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

Recently, however, when I open Fiddler and have it capture web traffic, my Internet connectivity dies. The error I get when I open IE is "This page cannot be displayed." In Chrome, I get "The web page is not available" with the following error: Error code: ERR_CONNECTION_RESET.

Fiddler doesn't even capture any of the requests going out. The weird thing is, this problem randomly starts, but then in an hour or two, it's fine. I don't restart Fiddler; it just suddenly starts working again.

I tried following the steps in Fiddler and Internet Explorer 11 on Windows 8.1, but that didn't help.

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

J0e3gan
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user1763965
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    Without Fiddler running, what do you see inside IE's Tools > Internet Options > Connections > LAN Settings? – EricLaw Oct 24 '13 at 19:52
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    The Automatically detect settings box is checked. When fiddler is running, the Use Proxy server check box is checked. – user1763965 Oct 25 '13 at 17:16
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    Based on the description, the most likely explanation is that your local firewall is blocking access to Fiddler. Please follow these steps: https://groups.google.com/forum/#!topic/httpfiddler/SsZnGxdxklg and create a new post in that forum. StackOverflow isn't really meant for non-programming questions. – EricLaw Oct 25 '13 at 20:37
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    I sort of get this issue. Mine seemed to be slightly different. I'd get the "ERR_CONNECTION_RESET" until I finally started up Fiddler. Once I started Fiddler, everything started working again...Fiddler of course does set proxy settings to run through localhost. When Fiddler is off, those settings are cleared and reset to automatic. I checked DNS, DHCP, and Proxy settings. The rest of my Internet access works fine - only HTTP access was not working. It seemed like there was a proxy configured, but there wasn't when I went to LAN settings. Very strange indeed. – crush Oct 16 '14 at 14:12

16 Answers16

29

Go to Tools -> Internet options -> Connections -> LAN Settings, and then untick 'use a proxy server' for your LAN, and tick 'automatically detect settings'.

TylerH
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user4417767
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    This worked on Mac OS X 2.13 for Network -> Advanced -> 'Auto Proxy Discovery' -> Apply. My Problem was that shutting off Fiddler would actually screw up my whole connection (either reset it? or change the Lookup to some other address? haven't figured that bit out yet) – GPP May 30 '17 at 06:32
  • I crashed Fiddler in some unexpected way, kept using my machine for a while, then rebooted and suddenly internet access was gone. This setting was hidden and Fiddler logged me out (requiring internet to log back in) - a total disaster. Thanks for helping me get my internet back – Dagrooms Sep 28 '21 at 22:58
  • FYI this works by disabling the method Fiddler uses to capture traffic – AFOC Mar 05 '22 at 17:50
24

I had the same issue with Windows 7 and Fiddler v4.6.1.4.

Whenever Fiddler was running, Google Chrome refused to let me connect to gmail, complaining about the Privacy settings.

My solution was to go into Tools\Fiddler Options, and untick the box to capture HTTPS connections.

enter image description here

Disabling the Proxy settings in Network settings also solved this issue.

Mike Gledhill
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    Sounds a bit strange that you'd have to disable HTTPS trafic in order to get Fiddler working? – Jim Aho Jan 08 '16 at 12:46
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    Actually I found another solution [here](http://stackoverflow.com/a/33818661/2874896) that worked well for me. In short my root certificate from Fiddler was not valid, hence no browser trusted it and wouldn't send HTTPS traffic. Re-installing Fiddlers root certificate did the job. – Jim Aho Jan 08 '16 at 13:00
8

This is the correct answer from the Telerik, the makers of Fiddler:
http://www.telerik.com/blogs/fiddler-and-internet-explorer-11-on-windows-8-1

From their site:

If you’re using Fiddler with Internet Explorer 11 on the now-in-beta Windows 8.1 OS, some simple additional configuration is required.

IE 11, when running on Windows 8.1, enables the Enhanced Protected Mode (EPM) feature by default. EPM was available in IE10 on Win8, but was off by default for Desktop IE and only enabled for the "non-desktop” mode of IE.

EPM is based on an isolation technology known as AppContainer; AppContainer restricts the destinations to which network traffic can be sent. Unfortunately for Fiddler users, “Internet-Zone” AppContainers are forbidden from sending traffic to the local computer. By default, IE instances in EPM cannot send their traffic to Fiddler, because it’s a proxy that runs on the local computer.

If you attempt to open any Internet-Zone page with Fiddler running, you’ll see an error message complaining “The proxy server isn’t responding”.

Fortunately, working around this problem is simple; just follow these steps:

  1. Click the Win8 Config button in the Fiddler toolbar.

  2. Click Yes in the Elevation prompt (shown if Fiddler is not running as Admin)

  3. In the window that appears, either click Exempt All or manually tick the checkbox next to the AppContainers with the display name windows_ie_ac_###. (IE may create more than one AppContainer to handle certain scenarios).

