Unintended side effects of debugging with Fiddler

If you use Fiddler to debug your web-based applications, there may be an unintended side effect of using Fiddler that you may not know about.

First of all, the way in which Fiddler works is by intercepting the traffic that is normally sent between your client browser and the target server.  How does it do that?  By inserting itself as a proxy between itself and the browser!

You can verify this for yourself by checking the proxy settings of your Internet Connection through Internet Explorer:

1. Open up Internet Explorer
2. Click on Tools-->Internet Options
3. Click on the Connections tab
4. Click on the LAN Settings button
5. You should see a checkbox next to "Use a proxy server for your LAN"
6. If you click on the Advanced button, you should now see the proxy server settings that Fiddler uses:
• HTTP Proxy Address: 127.0.0.1 Port: 8888
• Secure Proxy Address: 127.0.0.1 Port: 8888

Well, one of the unintended side effects of this is that you may not be able to get to content on the Internet if Fiddler is not running!  This will particularly occur if you are working on a virtual machine that is configured to use a static IP address rather than a DHCP Bridged or NAT IP Address.

Of course, the solution to this is to simply remove the proxy settings from Internet Explorer's Internet Connection properties, but if you have not encountered this before, you may be wondering what just happened to your Internet Connectivity!

So hopefully if you stumble across this situation in the midst of one of your debugging sessions, now you will know how to resolve it quickly and efficiently!