Update documentation on CoreDNS (#10923)

* update documentation on CoreDNS

* Update dns-custom-nameservers.md

Fixed nit noted in comments and cleaned up a few more things while I was in there. This whole topic needs a complete redo, but this PR is an iterative improvment. Thanks!

content/en/docs/concepts/services-networking/connect-applications-service.md

@@ -122,7 +122,7 @@ about the [service proxy](/docs/concepts/services-networking/service/#virtual-ip
 
 Kubernetes supports 2 primary modes of finding a Service - environment variables
 and DNS. The former works out of the box while the latter requires the
-[kube-dns cluster addon](http://releases.k8s.io/{{< param "githubbranch" >}}/cluster/addons/dns/kube-dns/README.md).
+[CoreDNS cluster addon](http://releases.k8s.io/{{< param "githubbranch" >}}/cluster/addons/dns/coredns).
 
 ### Environment Variables
 
@@ -178,7 +178,7 @@ kube-dns   ClusterIP   10.0.0.10    <none>        53/UDP,53/TCP   8m
 
 If it isn't running, you can [enable it](http://releases.k8s.io/{{< param "githubbranch" >}}/cluster/addons/dns/README.md#how-do-i-configure-it).
 The rest of this section will assume you have a Service with a long lived IP
-(my-nginx), and a DNS server that has assigned a name to that IP (the kube-dns
+(my-nginx), and a DNS server that has assigned a name to that IP (the CoreDNS
 cluster addon), so you can talk to the Service from any pod in your cluster using
 standard methods (e.g. gethostbyname). Let's run another curl application to test this:
 

content/en/docs/setup/independent/troubleshooting-kubeadm.md

@@ -258,6 +258,9 @@ kubectl -n kube-system get deployment coredns -o yaml | \
   kubectl apply -f -
 ```
 
+Another cause for CoreDNS to have `CrashLoopBackOff` is when a CoreDNS Pod deployed in Kubernetes detects a loop. [A number of workarounds](https://github.com/coredns/coredns/tree/master/plugin/loop#troubleshooting-loops-in-kubernetes-clusters)
+are available to avoid Kubernetes trying to restart the CoreDNS Pod every time CoreDNS detects the loop and exits.
+
 {{< warning >}}
 Disabling SELinux or setting `allowPrivilegeEscalation` to `true` can compromise
 the security of your cluster.

content/en/docs/setup/turnkey/gce.md

@@ -134,11 +134,11 @@ You'll see a list of pods that looks something like this (the name specifics wil
 
 ```shell
 NAMESPACE     NAME                                           READY     STATUS    RESTARTS   AGE
+kube-system   coredns-5f4fbb68df-mc8z8                       1/1       Running   0          15m
 kube-system   fluentd-cloud-logging-kubernetes-minion-63uo   1/1       Running   0          14m
 kube-system   fluentd-cloud-logging-kubernetes-minion-c1n9   1/1       Running   0          14m
 kube-system   fluentd-cloud-logging-kubernetes-minion-c4og   1/1       Running   0          14m
 kube-system   fluentd-cloud-logging-kubernetes-minion-ngua   1/1       Running   0          14m
-kube-system   kube-dns-v5-7ztia                              3/3       Running   0          15m
 kube-system   kube-ui-v1-curt1                               1/1       Running   0          15m
 kube-system   monitoring-heapster-v5-ex4u3                   1/1       Running   1          15m
 kube-system   monitoring-influx-grafana-v1-piled             2/2       Running   0          15m

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

@@ -63,6 +63,11 @@ value to `false`:
 kubeadm init --feature-gates=CoreDNS=false
 ```
 
+## Tuning CoreDNS
+
+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)).
+
 {{% /capture %}}
 
 {{% capture whatsnext %}}

content/en/docs/tasks/administer-cluster/dns-custom-nameservers.md

@@ -103,10 +103,10 @@ The Corefile configuration includes the following [plugins](https://coredns.io/p
 * [proxy](https://coredns.io/plugins/proxy/): Any queries that are not within the cluster domain of Kubernetes will be forwarded to predefined resolvers (/etc/resolv.conf).
 * [cache](https://coredns.io/plugins/cache/): This enables a frontend cache.
 * [loop](https://coredns.io/plugins/loop/): Detects simple forwarding loops and halts the CoreDNS process if a loop is found.
-* [reload](https://coredns.io/plugins/reload): Allows automatic reload of a changed Corefile.
-* [loadbalance](https://coredns.io/plugins/loadbalance): This is a round-robin DNS loadbalancer by randomizing the order of A, AAAA, and MX records in the answer.
+* [reload](https://coredns.io/plugins/reload): Allows automatic reload of a changed Corefile. After you edit the ConfigMap configuration, allow two minutes for your changes to take effect.
+* [loadbalance](https://coredns.io/plugins/loadbalance): This is a round-robin DNS loadbalancer that randomizes the order of A, AAAA, and MX records in the answer.
 
-We can modify the default behavior by modifying this configmap.
+You can modify the default CoreDNS behavior by modifying the ConfigMap.
 
 ### Configuration of Stub-domain and upstream nameserver using CoreDNS
 
@@ -132,7 +132,7 @@ proxy .  172.16.0.1
 upstream 172.16.0.1
 ```
 
-So, the final ConfigMap along with the default `Corefile` configuration will look like:
+The final ConfigMap along with the default `Corefile` configuration looks like:
 
 ```yaml
 apiVersion: v1
@@ -307,9 +307,9 @@ data:
 
 ## CoreDNS configuration equivalent to kube-dns
 
-CoreDNS supports all the functionalities and more that is provided by kube-dns.
+CoreDNS supports the features of kube-dns and more.
 A ConfigMap created for kube-dns to support `StubDomains`and `upstreamNameservers` translates to the `proxy` plugin in CoreDNS.
-Similarly, the `Federation` plugin translates to the `federation` plugin in CoreDNS.
+Similarly, the `Federations` plugin in kube-dns translates to the `federation` plugin in CoreDNS.
 
 ### Example
 
@@ -380,12 +380,10 @@ The complete Corefile with the default plugins:
     }
 ```
 
-In Kubernetes version 1.10 and later, kubeadm supports automatic translation of the CoreDNS ConfigMap from the kube-dns ConfigMap.
-
 ## Migration to CoreDNS
 
 To migrate from kube-dns to CoreDNS, [a detailed blog](https://coredns.io/2018/05/21/migration-from-kube-dns-to-coredns/) is available to help users adapt CoreDNS in place of kube-dns.
-A cluster administrator can also migrate using [the deploy script](https://github.com/coredns/deployment/blob/master/kubernetes/deploy.sh), which  will also help you translate the kube-dns configmap to the equivalent CoreDNS one.
+A cluster administrator can also migrate using [the deploy script](https://github.com/coredns/deployment/blob/master/kubernetes/deploy.sh).
 
 ## What's next
 - [Debugging DNS Resolution](/docs/tasks/administer-cluster/dns-debugging-resolution/).