Fiddler blocking all requests with an error message

[Barak Gall]

I am attempting to debug my own web application. I have used Fiddler extensively throughout the last few years to great effect. I find it a useful and informative tool.

However, I haven't used it in a bit, but now whenever I turn it on, and visit a non-HTTPS page I notice the following:

1) NO session is captured (The filters page has no filters active AND is disabled)

2) The webpage shows up in the browser as the following plaintext: {"reason":"Unknown command","result":"error"}

Going to an HTTPS page simply gives me an SSL connection error page (probably because fiddler is blocking the communication and returning the garbage error above).

I am using Fiddler v4.4.4.8 and the latest versions of firefox and chrome.

I googled the error and I found nothing, so I'm assuming there is something wrong with my computer configuration. Please help me sort it out.

Thanks for this ace product. Please help me get back to using it!

[EricLaw]

On the HELP menu, click Troubleshoot Filters. This will help ensure that you're not missing any traffic.

 

The error message in question sounds like it's coming from the SAHI web automation test tool. Are you using that?

[Barak Gall]

Ok, Eric.

I tried what you said, no sessions pop up when I enable troubleshoot filters. Additionally I tried this test.

1. Disable "Capture Traffic"

2. Help > Check for Updates...

This returns a "Your version of Fiddler is up-to-date" as expected. However:

1. Enable "Capture Traffic"
2. Help > Check for Updates...

This returns the following:

There was an error retrieving version information. You can check for updates manually by visiting https://fiddler2.com/

System.FormatException: Input string was not in a correct format.
  at System.Number.StringToNumber(String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal)
  at System.Number.ParseInt32(String s, NumberStyles style, NumberFormatInfo info)
  at Fiddler.Updater.GetLatestVersion(Boolean isBeta)
  at Fiddler.Updater.CheckForUpdates(BooleanbVerbose)

I don't know what but something is definitely wrong. In addition I don't know what the SAHI web automation test tool is... Sorry :S.

Lastly I found out something interesting yesterday.

The following is all the lines that involve port 9999 (my fiddler port), when I run "netstat -ano" when "Capture Traffic" is disabled

  Proto  Local Address          Foreign Address        State           PID
  TCP    0.0.0.0:9999           0.0.0.0:0              LISTENING       3252
  TCP    127.0.0.1:9999         0.0.0.0:0              LISTENING       12988
  TCP    [::]:9999              [::]:0                 LISTENING       3252

I also took note of the PIDs and looked for any other entries with the same PIDs:

  Proto  Local Address          Foreign Address        State           PID

 TCP    127.0.0.1:5555         0.0.0.0:0              LISTENING       12988

 TCP    127.0.0.1:6666         0.0.0.0:0              LISTENING       12988

 TCP    127.0.0.1:7777         0.0.0.0:0              LISTENING       12988

But when I turn on "Capture Filters" I see the following extra entries appear:

  Proto  Local Address          Foreign Address        State           PID

  TCP    127.0.0.1:9999         127.0.0.1:61870        TIME_WAIT       0

  TCP    127.0.0.1:9999         127.0.0.1:61871        TIME_WAIT       0

  TCP    127.0.0.1:9999         127.0.0.1:61872        TIME_WAIT       0

  TCP    127.0.0.1:9999         127.0.0.1:61873        TIME_WAIT       0

  TCP    127.0.0.1:9999         127.0.0.1:61874        TIME_WAIT       0

  TCP    127.0.0.1:9999         127.0.0.1:61875        TIME_WAIT       0

  TCP    127.0.0.1:9999         127.0.0.1:61876        TIME_WAIT       0

  TCP    127.0.0.1:9999         127.0.0.1:61877        TIME_WAIT       0

  TCP    127.0.0.1:9999         127.0.0.1:61878        TIME_WAIT       0

  TCP    127.0.0.1:9999         127.0.0.1:61879        TIME_WAIT       0

  TCP    127.0.0.1:9999         127.0.0.1:61880        TIME_WAIT       0

  TCP    127.0.0.1:9999         127.0.0.1:61881        TIME_WAIT       0

This is a list of entries that seems to continue growing and growing until it reaches some maximum. The older entries seem to dissipate slowly. This list of entries will reach a constant length, and then will remain at that length until I turn off fiddler or disable "Capture Traffic".

Thoughts?

[Barak Gall]

Ok,

I fixed it by uninstalling fiddler completely, and then reinstalling it. I wonder what went wrong?

[EricLaw]

Curious: You moved Fiddler off its default port. Why?

 

The screenshot you've provided above suggests that some process other than Fiddler is also listening on port 9999.

 

As I mentioned previously, Fiddler was not generating the error message in question; that string doesn't exist anywhere in the code. It would have been useful, before uninstalling, for you to tell me what you see inside Help > About in the GATEWAY field. I'm guessing that field is now blank.

[Barak Gall]

Hmm... Sorry, I really need this tool, so I kept playing with it until I got it working...

Now in the gateway field it says:

Auto-Config

Offline/disabled

I'm not sure what it said before though.

To be honest I thought 9999 was the default port.. I definitely did not change it, or if I did, I did not mean to. It is now set to 8888.

Just now I checked netstat again and 9999 is still open. However I went through the open ports and checked the PIDs and discovered the clash came from BlueStacks (an android emulator).