Configure serial or COM port redirection over the Remote Desktop Protocol

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You can configure the redirection behavior of serial or COM ports between a local device and a remote session over the Remote Desktop Protocol (RDP).

For Azure Virtual Desktop, we recommend you enable serial or COM port redirection on your session hosts using Group Policy, then control redirection using the host pool RDP properties.

For Windows 365, you can configure your Cloud PCs using Group Policy.

For Microsoft Dev Box, you can configure your dev boxes using Group Policy.

This article provides information about the supported redirection methods and how to configure the redirection behavior serial or COM ports. To learn more about how redirection works, see Redirection over the Remote Desktop Protocol.

Prerequisites

Before you can configure serial or COM port redirection, you need:

  • An existing Cloud PC.
  • An existing dev box.

  • A serial or COM port on a local device and a peripheral that connects to the port. Serial or COM port redirection uses opaque low-level redirection, so drivers need to be installed in the remote session for the peripheral to function correctly.

  • To configure Group Policy, you need:

    • A domain account that has permission to create or edit Group Policy objects.
    • A security group or organizational unit (OU) containing the devices you want to configure.

  • You need to connect to a remote session from a supported app and platform. To view redirection support in Windows App and the Remote Desktop app, see Compare Windows App features across platforms and devices and Compare Remote Desktop app features across platforms and devices.

Serial or COM port redirection

Configuration of a session host using Group Policy, or setting an RDP property on a host pool governs the ability to redirect serial or COM ports from the local device to the remote session, which is subject to a priority order.

The default configuration is:

  • Windows operating system: Serial or COM port redirection isn't blocked.
  • Azure Virtual Desktop host pool RDP properties: Serial or COM ports are redirected from the local device to the remote session.
  • Resultant default behavior: Serial or COM ports are redirected from the local device to the remote session.

Important

Take care when configuring redirection settings as the most restrictive setting is the resultant behavior. For example, if you disable serial or COM port redirection on a session host with Group Policy, but enable it with the host pool RDP property, redirection is disabled.

Configuration of a Cloud PC governs the ability to redirect the serial or COM ports from the local device to the remote session, and is set using Group Policy.

The default configuration is:

  • Windows operating system: Serial or COM port redirection isn't blocked.
  • Windows 365: Serial or COM ports are redirected from the local device to the remote session.
  • Resultant default behavior: Serial or COM ports are redirected from the local device to the remote session.

Configuration of a dev box governs the ability to redirect Serial or COM port from the local device to the remote session, and is set using Group Policy.

The default configuration is:

  • Windows operating system: Serial or COM port redirection isn't blocked.
  • Microsoft Dev Box: Serial or COM ports are redirected from the local device to the remote session.
  • Resultant default behavior: Serial or COM ports are redirected from the local device to the remote session.

Configure serial or COM port redirection using host pool RDP properties

The Azure Virtual Desktop host pool setting COM ports redirection controls whether to redirect the serial or COM ports between the remote session and the local device. The corresponding RDP property is redirectcomports:i:<value>. For more information, see Supported RDP properties.

To configure serial or COM port redirection using host pool RDP properties:

  1. Sign in to the Azure portal.

  2. In the search bar, type Azure Virtual Desktop and select the matching service entry.

  3. Select Host pools, then select the host pool you want to configure.

  4. Select RDP Properties, then select Device redirection.

    A screenshot showing the host pool device redirection tab in the Azure portal.

  5. For COM ports redirection, select the drop-down list, then select one of the following options:

    • COM ports on the local computer are not available in the remote session
    • COM ports on the local computer are available in the remote session (default)
    • Not configured

  6. Select Save.

Configure serial or COM port redirection using Group Policy

Configure serial or COM port redirection using Group Policy

Select the relevant tab for your scenario.

To allow or disable serial or COM port redirection using Group Policy:

  1. Open the Group Policy Management console on device you use to manage the Active Directory domain.

  2. Create or edit a policy that targets the computers providing a remote session you want to configure.

  3. Navigate to Computer Configuration > Policies > Administrative Templates > Windows Components > Remote Desktop Services > Remote Desktop Session Host > Device and Resource Redirection.

    A screenshot showing the device and resource redirection options in the Group Policy editor.

  4. Double-click the policy setting Do not allow COM port redirection to open it.

    • To allow serial or COM port redirection, select Disabled or Not configured, then select OK.

    • To disable serial or COM port redirection, select Enabled, then select OK.

  5. Ensure the policy is applied to the computers providing a remote session, then restart them for the settings to take effect.

Test serial or COM port redirection

When using serial or COM port redirection, consider the following behavior:

  • Drivers for redirected peripherals connected to a serial or COM port need to be installed in the remote session using the same process as the local device. Ensure that Windows Update is enabled in the remote session, or that drivers are available for the peripheral.

  • Opaque low-level redirection is designed for LAN connections; with higher latency, some peripherals connected to a serial or COM port might not function properly, or the user experience might not suitable.

  • Peripherals connected to a serial or COM port aren't available on the local device locally while it's redirected to the remote session.

  • Peripherals connected to a serial or COM port can only be used in one remote session at a time.

  • Serial or COM port redirection is only available from a local Windows device.

To test serial or COM port redirection from a local Windows device:

  1. Plug in the supported peripherals you want to use in a remote session to a serial or COM port.

  2. Connect to a remote session using Window App or the Remote Desktop app on a platform that supports drive redirection. For more information, see Compare Windows App features across platforms and devices and Compare Remote Desktop app features across platforms and devices.

  3. Check the device is functioning correctly in the remote session. As serial or COM ports are redirected using opaque low-level redirection, the correct driver needs to be installed in the remote session, which you need to do if it's not installed automatically.

    Here are some ways to check the USB peripherals are available in the remote session, depending on the permission you have in the remote session:

    1. Open Device Manager in the remote session from the start menu, or run devmgmt.msc from the command line. Check the redirected peripherals appear in the expected device category and don't show any errors.

      A screenshot showing device manager in a remote session.

    2. Open a Command Prompt or PowerShell prompt on both the local device and in the remote session, then run the following command in both locations. This command shows the serial or COM ports available locally and enable you to verify that they're available in the remote session.

      chgport

      The output is similar to the following output:

      • On the local device:

        COM3 = \Device\Serial0
COM4 = \Device\Serial1

      • In the remote session:

        COM3 = \Device\RdpDrPort\;COM3:2\tsclient\COM3
COM4 = \Device\RdpDrPort\;COM4:2\tsclient\COM4

  4. Once the peripherals are redirected and functioning correctly, you can use them as you would on a local device.

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