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Kubernetes has become the standard for container orchestration in modern cloud-native environments. It automates the deployment, scaling, and management of containerized applications. However, with great flexibility comes the need for robust governance and control. That’s where admission controllers come into play.
What is an Admission Controller?An admission controller in Kubernetes is a key component of the
control plane that intercepts requests to the Kubernetes API server after
authentication and authorization, but before the data is persisted to
the cluster store (etcd). Its primary purpose is to validate or modify requests
to ensure they comply with certain policies or constraints.
Think of it as a gatekeeper or filter that enforces rules on the
operations users or systems are trying to perform on Kubernetes resources like
pods, deployments, services, and namespaces.
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Where Admission
Controllers Fit in the Kubernetes Workflow
When a request is made to the Kubernetes API (for example, to create a
new pod), the following sequence occurs:
- Authentication: The system verifies the identity of the
requester.
- Authorization: It checks if the authenticated user has
permission to perform the requested action.
- Admission Control: The admission
controller evaluates the request and either allows it, modifies it, or
rejects it based on pre-configured policies.
- Persistence: If the request is approved, the change is
stored in etcd and applied to the cluster.
Thus, admission controllers are a critical part of the security and
governance mechanism in Kubernetes.
Types of Admission
Controllers
There are two main types of admission controllers in Kubernetes:
- Validating Admission Controllers: These
check whether a request complies with certain rules. If the request
violates any rule, it’s rejected. For instance, if a pod request tries to
use a disallowed container image, the request will be denied.
- Mutating Admission Controllers: These
modify or "mutate" incoming requests to enforce defaults or
transform configurations before they're stored. For example, they can
automatically inject environment variables or sidecar containers into a
pod definition. Docker
and Kubernetes Course
Kubernetes allows you to chain multiple admission controllers together,
and the order of execution matters—mutating admission controllers are run
before validating ones.
Why Admission
Controllers Matter
Admission controllers are not just optional enhancements—they play a
pivotal role in:
- Security Enforcement: Prevent the
deployment of insecure configurations such as containers running as root,
or disallowed host paths.
- Policy Compliance: Enforce
custom organizational policies, like restricting certain namespaces or
enforcing specific labeling conventions.
- Resource Optimization: Ensure
resource requests and limits are properly defined to avoid resource
starvation or waste.
- Automation and Consistency: Automatically
add or modify configurations for consistent deployments across
environments.
Commonly Used Admission
Controllers
Some of the most widely used admission controllers include:
- NamespaceLifecycle: Prevents
deletion of namespaces that are still in use.
- LimitRanger: Ensures resource limits and requests are
applied to pods and containers.
- ServiceAccount: Automates the mounting of service account
credentials.
- PodSecurity: Enforces Pod Security Standards like
privileged mode or running as non-root. Docker
and Kubernetes Training
- NodeRestriction: Restricts
what kubelets can modify, enhancing node security.
- ValidatingAdmissionWebhook & MutatingAdmissionWebhook: Allow users to plug in their own external validation and mutation
logic via webhooks.
Webhooks and
Extensibility
One of the most powerful aspects of admission controllers is their
extensibility through webhooks. Kubernetes allows cluster administrators
to define custom admission logic by deploying external services (webhooks) that
validate or mutate requests. This means organizations can enforce unique
business or security rules not covered by the built-in admission controllers.
For example, a company could write a webhook that blocks the creation of
resources in a “production” namespace during certain hours or checks container
images against an internal vulnerability database.
Configuring
Admission Controllers
Admission controllers are enabled and configured through the API server.
In managed Kubernetes environments like Google Kubernetes Engine (GKE) or Azure
Kubernetes Service (AKS), many essential admission controllers are enabled by
default for convenience and security. However, in self-managed clusters,
operators have full control over which controllers to enable. Kubernetes
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It’s important to carefully configure and test admission controllers, as
an incorrect setup can unintentionally block legitimate operations or open
security holes.
Conclusion
Admission controllers are essential to managing a secure, efficient, and
policy-compliant Kubernetes
environment. By acting as the final checkpoint before changes are committed to
the cluster, they help enforce security standards, operational rules, and
business policies. Whether through built-in features or custom webhooks,
admission controllers provide a flexible and powerful mechanism to control how
Kubernetes resources are created and modified.
In a rapidly growing ecosystem where governance and automation are
crucial, mastering the use of admission controllers is a foundational step for
any Kubernetes administrator or platform engineer.
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