RHCOS

Red Hat CoreOS (RHCOS) is Red Hat’s minimal, immutable operating system designed to run containerized workloads on OpenShift clusters. It serves as the node operating system for Red Hat OpenShift 4, providing a stable foundation for both control plane and worker nodes. Built from Fedora CoreOS with Red Hat updates and support, RHCOS is optimized for Kubernetes workloads and container runtimes.

RHCOS uses an OSTree-based, transactional image model with an immutable root filesystem. System changes are performed through the Machine Config Operator (MCO) and, when needed, controlled rpm-ostree overlays that are coordinated by OpenShift. The design emphasizes security and reliability, including SELinux in enforcing mode and a minimal surface area to reduce potential attack vectors.

Provisioning and updates: nodes are provisioned via Ignition on first boot, after which OpenShift configures and enforces the desired state. Updates are delivered as curated image updates by Red Hat and are applied atomically, typically requiring a reboot to complete. Direct manual package management is discouraged on RHCOS; configuration and customization are achieved primarily through machine configurations rather than ad hoc changes.

Role within OpenShift: RHCOS hosts run the container runtime (CRI-O) and Kubernetes components, providing the foundation for OpenShift clustering. Updates, security patches, and lifecycle management are coordinated by OpenShift tooling and operators, enabling predictable, cluster-driven maintenance while minimizing downtime and drift.

In summary, Red Hat CoreOS is the standardized, immutable host OS for OpenShift 4, designed for reliable, scalable container orchestration with integrated update and configuration management.