Why would Fiddler fix my connection issues to my WCF server?

Question

I have a WCF server hosting clientaccesspolicy.xml (for Silverlight clients) and I was testing access to it using Internet Explorer. When I accessed it using http://localhost/clientaccesspolicy.xml everything worked correctly, but when I used http://machinename/clientaccesspolicy.xml it wasn't retrieved.

I downloaded, installed and ran Fiddler and now the http://machinename/clientaccesspolicy.xml URL works correctly - even without Fiddler running.

Obviously this isn't a scalable solution to the problem so I was wondering if anyone know what Fiddler might have done to fix my issue, or what might have been causing my issue in the first place? Windows Firewall has been disabled throughout my testing.

Answer 1

It turns out our testing systems have Privoxy installed on them incorrectly - it successfully blocked the connection attempts because they weren't whitelisted, but didn't display the error message it should be configured to do so. Fiddler replaced the proxy with its own settings temporarily and hence let it work. Thanks (and an upvote) to Ladislav for the hint.