Revert "Revert "Add CoreDNS details to DNS Debug docs (#10201)""

This reverts commit bb30f4d.

Author: zparnold

content/en/docs/tasks/administer-cluster/dns-debugging-resolution.md

@@ -13,7 +13,7 @@ This page provides hints on diagnosing DNS problems.
 {{% capture prerequisites %}}
 * {{< include "task-tutorial-prereqs.md" >}} {{< version-check >}}
 * Kubernetes version 1.6 and above.
-* The cluster must be configured to use the `kube-dns` addon.
+* The cluster must be configured to use the `coredns` (or `kube-dns`) addons.
 {{% /capture %}}
 
 {{% capture steps %}}
@@ -68,7 +68,7 @@ nameserver 10.0.0.10
 options ndots:5
 ```
 
-Errors such as the following indicate a problem with the kube-dns add-on or
+Errors such as the following indicate a problem with the coredns/kube-dns add-on or
 associated Services:
 
 ```
@@ -93,6 +93,17 @@ nslookup: can't resolve 'kubernetes.default'
 
 Use the `kubectl get pods` command to verify that the DNS pod is running.
 
+For CoreDNS:
+```shell
+kubectl get pods --namespace=kube-system -l k8s-app=kube-dns
+NAME                       READY     STATUS    RESTARTS   AGE
+...
+coredns-7b96bf9f76-5hsxb   1/1       Running   0           1h
+coredns-7b96bf9f76-mvmmt   1/1       Running   0           1h
+...
+```
+
+Or for kube-dns:
 ```shell
 kubectl get pods --namespace=kube-system -l k8s-app=kube-dns
 NAME                    READY     STATUS    RESTARTS   AGE
@@ -107,8 +118,26 @@ have to deploy it manually.
 
 ### Check for Errors in the DNS pod
 
-Use `kubectl logs` command to see logs for the DNS daemons.
+Use `kubectl logs` command to see logs for the DNS containers.
 
+For CoreDNS:
+```shell
+for p in $(kubectl get pods --namespace=kube-system -l k8s-app=kube-dns -o name); do kubectl logs --namespace=kube-system $p; done
+```
+
+Here is an example of a healthy CoreDNS log:
+
+```
+.:53
+2018/08/15 14:37:17 [INFO] CoreDNS-1.2.2
+2018/08/15 14:37:17 [INFO] linux/amd64, go1.10.3, 2e322f6
+CoreDNS-1.2.2
+linux/amd64, go1.10.3, 2e322f6
+2018/08/15 14:37:17 [INFO] plugin/reload: Running configuration MD5 = 24e6c59e83ce706f07bcc82c31b1ea1c
+```
+
+
+For kube-dns, there are 3 sets of logs:
 ```shell
 kubectl logs --namespace=kube-system $(kubectl get pods --namespace=kube-system -l k8s-app=kube-dns -o name | head -1) -c kubedns
 
@@ -117,8 +146,8 @@ kubectl logs --namespace=kube-system $(kubectl get pods --namespace=kube-system
 kubectl logs --namespace=kube-system $(kubectl get pods --namespace=kube-system -l k8s-app=kube-dns -o name | head -1) -c sidecar
 ```
 
-See if there is any suspicious log. Letter '`W`', '`E`', '`F`' at the beginning
-represent Warning, Error and Failure. Please search for entries that have these
+See if there are any suspicious error messages in the logs. In kube-dns, a '`W`', '`E`' or '`F`' at the beginning
+of a line represents a Warning, Error or Failure. Please search for entries that have these
 as the logging level and use
 [kubernetes issues](https://github.com/kubernetes/kubernetes/issues)
 to report unexpected errors.
@@ -135,6 +164,8 @@ kube-dns     ClusterIP   10.0.0.10      <none>        53/UDP,53/TCP        1h
 ...
 ```
 
+
+Note that the service name will be "kube-dns" for both CoreDNS and kube-dns deployments.
 If you have created the service or in the case it should be created by default
 but it does not appear, see 
 [debugging services](/docs/tasks/debug-application-cluster/debug-service/) for
@@ -158,20 +189,83 @@ For additional Kubernetes DNS examples, see the
 [cluster-dns examples](https://github.com/kubernetes/examples/tree/master/staging/cluster-dns)
 in the Kubernetes GitHub repository.
 
