Web Debugging with Fiddler

 

Fiddler is a Web Debugging Proxy which logs all HTTP(S) traffic between your computer and the Internet. Fiddler allows you to inspect traffic, set breakpoints, and "fiddle" with incoming or outgoing data. Fiddler includes a powerful event-based scripting subsystem, and can be extended using any .NET language.

Fiddler is freeware and can debug traffic from virtually any application that supports a proxy, including Internet Explorer, Google Chrome, Apple Safari, Mozilla Firefox, Opera, and thousands more. You can also debug traffic from popular devices like Windows Phone, iPod/iPad, and others.

Below is the home screen for Fiddler:

Let's go through some of the most useful features of Fiddler quickly:

1. Web Sessions - On the left pane, all the traffic is captured. Using this session, we can compare the sessions by simply selecting two sessions, right clicking and choosing compare command.

1. Statistics - Statistics seen in the right pane shows the information for selected sessions in text and graphic format as seen below.

Inspectors

Inspectors allow you to visualize requests and responses in meaningful ways. When you double click on a session fiddler takes you to the Inspectors tab for that session. Below is a snapshot

QuickExec

There is a little textbox with black background color and green forecolor on the bottom of Fiddler form where you can run some very useful commands. To see a complete list of available commands type 'help' and enter. Its very handy in using some quick commands like setting and removing the breakpoints.

Breakpoints

We can put breakpoints on request or response using Web Session List. Breakpoints can be put using Rules menu>Auto Breakpoints or status bar or QuickExec. In the QuickExec box type in 'bp' and enter. This will show you a handy help popup dialog that lists the breakpoint commands.

When the breakpoint is set the next time you make a request or receive a response that fulfils the breakpoint's requirements the breakpoint is hit and you get the opportunity to change the traffic.

AutoResponder

AutoResponder is a very handy tool. It allows you to fake server's responses. If you think about it, Fiddler is sitting between you and the web server and nothing can stop it from returning the response locally without hitting the server, and that is basically what AutoResponder does. You can configure AutoResponder to respond to specific requests and let the others through, or you can ask it to only respond to the traffic you expect and block the rest. AutoResponder uses some rules to match the request. From Fiddler's reference

Request Composer

The Composer allows you to craft custom requests to send to a server. You can either create a new request manually, or you can drag and drop a session from the Web Sessions list to create a new request based on the existing request.

Filters

The Filters tab enables you to quickly filter out traffic that is not of interest.

Fiddler Extensions

There are some great Fiddler free to use extensions at http://www.fiddler2.com/fiddler2/extensions.asp

Fiddler Timeline

Fiddler's Timeline View allows you to visualize the HTTP(S) traffic on a "waterfall" diagram. Hovering over any entry will show more information about the entry. Double-clicking the entry will inspect that session.

Conclusion

I hope this gives a headstart for the ones who have not yet explored a web debugging tool powerful as Fiddler. If you want to dig deeper then Fiddler website and Fiddler Blog will be the place to look at.

References

http://www.fiddler2.com/Fiddler/help/

http://www.mehdi-khalili.com/fiddler-in-action/part-1

http://www.fiddlercap.com/FiddlerCap/

http://www.developer.com/lang/jscript/article.php/3631066/Fiddler-Can-Make-Debugging-Easy.htm