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extenal-name-service-support.md

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Support for external name service across namespaces

Namespaces are used to isolate resources within a Kubernetes cluster. Sometimes, services in a different namespace might have to access a service located in another namespace. In such scenarios, you can use the ExternalName service provided by Kubernetes. An ExternalName service is a special service that does not have selectors and instead uses DNS names.

In the service definition, the externalName field must point to the namespace and also to the service which we are trying to access on that namespace. Netscaler ingress controller supports services of type ExternalName when you have to access services within the cluster.

When you create the ExternalName service, the following criteria must be met:

  • The externalName field in the service definition must follow the format: svc: <name-of-the-service>.<namespace-of-the-service>.svc.cluster.local
  • The port number in the ExternalName service must exactly match the port number of the targetted service.

Note: When the service of an application is outside the Kubernetes cluster and you have created an ExternalName service, you can resolve the domain name using the Traffic management for external services feature.

Sample ExternalName service

In this example, a mysql service is running in the default namespace and a sample ExternalName service is created to access the mysql service from the namespace1 namespace.

The following is a sample service definition for a MySQL service running in the default namespace.

apiVersion: v1
kind: Service
metadata:
  name: mysql
  namespace: default
spec:
  clusterIP: None
  ports:
  - port: 3306
    protocol: TCP
    targetPort: 3306
  selector:
    app: mysql
  type: ClusterIP

The following is a sample ExternalName service definition to access the mysql service from the namespace1 namespace.

kind: Service
apiVersion: v1
metadata:
  name: dbservice
  namespace: namespace1
spec:
  type: ExternalName
  externalName: mysql.default.svc.cluster.local
  ports:
  - port: 3306 
    protocol: TCP
    targetPort: 3306

In the example, the service points to the namespace where mysql is deployed as specified in the field externalName: mysql.default.svc.cluster.local. Here mysql is the service name and default is the namespace. You can see that the port name is also the same as the mysql service.