Fiddler causes my Internet access to stop working

[Luis]

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?

[Steven]

I'm facing issue with Zoom meetings. Whenever I start/join a call it takes a minute-two instead of a few seconds.

OS: Windows 10

[Dimitar Tachev]

Hi,

According to the Zoom documentation, they recommend bypassing the zoom.us and *.zoom.us hosts when using a proxy.

In Fiddler, you can bypass hosts through the Connections options.

I hope this helps,
Dimitar

[Мелетий]

I've faced the same issue and in the end it turned out that I had proxy configured to virtual machine that was stopped. All browsers automatically ignored it, but fiddler didn't and didn't show any notification that proxy server is unreachable. When I turned proxy off it all started to work as usual. Hope it will be helpful for someone

[Buck]

This helped me.  I forced quit fiddler on a mac, and it kept my wifi locked in proxy mode for http and https.  no wifi would work until i turned the proxy off.

thanks

[Ross]

Awesome.  For completeness, I had my Android phone plugged in and it also blocked Fiddler from working correctly.  Presumably something to do with blocking ports.

[Greg]

I have the same problem intermittently with Windows 10.  Sometimes when Fiddler exits it does not remove the proxy settings and they are left grayed out so you can't change them via the UI. Have to edit registry settings.

 

 

[Max]

I have a similar issue. I have started to use Fiddler recently, and today the previous version of F is not working anymore. It automatically downloaded Fiddler 2.0 and broken my Internet connection. It was on Win10. I can ping any destination by ip or url, but none of my browsers don't work anymore. And Fiddler just showing me blank white screen. I had to uninstall it, reboot, nothing is working. Reinstalling gives the same result.

My fiddler account doesn't work on this site, and I had to create account to write this message.

Before I decided to capture traffic on Windows I have figured out, that my Debian 10 device also don't want to work with old version of F, and decided to update. No traffic is capturing.

Nothing at all is working, Internet is broken. I need to do a lot of urgent work now, but nothing is working! So much time spent! I'm very angry and unsatisfied. The worst software experience for the last years!!!

[Nick Iliev]

Hello Max,

I am sorry to hear about your experience with the Fiddler proxy! The thing is that both Fiddler Classic and Fiddler Everywhere are setting the system proxy to manual proxy. In some corner cases, the Fiddler application cannot reset the OS proxy setting to its initial state. In such a case, the OS proxy will continue to use the manual configuration, and this will cause you to lose your internet connectivity. Restart/reboot of the OS won't help as the OS proxy setting are not reset, and the connection will continue to use the manual proxy configuration (which won't be valid when Fiddler is not working).

The solution should be to reset your OS proxy settings:

- On Windows 10, go to Proxy Settings > Manual Proxy Setup and remove the Fiddler proxy.
- On macOS go to System Preferences > Network > <active network adapter> Advanced > Proxies and reset the Web Proxy (HTTP) and the Secure Web Proxy (HTTPS).
- On Linux, go to the related OS proxy and explicitly reset the manual proxy configuration.

If you are still experiencing a leaked proxy (the proxy setting not being reset when Fiddler is stopped) you could send us the FIddler Everywhere log files (Help > Open Application Logs Folder) so that we could investigate your case and improve the product. My email is nikolay.iliev at progress.com - in case the Fiddler log files are containing sensitive information that you don't want to post in this public thread.

 

The solution is to explicit

 

[Eric Lawrence]

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

[Luis]

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.

[Luis]

Is there any way to know if Fiddler is receiving any signal?

[Eric Lawrence]

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

[Luis]

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.

[Luis]

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?

[Luis]

My mistake, this option "DefaultLAN" is enabled automatically when you enable the traffic capture.

[Eric Lawrence]

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