Fiddler2 and the ASP.NET Development server (Cassini)

If you’re debugging ASP.NET apps, it can sometimes be an advantage to be able to see the actual requests, and analyze file sizes, run-time rendered code and being able to ‘fiddle’ with the code on run-time. Well. Fiddler2 provides extremely useful features for doing so, but for people using the ASP.NET development server (Cassini) you have to do a few workarounds…

I’m running IE7 on Windows Vista (a great OS). IE7 automatically bypasses proxies for localhost, which is our main problem. When using the ASP.NET development server (Cassini), the URL looks like this one: http://localhost:49950/app/Default.aspx. When you open Fiddler2, you see that it hasn’t monitored the traffic to the localhost address. This can be fixed by applying a period (.) after localhost. So change the URL to this:  http://localhost.:49950/app/Default.aspx

In my case, that wasn’t enough. I got an error from Fiddler2 in my browser. If I disabled Fiddler2, it worked just fine. Then I added a rule to Fiddler2. To do this, in Fiddler2 go to Rules > Customize rules. (or hit CTRL + R). Find the OnBeforeRequest event-handler, and add the following code:

if (, 10)==”localhost.”)
{“localhost.”, “”);

Now – when you want to use Fiddler, just add a dot right after localhost in the URL, and Fiddler will start working!

That worked for me – hope it works for you too, if you’re having problems.