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Windows kubeadm-based worker nodes support

Windows kubeadm-based worker nodes support

Table of Contents


Refer to the Cluster API Book Glossary.


This proposal is for the support of Windows OS worker nodes in Cluster API and infrastructure providers that wish to support Windows. Cluster API will support Windows by using kubeadm to add Windows nodes to a workload cluster.

Windows support has been stable in Kubernetes since 1.14 and is supported in clusters that run Linux for the Control Plane. The Worker nodes can be any combination of Windows or Linux.

Windows node support has some unique challenges because of the current limitations of Windows Containers. Windows containers do not support privileged operations which means that configuration and access to the host machine must be done at provisioning time.

An example of this limitation is how kube-proxy gets configured on Windows nodes. Kube-proxy typically runs as a Windows service on the host machine and it cannot be deployed as a DaemonSet as it is on Linux.
To address this limitation the community has built tools such as the CSI-Proxy, which is a CSI driver specific proxy. This proposal will address how to approach the configuration of components that are typically deployed as daemon sets when bootstrapping Windows nodes in CAPI.


Kubernetes has supported Windows workloads since the release of Windows support in Kubernetes 1.14. The motivation of this proposal is to enable Cluster API users to deploy Windows as part of a mixed OS cluster via the Cluster API automation built for platform operators. This will enable cluster operators to define Windows machines in the same consistent and repeatable fashion.


  • Enable the creation and management of Windows worker nodes on workload clusters by adding support via the Kubeadm bootstrap provider and infrastructure providers
  • Provide community guidance and scripts for building base images for Windows nodes
  • Re-use of the existing Cluster API Bootstrap Provider Kubeadm and other tools where appropriate

Non-Goals/Future Work

  • Provide a way to run control plane nodes as Windows
  • Support for Windows versions outside of the Kubernetes support versions
  • Support for Windows nodes on the management or bootstrap clusters
  • Provide a way to configure Windows nodes with non-Kubeadm based bootstrap providers


Cluster API Bootstrap Provider Kubeadm

cloud-init and cloudbase-init

For Linux, when using the Kubeadm bootstrap provider, the bootstrap script is provided to the infrastructure provider as a cloud-init script. The infrastructure provider is responsible for putting the cloud-init script in the right location. When the VM is booted, the cloud-init script runs automatically.

Cloud-init does not have Windows support. An alternative product is cloudbase-init. Cloudbase-init functions in the same way as cloud-init and can consume cloud-init scripts as provided by the Cluster API Bootstrap Provider Kubeadm.
By using cloudbase-init, Windows can leverage the existing solutions and stay up to date with the latest changes in CABPK. Refer to the cloudbase-init documentation for features that are supported.

Image Creation

Using cloudbase-init requires the creation of an image with the tooling installed on it since it is not provided out of the box by any cloud providers. We'll provide packer scripts as part of the image-builder project that pre-installs cloudbase-init. It is important to note that while scripts can be provided to build an image, all images built need to adhere to Windows licensing requirements.

There is prior art for building Windows base images. For example, AKS-Engine has an example implementation for using packer and scripts to do image configuration:
Another example is the the sig-windows-tools which provide scripts for image configuration when using Kubeadm.

Although the Linux implementation in image-builder uses Ansible for configuration, Windows isn't going to share the same configuration because Ansible requires Windows specific modules to do the configuration.

Kubelet and other component configuration

Due to the lack of privileged containers in Windows, a combination of PreKubeadmCommands/PostKubeadmCommands scripts and wins.exe can be used to configure the nodes. Wins.exe is currently provided as a way to bootstrap nodes along with kubeadm in the Kubernetes documentation for adding Windows nodes.
The components from the preparenode script can be used during image creation.

In the future, when support for Privileged Containers for Windows containers is merged, we might be able to revisit this proposal and use privileged containers in place of wins.exe enabled containers.

Each infrastructure providers must provide their own PreKubeadmCommands/PostKubeadmCommands scripts that are required for additional configuration for the node. During planning for Beta we will be able to identify common overlapping features that can be added into the the base images in image-builder and for re-use

netbios names

Cluster API currently generates the machine deployment name which can result in long machine names. This was a was concern for Netbios on Windows which requires Windows computer names to be 15 characters or fewer ( Attempting to set a hostname with more than 15 characters on a windows machine will result in only the first 15 being used.

The conclusion of the issue was NETBIOS name resolution is mostly unused today and is not required to join an AD domain since Windows 2000. If DNS is properly configured then the long host names generated by Cluster API will be usable.

Infrastructure provider implementation

By leveraging cloudbase-init, an infrastructure provider implementation will require only a few changes which include:

From the infrastructure provider perspective, there are no known required changes to the CAPI API to support Windows nodes at this time. If during alpha we identify changes we will open issues pertaining to the changes required.

User Stories

As an operator, I would like to create Windows OS worker nodes with the CAPI API.

As an operator, I would like to manage Windows OS worker nodes with the CAPI API.

