Service principals with Azure Kubernetes Service (AKS)

To interact with Azure APIs, an AKS cluster requires an Azure Active Directory (AD) service principal. The service principal is needed to dynamically create and manage other Azure resources such as an Azure load balancer or container registry (ACR).

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

Before you begin

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

If you are using a service principal from a different Azure AD tenant, there are additional 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 Azure Active Directory?

You also need the Azure CLI version 2.0.59 or later installed and configured. RunĀ az --version to find the version. If you need to install or upgrade, seeĀ Install Azure CLI.

Automatically create and use a service principal

When you create an AKS cluster in the Azure portal or using the az aks create command, Azure can automatically generate a service principal.

In the following Azure CLI example, a service principal is not specified. In this scenario, the Azure CLI creates a service principal for the AKS cluster. To successfully complete the operation, your Azure account must have the proper rights to create a service principal.

az aks create --name myAKSCluster --resource-group myResourceGroup

Manually create a service principal

To manually create a service principal with the Azure CLI, use the az ad sp create-for-rbac command. In the following example, the --skip-assignment parameter prevents any additional default assignments being assigned:

az ad sp create-for-rbac --skip-assignment

The output is similar to the following example. Make a note of your own appId and password. These values are used when you create an AKS cluster in the next section.

{
  "appId": "559513bd-0c19-4c1a-87cd-851a26afd5fc",
  "displayName": "azure-cli-2019-03-04-21-35-28",
  "name": "http://azure-cli-2019-03-04-21-35-28",
  "password": "e763725a-5eee-40e8-a466-dc88d980f415",
  "tenant": "72f988bf-86f1-41af-91ab-2d7cd011db48"
}

Specify a service principal for an AKS cluster

To use an existing service principal when you create an AKS cluster using the az aks create command, use the --service-principal and --client-secret parameters to specify the appId and password from the output of the az ad sp create-for-rbac command:

az aks create \
    --resource-group myResourceGroup \
    --name myAKSCluster \
    --service-principal <appId> \
    --client-secret <password>

If you deploy an AKS cluster using the Azure portal, on the Authentication page of the Create Kubernetes cluster dialog, choose to Configure service principal. Select Use existing, and specify the following values:

  • Service principal client ID is your appId
  • Service principal client secret is the password value

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Delegate access to other Azure resources

The service principal for the AKS cluster can be used 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.

To delegate permissions, 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. A role then defines what permissions the service principal has on the resource, as shown in the following example:

az role assignment create --assignee <appId> --scope <resourceScope> --role Contributor

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

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

Azure Container Registry

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. Currently, the recommended configuration is to use the az aks create or [az aks update][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.

Networking

You may use advanced networking where the virtual network and subnet or public IP addresses are in another resource group. Assign one of the following set of role permissions:

  • Create a custom role and define the following role permissions:
    • Microsoft.Network/virtualNetworks/subnets/join/action
    • Microsoft.Network/virtualNetworks/subnets/read
    • Microsoft.Network/virtualNetworks/subnets/write
    • Microsoft.Network/publicIPAddresses/join/action
    • Microsoft.Network/publicIPAddresses/read
    • Microsoft.Network/publicIPAddresses/write
  • Or, assign the Network Contributor built-in role on the subnet within the virtual network

Storage

You may need to access existing Disk resources in another resource group. Assign one of the following set of role permissions:

  • Create a custom role and define the following role permissions:
    • Microsoft.Compute/disks/read
    • Microsoft.Compute/disks/write
  • Or, assign the Storage Account 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 to the AKS cluster, the AKS service principal must be granted Contributor permissions on the ACI resource group.

Additional considerations

When using AKS and Azure AD service principals, keep the following considerations in mind.

  • The service principal for Kubernetes is a part of the cluster configuration. However, don't use the 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 an Azure AD application. The service principal for a Kubernetes cluster can be associated with any valid Azure AD 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 file /etc/kubernetes/azure.json
  • When you use the az aks create command to generate the service principal automatically, the service principal credentials are written to the file ~/.azure/aksServicePrincipal.json on the machine used to run the command.
  • When you delete an AKS cluster that was created by az aks create, the service principal that was created automatically is not deleted.
    • To delete the service principal, query for your cluster servicePrincipalProfile.clientId and then delete with az ad app delete. Replace the following resource group and cluster names with your own values:

      az ad sp delete --id $(az aks show -g myResourceGroup -n myAKSCluster --query servicePrincipalProfile.clientId -o tsv)
      

Troubleshoot

The service principal credentials for an AKS cluster are cached by the Azure CLI. If these credentials have expired, you encounter errors deploying AKS clusters. The following error message when running az aks create 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'.

Check the age of the credentials file using the following command:

ls -la $HOME/.azure/aksServicePrincipal.json

The default expiration time for the service principal credentials is one year. If your aksServicePrincipal.json file is older than one year, delete the file and try to deploy an AKS cluster again.

Next steps

For more information about Azure Active Directory service principals, see Application and service principal objects.

For information on how to update the credentials, see Update or rotate the credentials for a service principal in AKS.