Frequently asked questions for Windows Server node pools in AKS

In Azure Kubernetes Service (AKS), you can create a node pool that runs Windows Server as the guest OS on the nodes. These nodes can run native Windows container applications, such as those built on the .NET Framework. There are differences in how the Linux and Windows OS provides container support. Some common Linux Kubernetes and pod-related features are not currently available for Windows node pools.

This article outlines some of the frequently asked questions and OS concepts for Windows Server nodes in AKS.

Which Windows operating systems are supported?

AKS uses Windows Server 2019 and Windows Server 2022 as the host OS version and only supports process isolation. Container images built by using other Windows Server versions are not supported. For more information, see Windows container version compatibility.

Is Kubernetes different on Windows and Linux?

Windows Server node pool support includes some limitations that are part of the upstream Windows Server in Kubernetes project. These limitations are not specific to AKS. For more information on the upstream support from the Kubernetes project, see the Supported functionality and limitations section of the Intro to Windows support in Kubernetes document.

Historically, Kubernetes is Linux-focused. Many examples used in the upstream Kubernetes.io website are intended for use on Linux nodes. When you create deployments that use Windows Server containers, the following considerations at the OS level apply:

  • Identity: Linux identifies a user by an integer user identifier (UID). A user also has an alphanumeric user name for logging on, which Linux translates to the user's UID. Similarly, Linux identifies a user group by an integer group identifier (GID) and translates a group name to its corresponding GID. Windows Server uses a larger binary security identifier (SID) that's stored in the Windows Security Access Manager (SAM) database. This database is not shared between the host and containers, or between containers.
  • File permissions: Windows Server uses an access control list based on SIDs, rather than a bitmask of permissions and UID+GID.
  • File paths: The convention on Windows Server is to use \ instead of /. In pod specs that mount volumes, specify the path correctly for Windows Server containers. For example, rather than a mount point of /mnt/volume in a Linux container, specify a drive letter and location such as /K/Volume to mount as the K: drive.

What kind of disks are supported for Windows?

Azure Disks and Azure Files are the supported volume types, and are accessed as NTFS volumes in the Windows Server container.

Can I run Windows only clusters in AKS?

The master nodes (the control plane) in an AKS cluster are hosted by the AKS service. You won't be exposed to the operating system of the nodes hosting the master components. All AKS clusters are created with a default first node pool, which is Linux-based. This node pool contains system services that are needed for the cluster to function. We recommend that you run at least two nodes in the first node pool to ensure the reliability of your cluster and the ability to do cluster operations. The first Linux-based node pool can't be deleted unless the AKS cluster itself is deleted.

How do I patch my Windows nodes?

To get the latest patches for Windows nodes, you can either upgrade the node pool or upgrade the node image. Windows Updates are not enabled on nodes in AKS. AKS releases new node pool images as soon as patches are available, and it's the user's responsibility to upgrade node pools to stay current on patches and hotfixes. This patch process is also true for the Kubernetes version being used. AKS release notes indicate when new versions are available. For more information on upgrading the Windows Server node pool, see Upgrade a node pool in AKS. If you're only interested in updating the node image, see AKS node image upgrades.

Note

The updated Windows Server image will only be used if a cluster upgrade (control plane upgrade) has been performed prior to upgrading the node pool.

What network plug-ins are supported?

AKS clusters with Windows node pools must use the Azure Container Networking Interface (Azure CNI) (advanced) networking model. Kubenet (basic) networking is not supported. For more information on the differences in network models, see Network concepts for applications in AKS. The Azure CNI network model requires extra planning and consideration for IP address management. For more information on how to plan and implement Azure CNI, see Configure Azure CNI networking in AKS.

Windows nodes on AKS clusters also have Direct Server Return (DSR) enabled by default when Calico is enabled.

Is preserving the client source IP supported?

At this time, client source IP preservation is not supported with Windows nodes.

Can I change the maximum number of pods per node?

Yes. For the implications of making a change and the options that are available, see Maximum number of pods.

Why am I seeing an error when I try to create a new Windows agent pool?

If you created your cluster before February 2020 and have never done any cluster upgrade operations, the cluster still uses an old Windows image. You may have seen an error that resembles:

"The following list of images referenced from the deployment template is not found: Publisher: MicrosoftWindowsServer, Offer: WindowsServer, Sku: 2019-datacenter-core-smalldisk-2004, Version: latest. Please refer to Find and use Azure Marketplace VM images with Azure PowerShell for instructions on finding available images."

To fix this error:

  1. Upgrade the cluster control plane to update the image offer and publisher.
  2. Create new Windows agent pools.
  3. Move Windows pods from existing Windows agent pools to new Windows agent pools.
  4. Delete old Windows agent pools.

Why is there an unexpected user named "sshd" on my VM node?

AKS adds a user named "sshd" when installing the OpenSSH service. This user is not malicious. We recommend that customers update their alerts to ignore this unexpected user account.

How do I rotate the service principal for my Windows node pool?

Windows node pools do not support service principal rotation. To update the service principal, create a new Windows node pool and migrate your pods from the older pool to the new one. After your pods are migrated to the new pool, delete the older node pool.

