScript
- Port Swigger's Burp Proxy - an HTTP(S) proxy similar to Fiddler but more security focused
- Charles Web Debugging Proxy - an HTTP proxy similar to Fiddler but with more features focused on Adobe Flash development