Implementation Details/Notes/Constraints

Due to the lack of privileged containers in Windows there are two options for configuring the components such as kube-proxy, kubelet. The above solution using wins is preferred because it makes the move to privileged containers straightforward as a drop and replace.

While this is the best choice for the alpha and the community direction there are some infrastructure providers that may not be able to use wins due to signing or security concerns since wins allows the execution of any arbitrary command on the host. Pre/post commands can be used as an alternative with additional scripts cached on the image that enable the configuration.

Signing of the components.

Some infrastructure providers will require any scripts and binaries are signed before deployment.
This will be managed by providing the ability to provide url's to override external scripts and binaries during the image building process. An example of how this is could be accomplished is in the Linux implementation is the containerd_url. In this case, the containerd_url could point to a location that would contain a packaged with signed binaries from the infrastructure provider.

Known prototypes and prior work:

Security Model

Wins.exe is currently the recommended way to use kubeadm.
Limiting access to the named pipes that are required is one way to mitigate access. Wins is currently required during and after provisioning for running kube-proxy and the CNI daemonset.

The security model for Privileged containers is still being discussed in KEP and is still early. The security concerns for Privileged containers will be addresses in the Beta phase of this proposal after the Privileged containers KEP progresses.

Kubeadm bootstrap token should be able to use multi-part mime documents for cloudbase-init as done for Linux in CAPA. This will require an update to Cloudbase-init which does support mutli-part mime documents but is missing the boothook functionality.
Support for cloudhooks can be added to cloudbase-init to meet the AWS provider requirement that would bring parity to Linux implementation during the Beta phase.

There is no known requirement for managing the Admin Kubeconfig or domain passwords in the Windows configuration. Domain passwords should be managed outside the scope of CAPI and only Kubeadm bootstrap tokens, which have limited lifetime, should be used for the joining the Windows nodes to the cluster. The joining of Windows nodes to a Domain Controller can be accomplished through pre/post kubeadm commands. Future support could be added via a separate controller that supports composable bootstrapping which is outside of the scope of this CAEP. Refer to issue #3761 for more details.

Risks and Mitigations

  • Privileged containers are not implemented.
    • There is an active discussion and KEP in place. At the Beta stage the community can do a checkpoint to determine if the solution fits user needs
  • Cloudbase-init is a third party dependency
  • Windows image Distribution
    • Infrastructure providers can provide the ability to use user provided images and images provided by image-promoter are recommended for testing and demonstration purposes. It is recommended the user creates their own image.
    • Users using the image scripts must ensure they are following Windows licensing requirements
  • Wins.exe is a third party dependency
    • The project is under the Apache 2.0 License


  1. An alternative to using wins.exe and DaemonSets to do the configuration to download and configure components as services using the kubeadm pre/post commands. This would require the infrastructure providers to have the ability to pass configuration through the use of these commands which is already done today. During the Alpha phase with the pre/post scripts being developed by individual infra providers this will not be an issue. With the move to Windows privileged containers in Beta, this becomes a non issue as wins will no longer be required.

  2. Create a separate bootstrap provider for Windows. This would require re-implementing a lot of the logic that is already in CABPK.
    When bugs are fixed or changes in behavior occur, Windows would risk being out of sync with the Linux implementation.

  3. Modified CABPK provider to have a different output format than cloud-init for Windows nodes. With Cloudbase-init there are no requirements to change CABPK which makes the adaption for Windows straightforward. If we were to adapt the output format of CABPK there is a potential for introduction of bugs and variation in the logic that would be created for Windows nodes. This would cause the Windows implementation to differ from others which could lead to confusion when debugging differences.

Upgrade Strategy

Nodes that use this pattern will require the infrastructure to be immutable as specified by CAPI documentation.

Additional Details

Test Plan [optional]

Note: Section not required until targeted at a release.

There are no changes to CAPI core proposed at this time. End to end tests in CAPI test suit use Kind and Windows does not support Docker in Docker so end to end test can not be added for Windows specific behavior. If changes are required during development unit tests will be required.

For infrastructure providers the testing plan is left up to each infrastructure provider. It is recommended to leverage the existing upstream Kubernetes Windows tests to show that Windows nodes are operating effectively.

Graduation Criteria [optional]

Note: Section not required until targeted at a release.


  • PreKubeadmCommands/PostKubeadmCommands command scripts are created per infrastructure provider if required
  • Initially implemented with wins.exe
  • Windows Packer scripts to create image with cloudbase-init added to the image builder scripts


  • Pre/Post commands moved to bootstrap provider if identified as re-usable
  • Adopt privileged containers (dependant on Privileged containers KEP)
  • kubeadm bootstrap token can be kept secret via multi-part mime documents for cloudbase-init.


Use of privileged containers.

Version Skew Strategy

The version of support for the Windows operating system is outside the scope of cluster creation. Please refer to the Kubernetes Windows documentation for the latest version skew support, features, and functionality.

Implementation History