Configure USB redirection

Version 1.4 of the Linux VDA provides support for USB redirection. With this functionaliy, USB devices are shared among Citrix Receiver and the Linux VDA desktop. Once a USB device is redirected to the desktop, the user can use the USB device as if it were locally connected.  

USB redirection includes three main areas of functionality:

• open soure project implementation (VHCI)
• VHCI service
• USB service

Open source VHCI

This portion of the USB redirection feature develops a general USB device sharing system over an IP network. It consists of a Linux kernel driver and some user mode libraries, which allows you to communicate with the kernel driver to get all the USB data.  In the Linux VDA implementation, Citrix reuses the kernel driver of VHCI, however all the USB data transfers between Linux VDA and Citrix Receiver are encapsulated in the Citrix ICA protocol package.

VHCI service

The VHCI service is an open source service provided by Citrix to communicate with VHCI kernel module. This service works as a gateway between VHCI and the Citrix USB service.

USB service

The USB service represents a Citrix module that manages all the virtualization and data transfers on the USB device.

This article contains the following sections:

• How USB redirection works
• Configuring USB redirection
• Troubleshoot USB redirection issues
• Building the VHCI kernel module
• Known issues
• Supported USB devices

Typically, if a USB device is redirected successfully to the Linux VDA client, one or more device nodes are created in the system /dev path. However, in some cases the redirected device cannot be used by an active Linux VDA session. USB devices rely on drivers to function properly (and some devices require special drivers), and in some cases drivers are not provided, which results in some redirected USB devices being inaccessible to an active Linux VDA session. When situations like these occur, the drivers must be installed and the system must be configured properly to ensure USB device connectivity.

With version 1.4, the Linux VDA supports a list of USB devices that are sucessfully redirected to the client. In addition, the device is properly mounted, especially the USB disk, allowing the user to access the disk without any additional configuration.

A Citrix policy controls whether USB device redirection is enabled or disabled. In addition, the type of device can also be specified using s DDC policy. When configuring USB redirection for the Linux VDA, the following must be configured:

• client USB device redirection policy
• client USB device redirection rules

In Citrix Studio, enable (or disable) USB device redirection to and from the client (for workstation hosts only).

In the Edit Setting dialog:

1. Select Allowed.
2. Click OK.

After enabling the USB redirection policy, set redirection rules using Citrix Studio by specifying which devices are allowed (or denied) on the Linux VDA client. 

In the Client USB device redirection rules dialog:

1. Click New to add a redirection rule, or click Edit to review an existing rule. 
2. After creating (or modifying) a rule, click OK.

Use the information in this section to troubleshoot various issues that you might encounter when using the Linux VDA.

No devices on the Receiver toolbar

In some cases, you might not be able to see devices listed on the Citrix Receiver toolbar, which indicates that no USB redirection is taking place. If this occurs, verify the following:

• the policy is configured to allow USB redirection
• the Kernel module is compatible with your kernel

USB devices can be seen in the Receiver toolbar, but are labeled policy restricted, which results in failed redirection

This issue occurs because of the device's policy configuration. In such cases:

• configure the Linux VDA policy to enable redirection
• Verify if any additional policy restrictions are configured in the Receiver registry; a device might be blocked by the Receiver registry setting. Check DeviceRules in the registry path to ensure that the device is not denied access by this setting:

HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Citrix\ICA Client\GenericUSB

A USB device is redirected successfully, but I cannot use it in my session 

In most cases, only USB devices in the supported devices list can be redirected, however, in some cases other kinds of devices may be redirected into an active Linux VDA session. In these situations, for every redirected device, a node owned by the user is created in the system /dev path. However, it is the drivers and the configuration that determine whether the user can use the device successfully. If you find a device owned (plugged in) but inaccessible, add the device to an unrestricted policy.

USB redirection depends on the VHCI kernel modules (usb-vhci-hcd.ko and usb-vhci-iocif.ko). These modules are part of the Linux VDA distribution (as part of the RPM package). They are compiled based on the official Linux distribution kernels and are noted in the table below:

On the command line, execute the following command to verify if the kernel is consistent:

If the command executes successfully, it means that the kernel module has loaded successfully and that the version is consistent with the one installed by Citrix. 

If the command executes with errors, it means that the kernel is inconsistent with the Citrix module and must be rebuilt.

If the kernel module is inconsistent with the Citrix version, perform the following:

1. Build the VHCI kernel module. Download the VHCI source code from the project page.

2. Build the kernel module based on the header files and the Module.symvers file. Use the following steps to install the kernel header files and create Module.symvers based on the appropriate Linux distribution:

RHEL 7.2

RHEL 6.8

SUSE 12.1

SUSE 11.4

3.  In the folder (/usr/src/kernels/3.10.0-327.10.1.el7.x86_64) verify that the file Module.symvers is present. If this file is not in the directory, you must build the kernel to get this file (e.g., make oldconfig; make prepare;make modules;make) or copy it from /usr/src/kernels/3.10.0-327.10.1.el7.x86_64-obj/x86_64/defaults/module.*  

4.  vhci-hcd-1.15/Makefile, change the Makefile of VCHI, set KDIR to the kernel directory: 

5.  cd vhci-hcd-1.15/ and make; build the VHCI kernel.

6. Replace the kernel module with the newly built one: cp -f usb-vhci-*.ko /opt/Citrix/VDA/lib64/

7. Restart the USB service:  service ctxusbsd restart

8. Log off and log on to the the session again. Check to see if USB redirection is functioning.

• Unable to unmount the redirected USB disk. For the access control of all USB disks redirected from Citrix Receiver, the Linux VDA manages all of these devices under administrative privilege to ensure that only the owner can access the redirected device. As a result, the user is not permitted to unmount the device without the administrative privilege.

• File lost when you stop redirecting a USB disk.  If you redirect a USB disk into a session and try to modify it (for example, create some files on the disk), then stop redirecting it immediately using the Receiver toolbar, the file you modified (or created) can be lost. 

This issue occurs because when you write data to a file system, the system mounts the memory cache in the file system, the data is not actually written to the disk itself. If you stop redirecting using the Receiver toolbar, there is no time remaining for the data being flushed into the disk, which results in lost data. 

To resolve this issue, whenever you write data into the disk, use the sync command in a terminal to flush data into the disk, then stop USB redirection.

The following devices have been verified to work with this version of the Linux VDA. Other devices may be freely used, with unexpected results: