Remove the cluster_domain_suffix variable

* Drop support for `cluster_domain_suffix` customization and
always use `cluster.local`. Many components in the Kubernetes
ecosystem assume this default suffix and its very rare to be
setting a special value here these days
* Cleanup a few variables that are seldom used

digital-ocean/flatcar-linux/kubernetes/bootstrap.tf

@@ -11,11 +11,10 @@ module "bootstrap" {
   network_encapsulation = "vxlan"
   network_mtu           = "1450"
 
-  pod_cidr              = var.pod_cidr
-  service_cidr          = var.service_cidr
-  cluster_domain_suffix = var.cluster_domain_suffix
-  enable_reporting      = var.enable_reporting
-  enable_aggregation    = var.enable_aggregation
-  components            = var.components
+  pod_cidr           = var.pod_cidr
+  service_cidr       = var.service_cidr
+  enable_reporting   = var.enable_reporting
+  enable_aggregation = var.enable_aggregation
+  components         = var.components
 }
 

digital-ocean/flatcar-linux/kubernetes/butane/controller.yaml

@@ -153,7 +153,7 @@ storage:
           cgroupDriver: systemd
           clusterDNS:
             - ${cluster_dns_service_ip}
-          clusterDomain: ${cluster_domain_suffix}
+          clusterDomain: cluster.local
           healthzPort: 0
           rotateCertificates: true
           shutdownGracePeriod: 45s

digital-ocean/flatcar-linux/kubernetes/butane/worker.yaml

@@ -103,7 +103,7 @@ storage:
           cgroupDriver: systemd
           clusterDNS:
             - ${cluster_dns_service_ip}
-          clusterDomain: ${cluster_domain_suffix}
+          clusterDomain: cluster.local
           healthzPort: 0
           rotateCertificates: true
           shutdownGracePeriod: 45s

digital-ocean/flatcar-linux/kubernetes/controllers.tf

@@ -79,7 +79,6 @@ data "ct_config" "controllers" {
       for i in range(var.controller_count) : "etcd${i}=https://${var.cluster_name}-etcd${i}.${var.dns_zone}:2380"
     ])
     cluster_dns_service_ip = cidrhost(var.service_cidr, 10)
-    cluster_domain_suffix  = var.cluster_domain_suffix
   })
   strict   = true
   snippets = var.controller_snippets

digital-ocean/flatcar-linux/kubernetes/variables.tf

@@ -98,13 +98,7 @@ variable "enable_aggregation" {
   default     = true
 }
 
-# unofficial, undocumented, unsupported
-
-variable "cluster_domain_suffix" {
-  type        = string
-  description = "Queries for domains with the suffix will be answered by coredns. Default is cluster.local (e.g. foo.default.svc.cluster.local) "
-  default     = "cluster.local"
-}
+# advanced
 
 variable "components" {
   description = "Configure pre-installed cluster components"

digital-ocean/flatcar-linux/kubernetes/workers.tf

@@ -60,7 +60,6 @@ resource "digitalocean_tag" "workers" {
 data "ct_config" "worker" {
   content = templatefile("${path.module}/butane/worker.yaml", {
     cluster_dns_service_ip = cidrhost(var.service_cidr, 10)
-    cluster_domain_suffix  = var.cluster_domain_suffix
   })
   strict   = true
   snippets = var.worker_snippets