CRI-O: The Lightweight Container Runtime for Kubernetes

Purpose

When Kubernetes first became popular, it relied on Docker as the default container runtime. However, Docker was built with a broader focus, including features Kubernetes didn’t need (such as image building and registry management). This led to inefficiencies, prompting the Kubernetes community to create CRI-O—a runtime that does only one thing: run and manage containers in Kubernetes.

Think of it as a lean and optimized engine, built specifically for Kubernetes, without the extra weight of Docker’s full toolset.

How Does it Works

CRI-O acts as the middleman between Kubernetes and low-level container runtimes like runc (which actually runs the container process). Here’s how it fits into the ecosystem:

1. Kubernetes API Server → Kubelet
   • Kubernetes schedules workloads to nodes and tells the kubelet (node agent) to run a container.
2. Kubelet → CRI-O (via CRI)
   • Instead of talking to Docker, the kubelet communicates with CRI-O using the Container Runtime Interface (CRI).
3. CRI-O → runc (or another OCI-compliant runtime)
   • CRI-O then uses runc to actually launch the container.

This streamlined approach eliminates unnecessary layers, reducing overhead, attack surface, and resource consumption.

Key Features

1. Lightweight and Kubernetes-Native

CRI-O is built only for Kubernetes, making it smaller, faster, and more efficient than other container runtimes.

Example:
A typical Kubernetes cluster using Docker requires extra components like the Docker daemon. With CRI-O, this is unnecessary—everything is streamlined, reducing CPU and memory overhead.

2. Supports Open Container Initiative (OCI) Standards

It supports any OCI-compliant container images and runtimes (such as runc and crun), making it flexible.

Example:
You can run an OCI-compliant image from Docker Hub without modifying it.

crictl pull nginx
crictl runp nginx.json
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This pulls and runs an Nginx container without requiring Docker.

3. No Unnecessary Features (Unlike Docker)

CRI-O removes features that Kubernetes doesn’t need, such as:

• Building images
• Managing registries
• Swarm orchestration

Instead, it focuses purely on running containers efficiently.

4. Security Enhancements

CRI-O integrates seccomp, SELinux, and AppArmor for hardened security. Since it doesn’t require a long-running daemon (unlike Docker), it reduces the attack surface.

Example:
If a container escapes, the absence of a centralized Docker daemon makes it harder for an attacker to gain control over the host system.

5. Faster Pod Startups

Since CRI-O is optimized for Kubernetes, it can launch pods faster than Docker.

Example:
In large clusters, switching from Docker to CRI-O can reduce pod startup times by 30-40%.

Comparing CRI-O with Docker and containerd

Use Cases

Best choice for Kubernetes clusters

• If you run Kubernetes, especially large-scale clusters, it improves efficiency and performance.

Not for local development

• Unlike Docker, CRI-O does not include image-building tools. You would still need Buildah or Podman for that.

Installation guide

1. Install CRI-O

On a RedHat-based system (like CentOS, Fedora, RHEL):

sudo dnf install cri-o
sudo systemctl enable --now crio
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On Ubuntu/Debian:

sudo apt-get update
sudo apt-get install cri-o
sudo systemctl enable --now crio
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2. Check if CRI-O is Running

systemctl status crio
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3. Use CRI-O with Kubernetes

Tell kubelet to use CRI-O instead of Docker:

kubeadm init --cri-socket=/var/run/crio/crio.sock
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4. Run a Container Using CRI-O

Use crictl (CRI-O’s CLI tool) to pull and run a container:

crictl pull alpine
crictl runp alpine.json
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Common Issues and Fixes

Pods Stuck in ContainerCreating

Possible Cause: Kubelet is not configured to use CRI-O
Fix: Check if CRI-O is running and kubelet is using the correct CRI socket:

ps aux | grep kubelet
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If needed, restart kubelet with the CRI-O socket:

systemctl restart kubelet
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Can’t Pull Container Images

Possible Cause: Registry authentication issue
Fix: Make sure your authentication credentials are set up in /etc/crio/crio.conf.d/

cat /etc/crio/crio.conf.d/00-default
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Future of CRI-O

• With Docker support removed from Kubernetes (since v1.20), CRI-O and containerd have become the standard choices.
• Adoption is growing, especially in RedHat OpenShift and other Kubernetes-focused environments.
• Continuous security enhancements and performance optimizations make CRI-O one of the best choices for running Kubernetes workloads.