Merge pull request #32126 from tengqm/update-coredns

Improve coredns page

content/en/docs/tasks/administer-cluster/coredns.md

@@ -19,11 +19,14 @@ This page describes the CoreDNS upgrade process and how to install CoreDNS inste
 
 ## About CoreDNS
 
-[CoreDNS](https://coredns.io) is a flexible, extensible DNS server that can serve as the Kubernetes cluster DNS.
-Like Kubernetes, the CoreDNS project is hosted by the {{< glossary_tooltip text="CNCF" term_id="cncf" >}}.
+[CoreDNS](https://coredns.io) is a flexible, extensible DNS server
+that can serve as the Kubernetes cluster DNS.
+Like Kubernetes, the CoreDNS project is hosted by the
+{{< glossary_tooltip text="CNCF" term_id="cncf" >}}.
 
-You can use CoreDNS instead of kube-dns in your cluster by replacing kube-dns in an existing
-deployment, or by using tools like kubeadm that will deploy and upgrade the cluster for you.
+You can use CoreDNS instead of kube-dns in your cluster by replacing
+kube-dns in an existing deployment, or by using tools like kubeadm
+that will deploy and upgrade the cluster for you.
 
 ## Installing CoreDNS
 
@@ -34,51 +37,44 @@ For manual deployment or replacement of kube-dns, see the documentation at the
 
 ### Upgrading an existing cluster with kubeadm
 
-In Kubernetes version 1.10 and later, you can also move to CoreDNS when you use `kubeadm` to upgrade
-a cluster that is using `kube-dns`. In this case, `kubeadm` will generate the CoreDNS configuration
+In Kubernetes version 1.21, kubeadm removed its support for `kube-dns` as a DNS application.
+For `kubeadm` v{{< skew currentVersion >}}, the only supported cluster DNS application
+is CoreDNS.
+
+You can move to CoreDNS when you use `kubeadm` to upgrade a cluster that is
+using `kube-dns`. In this case, `kubeadm` generates the CoreDNS configuration
 ("Corefile") based upon the `kube-dns` ConfigMap, preserving configurations for
 stub domains, and upstream name server.
 
-If you are moving from kube-dns to CoreDNS, make sure to set the `CoreDNS` feature gate to `true`
-during an upgrade. For example, here is what a `v1.11.0` upgrade would look like:
-```
-kubeadm upgrade apply v1.11.0 --feature-gates=CoreDNS=true
-```
-
-In Kubernetes version 1.13 and later the `CoreDNS` feature gate is removed and CoreDNS
-is used by default.
-
-In versions prior to 1.11 the Corefile will be **overwritten** by the one created during upgrade.
-**You should save your existing ConfigMap if you have customized it.** You may re-apply your
-customizations after the new ConfigMap is up and running.
-
-If you are running CoreDNS in Kubernetes version 1.11 and later, during upgrade,
-your existing Corefile will be retained.
+## Upgrading CoreDNS
 
-In Kubernetes version 1.21, support for `kube-dns` is removed from kubeadm.
+You can check the version of CoreDNS that kubeadm installs for each version of
+Kubernetes in the page
+[CoreDNS version in Kubernetes](https://github.com/coredns/deployment/blob/master/kubernetes/CoreDNS-k8s_version.md).
 
-## Upgrading CoreDNS
+CoreDNS can be upgraded manually in case you want to only upgrade CoreDNS
+or use your own custom image.
+There is a helpful [guideline and walkthrough](https://github.com/coredns/deployment/blob/master/kubernetes/Upgrading_CoreDNS.md)
+available to ensure a smooth upgrade.
+Make sure the existing CoreDNS configuration ("Corefile") is retained when
+upgrading your cluster.
 
-CoreDNS is available in Kubernetes since v1.9.
-You can check the version of CoreDNS shipped with Kubernetes and the changes made to CoreDNS [here](https://github.com/coredns/deployment/blob/master/kubernetes/CoreDNS-k8s_version.md).
+If you are upgrading your cluster using the `kubeadm` tool, `kubeadm`
+can take care of retaining the existing CoreDNS configuration automatically.
 
-CoreDNS can be upgraded manually in case you want to only upgrade CoreDNS or use your own custom image.
-There is a helpful [guideline and walkthrough](https://github.com/coredns/deployment/blob/master/kubernetes/Upgrading_CoreDNS.md) available to ensure a smooth upgrade.
 
 ## Tuning CoreDNS
 
-When resource utilisation is a concern, it may be useful to tune the configuration of CoreDNS. For more details, check out the
+When resource utilisation is a concern, it may be useful to tune the
+configuration of CoreDNS. For more details, check out the
 [documentation on scaling CoreDNS](https://github.com/coredns/deployment/blob/master/kubernetes/Scaling_CoreDNS.md).
 
-
-
 ## {{% heading "whatsnext" %}}
 
-
 You can configure [CoreDNS](https://coredns.io) to support many more use cases than
-kube-dns by modifying the `Corefile`. For more information, see the
-[CoreDNS site](https://coredns.io/2017/05/08/custom-dns-entries-for-kubernetes/).
-
-
-
+kube-dns does by modifying the CoreDNS configuration ("Corefile").
+For more information, see the [documentation](https://coredns.io/plugins/kubernetes/)
+for the `kubernetes` CoreDNS plugin, or read the 
+[Custom DNS Entries for Kubernetes](https://coredns.io/2017/05/08/custom-dns-entries-for-kubernetes/).
+in the CoreDNS blog.