This is a Cluster Administrator guide to service accounts. It assumes knowledge of the User Guide to Service Accounts.
Support for authorization and user accounts is planned but incomplete. Sometimes incomplete features are referred to in order to better describe service accounts.
User accounts vs service accounts
Kubernetes distinguished between the concept of a user account and a service accounts for a number of reasons:
- User accounts are for humans. Service accounts are for processes, which run in pods.
- User accounts are intended to be global. Names must be unique across all namespaces of a cluster, future user resource will not be namespaced. Service accounts are namespaced.
- Typically, a cluster’s User accounts might be synced from a corporate database, where new user account creation requires special privileges and is tied to complex business processes. Service account creation is intended to be more lightweight, allowing cluster users to create service accounts for specific tasks (i.e. principle of least privilege).
- Auditing considerations for humans and service accounts may differ.
- A config bundle for a complex system may include definition of various service accounts for components of that system. Because service accounts can be created ad-hoc and have namespaced names, such config is portable.
Service account automation
Three separate components cooperate to implement the automation around service accounts:
- A Service account admission controller
- A Token controller
- A Service account controller
Service Account Admission Controller
The modification of pods is implemented via a plugin called an Admission Controller. It is part of the apiserver. It acts synchronously to modify pods as they are created or updated. When this plugin is active (and it is by default on most distributions), then it does the following when a pod is created or modified:
- If the pod does not have a
ServiceAccountset, it sets theServiceAccounttodefault. - It ensures that the
ServiceAccountreferenced by the pod exists, and otherwise rejects it. - If the pod does not contain any
ImagePullSecrets, thenImagePullSecretsof theServiceAccountare added to the pod. - It adds a
volumeto the pod which contains a token for API access. - It adds a
volumeSourceto each container of the pod mounted at/var/run/secrets/kubernetes.io/serviceaccount.
Token Controller
TokenController runs as part of controller-manager. It acts asynchronously. It:
- observes serviceAccount creation and creates a corresponding Secret to allow API access.
- observes serviceAccount deletion and deletes all corresponding ServiceAccountToken Secrets
- observes secret addition, and ensures the referenced ServiceAccount exists, and adds a token to the secret if needed
- observes secret deletion and removes a reference from the corresponding ServiceAccount if needed
To create additional API tokens
A controller loop ensures a secret with an API token exists for each service
account. To create additional API tokens for a service account, create a secret
of type ServiceAccountToken with an annotation referencing the service
account, and the controller will update it with a generated token:
secret.json:
{
"kind": "Secret",
"apiVersion": "v1",
"metadata": {
"name": "mysecretname",
"annotations": {
"kubernetes.io/service-account.name": "myserviceaccount"
}
},
"type": "kubernetes.io/service-account-token"
}
kubectl create -f ./secret.json
kubectl describe secret mysecretname
To delete/invalidate a service account token
kubectl delete secret mysecretname
Service Account Controller
Service Account Controller manages ServiceAccount inside namespaces, and ensures a ServiceAccount named “default” exists in every active namespace.