Q:

Fiddler Causes Error to Go Away

I have an application set up in IIS that has Windows Authentication enabled. When I browse to this site in IE11 it prompts me for login credentials (which is strange because I'm logged on to the domain)
However, when I open up Fiddler it starts working perfectly fine, no prompts for authentication. When I close Fiddler again, it starts prompting me again. Any idea what could cause this? I'm trying to narrow down what exactly Fiddler is doing so I can figure out what I need to do to make the prompts on my app go away.

A:

I cover many possible culprits in my post Help, Running Fiddler Fixes My App. My guess here, though, is that maybe your server is trying to use Kerberos authentication when going direct and NTLM when you go through a proxy. 
To better debug this, a few things we need to know: Which browser? What's the hostname of the target server? Is it hosted on your current machine? If you disable Tools > Internet Options > Advanced > Use Windows Integrated Authentication (and restart) does it make a difference? What Zone (Right-click the page in IE, choose Properties) does the target run in?

