How to add custom host entries to kubernetes Pods? - docker

Question

My application communicates to some services via hostnames.
When running my application as a docker container i used to add hostnames to the /etc/hosts of the hostmachine and run the container using --net=host.
Now I'm running my containers in kubernetes cluster. I would like to know how can i add the /etc/hosts entries to the pod via yaml.
I'm using kubernetes v1.5.3.

Answer 1

From k8s 1.7 you can add hostAliases. Example from the docs:
apiVersion: v1
kind: Pod
metadata:
name: hostaliases-pod
spec:
restartPolicy: Never
hostAliases:
- ip: "127.0.0.1"
hostnames:
- "foo.local"
- "bar.local"
- ip: "10.1.2.3"
hostnames:
- "foo.remote"
- "bar.remote"

Answer 2

Host files are going to give you problems, but if you really need to, you could use a configmap.
Add a configmap like so
apiVersion: v1
kind: ConfigMap
metadata:
name: my-app-hosts-file-configmap
data:
hosts: |-
192.168.0.1 gateway
127.0.0.1 localhost
Then mount that inside your pod, like so:
volumeMounts:
- name: my-app-hosts-file
mountPath: /etc/
volumes:
- name: my-app-hosts-file
configMap:
name: my-app-hosts-file-configmap

Answer 3

This works and also looks simpler:
kind: Service
apiVersion: v1
metadata:
name: {HOST_NAME}
spec:
ports:
- protocol: TCP
port: {PORT}
targetPort: {PORT}
type: ExternalName
externalName: {EXTERNAL_IP}
Now you can use the HOST_NAME from the pod directly to access the external machine.

Answer 4

Another approach could be to use postStart hook on the pod lifecycle as below:
lifecycle:
postStart:
exec:
command: ["/bin/sh", "-c", "echo '192.168.1.10 weblogic-jms1.apizone.io' >> /etc/hosts; echo '192.168.1.20
weblogic-jms2.apizone.io' >> /etc/hosts; echo '192.168.1.30
weblogic-jms3.apizone.io' >> /etc/hosts; echo '192.168.1.40
weblogic-jms4.apizone.io' >> /etc/hosts"]