Merge pull request #2 from timoreimann/add-deployment-guide

Add deployment guide

README.md

@@ -25,6 +25,10 @@ To increase the amount of logging, add `-v=3` as parameter.
 - `make container`: create a Docker image (set the `VERSION` environment variable to choose a custom image tag)
 - `make push`: push the Docker image
 
+## Deploying
+
+See [the deployment guide](/deploy/README.md).
+
 ## Releasing
 
 1. Update the `VERSION` variable in the Makefile.

deploy/README.md

@@ -0,0 +1,16 @@
+# Deployment
+
+Deploying coredns-sidecar requires the following modifications to an existing CoreDNS installation:
+
+1. add the sidecar to the CoreDNS deployment (see [this patch](coredns-deploy-patch.yml) that can be applied running `kubectl patch --namespace kube-system deploy coredns --patch "$(cat coredns-deploy-patch.yml)"`)
+1. expand existing RBAC rules for CoreDNS to permit listing and watching nodes
+1. amend the CoreDNS Corefile by a piece of configuration for the hosts plugin:
+
+```
+hosts /shared/hosts {
+    ttl 5
+    fallthrough
+}
+```
+
+`fallthrough` is needed so that lookups which cannot be fulfilled by the hosts file get relayed to other resolution mechanisms provided by CoreDNS. `tls` can be adjusted per your own discretion.

deploy/coredns-deploy-patch.yml

@@ -0,0 +1,35 @@
+spec:
+  template:
+    spec:
+      containers:
+      - name: coredns
+        volumeMounts:
+        - mountPath: /shared
+          name: hostsfile
+      - args:
+        - -hostsfile
+        - /shared/hosts
+        image: timoreimann/coredns-sidecar:0.0.1
+        name: hostsfile-updater
+        resources:
+          limits:
+            memory: 100Mi
+          requests:
+            cpu: 100m
+            memory: 100Mi
+        volumeMounts:
+        - mountPath: /shared
+          name: hostsfile
+      initContainers:
+      # Use an init container to create an empty hosts file with proper file permissions. We need this because
+      # the sidecar implementation copies over the permissions from an existing file.
+      - name: hostsfile-creator
+        image: busybox:1.30.1
+        command: ['sh', '-c', 'touch /shared/hosts && chmod 755 /shared/hosts']
+        volumeMounts:
+        - mountPath: /shared
+          name: hostsfile
+      volumes:
+      - emptyDir: {}
+        name: hostsfile
+