Debug Web Sites With Microsoft's Fiddler Powertoy


Techie Warning! This is complicated stuff.

Most people couldn't tell the difference between a web page view-source listing and a bowl of alphabet soup. In fact, the truth of what a web page is doing is far more complicated than what is in the source that your browser sees. But there are tools that let you see a lot more detail.

You may be suspicious about where a page is drawing some of its contents. What headers did the page send? Did it send cookies? Maybe the page is slow and you want to see which element on the page is responsible.

Microsoft's Fiddler Powertoy is a web page debugger that lets you do all this and more. Install it and it registers itself as the system proxy for , and all HTTP communications from WinInet applications goes through Fiddler. (Note that WinInet is only one way that programs may perform HTTP communications, albeit a popular one.)

One quick gripe: The executable for installing Fiddler is not digitally signed and you may have noticed that it doesn't come from The About box clearly identifies the author as Eric Lawrence, a program manager on the Internet Explorer development team to whom the site is also registered, but it uses a Microsoft copyright. I've chosen to trust the executable and to recommend it to others because I read about it in an MSDN article on Microsoft's site, but I wish Microsoft and Eric Lawrence had gone to the trouble of signing the program.

To repeat, since it's the system proxy, **ANY** program that uses WinInet will have to deal with Fiddler. This includes a lot of adware, for example, and you can use Fiddler to check on whether a program is communicating when it shouldn't and with whom. Since Fiddler only shows HTTP it is easier to use for these purposes than more general network monitoring tools.

Updated: By default only WinInet apps will proxy through Fidder, but almost any program, including Mozilla and Firefox, can be set to do so.

Fiddler installs a toolbar icon in IE to launch it, or you can run it from Start->Programs or from Tools-Fiddler in IE. The big pane on the left shows your HTTP sessions, and there can be a lot of them. Click here to see a sample screen. You click on a session on the right and view information about it on the right side.

The Performance Statistics tab on the right shows the total size of the HTTP request and the number of requests in it, along with some fishy performance numbers. Their derivation is not explained. The Session Inspector tab is more interesting and complicated. You can view the headers, text and graphics for any individual request, and view data in hex if you wish. There's a request builder for you to custom-up an HTTP transaction with a server and view it.

There's a lot in this program and there's no point in explaining all the details of how to use it when the help file does that. Also see the MSDN article written by the program's author for a good intro to the program.

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