+
+### Are DNS queries being received/processed?
+
+You can verify if queries are being received by CoreDNS by adding the `log` plugin to the CoreDNS configuration (aka Corefile).
+The CoreDNS Corefile is held in a ConfigMap named `coredns`. To edit it, use the command ...
+
+```
+kubectl -n kube-system edit configmap coredns
+```
+
+Then add `log` in the Corefile section per the example below.
+
+```
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: coredns
+  namespace: kube-system
+data:
+  Corefile: |
+    .:53 {
+        log
+        errors
+        health
+        kubernetes cluster.local in-addr.arpa ip6.arpa {
+          pods insecure
+          upstream
+          fallthrough in-addr.arpa ip6.arpa
+        }
+        prometheus :9153
+        proxy . /etc/resolv.conf
+        cache 30
+        loop
+        reload
+        loadbalance
+    }
+
+```
+
+After saving the changes, it may take up to minute or two for Kubernetes to propagate these changes to the CoreDNS pods.
+
+Next, make some queries and view the logs per the sections above in this document. If CoreDNS pods are receiving the queries, you should see them in the logs.
+
+Here is an example of a query in the log.
+
+```
+.:53
+2018/08/15 14:37:15 [INFO] CoreDNS-1.2.0
+2018/08/15 14:37:15 [INFO] linux/amd64, go1.10.3, 2e322f6
+CoreDNS-1.2.0
+linux/amd64, go1.10.3, 2e322f6
+2018/09/07 15:29:04 [INFO] plugin/reload: Running configuration MD5 = 162475cdf272d8aa601e6fe67a6ad42f
+2018/09/07 15:29:04 [INFO] Reloading complete
+172.17.0.18:41675 - [07/Sep/2018:15:29:11 +0000] 59925 "A IN kubernetes.default.svc.cluster.local. udp 54 false 512" NOERROR qr,aa,rd,ra 106 0.000066649s
+
+```
+
 ## Known issues
 
-Kubernetes installs do not configure the nodes' resolv.conf files to use the
-cluster DNS by default, because that process is inherently distro-specific.
+Some Linux distributions (e.g. Ubuntu), use a local DNS resolver by default (systemd-resolved).
+Systemd-resolved moves and replaces `/etc/resolv.conf` with a stub file that can cause a fatal forwarding
+loop when resolving names in upstream servers. This can be fixed manually by using kubelet's `--resolv-conf` flag
+to point to the correct `resolv.conf` (With `systemd-resolved`, this is `/run/systemd/resolve/resolv.conf`).
+kubeadm 1.11 automatically detects `systemd-resolved`, and adjusts the kubelet flags accordingly.
+
+Kubernetes installs do not configure the nodes' `resolv.conf` files to use the
+cluster DNS by default, because that process is inherently distribution-specific.
 This should probably be implemented eventually.
 
 Linux's libc is impossibly stuck ([see this bug from
 2005](https://bugzilla.redhat.com/show_bug.cgi?id=168253)) with limits of just
-3 DNS `nameserver` records and 6 DNS `search` records.  Kubernetes needs to
-consume 1 `nameserver` record and 3 `search` records.  This means that if a
+3 DNS `nameserver` records and 6 DNS `search` records. Kubernetes needs to
+consume 1 `nameserver` record and 3 `search` records. This means that if a
 local installation already uses 3 `nameserver`s or uses more than 3 `search`es,
-some of those settings will be lost.  As a partial workaround, the node can run
+some of those settings will be lost. As a partial workaround, the node can run
 `dnsmasq` which will provide more `nameserver` entries, but not more `search`
-entries.  You can also use kubelet's `--resolv-conf` flag.
+entries. You can also use kubelet's `--resolv-conf` flag.
 
 If you are using Alpine version 3.3 or earlier as your base image, DNS may not
 work properly owing to a known issue with Alpine.