Azure Private Link Service Integration

Azure Private Link Service (PLS) is an infrastructure component that allows users to privately connect via a Private Endpoint (PE) in a VNET in Azure and a Frontend IP Configuration associated with an Azure Load Balancer (ALB). With Private Link, users as service providers can securely provide their services to consumers who can connect from within Azure or on-premises without data exfiltration risks.

Before Private Link Service integration, users who wanted private connectivity from on-premises or other VNETs to their services in the Azure Kubernetes cluster were required to create a Private Link Service (PLS) to reference the Azure LoadBalancer. The user would then create a Private Endpoint (PE) to connect to the PLS to enable private connectivity. With this feature, a managed PLS to the LB would be created automatically, and the user would only be required to create PE connections to it for private connectivity.

Note: When PLS has TCP proxy protocol V2 enabled (service.beta.kubernetes.io/azure-pls-proxy-protocol: true) and service externalTrafficPolicy is set to Local, LB health probe is down. This is because when PLS has proxy protocol enabled, the corresponding LB HTTP health probe would use proxy protocol as well. When service’s externalTrafficPolicy is set to Local, health probe depends on kube-proxy’s health check service which does not accept proxy protocol and all health probes fail. PR #3931 allows users to customize health probe when externalTrafficPolicy is set to Local and thus provides the workaround. It will be released soon.

PrivateLinkService annotations

Below is a list of annotations supported for Kubernetes services with Azure PLS created:

For more details about each configuration, please refer to Azure Private Link Service Documentation.

Design Details

Creating managed PrivateLinkService

When a LoadBalancer typed service is created without the annotations service.beta.kubernetes.io/azure-load-balancer-ipv4, service.beta.kubernetes.io/azure-load-balancer-ipv6 or field Service.Spec.LoadBalancerIP set, an LB frontend IP configuration is created with a dynamically generated IP. If the service has the annotation service.beta.kubernetes.io/azure-load-balancer-ipv4 or service.beta.kubernetes.io/azure-load-balancer-ipv6 set, an existing LB frontend IP configuration may be reused if one exists; otherwise a static configuration is created with the specified IP. When a service is created with annotation service.beta.kubernetes.io/azure-pls-create set to true or updated later with the annotation added, a PLS resource attached to the LB frontend is created in the default resource group or the resource group user set in config file with key PrivateLinkServiceResourceGroup.

The Kubernetes service creating the PLS is assigned as the owner of the resource. Azure cloud provider tags the PLS with cluster name and service name kubernetes-owner-service: <namespace>/<service name>. Only the owner service can later update the properties of the PLS resource.

If there’s a managed PLS already created for the LB frontend, the same PLS is reused automatically since each LB frontend can be referenced by only one PLS. If the LB frontend is attached to a user defined PLS, service creation should fail with proper error logged.

For now, Azure cloud provider does not manage any Private Link Endpoint resources. Once a PLS is created, users can create their own PEs to connect to the PLS.

Deleting managed PrivateLinkService

Once a PLS is created, it shares the lifetime of the LB frontend IP configuration and is deleted only when its corresponding LB frontend gets deleted. As a result, a PLS may still exist even when its owner service is deleted. This is out of the consideration that multiple Kubernetes services can share the same LB frontend IP configuration and thus share the PLS automatically. More details are discussed in next section.

If there are active PE connections to the PLS, all connections are removed and the PEs become obsolete. Users are responsible for cleaning up the PE resources.

Sharing managed PrivateLinkService

Multiple Kubernetes services can share the same LB frontend by specifying the same annotations service.beta.kubernetes.io/azure-load-balancer-ipv4, service.beta.kubernetes.io/azure-load-balancer-ipv6 or field Service.Spec.LoadBalancerIP (for more details, please refer to Multiple Services Sharing One IP Address). Once a PLS is attached to the LB frontend, these services automatically share the PLS. Users can access these services via the same PE but different ports.

Azure cloud provider tags the service creating the PLS as the owner (kubernetes-owner-service: <namespace>/<service name>) and only allows that service to update the configurations of the PLS. If the owner service is deleted or if user wants some other service to take control, user can modify the tag value to a new service in <namespace>/<service name> pattern.

PLS is only automatically deleted when the LB frontend IP configuration is deleted. One can delete a service while preserving the PLS by creating a temporary service referring to the same LB frontend.

Managed PrivateLinkService Creation example

Below we provide an example for creating a Kubernetes service object with Azure ILB and PLS created:

apiVersion: v1
kind: Service
metadata:
  name: myService
  annotations:
    service.beta.kubernetes.io/azure-load-balancer-internal: "true" # Use an internal LB with PLS
    service.beta.kubernetes.io/azure-pls-create: "true"
    service.beta.kubernetes.io/azure-pls-name: myServicePLS
    service.beta.kubernetes.io/azure-pls-ip-configuration-subnet: pls-subnet
    service.beta.kubernetes.io/azure-pls-ip-configuration-ip-address-count: "1"
    service.beta.kubernetes.io/azure-pls-ip-configuration-ip-address: 10.240.0.9 # Must be available in pls-subnet
    service.beta.kubernetes.io/azure-pls-fqdns: "fqdn1 fqdn2"
    service.beta.kubernetes.io/azure-pls-proxy-protocol: "false"
    service.beta.kubernetes.io/azure-pls-visibility: "*"
    service.beta.kubernetes.io/azure-pls-auto-approval: "subId1"
spec:
  type: LoadBalancer
  selector:
    app: myApp
  ports:
    - name: myAppPort
      protocol: TCP
      port: 80
      targetPort: 80

Restrictions

• PLS does not support basic Load Balancer or IP-based Load Balancer.
• PLS connectivity is broken with Azure external Standard Load Balancer and floating ip enabled (default). To use managed private link service, users can either create an internal service by setting annotation service.beta.kubernetes.io/azure-load-balancer-internal to true or disable floating ip by setting annotation service.beta.kubernetes.io/azure-disable-load-balancer-floating-ip to true (more details here).
• Due to limitation of kubernetes#95555, when the service’s externalTrafficPolicy set to Local, PLS need to use a different subnet from Pod’s subnet. If the same subnet is required, then the service should use Cluster externalTrafficPolicy.
• PLS only works with IPv4 and cannot be deployed to an SLB with IPv6 frontend ipConfigurations. In dual-stack clusters, users cannot create a service with PLS if there’s existing IPv6 service deployed on the same load balancer.
• For other limitations, please check Azure Private Link Service Doc.