When changing the service_cidr, the cluster_dns address should also be changed

[jpetazzo]

service_cidr and cluster_dns have default values (10.43.0.0/16 and 10.43.0.10 respectively).

If they are both absent from the configuration, the default values are used, and the resulting cluster will have a functioning kube-dns Service. Yay!

However, if I set service_cidr (for instance, to 10.10.0.0/16) without setting cluster_dns, then it looks like cluster_dns remains to its default value of 10.43.0.10, and since the default value is outside of the service CIDR, the creation of the kube-dns Service fails, and the resulting cluster has a broken DNS configuration (the pods are configured with 10.43.0.10 but there is no ClusterIP service at that address).

I'm going to submit a PR to add a warning in the sample configuration file, to remind users that if they change the service_cidr, they should also change the cluster_dns.

Do you think it would be necessary to add a check in the configuration validation as well?
Or, alternatively, if cluster_dns is not set, should it be automatically computed from service_cidr?

[vitobotta]

A PR would be nice. I think it's better to keep settings explicit IMO. BTW I am in the middle of a massive refactoring, and a lot of code has changed already for v2.0 which I am going to release soon. So you may want to wait for the PR. Otherwise you will need to adapt it to the new code later.