Configure GMSA for Windows Pods and containers [page]deterministic

This page shows how to configure [Group Managed Service Accounts](https://docs.microsoft.com/en-us/windows-server/security/group-managed-service-accounts/group-managed-service-accounts-overview) (GMSA) for Pods and containers that will run on Windows nodes. Group Managed Service Accounts are a specific type of Active Directory account that provides automatic password management, simplified service principal name (SPN) management, and the ability to delegate the management to other administrators across multiple servers.

In Kubernetes, GMSA credential specs are configured at a Kubernetes cluster-wide scope as Custom Resources. Windows Pods, as well as individual containers within a Pod, can be configured to use a GMSA for domain based functions (e.g. Kerberos authentication) when interacting with other Windows services.

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You need to have a Kubernetes cluster and the `kubectl` command-line tool must be configured to communicate with your cluster. The cluster is expected to have Windows worker nodes. This section covers a set of initial steps required once for each cluster:

### Install the GMSACredentialSpec CRD

A [CustomResourceDefinition](/docs/tasks/extend-kubernetes/custom-resources/custom-resource-definitions/)(CRD) for GMSA credential spec resources needs to be configured on the cluster to define the custom resource type `GMSACredentialSpec`. Download the GMSA CRD [YAML](https://github.com/kubernetes-sigs/windows-gmsa/blob/master/admission-webhook/deploy/gmsa-crd.yml) and save it as gmsa-crd.yaml. Next, install the CRD with `kubectl apply -f gmsa-crd.yaml`

### Install webhooks to validate GMSA users

Two webhooks need to be configured on the Kubernetes cluster to populate and validate GMSA credential spec references at the Pod or container level:

1. A mutating webhook that expands references to GMSAs (by name from a Pod specification) into the full credential spec in JSON form within the Pod spec.

1. A validating webhook ensures all references to GMSAs are authorized to be used by the Pod service account.

Installing the above webhooks and associated objects require the steps below:

1. Create a certificate key pair (that will be used to allow the webhook container to communicate to the cluster)

1. Install a secret with the certificate from above.

1. Create a deployment for the core webhook logic.

1. Create the validating and mutating webhook configurations referring to the deployment.

A [script](https://github.com/kubernetes-sigs/windows-gmsa/blob/master/admission-webhook/deploy/deploy-gmsa-webhook.sh) can be used to deploy and configure the GMSA webhooks and associated objects mentioned above. The script can be run with a `--dry-run=server` option to allow you to review the changes that would be made to your cluster.

The [YAML template](https://github.com/kubernetes-sigs/windows-gmsa/blob/master/admission-webhook/deploy/gmsa-webhook.yml.tpl) used by the script may also be used to deploy the webhooks and associated objects manually (with appropriate substitutions for the parameters)

## Configure GMSAs and Windows nodes in Active Directory

Before Pods in Kubernetes can be configured to use GMSAs, the desired GMSAs need to be provisioned in Active Directory as described in the [Windows GMSA documentation](https://docs.microsoft.com/en-us/windows-server/security/group-managed-service-accounts/getting-started-with-group-managed-service-accounts#BKMK_Step1). Windows worker nodes (that are part of the Kubernetes cluster) need to be configured in Active Directory to access the secret credentials associated with the desired GMSA as described in the [Windows GMSA documentation](https://docs.microsoft.com/en-us/windows-server/security/group-managed-service-accounts/getting-started-with-group-managed-service-accounts#to-add-member-hosts-using-the-set-adserviceaccount-cmdlet).

## Create GMSA credential spec resources

With the GMSACredentialSpec CRD installed (as described earlier), custom resources contain …(trimmed)