Fiddler not forwarding traffic if using "System Proxy"

4 posts, 0 answers
  1. Nicholas Weise
    Nicholas Weise avatar
    2 posts
    Member since:
    Aug 2019

    Posted 06 Aug Link to this post

    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???

  2. Eric R | Technical Support Engineer
    Admin
    Eric R | Technical Support Engineer avatar
    230 posts

    Posted 08 Aug Link to this post

    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
    Progress Telerik
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  3. Nicholas Weise
    Nicholas Weise avatar
    2 posts
    Member since:
    Aug 2019

    Posted 12 Aug in reply to Eric R | Technical Support Engineer Link to this post

    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.

  4. Eric R | Technical Support Engineer
    Admin
    Eric R | Technical Support Engineer avatar
    230 posts

    Posted 19 Aug Link to this post

    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
    Progress Telerik
    Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Feedback Portal and vote to affect the priority of the items
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