Kubernetes Admission Controllers
Tags: kubernetes, docker, devops
Published: 2020-07-23 18:29:15

Kubernetes supports over 30 Admission Controllers. Subsequent to **Authorization** and **Authentication**, **Admission Controllers** are the final step in a 3-step process before Kubernetes persists the resource in `etcd` (a consistent and highly-available key value store used as Kubernetes’ backing store for all cluster data). Some relevant Admission Controllers to secure running containers are:

* **PodSecurityPolicy**: this option implements pod admission based on security context and available policies.
* **DenyEscalatingExec**: when hackers open shells in privileged containers, they have access to the host. This option ensures that `exec` and `attach` commands from privileged containers are blocked.
* **AlwaysPullImages**: while there is a performance advantage to storing and reusing image on a node, hygiene and the assurance that you always run up-to-date container images may be important. Since vulnerabilities are patched upstream, pulling images ensures that the latest remediation are always downloaded.
* **LimitRange and ResourceQuota**: to prevent denial of service attacks, and any spawning of unauthorized processes from established pods, this option observes incoming requests for violation of these limits.
* **NodeRestriction**: this limits the permissions of each `kubelet`, ensuring that it can only modify pods that are bound to it and its own Node object.

###NodeRestriction

This admission controller limits the `Node` and `Pod` objects a `kubelet` can modify. In order to be limited by this admission controller, `kubelet` must use credentials in the `system:node` group, with a username in the form `system:node:<NodeName>`. Such `kubelet` will only be allowed to modify their own `Node API` object.

`NodeRestriction` admission plugin prevents `kubelet` from deleting its `Node API` object, and enforces `kubelet` modification of labels under the `kubernetes.io/` or `k8s.io/` prefixes as follows:

* **Allows** `kubelet` to add/remove/update these labels and label prefixes:
* **Prevents** `kubelet` from adding/removing/updating labels with a `node-restriction.kubernetes.io/` prefix. This label prefix is reserved for administrators to label their `Node` objects for workload isolation purposes, and `kubelet` will not be allowed to modify labels with that prefix.

Use of any other labels under the `kubernetes.io` or `k8s.io` prefixes by kubelet is reserved, and may be disallowed or allowed by the `NodeRestriction` admission plugin in the future.

Future version may add additional restrictions to ensure `kubelets` have the minimal set of permissions required to operate correctly.

###Do we need them?

Many advanced features in Kubernetes require an Admission Controller to be enabled in order to properly support the feature. As a result, a Kubernetes API server, that is not properly configured with the right set of Admission Controllers is an incomplete server and will not support all the features you expect. 

###Turn on an Admission Controller

The Kubernetes API server flag `enable-admission-plugins` takes a comma-delimited list of admission control plugins to invoke prior to modifying objects in the cluster. For instance, the following command line enables the `NamespaceLifecycle` and the `LimitRanger` admission control plugins:

```
kube-apiserver --enable-admission-plugins=NamespaceLifecycle,LimitRanger
```

*Note: Depending on the way your Kubernetes cluster is deployed and how the API server is started, you may need to apply the setting in different ways. For instance, you may have to modify the `systemd` unit file if the API is deployed as a `systemd` service; while you may have to modify the manifest file for the API server if Kubernetes is deployed in a self-hosted way*.

###Recommended set of Admission Controllers

For Kubernetes version 1.10 and later, it is recommended running the set of Admission Controllers using the `--enable-admission-plugins flag`.

For Kubernetes 1.9 and earlier, it is recommended running the set of Admission Controllers using the `--admission-control` flag.