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Fix missing ScrewDrivers printers and scanners in Citrix VDA 2109 or newer

In Citrix Virtual Apps and Desktops 2109 or newer, ScrewDrivers client printers and scanners may not appear in user sessions because Citrix blocks third-party virtual channels unless they're allowed by policy.

This article explains how to restore ScrewDrivers Endpoint Client printer and scanner mapping by configuring the Citrix Virtual channel allow list policy.

Applies to

This issue applies to client printers and scanners mapped through the ScrewDrivers Endpoint Client. Print Server printers, Direct IP printers, and other printing methods are not affected by this Citrix virtual channel policy change.

Symptoms​

Users connect to a Citrix session, but one or more of the following are missing:

  • ScrewDrivers client printers
  • ScrewDrivers scanner devices
  • Local printers mapped through the ScrewDrivers Endpoint Client

The ScrewDrivers Endpoint Client may be installed correctly, but Citrix does not load the required virtual channels in the session.

Cause​

Citrix introduced the Virtual channel allow list policy in Citrix Virtual Apps and Desktops 2006. Starting with Citrix VDA 2109, also known as 7.31, the policy defaults to allowing only approved third-party virtual channels.

If the policy is enabled, and the ScrewDrivers virtual channels are not included in the allow list, Citrix blocks the channels that ScrewDrivers uses for client printer and scanner mapping.

note

If the policy is not explicitly configured, Citrix treats it as enabled in affected versions. To allow all virtual channels, create a Citrix policy and set Virtual channel allow list to Disabled.

Resolution options​

You can resolve this issue in either of these ways:

  1. Disable the Virtual channel allow list policy.
  2. Keep the policy enabled and add the ScrewDrivers virtual channels to the allow list.

For environments that use ScrewDrivers Scanning, disabling the policy is the simpler option because scanning applications can vary by environment.

Option 1: Disable the Virtual channel allow list policy​

Use this option when you want Citrix to allow all third-party virtual channels, or when you need the simplest configuration for ScrewDrivers Scanning.

  1. Open Citrix Studio.
  2. Go to Policies.
  3. Create a new policy or edit an existing policy.
  4. Search for virtual channel.
  5. Select Virtual channel allow list.
  6. Set the policy to Disabled.
  7. Select Next.
  8. Assign the policy to the appropriate users and machines, or select All objects in the site to apply it universally.
  9. Name the policy.
  10. Select Enable Policy, then select Finish.
  11. Have affected users sign out of their Citrix sessions and reconnect.
  12. Confirm that ScrewDrivers printers or scanners appear in the session.

Option 2: Add ScrewDrivers channels to the allow list​

Use this option when your environment requires the Citrix allow list policy to remain enabled.

  1. Open Citrix Studio.
  2. Go to Policies.
  3. Create a new policy or edit an existing policy.
  4. Search for virtual channel.
  5. Select Virtual channel allow list.
  6. Set the policy to Enabled.
  7. Add the required ScrewDrivers virtual channel entries.

For ScrewDrivers client printers, add these entries:

SCREW04,C:\Program Files\Tricerat\ScrewDrivers\Common\ScrewDrivers Notification Service.exe
SCREWD4,C:\Windows\System32\spoolsv.exe

If you use ScrewDrivers Scanning, also add these entries:

VSCAN01,C:\Program Files\Tricerat\ScrewDrivers\Common\ScrewDrivers Notification Service.exe
VSCAN01,C:\Program Files\TWAIN Working Group\Twack_32.exe
VSCAN01,C:\Program Files\TWAIN Working Group\Twack_64.exe
VSCAN01,<The path to your scanning application.exe>
  1. Assign the policy to the appropriate users and machines.
  2. Apply the policy.
  3. Have affected users sign out of their Citrix sessions and reconnect.
  4. Confirm that ScrewDrivers printers or scanners appear in the session.
Scanning applications

For scanning workflows, add the executable path for each scanning application that needs to use the ScrewDrivers scanning virtual channel. If several applications use scanning, each application may need its own VSCAN01 entry.

Verify the fix​

After you configure the policy:

  1. Confirm the policy applies to the affected user and VDA machine.
  2. Have the user start a new Citrix session.
  3. Check whether ScrewDrivers client printers appear in the remote session.
  4. If scanning is used, test the scanner from the expected scanning application.
  5. If devices are still missing, verify the ScrewDrivers Endpoint Client and Citrix client plugin configuration. See Troubleshooting Citrix Client Issues in ScrewDrivers v6 and v7.