What Operation System parameters can be changed by Fiddler while it's running?

[Yamato]

I'm seeking the difference of Operation System (Windows10)  parameters between when Fiddler is running and not.
The behavior of secure network traffics can change somehow, like proxy setting, while Fiddler is running.
If someone knows more about this, please give me an assistant. Thanks!

[Nick Iliev]

Hello Yamato,

 

Fiddler acts as an MITM proxy, and as such, it could change the default TLS version (and the used cipher) and can also introduce a small delay in transitioning the HTTP requests and responses. Those and some other reasons why a request might behave differently when Fiddler is in the middle are highlighted in the following blog post:

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

Fiddler can also explicitly modify the default request and response through its AutoResponder feature (the alternative in Fiddler Everywhere is called "Rules").

Another possible difference is that Fiddler Classic supports older versions of TLS (TLS 1.1, TLS 1.2) and HTTP/1. In contrast, the actual request might be using newer protocol versions like TLS 1.3 and HTTP/2 (these versions are supported in Fiddler Everywhere).

Please be aware that Fiddler Classic is not in active development and offers no commitments for releases, patches, or tech support. By using this product, you assume all associated risks.

 

Regards,
Nick Iliev
Progress Telerik