Fiddler causes my Internet access to stop working

9 posts, 0 answers
  1. Luis
    Luis avatar
    30 posts
    Member since:
    Aug 2009

    Posted 04 Nov 2015 Link to this post

    I have Windows 8.1 installed on my computer and regularly use Fiddler to capture web traffic.

    Recently, however, when I open Fiddler and it strats to capture web traffic, my Internet connectivity dies.

    The error I get when I open IE is "the proxy server isn't responding." In Chrome, I get "Could not connect to proxy server" with the following error: Error code: ERR_PROXY_CONNECTION_FAILED.

    Fiddler doesn't even capture any of the requests going out. The weird thing is that Fiddler was working ok just some days ago and nothing was recently installed on my system.

    Searching the Internet for 5 hours, trying everything, and no effective response.

    This also had no effect: http://www.telerik.com/blogs/fiddler-and-internet-explorer-11-on-windows-8-1​

    It seems that the proxy server created by Fiddler is simply not attending to any traffic.

    If I close Fiddler or disable the capture mode my internet come back to normal.

    Uninstall and reinstall Fiddler does not solve the problem, neither restart Windows.​

    Why is Fiddler having this problem, and how can I fix it?

  2. Eric Lawrence
    Admin
    Eric Lawrence avatar
    833 posts

    Posted 05 Nov 2015 Link to this post

    99% of the time, this is caused by running a 3rd-party firewall which is blocking access to Fiddler.

    1% of the time, this is caused by plugging a Windows Phone device into your PC over USB. The Windows Phone team steals Fiddler's default port (8888) from it.

    Running Help > Troubleshoot and updating your question with its output may help.

    Regards,
    Eric Lawrence
    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
  3. Luis
    Luis avatar
    30 posts
    Member since:
    Aug 2009

    Posted 05 Nov 2015 in reply to Eric Lawrence Link to this post

    Hello Eric Lawrence, thanks for the support.
    I saw that you also answered on StackOverflow, I appreciate the help.

    As for the firewall I assure you that it is disabled and still not solved the problem.

    Also checked the port 8888, no other process is taking up the same and also tried to change this port in the Fiddler configuration, which also failed to resolve.

    Running Help> Troubleshoot had no effect, simply appears the same error in any browser, it seems that even the local traffic is compromised.

    I do not know what to do.

  4. Luis
    Luis avatar
    30 posts
    Member since:
    Aug 2009

    Posted 05 Nov 2015 in reply to Luis Link to this post

    Is there any way to know if Fiddler is receiving any signal?
  5. Eric Lawrence
    Admin
    Eric Lawrence avatar
    833 posts

    Posted 05 Nov 2015 Link to this post

    I'm not sure what you mean by "Running Help> Troubleshoot had no effect", what, if anything, did you see in Fiddler's Web Sessions list when you ran this command? If you use Fiddler's Composer tab to send a request, what do you see?

    What firewall do you have installed but "disabled"? What security software do you have installed? Do you have any custom networking (e.g. VPN/RAS) software installed?

    Regards,
    Eric Lawrence
    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
  6. Luis
    Luis avatar
    30 posts
    Member since:
    Aug 2009

    Posted 05 Nov 2015 in reply to Eric Lawrence Link to this post

    Sorry if I was not clear enough, English is not my native language.

    Running Help> Troubleshoot displays the same error message that any other address. On Google Chrome that is "ERR_NETWORK_ACCESS_DENIED" and something similar on other browsers like IE and Firefox.

    The only one Web Session I see is when opening Fiddler and it tries to check for updates (see the attached image).

    Using Fiddler's Composer tab to send a request to www.google.com indeed come with some results (see the attached image).

    My security software is Kaspersky Internet Security 2016 which I disabled and shut up the process during the tests, besides I also deactivated the Windows firewall.

    I don't have any custom networking (e.g. VPN/RAS) software installed.

  7. Luis
    Luis avatar
    30 posts
    Member since:
    Aug 2009

    Posted 05 Nov 2015 in reply to Luis Link to this post

    I just notice something strange the option "Fiddler Options>Connections>DefaultLAN " is disabled and greyed out, so I can not enable it (see the attached image).

    Do you think this has something to do with the problem?

  8. Luis
    Luis avatar
    30 posts
    Member since:
    Aug 2009

    Posted 05 Nov 2015 in reply to Luis Link to this post

    My mistake, this option "DefaultLAN" is enabled automatically when you enable the traffic capture.
  9. Eric Lawrence
    Admin
    Eric Lawrence avatar
    833 posts

    Posted 06 Nov 2015 Link to this post

    Hi, Luis--

    ERR_NETWORK_ACCESS_DENIED is the sort of error message you would see if access were blocked by your firewall software.

    We've never seen this error occur without a firewall blocking access, and all of the pages I've seen on Google for "Chrome ERR_NETWORK_ACCESS_DENIED" relate to a firewall.

    From the Chrome source code:

      int MapConnectError(int os_error) {  
          switch (os_error) {  
            // connect fails with WSAEACCES when Windows Firewall blocks the  
            // connection.  
            case WSAEACCES:  return ERR_NETWORK_ACCESS_DENIED;

    Can you please elaborate on how specifically you disabled each firewall?

    Regards,
    Eric Lawrence
    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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