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

ghz 1years ago ⋅ 1913 views

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

UPDATE: 2017-07-03 Kunbernetes 1.7 now support [Adding entries to Pod /etc/hosts with HostAliases](https://kubernetes.io/docs/concepts/services- networking/add-entries-to-pod-etc-hosts-with-host-aliases/).

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.

  1. 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

  1. 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 ...

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