Chaining Fiddler Everywhere to other upstream proxies?

[Jang]

Hello, for some tests, I'd like to use Fiddler to route through the corporate proxy network and, in a specific case, process requests/responses directly with mitmproxy scripts (because Fiddler Everywhere doesn't yet offer the same functionality as FC).

So I connect Fiddler to mitmproxy by setting the gateway proxy to mitmproxy listening at localhost:9090.

But it seems that Fiddler doesn't respect this setting. It always routes through the internet normally.
Even if I intentionally configure the wrong proxy port, the connection persists.

What should I do? Maybe I got wrong something??

 

I'm using Fiddler in Linux and already added certificates of Fiddler and mitmproxy to the system (actually i'm using the same one).

[Nick Iliev]

Hello Jang,

 

The first thing to do is add the http:// protocol prefix to your proxy addresses. For example, as follows:

http=http://127.0.0.1:9090;https=http://127.0.0.1:9090

Ensure you have started the upstream proxy before setting up the gateway in Fiddler. After setting up the gateway, restart the Fiddler Eveyrwhere application with elevated privileges. They also ensure that you are not using the Fiddler's system capturing mode, which (when used) will try to change the OS proxy settings and effectively override your MITM proxy settings. Instead, use one of the other available capturing modes while leaving the MITM as a system proxy.

If that does not help, note that depending on the specific Linux distro, Fiddler handles the manual proxy configuration differently. So, you could be using a distro, and the gateway setting is not working correctly. If that is the case, please provide more details on the Linux distribution and desktop environment you are using.

 

Regards,
Nick Iliev
Progress Telerik

[Jang]

Hello, I tried your suggestion, but it doesn't work. :)

I manually edit the proxy configuration, close and reopen Fiddler, but the problem persists. (I tried repeating some capture requests from a snapshot, It still got direct connections with real host certificates)

I'm not getting about "elevated privileges".
Since Fiddler Linux is an AppImage, do I need to open it with sudo?

I'm using Debian Testing/Ubuntu 24.04 and and both use XFCE4.

[Jang]

Btw, as you had suggested, proxy string placeholder in Fiddler quiet confusing.

[Zhivko]

Hello Jang,

From the first attached screenshot, I see that you have correctly set the manual proxy configuration string to "http=127.0.0.1:9090;https=127.0.0.1:9090".  Please revert it back to that, without the URI scheme.

I believe there was a slight misunderstanding in the previous reply, so let's start over.

First of all, let's talk about the manual proxy config. When you set it to a value, Fiddler will try to connect to the gateway and if the connection fails, we will ignore that gateway. You've already observed that behaviour by putting the incorrect port number and Fiddler was still able to capture traffic. That's why it's important to start the mitm proxy first and then Fiddler  (or before you change the Gateway option to Manual proxy config).

As for the second suggestion, I do believe that you can use all capturing modes without an issue, since we will send that traffic to MITM proxy anyways. The question is which MITM capturing mode are you using? If you are using some of the transparent options, it might be possible that they are intercepting the traffic before it even gets to Fiddler. 

You can disregard the comment about elevated privileges for now.

If you are still having issues, after setting the string back to ""http=127.0.0.1:9090;https=127.0.0.1:9090" and making sure mitmproxy is already listening on that port., please attach the Fiddler logs, so I can investigate further. You can access them by following the instructions here: https://docs.telerik.com/fiddler-everywhere/knowledge-base/accessing-fiddler-logs#accessing-the-logs-through-the-ui

Regards, Zhivko Progress Telerik