Connection to a PLC not possible

[ar0707]

I tried to run the example code, but the window remains black. Do you have any ideas what the problem could be?

[QVNCWidget]` DEBUG: backbuffer is None
[RFB Client <-] DEBUG: len: 12 | b'RFB 003.003\n'
[RFB Client] INFO: RFB from server: 3.3
[RFB -> Server] DEBUG: b'RFB 003.003\n'
[RFB Client] INFO: VNC connected
[RFB Client <-] DEBUG: len: 4 | b'\x00\x00\x00\x01'
[RFB -> Server] DEBUG: b'\x01'
[RFB Client <-] DEBUG: len: 24 | 
```b'\x02\x80\x01\xe0\x08\x08\x00\x00\x00\x07\x00\x07\x00\x03\x00\x03\x06\x00\x00\x00\x00\x00\x00\x11'
[RFB Client <-] DEBUG: len: 17 | b"VC Project 'Visu'"
[RFB Client] DEBUG: Connecting to "VC Project 'Visu'"
[RFB Client] DEBUG: Server Pixelformat: 8;8;0;0;7;7;3;0;3;6
[RFB Client] DEBUG: Resolution: 640x480
[RFB -> Server] DEBUG: b'\x02\x00\x00\x01'
[RFB -> Server] DEBUG: b'\x00\x00\x00\x00'
[QVNCWidget] INFO: VNC handshake done
[RFB -> Server] DEBUG: b'\x00\x00\x00\x00  \x00\x01\x00\xff\x00\xff\x00\xff\x10\x08\x00\x00\x00\x00'
[RFB -> Server] DEBUG: b'\x03\x00\x00\x00\x00\x00\x02\x80\x01\xe0'
Exception in thread Thread-1 (__start):
Traceback (most recent call last):
  File "C:\Users\test\AppData\Local\Programs\Python\Python311\Lib\threading.py", line 1038, in _bootstrap_inner
    self.run()
  File "C:\Users\test\AppData\Local\Programs\Python\Python311\Lib\threading.py", line 975, in run
    self._target(*self._args, **self._kwargs)
  File "C:\repo\venv\Lib\site-packages\qvncwidget\rfb.py", line 108, in __start
    self._handleInitial()
  File "C:\repo\venv\Lib\site-packages\qvncwidget\rfb.py", line 148, in _handleInitial
    self._handleAuth33(self.__recv(4))
  File "C:\repo\venv\Lib\site-packages\qvncwidget\rfb.py", line 163, in _handleAuth33
    self._doClientInit()
  File "C:\repo\venv\Lib\site-packages\qvncwidget\rfb.py", line 173, in _doClientInit
    self._handleServerInit(self.__recv(24))
  File "C:\repo\venv\Lib\site-packages\qvncwidget\rfb.py", line 200, in _handleServerInit
    self._mainRequestLoop()
  File "C:\repo\venv\Lib\site-packages\qvncwidget\rfb.py", line 269, in _mainRequestLoop
    self.onFatalError(e)
  File "C:\repo\venv\Lib\site-packages\qvncwidget\rfb.py", line 482, in onFatalError
    raise error
  File "C:\repo\venv\Lib\site-packages\qvncwidget\rfb.py", line 251, in _mainRequestLoop
    dType = self.__recv(1)
            ^^^^^^^^^^^^^^
  File "C:\repo\venv\Lib\site-packages\qvncwidget\rfb.py", line 92, in __recv
    buffer = self.connection.recv(expectedSize, socket.MSG_WAITALL)    
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ConnectionResetError: [WinError 10054] Eine vorhandene Verbindung wurde vom Remotehost geschlossen
[RFB Client] DEBUG: Closing connection

Process finished with exit code 0`

[zocker-160]

Thank you for reporting and the debug log, I am sadly currently a bit short on time, but I will take a look at it.
From a brief look, it seems like after the pixelformat request the server is simply closing the connection.

Do other VNC clients work with that server?

[ar0707]

Thank you for taking the time. The UltraVNC Viewer is used for the connection. It works without any problems.

[zocker-160]

Could you try connecting using the following settings:

[zocker-160]

So I did investigate this a bit more today and the only explanation I could come up with, is that the server is not supporting the requested pixelformat.

From your log, after a successful handshake, the client is requesting the pixel format and then requesting a new frame (the first frame actually) at which point the server closes the connection.

What I find interesting is the pixel format offered by the server, which is 8;8;0;0;7;7;3;0;3;6 according to your log. This means the server offers 8 bpp und a color depth of 8 bit, true color is set to false and a max color value of 7.

This is a terrible format, the image must look really bad. My client is always requesting 32;32;0;1;255;255;255;16;8;0 but your server does not seem to like it.

Sadly there is not much I can do tbh, I would need a Wireshark capture to see how a successful connection looks like and how it differs.

UPDATE:

After doing some more testing, I found out that sending an unsupported pixel format does make many VNC server behave exactly this way.

So one simply needs to find out which pixel formats your PLC supports.

[ar0707]

I tried your suggested settings and it seems to be working without any issues. Thank you for your help. When I have time, I will try to debug it myself.

[zocker-160]

Would it be possible for you to share a Wireshark capture of the working connection (using the settings with RAW encoding and no compression)?

It would help me a lot to debug the problem.

[ar0707]

I will try it out soon.