Troubleshooting: Initial VDI Connection Failures with Per-User Remote Desktop Licensing

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When utilizing per-user Remote Desktop licensing within a Virtual Desktop Infrastructure (VDI) environment that relies on non-persistent virtual machines (VMs), you might encounter an error message upon a user’s initial connection attempt. This error typically manifests as:

The remote session disconnected because there are no Remote Desktop License Servers available.

This article aims to clarify the underlying reasons for this error and provide actionable steps to effectively mitigate it.

Understanding Persistent vs. Non-Persistent VDI¶

To grasp the context of this issue, it’s crucial to differentiate between persistent and non-persistent VDI environments. These two approaches handle user sessions and VM states in fundamentally different ways.

• Persistent VDI: In a persistent VDI setup, each user essentially has a dedicated virtual machine that retains its state across sessions. Any modifications, installations, or personalizations made by the user within their VM are preserved even after shutdown or restart. Think of persistent VDI much like a traditional physical desktop – it’s your machine, and it remembers your changes.

• Non-Persistent VDI: Conversely, non-persistent VDI environments offer a more streamlined and centrally managed approach. With non-persistent VMs, each session starts from a clean, standardized image. Once a user logs off or the VM is restarted, all changes made during that session are discarded. The VM reverts to its original, pristine state. This model is highly efficient for scenarios requiring consistent environments and simplified management, often leveraging a shared custom image for VM deployment.

Symptoms¶

The primary symptom of this issue is the error message: “The remote session disconnected because there are no Remote Desktop License Servers available.” This error arises specifically when a user attempts the very first connection to a non-persistent VM after it has been powered on. This occurs under a specific set of conditions:

• Remote Desktop Session Host (RDSH) Role: The Windows VM in question must have the Remote Desktop Session Host (RDSH) role installed. This role is essential for enabling remote desktop services on the VM.
• Per-User Licensing: The Remote Desktop licensing server that the VM is configured to use must be operating in per-user licensing mode. This licensing model assigns licenses to individual users, rather than devices.
• Expired Grace Period: The 120-day licensing grace period for Remote Desktop Services must have expired. During this initial grace period, a license server is not required, masking the underlying issue.
• First-Time Connection: Critically, the VM must never have successfully connected to the designated Remote Desktop licensing server before.

While this scenario can theoretically occur in both persistent and non-persistent VDI environments, it is significantly more prevalent in non-persistent setups. This is because persistent VMs often establish a connection to the licensing server and obtain necessary licenses during the initial grace period, thus avoiding the error after the grace period expires.

It’s important to note that this particular behavior is primarily observed in third-party VDI solutions. Microsoft Remote Desktop Services deployments do not natively offer non-persistent VMs as a standard configuration option.

Cause: X509 Certificate Retrieval Delay¶

The root cause of this initial connection failure lies in the process of acquiring an X509 certificate from the Remote Desktop licensing server. When a user initiates the first connection to a newly powered-on VM, the VM immediately attempts to contact the licensing server to request an X509 certificate. This certificate is crucial for establishing a secure and licensed Remote Desktop session.

This certificate retrieval process is not instantaneous. It can take several seconds for the VM to successfully communicate with the licensing server, process the request, and receive the X509 certificate. During this brief period, the connection attempt is likely to fail, resulting in the “no Remote Desktop License Servers available” error message being displayed to the user.

Once the VM has successfully obtained and cached the X509 certificate, subsequent connection attempts from any user will typically succeed without issue. The certificate remains valid for a period, eliminating the need for immediate re-acquisition for each connection.

However, the nature of non-persistent VMs exacerbates this issue. Due to their stateless design, non-persistent VMs revert to their original image upon each restart or session end. This means that the cached X509 certificate is also discarded when the VM resets. Consequently, the next time the VM is powered on, it will again need to retrieve a new X509 certificate. This cycle repeats every time a non-persistent VM starts, making the initial connection failure a recurring problem for first-time users after each VM startup.

Resolution: Pre-Provisioning the X509 Certificate¶

To effectively resolve this issue and ensure seamless user connections to non-persistent VMs, the key is to ensure that the VM already possesses a valid X509 certificate before users attempt to connect. Since the delay in obtaining the certificate during the initial connection is the root cause, pre-provisioning the certificate circumvents this delay.

The methodology for achieving this pre-provisioning revolves around the concept of the custom image used to create non-persistent VMs. Non-persistent VDI environments are typically built upon a base custom image. Multiple VMs are provisioned from this single, standardized image. Administrators manage and update this custom image, rather than individually servicing each non-persistent VM. This image-based management is central to the efficiency of non-persistent VDI.

Therefore, the resolution strategy focuses on servicing the custom image itself to include the X509 certificate. By incorporating the certificate into the base image, any VM deployed from that image will inherit the certificate upon creation. This eliminates the need for each VM to individually request and retrieve the certificate upon its first startup.

Servicing the Custom Image¶

Servicing the custom image to include the X509 certificate can be approached in various ways, depending on the specific VDI platform and administrative workflows in place. Here are two common approaches:

1. Manual Certificate Acquisition and Image Update:

   • Provision a Temporary VM: Create a temporary, persistent VM from the custom image.
   • Connect to the Licensing Server: Manually establish a Remote Desktop connection from this temporary VM to the designated Remote Desktop licensing server. This action will force the VM to obtain and cache the X509 certificate.
   • Capture the Image: Once the connection is successful and the certificate is acquired, shut down the temporary VM and capture it as a new version of the custom image.
   • Deploy New VMs: Use this newly captured image to deploy or update the non-persistent VMs in the VDI environment. VMs created from this updated image will now inherit the pre-provisioned X509 certificate.

2. Automated Image Servicing Pipeline:

   • Scripted Certificate Acquisition: Integrate a script into the image servicing pipeline that automatically triggers the X509 certificate acquisition process during image creation or update. This script could involve programmatically initiating a Remote Desktop connection or directly interacting with the Remote Desktop licensing components on the VM.
   • Image Update and Deployment: The automated pipeline will then incorporate the acquired certificate into the custom image and automatically deploy or update non-persistent VMs with the serviced image. This approach streamlines the process and ensures consistent certificate provisioning with each image update.

Considerations for Servicing Approaches¶

Approach	Pros	Cons	Best Suited For
Manual Certificate Acquisition	Simpler to implement initially, good for smaller environments.	More manual effort, prone to errors, less scalable for large environments.	Smaller VDI deployments, less frequent image updates.
Automated Image Servicing	Highly efficient, scalable, consistent, reduces manual intervention.	Requires initial setup and scripting, more complex to implement initially.	Larger VDI deployments, frequent image updates, DevOps-oriented environments.

Choosing the appropriate servicing approach depends on the scale of the VDI environment, the frequency of image updates, and the level of automation desired. Regardless of the method chosen, the fundamental principle remains the same: proactively provisioning the X509 certificate within the custom image to eliminate the connection delay and the associated error for users connecting to non-persistent VMs for the first time.

By implementing these resolution steps, administrators can ensure a smoother and more reliable user experience in non-persistent VDI environments leveraging per-user Remote Desktop Licensing, eliminating the frustrating initial connection failures.

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If you have encountered this issue or have implemented other solutions, please share your experiences and insights in the comments below!