Question about how Fiddler works

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Fiddler Classic Windows
Ward
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Ward asked on 16 Sep 2022, 08:59 PM

Afternoon folks...

I have an application that keeps sending DNS traffic over HTTPS and ultimately fails because it resolves as external where it should be internal. So I'm tracing it down using Fiddler. The application is spawned by Edge WebView2 and I can see the requests to Google and CloudFlare over :443 for DNS (through WireShark). What is very odd, is when I am running Fiddler the application never fails. So I'm trying to understand what Fiddler does while it's capturing so I can pick it apart and see which bit is making this work.

I know it uses WPAD for a bit of its operations too, I have that enabled on my machine now too. I killed DoH on my laptop too as well as Secure DNS in Edge/Chrome.

Anyone know the innerworkings of Fiddler (obviously not at an intellectual property level) but more what is it doing while it's running.

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Nick Iliev
Telerik team
answered on 20 Sep 2022, 06:24 AM

Hello Ward,

 

The most common reasons why Fiddler "fixes" your application are listed in the following blog post:

https://www.telerik.com/blogs/help!-running-fiddler-fixes-my-app- 

 

Regards,
Nick Iliev
Progress Telerik

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Fiddler Classic Windows
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Ward
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Nick Iliev
Telerik team
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