Issues with Fiddler on Windows 8.1 and IE 11

Update from Eric Law

As per his comment below, Eric has posted an update to his EnableLoopback utility to revert to looking through the registry if it can’t access the AppContainers through the normal APIs. You can get the update from https://www.fiddler2.com/dl/EnableLoopbackUtility.exe . DEFINITELY use this if you can instead of doing the manual method I outline below.

The Issue

One of the most useful tools out there for anyone doing any kind of web development is Fiddler and I use it daily to troubleshoot all kinds of issues. Unfortunately when you run Windows 8.1 and IE 11, which I recently upgraded to, Fiddler doesn’t appear to play too happily and you can get some pretty cryptic errors. I don’t thoroughly understand the architectural changes from IE 10 to IE 11 that has resulted in this, though I suspect it’s probably a movement from IE 10 from being a true desktop application to IE 11 now being a desktop application based on the Windows App Store framework.

Thankfully though, Eric Law developed an application which you can download here but recently this application stopped working for me and I started getting the error “The utility was unable to collect the list of AppContainers on this system. [NetIso ErrorCode 0x6F4] – Unable to enumerate AppContainers.” (shown below). I honestly have no clue why this has started happening and whether it could be related to a recent Windows Update but all attempts at running the program have failed and resulted in this message for me. In addition to this problem, I’ve not been able to find a good article explaining a work around, hence this article.

FiddlerError

The Workaround

[N.B. this is pretty technically heavy so if you aren’t comfortable with working from a command line and in the registry, I recommend you just wait until someone comes up with an automated method like Eric’s software or you are so desperate that potentially killing your machine starts no longer appears like a bad idea – in addition to this, let me just point out that if you do screw things up for yourself, any posts to this article WILL BE IGNORED. I’m just showing you the edge of the cliff, it’s up to you to jump off it]

So before Eric Law wrote his software, there was a manual method to setup the loopback exemptions using checknetisolation from a command line. I looked more into it, and while it is a little painful, with a little elbow grease you can setup a command line batch file to setup the exemptions for you.

Step 1 – Enumerate the “AppContainers”

Go into the registry to the key [HKEY_CURRENT_USER\Software\Classes\Local Settings\Software\Microsoft\Windows\CurrentVersion\AppContainer\Mappings]. Under here you will find a bunch of keys named using the SID formatting. If you select one of these keys you’ll see the “DisplayName” value and data that says “Microsoft Visual C++ Runtime Package”, “@{Microsoft.XboxLIVEGamesXXXXXXXXX}”, etc. To put it simply you’re going to need ALL of these names from all the keys.

This is a pretty painstaking process.

The way I got around it was to export the [HKEY_CURRENT_USER\Software\Classes\Local Settings\Software\Microsoft\Windows\CurrentVersion\AppContainer\Mappings] key to a *.reg file. I renamed this file to *.csv and did a find for DisplayName=” and then copied all those cells into another spreadsheet. If you have no idea how to do this, sorry but I’m not going to explain it. Unfortunately you’ll have to do it the tedious typing way.

Step 2 – Add Exemptions

Once I had all those values out I then copied those values into notepad and did a search and replace on DisplayName=” and replaced it with the following string checknetisolation loopbackexempt -a -n= . So just to provide a little explanation of the command line arguments you’re adding in. The checknetisolation loopbackexempt part is telling the checknetisolation utility that you want to modify the loopback exemption list in some way. The -a part is the argument for adding a new exemption. The -n= part is the name of the AppContainer you’re wanting to exempt.

Once I did the search and replace, I saved the file as a *.bat file called “setloopbackexceptions.bat”.

Step 3 – Create Clear Exemptions

So just for good measure, I also created a “clearloopbackexceptions.bat” file which the following single command checknetisolation loopbackexempt -c to clear all the exemptions. As I see it, Microsoft has purposely blocked loopbacks for AppContainers and rather than screw with the powers that be, I like to keep my machine “default” whenever I’m not needing a particular feature. I advise you not to be such a lazy monkey and do the same too.

Final Comments

As far as I’m aware (please correct me if I’m wrong), but this is just manually doing what Eric Law’s application does. Hopefully there will be a fix soon and this method will become redundant.

3 thoughts on “Issues with Fiddler on Windows 8.1 and IE 11

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