  4. Click the Save Changes button.

After you make this configuration change, IE will no longer be forbidden from sending its traffic to Fiddler, and the tool should work correctly.

KyleK
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8

The following steps worked for me :

  1. Launch Fiddler
  2. Click menu Tools > Options, then click on the HTTPS tab. Under the HTTPS tab, you should see Certificates generated by CertEnroll engine
  3. Click the button names Actions and then choose Reset All Certificates and accept all ensuing prompts.
  4. Then select Decrypt HTTPS traffic again.
  5. Click OK

I referred to this link and it really helped.

ajaysinghdav10d
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3

I had the same problem so I stopped capturing traffic (File -> Capture Traffic, F12), and this fixed the connectivity issue, and I could still execute the local Rest URL.

Problem is if you actually want to capture the traffic. My issue was I was behind a proxy and as well as setting the LAN settings in Internet Properties, I also needed to set it in Fiddler (Tools -> Fiddler Options -> Gateway [Choose or configure proxy option] ), and that worked for me

EDIT

Choosing the 'Use System Proxy (Recommended)', from fiddler options worked for a bit and then stoppped again. So I used the Manual Proxy Configuration and that works better

http=proxy.[your proxy domain].net:80; https=proxy.[your proxy domain].net:80

Make sure you restart fiddler after each modification

beaumondo
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3

Change the port from 8888 to another one that is not already used (Tools -> Fiddler Options -> Connection)

I had to change the port because I installed the Windows Phone 8.1 SDK which seems to ship with a debugger listening on port 8888. So that was the reason for the connection failures.

0x8BADF00D
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1

I know this is old, but I've just had the same issue. Recently upgraded to Windows10 and Fiddler blocks all traffic. The Windows10 upgrade may be a coincidence but I thought I'd mention it.

None of the other solutions worked and I've wasted about an hour trying to resolve the problem...

Anyway - the only solution that worked for me - uninstall then reinstall... All working fine again now.

Percy
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If you find that web traffic works if you disable 'Decrypt HTTPS traffic' in Tools > Options > HTTPS, then you may need to do this; In the same HTTPS tab, click 'Actions', 'Trust Root Certificate', then accept the prompts.

guest
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I know this is very old question but providing my hard found resolution to help someone else.

Tick the "Unmatched requests passthrough" checkbox at AutoResponder tab of Fiddler(I'm using version 4.6)

If this is not checked fiddler doesn't know what to be done for any URLs where we haven't set any rule.

highfive
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The solution that worked for me is below: 1. Remove all the certificates from your system that were installed by Fiddler

enter image description here

  1. Launch Fiddler --> Tools --> Options --> HTTPS --> Actions (on the right) --> Trust Root Certificate --> Install the certificate.

This will install the certificate.

enter image description here

  1. Relaunch fiddler.
Aish
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This can still become an issue even in 2021. On my work PC managed by my organization, an update was pushed to my system which (unbeknownst to me) changed some of the Network options, which ultimately caused all network traffic to time out when capturing traffic with Fiddler.

System details: Windows 10x64 Fiddler v5.0.20204.45441 for .NET 4.6.1, Built: Tuesday, November 3, 2020

I tried performing all the steps to reset the certificates, re-trust the root certificate, and still things did not work.

After some trial and error, I noticed that the TLS settings under Internet Options -> Advanced did not match what was in Fiddler.

After making the following update in Fiddler to match the current Windows settings: Fiddler Windows 10 settings inconsistency issue

and restarting Fiddler, Fiddler was able to properly capture traffic without any issues.

cookch10msu
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I've found a solution to this after a few days troubled. When closed or stopped using Fiddler capture request, its still not change all the setting into previous settings that fit for windows 8/8.1/10 so all you have to do is following these steps:

  1. Open internet setting *run -> inetcpl.cpl or *search internet option

  2. Connection tab -> LAN setting -> untick the Proxy Server (this in case Fiddler wasn't restore the setting) -> OK

  3. Security tab -> Local Intranet -> Restore default -> OK

now all the universal apps have access to the internet again.

Zeelios
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Follow this steps and you'll have internet again : internet properties => connections => LAN setting => debifate "use a proxy server for your lan".

BaGz
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Go to Fiddler Options/ gateway and select

USE System proxy.

Taran
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I also found this issue and the reason is that webstorm has a build-in nodejs server which also listens on 8888. So when i open webstorm and fiddler at the same time i can't access the internet. Just change the port!

tomwang1013
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After closing Fiddler 4 I had no internet connection in Windows 10.

Going in the menu: Tools => WinlNET Options... and unchecking Use a proxy server for your LAN solved the issue in my case.

Note: I'm using Parallels Desktop with Shared Network here.

enter image description here

Another way of changing this option: https://stackoverflow.com/a/32855493/114029

Leniel Maccaferri
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