docs(gke): detail how to configure workload identity

docs/tutorials/gke.md

@@ -14,7 +14,7 @@ This solution will only work when both CloudDNS and GKE are provisioned in the s
 
 ### Configure Project Environment
 
-Setup your environment to work with Google Cloud Platform. Fill in your variables as needed, e.g. target project.
+Set up your environment to work with Google Cloud Platform. Fill in your variables as needed, e.g. target project.
 
 ```bash
 # set variables to the appropriate desired values
@@ -151,6 +151,91 @@ gcloud container clusters create $GKE_CLUSTER_NAME \
   --workload-pool "$GKE_PROJECT_ID.svc.id.goog"
 ```
 
+### Workload Identity
+
+[Workload Identity](https://cloud.google.com/kubernetes-engine/docs/how-to/workload-identity) allows workloads in your GKE cluster to [authenticate directly to GCP](https://cloud.google.com/kubernetes-engine/docs/concepts/workload-identity#credential-flow) using Kubernetes Service Accounts
+
+You have an option to chose from using the gcloud CLI or using Terraform.
+
+=== "gcloud CLI"
+
+    The below instructions assume you are using the default Kubernetes Service account name of `external-dns` in the namespace `external-dns`
+
+    Grant the Kubernetes service account DNS `roles/dns.admin` at project level
+
+    ```shell
+    gcloud projects add-iam-policy-binding projects/DNS_PROJECT_ID \
+        --role=roles/dns.admin \
+        --member=principal://iam.googleapis.com/projects/PROJECT_NUMBER/locations/global/workloadIdentityPools/PROJECT_ID.svc.id.goog/subject/ns/external-dns/sa/external-dns \
+        --condition=None
+    ```
+
+    Replace the following:
+
+    * `DNS_PROJECT_ID` : Project ID of your DNS project. If DNS is in the same project as your GKE cluster, use your GKE project.
+    * `PROJECT_ID`: your Google Cloud project ID of your GKE Cluster
+    * `PROJECT_NUMBER`: your numerical Google Cloud project number of your GKE cluster
+
+    If you wish to change the namespace, replace 
+
+    * `ns/external-dns` with `ns/<your namespace`  
+    * `sa/external-dns` with `sa/<your ksa>`
+
+
+
+=== "Terraform"
+
+    The below instructions assume you are using the default Kubernetes Service account name of `external-dns` in the namespace `external-dns`
+
+    Create a file called `main.tf` and place in it the below. _Note: If you're an experienced terraform user feel free to split these out in to different files_ 
+
+    ```hcl
+    variable "gke-project" {
+      type        = string
+      description = "Name of the project that the GKE cluster exists in"
+      default     = "GKE-PROJECT"
+    }
+
+    variable "ksa_name" {
+      type        = string
+      description = "Name of the Kubernetes service account that will be accessing the DNS Zones"
+      default     = "external-dns"
+    }
+
+    variable "kns_name" {
+      type        = string
+      description = "Name of the Kubernetes Namespace"
+      default     = "external-dns"
+    }
+
+    data "google_project" "project" {
+      project_id = var.gke-project
+    }
+
+    locals {
+      member = "principal://iam.googleapis.com/projects/${data.google_project.project.number}/locations/global/workloadIdentityPools/${var.gke-project}.svc.id.goog/subject/ns/${var.kns_name}/sa/${var.ksa_name}"
+    }
+
+    resource "google_project_iam_member" "external_dns" {
+      member  = local.member
+      project = "DNS-PROJECT"
+      role    = "roles/dns.admin"
+    }
+    ```
+
+    Replace the following
+
+    * `GKE-PROJECT` : Project that contains your GKE cluster
+    * `DNS-PROJECT` : Project that holds your DNS zones
+
+    You can also change the below if you plan to use a different service account name and namespace
+
+    * `variable "ksa_name"` : Name of the Kubernetes service account external-dns will use
+    * `variable "kns_name"` : Name of the Kubernetes Name Space that will have external-dns installed to
+
+
+
+
 ### Worker Node Service Account method
 
 In this method, the GSA (Google Service Account) that is associated with GKE worker nodes will be configured to have access to Cloud DNS.  
@@ -206,47 +291,17 @@ kubectl create secret generic "external-dns" --namespace ${EXTERNALDNS_NS:-"defa
 ```
 
 After this, follow the steps in [Deploy ExternalDNS](#deploy-externaldns).  Make sure to set the `--google-project` flag to match Cloud DNS project name. Make sure to uncomment out the section that mounts the secret to the ExternalDNS pods.
-### Workload Identity
-
-[Workload Identity](https://cloud.google.com/kubernetes-engine/docs/how-to/workload-identity) allows workloads in your GKE cluster to impersonate GSA (Google Service Accounts) using KSA (Kubernetes Service Accounts) configured during deployemnt.  These are the steps to use this feature with ExternalDNS.
-
-#### Create GSA for use with Workload Identity
-
-```bash
-DNS_SA_NAME="external-dns-sa"
-DNS_SA_EMAIL="$DNS_SA_NAME@${GKE_PROJECT_ID}.iam.gserviceaccount.com"
-
-gcloud iam service-accounts create $DNS_SA_NAME --display-name $DNS_SA_NAME
-gcloud projects add-iam-policy-binding $DNS_PROJECT_ID \
-   --member serviceAccount:$DNS_SA_EMAIL --role "roles/dns.admin"
-```
-
-#### Link KSA to GSA
 
-Add an IAM policy binding bewtween the workload identity GSA and ExternalDNS GSA.  This will link the ExternalDNS KSA to ExternalDNS GSA.
-
-```bash
-gcloud iam service-accounts add-iam-policy-binding $DNS_SA_EMAIL \
-  --role "roles/iam.workloadIdentityUser" \
-  --member "serviceAccount:$GKE_PROJECT_ID.svc.id.goog[${EXTERNALDNS_NS:-"default"}/external-dns]"
-```
 
 #### Deploy External DNS
 
 Deploy ExternalDNS with the following steps below, documented under [Deploy ExternalDNS](#deploy-externaldns).  Set the `--google-project` flag to the Cloud DNS project name.
 
-#### Link KSA to GSA in Kubernetes
-
-Add the proper workload identity annotation to the ExternalDNS KSA.
-
-```bash
-kubectl annotate serviceaccount "external-dns" \
-  --namespace ${EXTERNALDNS_NS:-"default"} \
-  "iam.gke.io/gcp-service-account=$DNS_SA_EMAIL"
-```
-
 #### Update ExternalDNS pods
 
+!!! note "Only required if not enabled on all nodes"
+    If you have GKE Workload Identity enabled on all nodes in your cluster, the below step is not necessary 
+
 Update the Pod spec to schedule the workloads on nodes that use Workload Identity and to use the annotated Kubernetes service account.
 
 ```bash