Kubernetes Access Service located in another namespace

Posted on Wednesday September 4, 2019

Introduction

In Kubernetes the namespace is used to isolate the resource, based on the use case services in different namespace may required to access the service located in another namespace and kubernetes allow this by using ExternalName concept.

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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