Merge pull request #70965 from prameshj/nodecache-readme

README for nodelocal dns cache
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# Nodelocal DNS Cache
This addon runs a node-local-dns pod on all cluster nodes. The pod runs CoreDNS as the dns cache. It runs with `hostNetwork:True` and creates a dedicated dummy interface with a link local ip(169.254.20.10/32 by default) to listen for DNS queries. The cache instances connect to clusterDNS in case of cache misses.
Design details [here](https://github.com/kubernetes/community/blob/master/keps/sig-network/0030-nodelocal-dns-cache.md)
## nodelocaldns addon template
This directory contains the addon config yaml - `nodelocaldns.yaml`
The variables will be substituted by the configure scripts when the yaml is copied into master.
### Network policy and DNS connectivity
When running nodelocaldns addon on clusters using network policy, additional rules might be required to enable dns connectivity.
Using a namespace selector for dns egress traffic as shown [here](https://docs.projectcalico.org/v2.6/getting-started/kubernetes/tutorials/advanced-policy)
might not be enough since the node-local-dns pods run with `hostNetwork: True`
One way to enable connectivity from node-local-dns pods to clusterDNS ip is to use an ipBlock rule instead:
```
spec:
egress:
- ports:
- port: 53
protocol: TCP
- port: 53
protocol: UDP
to:
- ipBlock:
cidr: <well-known clusterIP for DNS>/32
podSelector: {}
policyTypes:
- Ingress
- Egress
```