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Sometime when you're trying to track down a bug in a web application you need to
roll your sleeves up and get your hands dirty. I was recently forced to dust off a
tool I haven't used in a while to help me get to the root of a particularly
nasty little problem I was having. As I was installing the latest version of
the little gem, I realized that most of our visitors have probably never even heard of
Fiddler. This article is an attempt to rectify that situation.
What is Fiddler?
I'll leave the job of describing Fiddler to the pros... according to
Fiddler's Home Page:
Fiddler is a HTTP Debugging Proxy which logs all HTTP traffic between your
computer and the Internet. Fiddler allows you to inspect all HTTP 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,
including Internet Explorer, Mozilla Firefox, Opera, and thousands more.
So what does that mean? Well basically... while it's running, Fiddler will
collect almost every detail you could possibly want about every HTTP request
your computer makes.
Installation
Before we get ahead of ourselves, the first step is to download and install Fiddler.
As of this writing there are two different versions of Fiddler available:
Fiddler 1.x for .NET Framework version 1.1 users and
Fiddler 2.x for users with .NET Framework version 2.0 or later installed.
Unless your computer doesn't have .NET 2.0 or later, you should
download and install Fiddler 2.x.
Installation is fast and straight-forward.
What Does Fiddler Capture?
So exactly what information does Fiddler snag for you? Pretty much anything and everything.
I'm not going to list everything because I'm sure to miss some stuff and by the time you finished
reading the list you could have already downloaded the tool and seen for yourself, but here are a
few screen captures to show you some of the highlights.
Fiddler will give you a quick graph of data transferred during a session by file type.
It's great for examining the request and response headers for cookies and form data.
It doesn't do anything too exciting for images, but it does display them right within the tool so you know what image you're dealing with.
It'll show you just the raw data if that's what you want.
It even lets you make custom HTTP requests right from within Fiddler. Better yet, you can use a previous request as a template and then modify it however you'd like.
Conclusion
Fiddler is a great little tool to have in your web application debugging toolbox.
It lets you see details about HTTP requests and responses that you just can't get from a browser.
As such, it's invaluable when dealing with things you usually can't see like caching, cookies,
form data, and headers. I hope you'll not only find Fiddler useful, but will also help me
spread the word about this wonderful little debugging tool.
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