Can't Get Fiddler to Capture local traffic to IIS

Question

I have recently installed the latest Fiddler (Fiddler4) and absolutely nothing that I try is working to get it to capture local traffic. I have an MVC application that is connecting to an MVC WebApi app both running on IIS, here's what I've tried:

• Fiddler Options > Connections > Monitor All Connections & Use PAC Script
• Changed the WebClient C# call to use the following URLs with no success:
  • http://machinename/app-name
  • http://localhostproxy.com/app-name
  • all of the ipv4 / ipv6 etc urls on this page http://fiddler2.com/documentation/Configure-Fiddler/Tasks/MonitorLocalTraffic
  • also tried using the . fix - http://localhost.
• Changed my "Default Website" in IIS to run under a new AppPool that uses my local account, and ran Fiddler as administrator
• added <system.net> <defaultProxy> <proxy bypassonlocal="False" usesystemdefault="True" /> </defaultProxy> </system.net> to my web.config of the MVC app.

Nothing has worked, I have no idea what to try. Any advice is greatly appreciated. Thanks, Shawn

Answer 1

I know you mentioned this in your OP, but this worked for me and was super simple to do, so I thought I would share in case others run into the same problem and don't want to go through the trouble of setting up a reverse proxy if they don't need to.

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I got this to work by simply changing the IIS Application Pool whose traffic I wanted to inspect to run under my identity, since Fiddler seems to only capture traffic of the logged in user by default. Just make sure your user has all of the necessary permissions required by IIS; I'm an admin so this wasn't an issue for me. And of course, don't forget to change the IIS Application Pool identity back when you are done.

The other option of course is to instead log in as the user that the IIS Application Pool is running as (if it's not a service account like Network Service) and then run Fiddler while logged in as that other user.