OPA Gatekeeper
You set up opa gatekeeper by adding constraint templates and rules to your Kubernetes cluster. The admission webhook then checks every new resource against those rules before the cluster saves it. Many teams start by writing broad rules to block bad pod settings across namespaces. Still, broad cluster roles often let admins bypass these checks or run direct commands inside pods. How opa gatekeeper works in Kubernetes Kubernetes uses an admission step to check API requests before saving object
Gatekeeper / OPA (K8s): the complete guide
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You set up opa gatekeeper by adding constraint templates and rules to your Kubernetes cluster. The admission webhook then checks every new resource against those rules before the cluster saves it. Many teams start by writing broad rules to block bad pod settings across namespaces. Still, broad cluster roles often let admins bypass these checks or run direct commands inside pods.
How opa gatekeeper works in Kubernetes
Kubernetes uses an admission step to check API requests before saving objects to its store. This tool acts as a webhook that intercepts create and update requests in real time. It uses the Open Policy Agent engine to read rules written in a policy language. Platform teams define reusable rule logic inside custom resources called constraint templates.
Once a template is in place, the team creates individual constraints for specific namespaces or resources. For example, a constraint might require every new pod to have an owner label. Another common rule stops users from pulling container images from untrusted public registries. You can also set up mutation rules that change object fields before the cluster saves them.
Also, to checking new requests, the tool runs a background audit job. This audit job scans existing objects in the cluster to find older violations. It writes these findings to the status field of each constraint resource. Teams read these status reports to fix old workloads without breaking running apps.
Managing templates and audit workflows
Running admission rules across many clusters takes careful planning and testing. If a new rule has a flaw, it can block valid app deployments right away. To prevent outages, teams first set new rules to warn mode or dry run mode. This lets the cluster log rule hits without rejecting the deployment request.
Some rules need context from other objects that already exist in the cluster. For example, checking for unique ingress hostnames requires knowing all current hostnames across namespaces. You must sync those target resources into the local policy cache so the webhook can read them. If the cache misses data, the rule check can fail or pass bad objects.
Gatekeeper / OPA (K8s): architecture patterns and best practices
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What admission rules solve and what they miss
Admission control is great for keeping cluster manifests clean and consistent at deploy time. It stops bad pod specs, missing resource limits, and unsafe volume mounts before pods start. It also gives security teams a clear way to audit cluster state against baseline rules. Still, admission webhooks only see requests that create or change Kubernetes API objects.
Once a pod is running, admission rules have no view into live user actions. When an engineer runs an interactive shell session, the admission webhook does not inspect the commands typed inside. It also does not see SQL queries or API calls sent to databases and services. Native Kubernetes roles decide who can start a session, not what commands run in flight. Teams also have to protect sensitive data like emails, phone numbers, card numbers, and national IDs during live fixing.
Govern statements and responses across databases and APIs
Good config and scoped roles are needed setup. They do not check single queries in flight or mask sensitive fields in returned rows. Across Postgres, MySQL, SQL Server, MongoDB, ClickHouse, Kubernetes APIs, SSH, and internal HTTP endpoints, hoop.dev runs beside each supported service as an open-source proxy, a sidecar for your database or API.
First, hoop.dev guardrails block banned queries before they run. Then hoop.dev data masking rewrites matched sensitive values in responses in memory. The audit trail records every statement and verdict per resource. Furthermore, hoop.dev holds SOC 2 Type II and is open source on GitHub.
Frequently asked questions
What is the difference between Open Policy Agent and Gatekeeper?
Open Policy Agent is a general engine that checks rules across many different tools and apps. Gatekeeper is a specific Kubernetes project that wraps that engine into a native admission controller using custom resources.
Can admission rules stop an engineer from running commands inside a pod?
An admission webhook can block the initial request to open a pod shell if a rule forbids it. Once the shell session starts, it cannot read or block individual commands the user runs inside the container.
What happens to existing pods when you add a new constraint?
Adding a new constraint does not stop or delete pods that already run in the cluster. Instead, the background audit job flags those older pods as violations so your team can update them later.
Short role checks help teams work fast. Simple rules make each review clear. Good logs keep every team safe. Scoped keys stop bad changes early.