Dynamically assign DNS IP using cni-env

[sachinkumarsingh092]

Summary

Earlier we used to assign DNS IP statically. But now we use the CIDR in cni-env to assign a valid IP from that range removing hard-coded IP.
We add new utilities in the utils.sh to calculate and test a valid IP from the cidr. Not using external utils like ipcalc, as they may not be present in every system.

[neoaggelos]

Is it not better to simply deploy the service without a fixed IP address, and then query KubeAPIserver with the address?

I cannot see the reason to add all this logic just to pick a random IP of a valid range, when Kubernetes will do that for us either way.

[sachinkumarsingh092]

We now assign DNSIP after deploying the service and assign arguments in kubelet based on the Cluster IP assigned to the DNS service. We also remove DNSIPARGS and assign IP to the DNS service dynamically only.

[neoaggelos]

This works much better. However, setting the DNS IP address as a second argument must still be supported.

You can support both by setting clusterIP if $2 is provided, otherwise leave it as an empty value.

[sachinkumarsingh092]

But the DNSIP env variable cannot be an empty value inside the manifest. Otherwise we will get errors like:

The Service "kube-dns" is invalid: spec.clusterIPs[0]: Invalid value: "$DNSIP": must be a valid IP address, (e.g. 10.9.8.7 or 2001:db8::ffff)

[neoaggelos]

Yes, this is because $DNSIP is not replaced in the manifest, and is rejected because of this. Try the following:

# works
apiVersion: v1
kind: Service
metadata:
  name: svc-1
spec:
  clusterIP: 
  ports:
  - port: 80
    protocol: TCP
    targetPort: 80
---
# does not work
apiVersion: v1
kind: Service
metadata:
  name: svc-2
spec:
  clusterIP: "$DNSIP"
  ports:
  - port: 80
    protocol: TCP
    targetPort: 80