Use a service principal with Azure Kubernetes Service (AKS)

Article
10/12/2023

An AKS cluster requires either an Microsoft Entra service principal or a managed identity to dynamically create and manage other Azure resources, such as an Azure Load Balancer or Azure Container Registry (ACR).

Note

We recommend using managed identities to authenticate with other resources in Azure, and they're the default authentication method for your AKS cluster. For more information about using a managed identity with your cluster, see Use a system-assigned managed identity.

This article shows you how to create and use a service principal for your AKS clusters.

Before you begin

To create a Microsoft Entra service principal, you must have permissions to register an application with your Microsoft Entra tenant and to assign the application to a role in your subscription. If you don't have the necessary permissions, you need to ask your Microsoft Entra ID or subscription administrator to assign the necessary permissions or pre-create a service principal for you to use with your AKS cluster.

If you're using a service principal from a different Microsoft Entra tenant, there are other considerations around the permissions available when you deploy the cluster. You may not have the appropriate permissions to read and write directory information. For more information, see What are the default user permissions in Microsoft Entra ID?

Prerequisites

If using Azure CLI, you need Azure CLI version 2.0.59 or later. Run az --version to find the version. If you need to install or upgrade, see Install Azure CLI.
If using Azure PowerShell, you need Azure PowerShell version 5.0.0 or later. Run Get-InstalledModule -Name Az to find the version. If you need to install or upgrade, see Install the Azure Az PowerShell module.

Create a service principal using the az ad sp create-for-rbac command.

az ad sp create-for-rbac --name myAKSClusterServicePrincipal

Your output should be similar to the following example output:

{
  "appId": "559513bd-0c19-4c1a-87cd-851a26afd5fc",
  "displayName": "myAKSClusterServicePrincipal",
  "name": "http://myAKSClusterServicePrincipal",
  "password": "e763725a-5eee-40e8-a466-dc88d980f415",
  "tenant": "72f988bf-86f1-41af-91ab-2d7cd011db48"
}

Copy the values for appId and password from the output. You use these when creating an AKS cluster in the next section.

Create a service principal using the New-AzADServicePrincipal command.

New-AzADServicePrincipal -DisplayName myAKSClusterServicePrincipal -OutVariable sp

Your output should be similar to the following example output:

Secret                : System.Security.SecureString
ServicePrincipalNames : {559513bd-0c19-4c1a-87cd-851a26afd5fc, http://myAKSClusterServicePrincipal}
ApplicationId         : 559513bd-0c19-4c1a-87cd-851a26afd5fc
ObjectType            : ServicePrincipal
DisplayName           : myAKSClusterServicePrincipal
Id                    : 559513bd-0c19-4c1a-87cd-851a26afd5fc
Type                  :

The values are stored in a variable that you use when creating an AKS cluster in the next section.

Decrypt the value stored in the Secret secure string using the following command.

$BSTR = [System.Runtime.InteropServices.Marshal]::SecureStringToBSTR($sp.Secret)
[System.Runtime.InteropServices.Marshal]::PtrToStringAuto($BSTR)

Specify a service principal for an AKS cluster

Azure CLI
Azure PowerShell

Use an existing service principal for a new AKS cluster using the az aks create command and use the --service-principal and --client-secret parameters to specify the appId and password from the output you received the previous section.
```
az aks create \
    --resource-group myResourceGroup \
    --name myAKSCluster \
    --service-principal <appId> \
    --client-secret <password>
```
Note

If you're using an existing service principal with customized secret, make sure the secret isn't longer than 190 bytes.

Convert the service principal ApplicationId and Secret to a PSCredential object using the following command.

$Cred = New-Object -TypeName System.Management.Automation.PSCredential ($sp.ApplicationId, $sp.Secret)

Use an existing service principal for a new AKS cluster using the New-AzAksCluster command and specify the ServicePrincipalIdAndSecret parameter with the previously created PSCredential object as its value.
```
New-AzAksCluster -ResourceGroupName myResourceGroup -Name myAKSCluster -ServicePrincipalIdAndSecret $Cred
```
Note

If you're using an existing service principal with customized secret, make sure the secret isn't longer than 190 bytes.

Delegate access to other Azure resources

You can use the service principal for the AKS cluster to access other resources. For example, if you want to deploy your AKS cluster into an existing Azure virtual network subnet or connect to Azure Container Registry (ACR), you need to delegate access to those resources to the service principal. Permission granted to a cluster using a system-assigned managed identity may take up 60 minutes to populate.

Azure CLI
Azure PowerShell

Create a role assignment using the az role assignment create command. Assign the appId to a particular scope, such as a resource group or virtual network resource. The role defines what permissions the service principal has on the resource.

Note

The --scope for a resource needs to be a full resource ID, such as /subscriptions/<guid>/resourceGroups/myResourceGroup or /subscriptions/<guid>/resourceGroups/myResourceGroupVnet/providers/Microsoft.Network/virtualNetworks/myVnet.
```
az role assignment create --assignee <appId> --scope <resourceScope> --role Contributor
```

Create a role assignment using the New-AzRoleAssignment command. Assign the ApplicationId to a particular scope, such as a resource group or virtual network resource. The role defines what permissions the service principal has on the resource.

Note

The Scope for a resource needs to be a full resource ID, such as /subscriptions/<guid>/resourceGroups/myResourceGroup or /subscriptions/<guid>/resourceGroups/myResourceGroupVnet/providers/Microsoft.Network/virtualNetworks/myVnet
```
New-AzRoleAssignment -ApplicationId <ApplicationId> -Scope <resourceScope> -RoleDefinitionName Contributor
```

The following sections detail common delegations that you may need to assign.

