Fiddler not forwarding traffic if using "System Proxy"

[Nicholas Weise]

Hi,

Suddenly my Fiddler installation is no longer able to forward traffic to my upstream system proxy, if I use the Fiddler Gateway setting "Use System Proxy (recommended)". The proxy is manually set up in my Windows 10 Proxy settings to "http://devproxy.mycompany.net" port 8080 and works fine for e.g. requests from Chrome (without Fiddler).

In Wireshark I can see that if I use the the Fiddler Gateway setting "Manual Proxy Configuration", with "http=devproxy.mycompany.net:8080;https=devproxy.mycompany.net:8080" I can see that Fiddler forwards SSL "CONNECT devserver.mycompany.net:443" calls correctly to the proxy.

But if I use the Fiddler Gateway setting "Use System Proxy (recommended)", Wireshark shows that something, presumably Fiddler, instead tries to look up devserver.mycompany.net locally in DNS, where it cannot be found, and the CONNECT request results in a "502 Fiddler - DNS Lookup Failed" response.

Do you have any idea why Fiddler doesn't just forward the requests "raw" to the proxy in this case???

[Eric R | Senior Technical Support Engineer]

Hi Nicholas Weise,

I recommend configuring the WinINET settings for the upstream proxy. See the Chain to Upstream Proxy documentation for more details. Additional information is also available at the Fiddler Proxy Knowledgebase as well.

Please let me know if you need any additional information. Thank you for using the Fiddler Forums.

Regards,

Eric R | Technical Support Engineer
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[Nicholas Weise]

Hi Eric,

Thank you for your reply. I believe I have the proxy settings set correctly (see attached "WinINET settings.png"). It is just as if Fiddler doesn't "pick it up"...?

When the default "Use System Proxy (recommended)" is selected (see attached "Use System Proxy.png)", the Fiddler About dialog, shows "Gateway: No Gateway" (see attached "About with no Gateway.png") - and traffic is not passed through the proxy.

If instead I select "Manual Proxy Configuration" in Fiddler's Gateway Options (see attached "Manual Proxy Configuration.png"), the Fiddler About dialog correctly shows the IP-address of the Gateway proxy (see attached "About with Gateway.png) - and traffic is now correctly passed through the specified proxy.

[Eric R | Senior Technical Support Engineer]

Hi Nicholas Weise,

It appears that chaining to the Upstream Proxy configuration is working but these settings are persisting when choosing to use the System Proxy. I recommend ensuring the System Proxy settings are set to http=127.0.0.1:8888;https=127.0.0.1:8888 when both settings are configured. See below for reference.

System Proxy
Fiddler Use System Proxy checked
WinInet/System Proxy Value: http=127.0.0.1:8888;https=127.0.0.1:8888

Manual Gateway
Fiddler Manual Gateway checked Value: http=devproxy.mycompany.net:8080;https=devproxy.mycompany.net:8080
WinInet/System Proxy Value: http=127.0.0.1:8888;https=127.0.0.1:8888

Additionally, we are unable to provide support for WireShark and it is possible this is configuring something that conflicts with Fiddler.

Please let me know if this works as expected after confirming the recommended changes. Thank you and I look forward to your reply.

Regards,

Eric R | Technical Support Engineer
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