Kubernetes Access Service located in another namespace

by Anish

Posted on Wednesday September 4, 2019

Introduction

An ExternalName Service is a special case of Service that does not have selectors and uses DNS names instead.

To demonstrate the ExternalName lets assume we have a service of mysql which is running on defualt 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

And a test busybox pod located in the default namespace is accessing this service using DNS name mysql

root@kube-master-2:# kubectl exec -it busybox sh
/ # ping -c1 mysql
PING mysql (192.168.242.145): 56 data bytes
64 bytes from 192.168.242.145: seq=0 ttl=63 time=0.068 ms

--- mysql ping statistics ---
1 packets transmitted, 1 packets received, 0% packet loss
round-trip min/avg/max = 0.068/0.068/0.068 ms
/ # exit

If some other pod in another namespace is try to access the mysql service, it will not able to resolve the DNS mysql as shown below,

root@kube-master-2:# kubectl create ns playground
root@kube-master-2:# kubectl apply -f busybox.yml -n playground 
pod/busybox unchanged

root@kube-master-2:# kubectl exec -it busybox sh -n playground 
/ # 
/ # ping -c2 mysql
ping: bad address 'mysql'
/ # exit

To Access the mysql service in the default namespace we need to build a service with type ExternalName as shown below. The externalName must point to the namespace which we are trying to access on that namespace.

mysql.default.svc.cluster.local

kind: Service
apiVersion: v1
metadata:
  name: mysql
  namespace: playground
spec:
  type: ExternalName
  externalName: mysql.default.svc.cluster.local
  ports:
  - port: 3306

Apply the configuration and check for the svc

root@kube-master-2:# kubectl get svc -n playground 
NAME                   TYPE           CLUSTER-IP      EXTERNAL-IP                       PORT(S)    AGE
mysql                  ExternalName   <none>          mysql.default.svc.cluster.local   3306/TCP   8s

The ExternalName is configured properly, now different pod should be access the service mysql which is located in another namespace.

root@kube-master-2:# kubectl exec -it busybox sh -n playground 
/ # ping -c2 mysql
PING mysql (192.168.242.145): 56 data bytes
64 bytes from 192.168.242.145: seq=0 ttl=63 time=0.069 ms
64 bytes from 192.168.242.145: seq=1 ttl=63 time=0.075 ms

--- mysql ping statistics ---
2 packets transmitted, 2 packets received, 0% packet loss
round-trip min/avg/max = 0.069/0.072/0.075 ms
/ # exit

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