Azure Container Registry

Azure CLI
Azure PowerShell

If you use Azure Container Registry (ACR) as your container image store, you need to grant permissions to the service principal for your AKS cluster to read and pull images. We recommend using the az aks create or az aks update command to integrate with a registry and assign the appropriate role for the service principal. For detailed steps, see Authenticate with Azure Container Registry from Azure Kubernetes Service.

If you use Azure Container Registry (ACR) as your container image store, you need to grant permissions to the service principal for your AKS cluster to read and pull images. We recommend using the New-AzAksCluster or Set-AzAksCluster command to integrate with a registry and assign the appropriate role for the service principal. For detailed steps, see Authenticate with Azure Container Registry from Azure Kubernetes Service.

Networking

You may use advanced networking where the virtual network and subnet or public IP addresses are in another resource group. Assign the Network Contributor built-in role on the subnet within the virtual network. Alternatively, you can create a custom role with permissions to access the network resources in that resource group. For more information, see AKS service permissions.

Storage

If you need to access existing disk resources in another resource group, assign one of the following sets of role permissions:

Create a custom role and define the Microsoft.Compute/disks/read and Microsoft.Compute/disks/write role permissions, or
Assign the Virtual Machine Contributor built-in role on the resource group.

Azure Container Instances

If you use Virtual Kubelet to integrate with AKS and choose to run Azure Container Instances (ACI) in resource group separate from the AKS cluster, the AKS cluster service principal must be granted Contributor permissions on the ACI resource group.

Other considerations

Azure CLI
Azure PowerShell

When using AKS and a Microsoft Entra service principal, consider the following:

The service principal for Kubernetes is a part of the cluster configuration, but don't use this identity to deploy the cluster.
By default, the service principal credentials are valid for one year. You can update or rotate the service principal credentials at any time.
Every service principal is associated with a Microsoft Entra application. You can associate the service principal for a Kubernetes cluster with any valid Microsoft Entra application name (for example: https://www.contoso.org/example). The URL for the application doesn't have to be a real endpoint.
When you specify the service principal Client ID, use the value of the appId.
On the agent node VMs in the Kubernetes cluster, the service principal credentials are stored in the /etc/kubernetes/azure.json file.
When you delete an AKS cluster that was created using the az aks create command, the service principal created isn't automatically deleted.
- To delete the service principal, query for your cluster's servicePrincipalProfile.clientId and delete it using the az ad sp delete command. Replace the values for the -g parameter for the resource group name and -n parameter for the cluster name:
```
az ad sp delete --id $(az aks show -g myResourceGroup -n myAKSCluster --query servicePrincipalProfile.clientId -o tsv)
```

When using AKS and a Microsoft Entra service principal, consider the following:

The service principal for Kubernetes is a part of the cluster configuration, but don't use this identity to deploy the cluster.
By default, the service principal credentials are valid for one year. You can update or rotate the service principal credentials at any time.
Every service principal is associated with a Microsoft Entra application. You can associate the service principal for a Kubernetes cluster with any valid Microsoft Entra application name (for example: https://www.contoso.org/example). The URL for the application doesn't have to be a real endpoint.
When you specify the service principal Client ID, use the value of the ApplicationId.
On the agent node VMs in the Kubernetes cluster, the service principal credentials are stored in the /etc/kubernetes/azure.json file.
When you delete an AKS cluster that was created using the New-AzAksCluster, the service principal created isn't automatically deleted.
- To delete the service principal, query for your cluster's ServicePrincipalProfile.ClientId and delete it using the Remove-AzADServicePrincipal command. Replace the values for the -ResourceGroupName parameter for the resource group name and -Name parameter for the cluster name:
```
$ClientId = (Get-AzAksCluster -ResourceGroupName myResourceGroup -Name myAKSCluster ).ServicePrincipalProfile.ClientId
Remove-AzADServicePrincipal -ApplicationId $ClientId
```

Troubleshoot

Azure CLI
Azure PowerShell

Azure CLI caches the service principal credentials for AKS clusters. If these credentials expire, you encounter errors during AKS cluster deployment. If you run the az aks create command and receive an error message similar to the following, it may indicate a problem with the cached service principal credentials:

Operation failed with status: 'Bad Request'.
Details: The credentials in ServicePrincipalProfile were invalid. Please see https://aka.ms/aks-sp-help for more details.
(Details: adal: Refresh request failed. Status Code = '401'.

You can check the expiration date of your service principal credentials using the az ad app credential list command with the "[].endDateTime" query.

az ad app credential list --id <app-id> --query "[].endDateTime" -o tsv

The default expiration time for the service principal credentials is one year. If your credentials are older than one year, you can reset the existing credentials or create a new service principal.

General Azure CLI troubleshooting

The Azure CLI can run in several shell environments, but with slight format variations. If you have unexpected results with Azure CLI commands, see How to use the Azure CLI successfully.

Azure PowerShell caches the service principal credentials for AKS clusters. If these credentials expire, you encounter errors during AKS cluster deployment. If you run the New-AzAksCluster command and receive an error message similar to the following, it may indicate a problem with the cached service principal credentials:

Operation failed with status: 'Bad Request'.
Details: The credentials in ServicePrincipalProfile were invalid. Please see https://aka.ms/aks-sp-help for more details.
(Details: adal: Refresh request failed. Status Code = '401'.

You can check the expiration date of your service principal credentials using the Get-AzADAppCredential command. The output shows you the StartDateTime of your credentials.

Get-AzADAppCredential -ApplicationId <ApplicationId>