Auto-upgrade Azure Kubernetes Service cluster node OS images

AKS now supports the node OS auto-upgrade channel, an exclusive channel dedicated to controlling node-level OS security updates. This channel can't be used for cluster-level Kubernetes version upgrades.

How does node OS auto-upgrade work with cluster auto-upgrade?

Node-level OS security updates are released at a faster rate than Kubernetes patch or minor version updates. The node OS auto-upgrade channel grants you flexibility and enables a customized strategy for node-level OS security updates. Then, you can choose a separate plan for cluster-level Kubernetes version auto-upgrades. It's highly recommended to use both cluster-level auto-upgrades and the node OS auto-upgrade channel together. Scheduling can be fine-tuned by applying two separate sets of maintenance windows - aksManagedAutoUpgradeSchedule for the cluster auto-upgrade channel and aksManagedNodeOSUpgradeSchedule for the node OS auto-upgrade channel.

Use node OS auto-upgrade

The selected channel determines the timing of upgrades. When making changes to node OS auto-upgrade channels, allow up to 24 hours for the changes to take effect.

Note

Node OS image auto-upgrade won't affect the cluster's Kubernetes version. It only works for a cluster in a supported version.

The following upgrade channels are available. You're allowed to choose one of these options:

Channel	Description	OS-specific behavior
None	Your nodes don't have security updates applied automatically. This means you're solely responsible for your security updates.	N/A
Unmanaged	OS updates are applied automatically through the OS built-in patching infrastructure. Newly allocated machines are unpatched initially. The OS's infrastructure patches them at some point.	Ubuntu and Azure Linux (CPU node pools) apply security patches through unattended upgrade/dnf-automatic roughly once per day around 06:00 UTC. Windows doesn't automatically apply security patches, so this option behaves equivalently to None.
SecurityPatch	This channel is in preview and requires enabling the feature flag NodeOsUpgradeChannelPreview. Refer to the prerequisites section for details. AKS regularly updates the node's virtual hard disk (VHD) with patches from the image maintainer labeled "security only." There may be disruptions when the security patches are applied to the nodes. When the patches are applied, the VHD is updated and existing machines are upgraded to that VHD, honoring maintenance windows and surge settings. This option incurs the extra cost of hosting the VHDs in your node resource group. If you use this channel, Linux unattended upgrades are disabled by default.	Azure Linux doesn't support this channel on GPU-enabled VMs. SecurityPatch works on patch versions that are deprecated, so long as the minor Kubernetes version is still supported.
NodeImage	AKS updates the nodes with a newly patched VHD containing security fixes and bug fixes on a weekly cadence. The update to the new VHD is disruptive, following maintenance windows and surge settings. No extra VHD cost is incurred when choosing this option. If you use this channel, Linux unattended upgrades are disabled by default. Node image upgrades will work on patch versions that are deprecated, so long as the minor Kubernetes version is still supported.

To set the node OS auto-upgrade channel when creating a cluster, use the node-os-upgrade-channel parameter, similar to the following example.

az aks create --resource-group myResourceGroup --name myAKSCluster --node-os-upgrade-channel SecurityPatch

To set the node os auto-upgrade channel on existing cluster, update the node-os-upgrade-channel parameter, similar to the following example.

az aks update --resource-group myResourceGroup --name myAKSCluster --node-os-upgrade-channel SecurityPatch

Update ownership and cadence

The default cadence means there's no planned maintenance window applied.

Channel	Updates Ownership	Default cadence
Unmanaged	OS driven security updates. AKS has no control over these updates	Nightly around 6AM UTC for Ubuntu and Mariner, Windows every month.
SecurityPatch	AKS	Weekly
NodeImage	AKS	Weekly

Prerequisites

"The following prerequisites are only applicable when using the SecurityPatch channel. If you aren't using this channel, you can ignore these requirements.

• Must be using API version 11-02-preview or later

• If using Azure CLI, the aks-preview CLI extension version 0.5.127 or later must be installed

• The NodeOsUpgradeChannelPreview feature flag must be enabled on your subscription

Register the 'NodeOsUpgradeChannelPreview' feature flag

Register the NodeOsUpgradeChannelPreview feature flag by using the az feature register command, as shown in the following example:

az feature register --namespace "Microsoft.ContainerService" --name "NodeOsUpgradeChannelPreview"

It takes a few minutes for the status to show Registered. Verify the registration status by using the az feature show command:

az feature show --namespace "Microsoft.ContainerService" --name "NodeOsUpgradeChannelPreview"

When the status reflects Registered, refresh the registration of the Microsoft.ContainerService resource provider by using the az provider register command:

az provider register --namespace Microsoft.ContainerService

Limitations

• Currently, when you set the cluster auto-upgrade channel to node-image, it also automatically sets the node OS auto-upgrade channel to NodeImage. You can't change node OS auto-upgrade channel value if your cluster auto-upgrade channel is node-image. In order to set the node OS auto-upgrade channel value, make sure the cluster auto-upgrade channel value isn't node-image.

• The SecurityPatch channel isn't supported on Windows OS node pools.

Note

By default, any new cluster created with an API version of 06-01-2022 or later will set the node OS auto-upgrade channel value to NodeImage. Any existing clusters created with an API version earlier than 06-01-2022 will have the node OS auto-upgrade channel value set to None by default.

Node OS auto-upgrade with Planned Maintenance

Planned Maintenance for the node OS auto-upgrade starts at your specified maintenance window.

Note

To ensure proper functionality, use a maintenance window of four hours or more.

For more information on Planned Maintenance, see Use Planned Maintenance to schedule maintenance windows for your Azure Kubernetes Service (AKS) cluster.

FAQ

• How can I check the current nodeOsUpgradeChannel value on a cluster?

Run the az aks show command and check the "autoUpgradeProfile" to determine what value the nodeOsUpgradeChannel is set to:

az aks show --resource-group myResourceGroup --name myAKSCluster --query "autoUpgradeProfile"

• How can I monitor the status of node OS auto-upgrades?

To view the status of your node OS auto upgrades, look up activity logs on your cluster. You may also look up specific upgrade-related events as mentioned in Upgrade an AKS cluster. AKS also emits upgrade-related Event Grid events. To learn more, see AKS as an Event Grid source.

• Can I change the node OS auto-upgrade channel value if my cluster auto-upgrade channel is set to node-image ?

No. Currently, when you set the cluster auto-upgrade channel to node-image, it also automatically sets the node OS auto-upgrade channel to NodeImage. You can't change the node OS auto-upgrade channel value if your cluster auto-upgrade channel is node-image. In order to be able to change the node OS auto-upgrade channel values, make sure the cluster auto-upgrade channel isn't node-image.