Firewall block on Windows Server 2012

2 posts, 0 answers
  1. Brendan
    Brendan avatar
    1 posts
    Member since:
    Mar 2018

    Posted 01 Mar 2018 Link to this post

    I have installed FIDDLER on a Windows 1012 Server on a corporate network with very rigid Firewall rules. When I run Fiddler I receive this error:

    06:43:53:2458 Fiddler Running... 
    06:43:53:2771 Fiddler.Network.AutoProxy> AutoProxy Detection failed. 
    06:43:53:2771 AutoProxy failed. Disabling for this network. 
    06:44:34:8166> Session #2 raised exception An existing connection was forcibly closed by the remote host 


    I suspect Firewall rules. I need to submit a Firewall Change Request to allow exceptions for Fiddler to operate on that server, but I need details from Fiddler Tech Support. Can you please identify the external targets / requirements which Fiddler will try to reach out to? (We need to set our Firewall to allow Fiddler to function)

  2. Simeon
    Simeon avatar
    238 posts

    Posted 07 Mar 2018 Link to this post


    Fiddler gets the initial system proxy settings on startup. Then Fiddler sets the system proxy settings to point to Fiddler. The default behavior of Fiddler is using the initial system proxy settings, it has read, in order to send the captured requests to this proxy, if any. I.e. Fiddler becomes the system proxy and the initial system proxy becomes the proxy of Fiddler.

    However, you could modify the Fiddler's proxy from the Gateway tab of Tools\Options...

    In your case it seems that the initial system proxy settings are set to use the WPAD protocol for proxy auto-discovery. However, if for some reason the protocol fails, Fiddler falls back and tries to send the request without using a proxy.

    I hope that this information is useful to you.

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