add tutorial for coredns (#791)

There is no coredns tutorial for externalDNS. This pull request makes
coredns based on minikube for working with externalDNS.

README.md

@@ -55,6 +55,7 @@ The following tutorials are provided:
 * [AWS (Route53)](docs/tutorials/aws.md)
 * [AWS (Service Discovery)](docs/tutorials/aws-sd.md)
 * [Azure](docs/tutorials/azure.md)
+* [CoreDNS](docs/tutorials/coredns.md)
 * [Cloudflare](docs/tutorials/cloudflare.md)
 * [DigitalOcean](docs/tutorials/digitalocean.md)
 * [Infoblox](docs/tutorials/infoblox.md)

docs/tutorials/coredns.md

@@ -0,0 +1,164 @@
+# Setting up ExternalDNS for CoreDNS with minikube
+This tutorial describes how to setup ExternalDNS for usage within a [minikube](https://github.com/kubernetes/minikube) cluster that makes use of [CoreDNS](https://github.com/coredns/coredns) and [nginx ingress controller](https://github.com/kubernetes/ingress-nginx).  
+You need to:
+* install CoreDNS with [etcd](https://github.com/etcd-io/etcd) enabled
+* install external-dns with coredns as a provider
+* enable ingress controller for the minikube cluster
+
+
+## Creating a cluster
+```
+minikube start
+```
+
+## Installing CoreDNS with etcd enabled
+Helm chart is used to install etcd and CoreDNS.
+### Initializing helm chart
+```
+helm init
+```
+### Installing etcd
+[etcd operator](https://github.com/coreos/etcd-operator) is used to manage etcd clusters.
+```
+helm install stable/etcd-operator --name my-etcd-op
+```
+etcd cluster is installed with example yaml from etcd operator website.
+```
+kubectl  apply -f https://raw.githubusercontent.com/coreos/etcd-operator/master/example/example-etcd-cluster.yaml
+```
+
+### Installing CoreDNS
+In order to make CoreDNS work with etcd backend, values.yaml of the chart should be changed with corresponding configurations.
+```
+wget https://raw.githubusercontent.com/helm/charts/master/stable/coredns/values.yaml
+```
+
+You need to edit/patch the file with below diff
+```
+diff --git a/values.yaml b/values.yaml
+index 964e72b..e2fa934 100644
+--- a/values.yaml
++++ b/values.yaml
+@@ -27,12 +27,12 @@ service:
+
+ rbac:
+   # If true, create & use RBAC resources
+-  create: false
++  create: true
+   # Ignored if rbac.create is true
+   serviceAccountName: default
+
+ # isClusterService specifies whether chart should be deployed as cluster-service or normal k8s app.
+-isClusterService: true
++isClusterService: false
+
+ servers:
+ - zones:
+@@ -51,6 +51,12 @@ servers:
+     parameters: 0.0.0.0:9153
+   - name: proxy
+     parameters: . /etc/resolv.conf
++  - name: etcd
++    parameters: example.org
++    configBlock: |-
++      stubzones
++      path /skydns
++      endpoint http://10.105.68.165:2379
+
+ # Complete example with all the options:
+ # - zones:                 # the `zones` block can be left out entirely, defaults to "."
+```
+**Note**:  
+* IP address of etcd's endpoint should be get from etcd client service. It should be "example-etcd-cluster-client" in this example. This IP address is used through this document for etcd endpoint configuration.
+```
+$ kubectl get svc example-etcd-cluster-client
+NAME                          TYPE        CLUSTER-IP      EXTERNAL-IP   PORT(S)    AGE
+example-etcd-cluster-client   ClusterIP   10.105.68.165   <none>        2379/TCP   16m
+```
+* Parameters should configure your own domain. "example.org" is used in this example.
+
+
+After configuration done in values.yaml, you can install coredns chart.
+```
+helm install --name my-coredns --values values.yaml stable/coredns
+```
+
+## Installing ExternalDNS
+### Install external ExternalDNS
+ETCD_URLS is configured to etcd client service address.
+```
+$ cat external-dns.yaml
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: external-dns
+  namespace: kube-system
+spec:
+  strategy:
+    type: Recreate
+  selector:
+    matchLabels:                                                                                                                                                                                                                            
+      app: external-dns
+  template:
+    metadata:
+      labels:
+        app: external-dns
+    spec:
+      containers:
+      - name: external-dns
+        image: registry.opensource.zalan.do/teapot/external-dns:latest
+        args:
+        - --source=ingress
+        - --provider=coredns
+        - --log-level=debug # debug only
+        env:
+        - name: ETCD_URLS
+          value: http://10.105.68.165:2379
+$ kubectl apply -f external-dns.yaml
+```
+
+## Enable the ingress controller
+You can use the ingress controller in minikube cluster. It needs to enable ingress addon in the cluster.
+```
+minikube addons enable ingress
+```
+
+## Testing ingress example
+```
+$ cat ingress.yaml      
+kind: Ingress
+metadata:
+  name: nginx
+  annotations:
+    kubernetes.io/ingress.class: "nginx"
+spec:
+  rules:
+  - host: nginx.example.org
+    http:
+      paths:
+      - backend:
+          serviceName: nginx
+          servicePort: 80
+
+$ kubectl apply -f ingress.yaml
+ingress.extensions "nginx" created
+```
+
+
+Wait a moment until DNS has the ingress IP. The DNS service IP is from CoreDNS service. It is "my-coredns-coredns" in this example.
+```
+$ kubectl get svc my-coredns-coredns
+NAME                 TYPE        CLUSTER-IP     EXTERNAL-IP   PORT(S)   AGE
+my-coredns-coredns   ClusterIP   10.100.4.143   <none>        53/UDP    12m
+
+$ kubectl get ingress
+NAME      HOSTS               ADDRESS     PORTS     AGE
+nginx     nginx.example.org   10.0.2.15   80        2m
+
+$ kubectl run -it --rm --restart=Never --image=infoblox/dnstools:latest dnstools
+If you don't see a command prompt, try pressing enter.
+dnstools# dig @10.102.213.122 nginx.example.org +short
+dnstools# dig @10.102.213.122 nginx.example.org +short
+10.0.2.15
+dnstools#  
+```