initial cluster domain docs

Author: maltesander

modules/guides/nav.adoc

@@ -4,3 +4,4 @@
 ** xref:running-stackable-in-an-airgapped-environment.adoc[]
 ** xref:viewing-and-verifying-sboms.adoc[]
 ** xref:enabling-verification-of-image-signatures.adoc[]
+** xref:cluster-domain.adoc[]

modules/guides/pages/cluster-domain.adoc

@@ -0,0 +1,43 @@
+= Configuring the Kubernetes cluster domain
+:description: Configure Stackable operators to use a different cluster domain other than 'cluster.local'.
+:dns-custom-nameservers: https://kubernetes.io/docs/tasks/administer-cluster/dns-custom-nameservers/
+:dns-pod-service: https://kubernetes.io/docs/concepts/services-networking/dns-pod-service/
+
+Stackable operators allow to configure a non-default cluster domain as described in {dns-custom-nameservers}[`Customizing DNS Service`] (and more in {dns-pod-service}[`DNS for Services and Pods`]).
+The configuration is primarily done via and environment variable `KUBERNETES_CLUSTER_DOMAIN`, but there is also a mechansim to auto detect the cluster domain using the `/etc/resolv.conf` file.
+
+The following steps explain the use of the environment variable and the auto detection mechanism in detail.
+
+== Steps
+
+=== Setting `KUBERNETES_CLUSTER_DOMAIN` variable
+
+The environment variable `KUBERNETES_CLUSTER_DOMAIN` takes precedence over auto detection. This means that the desired cluster domain can be configured in a Helm values file via the property`kubernetesClusterDomain` or via the install command like:
+
+```
+helm install hive-operator stackable-stable/hive-operator --set kubernetesClusterDomain="my-cluster.local"
+```
+
+=== Using auto detection
+
+As a first step, the auto detection is checking the runtime environment of the operator. In most cases this is a clusterized environment like Kubernetes or Openshift.
+This is determined by checking if the `KUBERNETES_SERVICE_HOST` environment variable is set in the operator Pod.
+
+If clusterized, the auto detection parses the `/etc/resolv.conf` file for the last entry starting with the `search` keyword. Within that `search` entry the shortest option is selected.
+
+An example for the `/etc/resolv.conf` in a Kubelet looks like:
+
+```
+nameserver 10.32.0.10
+search <namespace>.svc.cluster.local svc.cluster.local cluster.local
+options ndots:5
+```
+
+Using this example as a base for the cluster domain auto detection, the operator would choose the `cluster.local` option.
+
+=== Running locally
+
+This section covers the case when both `KUBERNETES_CLUSTER_DOMAIN` and `KUBERNETES_SERVICE_HOST` are not set. This would be the case if the operator is not running in a clusterized environment like e.g. locally a Laptop or PC.
+Running the operator locally is mostly for development and therefore not relevant for most users.
+
+If running locally, and `KUBERNETES_CLUSTER_DOMAIN` and `KUBERNETES_SERVICE_HOST` are not set, the operator will default its cluster domain to `cluster.local`.