CRI-O

Introduction

CRI-O is a lightweight container runtime specifically designed for Kubernetes. It implements the Kubernetes Container Runtime Interface (CRI) using OCI-compatible runtimes, providing a minimal and stable alternative to Docker and containerd for running containers in Kubernetes clusters. CRI-O follows the Kubernetes release cycle, with each CRI-O minor version matching the corresponding Kubernetes minor version.

Key Features

• CRI-Native: Built exclusively to implement the Kubernetes CRI specification
• OCI-Compatible: Supports any OCI-compliant runtime (runc, crun, Kata Containers)
• Minimal Footprint: No daemon bloat -- only implements what Kubernetes needs
• Version Alignment: CRI-O 1.x.y is compatible with Kubernetes 1.x.y
• Image Management: Pulls images from any OCI-compliant or Docker registry
• Conmon: Lightweight container monitor that tracks container lifecycle

Architecture

CRI-O sits between the kubelet and the OCI runtime:

kubelet → CRI (gRPC) → CRI-O → OCI Runtime (runc/crun)
                           ↓
                        conmon (container monitor)

Components:

• CRI-O daemon: Receives CRI calls from kubelet
• conmon: Per-container process that monitors the container and handles logging
• OCI runtime: Actual container executor (default: runc)
• CNI plugins: Network configuration for pods
• containers/image: Image pull and management library

Installation

RHEL/CentOS/Fedora

# Set Kubernetes version
KUBERNETES_VERSION=v1.30
CRIO_VERSION=v1.30

# Add repositories
curl -fsSL https://pkgs.k8s.io/addons:/cri-o:/stable:/$CRIO_VERSION/rpm/repodata/repomd.xml.key | \
  gpg --dearmor -o /etc/pki/rpm-gpg/RPM-GPG-KEY-cri-o

# Install CRI-O
dnf install -y cri-o
systemctl enable --now crio

Verify Installation

# Check CRI-O status
systemctl status crio

# Verify with crictl
crictl info
crictl version

Configuration

CRI-O configuration is located at /etc/crio/crio.conf:

[crio]
  log_dir = "/var/log/crio/pods"
  log_level = "info"

[crio.api]
  listen = "/var/run/crio/crio.sock"
  grpc_max_send_msg_size = 83886080
  grpc_max_recv_msg_size = 83886080

[crio.runtime]
  default_runtime = "runc"
  conmon = "/usr/bin/conmon"
  cgroup_manager = "systemd"

  [crio.runtime.runtimes.runc]
    runtime_path = "/usr/bin/runc"
    runtime_type = "oci"

[crio.image]
  pause_image = "registry.k8s.io/pause:3.9"

[crio.network]
  network_dir = "/etc/cni/net.d/"
  plugin_dir = "/opt/cni/bin/"

Kubeadm Integration

Configure kubeadm to use CRI-O as the container runtime:

apiVersion: kubeadm.k8s.io/v1beta3
kind: InitConfiguration
nodeRegistration:
  criSocket: unix:///var/run/crio/crio.sock

Reference:

1. Official Website
2. Repository