Product Documentation

Desktop and application access if Delivery Controllers fail

Mar 11, 2014

If all Delivery Controllers in a Site fail, you can configure the Virtual Delivery Agent (VDA) (for Server OS machines and Desktop OS machines) to operate in high availability mode so that users can continue to access and use their desktops. In high availability mode, the VDA will accept direct ICA connections from users, rather than connections brokered by the controller.

Note: This feature is for use only on the rare occasion that communication with all Controllers fails; it is not an alternative to other high availability solutions, such as configuring database fault tolerance and site failover. Before using this feature, refer to the list of limitations below as these have security implications.

If communication with the Controller fails, high availability mode is initiated only after a set period of time has elapsed. By default, this is 300 seconds (5 minutes) but you can configure the time period.

Once in high availability mode (which is enabled for 30 days), the VDA attempts to register with a Controller for up to 30 days, while the user continues to use the desktop in this mode. When the Controller later becomes available, the desktop registers and the user's session continues uninterrupted, but any subsequent connection is brokered by the Controller as normal. If after 30 days the desktop is unable to register with the Controller, the desktop stops listening for connections and is no longer available. This means the administrator has 30 days in which to repair the Controller infrastructure and should not become reliant upon high availability mode.

High availability mode is suitable only for use with dedicated desktops, where the mapping between the user and the VDA is known. You cannot configure high availability mode for use with pooled desktops.

Caution: Editing the registry incorrectly can cause serious problems that may 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.

To enable high availability mode, you:

  1. Set the HighAvailability and HaRegistrarTimeout registry keys.
  2. Provide users with an ICA launch file that will enable them to make direct ICA connections. You have to create an ICA file for each user who requires this feature; Citrix does not create or distribute ICA files for this purpose.

Set the registry keys

To configure the VDA so that it operates in high availability mode when necessary, add the following registry key(s). You must do this after the VDA has been installed.

  1. Add the following registry entry to HKEY_LOCAL_MACHINE\SOFTWARE\Citrix\VirtualDesktopAgent:

    Name: HighAvailability

    Type: REG_DWORD


    1 = enables high availability mode

    0 (zero) = disables high availability mode

  2. To change the time period that the VDA tries registering with the Controller before initiating high availability mode, also add the following registry entry:

    Name: HaRegistrarTimeout

    Type: REG_DWORD

    Value: number of seconds. The default is 300 seconds.

  3. Restart the virtual desktop.

Prepare an ICA launch file

To establish a direct ICA connection to desktops, provide users with an ICA launch file that they can use should communication with the Controller fail. You must create an ICA launch file for each user who requires this feature; Citrix does not create or distribute ICA files for this purpose. For information on how to create ICA files, see

You will need to tell users when it is appropriate to use this ICA launch file and where they can access it from.

High availability mode limitations

High availability mode is suitable for use only with dedicated desktops; you cannot configure this for use with pooled desktops.

In high availability mode, some features are unavailable. These include:

  • User roaming. If a user device is already connected to the desktop, users are unable to connect from a different user device.
  • Power management. When the desktop powers up, it attempts to register, fails and, after the timeout, enters high availability mode.
  • Delivery Controller-originated policies. Policies originating on the Controller, such as those governing client drive mapping and access to the clipboard, will not function as there is no connection to the Controller. Policies originating from the Domain Controller and Local Group Policy are unaffected. Note that policies from a previous registration persist and are applied, so outdated policies might take affect.
  • NetScaler Gateway and Remote Access.

High availability mode persists for up to 30 days only, after which the desktop is no longer available.