Is there a way to add arbitrary records to kube-dns?

Question

I will use a very specific way to explain my problem, but I think this is better to be specific than explain in an abstract way...

Say, there is a MongoDB replica set outside of a Kubernetes cluster but in a network. The ip addresses of all members of the replica set were resolved by /etc/hosts in app servers and db servers.

In an experiment/transition phase, I need to access those mongo db servers from kubernetes pods. However, kubernetes doesn't seem to allow adding custom entries to /etc/hosts in pods/containers.

The MongoDB replica sets are already working with large data set, creating a new replica set in the cluster is not an option.

Because I use GKE, changing any of resources in kube-dns namespace should be avoided I suppose. Configuring or replace kube-dns to be suitable for my need are last thing to try.

Is there a way to resolve ip address of custom hostnames in a Kubernetes cluster?

It is just an idea, but if kube2sky can read some entries of configmap and use them as dns records, it colud be great. e.g. repl1.mongo.local: 192.168.10.100.

EDIT: I referenced this question from https://github.com/kubernetes/kubernetes/issues/12337

Answer 1

UPDATE: 2017-07-03 Kunbernetes 1.7 now support Adding entries to Pod /etc/hosts with HostAliases.

---

The solution is not about kube-dns, but /etc/hosts. Anyway, following trick seems to work so far...

EDIT: Changing /etc/hosts may has race condition with kubernetes system. Let it retry.

1) create a configMap

apiVersion: v1
kind: ConfigMap
metadata:
  name: db-hosts
data:
  hosts: |
    10.0.0.1  db1
    10.0.0.2  db2

2) Add a script named ensure_hosts.sh.

#!/bin/sh                                                                                                           
while true
do
    grep db1 /etc/hosts > /dev/null || cat /mnt/hosts.append/hosts >> /etc/hosts
    sleep 5
done

Don't forget chmod a+x ensure_hosts.sh.

3) Add a wrapper script start.sh your image

#!/bin/sh
$(dirname "$(realpath "$0")")/ensure_hosts.sh &
exec your-app args...

Don't forget chmod a+x start.sh

4) Use the configmap as a volume and run start.sh

apiVersion: extensions/v1beta1
kind: Deployment
...
spec:
  template:
    ...
    spec:
      volumes:
      - name: hosts-volume
        configMap:
          name: db-hosts
      ...
      containers:
        command:
        - ./start.sh
        ...
        volumeMounts:
        - name: hosts-volume
          mountPath: /mnt/hosts.append
        ...