This article is intended for Citrix administrators and technical teams only. Non-admin users must contact their company’s Help Desk/IT support team and can refer to CTX297149 for more information.


When connecting from a client machine that has Receiver 4.9 LTSR or Receiver 4.10 (upgraded from Receiver 4.9) to a Linux VDA, users may notice one of the following behaviours:

1. There will be a complete grey screen and the session window closes after some time with no error message or with the error "Citrix HDX Engine has stopped working"
2. ICA Connection opens correctly or sometimes with a partial grey screen when opened in Full Screen mode. If it is switched to Window mode and resized, the screen is not updated correctly and partial grey screen could be seen.

These problems are not seen when connecting to a Windows VDA from the same client machine.

Caution! Using Registry Editor incorrectly can cause serious problems that might require you to reinstall your operating system. Citrix cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk. Be sure to back up the registry before you edit it.

There are two possible workarounds for this issue:

1.  If the Receiver is a clean install and you see the partial grey screen issue, reducing the number of virtual channels on the Receiver side (so that they are less than or equal to 32 in number ) would resolve the problem. To reduce the virtual channels, perform the following steps:

1. Exit Receiver from System tray.
2. Open Registry on the client machine.
3. Browse to the following key:

HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Citrix\ICA Client\Engine\Configuration\Advanced\Modules\ICA 3.0

Modify the VirtualDriver key on the right hand pane and remove the entries that are not needed in your environment. Some candidates are:
 
For example, removing vd3d and Framehawk, following would be the contents of VirtualDriver key:
 
Thinwire3.0,  ClientDrive, ClientPrinterQueue, ClientPrinterPort, Clipboard, ClientComm, ClientAudio,  LicenseHandler, TWI,ZL_FONT,ZLC,SmartCard,Multimedia,ICACTL,SSPI,TwainRdr,UserExperience,VDTUI,DVCAdapter,GenericUSB, Flash,FlashV2,MobileReceiver,Sensor,Multitouch,VDIME,WebPageRedirection,PortForward,NSAP

2.  If the Receiver is an upgrade from 4.9 to 4.10, uninstalling the Receiver and performing a clean install (instead of upgrade) will resolve the issue. This is because the clean install has a different ordering of Virtual Channels compared to an upgrade.

Citrix team is currently working on enhancing the Linux VDA to support 64 Virtual channels. This support is expected to be included in the future version of Linux VDA.
 

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Problem Cause

This problem is caused by the current design limitation of support for a maximum of 32 virtual channels by Linux VDA. With Receiver 4.9 LTSR and Receiver 4.10, new virtual channel support has been added and this increases the overall virtual channel numbers to beyond 32. So when connecting to Linux VDA machine, because of the limitation, the virtual channels loaded last get unloaded resulting in the HDX Engine crash issue. Windows VDAs support up to 64 Virtual channels. So the problem is not seen when connecting to Windows VDAs.