Instead of service principals, use managed identities, which are essentially wrappers around service principals. For more information, see Use managed identities in Azure Kubernetes Service.

How do I change the administrator password for Windows Server nodes on my cluster?

When you create your AKS cluster, you specify the --windows-admin-password and --windows-admin-username parameters to set the administrator credentials for any Windows Server nodes on the cluster. If you didn't specify administrator credentials when you created a cluster by using the Azure portal or when setting --vm-set-type VirtualMachineScaleSets and --network-plugin azure by using the Azure CLI, the username defaults to azureuser and a randomized password.

To change the administrator password, use the az aks update command:

az aks update \
    --resource-group $RESOURCE_GROUP \
    --name $CLUSTER_NAME \
    --windows-admin-password $NEW_PW

Important

Performing the az aks update operation upgrades only Windows Server node pools. Linux node pools are not affected.

When you're changing --windows-admin-password, the new password must be at least 14 characters and meet Windows Server password requirements.

How many node pools can I create?

The AKS cluster can have a maximum of 100 node pools. You can have a maximum of 1,000 nodes across those node pools. For more information, see Node pool limitations.

What can I name my Windows node pools?

A Windows node pool can have a six-character name.

Are all features supported with Windows nodes?

Kubenet is currently not supported with Windows nodes.

Can I run ingress controllers on Windows nodes?

Yes, an ingress controller that supports Windows Server containers can run on Windows nodes in AKS.

Can my Windows Server containers use gMSA?

Group-managed service account (gMSA) support is generally available for Windows on AKS. See Enable Group Managed Service Accounts (GMSA) for your Windows Server nodes on your Azure Kubernetes Service (AKS) cluster

Can I use Azure Monitor for containers with Windows nodes and containers?

Yes, you can. However, Azure Monitor is in public preview for gathering logs (stdout, stderr) and metrics from Windows containers. You can also attach to the live stream of stdout logs from a Windows container.

Are there any limitations on the number of services on a cluster with Windows nodes?

A cluster with Windows nodes can have approximately 500 services before it encounters port exhaustion.

Can I use Azure Hybrid Benefit with Windows nodes?

Yes. Azure Hybrid Benefit for Windows Server reduces operating costs by letting you bring your on-premises Windows Server license to AKS Windows nodes.

Azure Hybrid Benefit can be used on your entire AKS cluster or on individual nodes. For individual nodes, you need to browse to the node resource group and apply the Azure Hybrid Benefit to the nodes directly. For more information on applying Azure Hybrid Benefit to individual nodes, see Azure Hybrid Benefit for Windows Server.

To use Azure Hybrid Benefit on a new AKS cluster, run the az aks create command and use the --enable-ahub argument.

az aks create \
    --resource-group myResourceGroup \
    --name myAKSCluster \
    --load-balancer-sku Standard \
    --windows-admin-password 'Password1234$' \
    --windows-admin-username azure \
    --network-plugin azure
    --enable-ahub

To use Azure Hybrid Benefit on an existing AKS cluster, run the az aks update command and use the update the cluster by using the --enable-ahub argument.

az aks update \
    --resource-group myResourceGroup
    --name myAKSCluster
    --enable-ahub

To check if Azure Hybrid Benefit is set on the Windows nodes in the cluster, run the az vmss show command with the --name and --resource-group arguments to query the virtual machine scale set. To identify the resource group the scale set for the Windows node pool is created in, you can run the az vmss list -o table command.

az vmss show --name myScaleSet --resource-group MC_<resourceGroup>_<clusterName>_<region>

If the Windows nodes in the scale set have Azure Hybrid Benefit enabled, the output of az vmss show will be similar to the following:

""hardwareProfile": null,
    "licenseType": "Windows_Server",
    "networkProfile": {
      "healthProbe": null,
      "networkApiVersion": null,

How do I change the time zone of a running container?

To change the time zone of a running Windows Server container, connect to the running container with a PowerShell session. For example:

kubectl exec -it CONTAINER-NAME -- powershell

In the running container, use Set-TimeZone to set the time zone of the running container. For example:

Set-TimeZone -Id "Russian Standard Time"

To see the current time zone of the running container or an available list of time zones, use Get-TimeZone.

Can I maintain session affinity from client connections to pods with Windows containers?

Although maintaining session affinity from client connections to pods with Windows containers will be supported in the Windows Server 2022 OS version, you achieve session affinity by client IP currently by limiting your desired pod to run a single instance per node and configuring your Kubernetes service to direct traffic to the pod on the local node.

Use the following configuration:

  1. Use an AKS cluster running a minimum version of 1.20.
  2. Constrain your pod to allow only one instance per Windows node. You can achieve this by using anti-affinity in your deployment configuration.
  3. In your Kubernetes service configuration, set externalTrafficPolicy=Local. This ensures that the Kubernetes service directs traffic only to pods within the local node.
  4. In your Kubernetes service configuration, set sessionAffinity: ClientIP. This ensures that the Azure Load Balancer gets configured with session affinity.

Next steps

To get started with Windows Server containers in AKS, see Create a node pool that runs Windows Server in AKS.