Immutable

RHCOS (Red Hat Enterprise Linux CoreOS) is the operating system that runs on every OpenShift control plane and worker node. It is not a general-purpose Linux distribution — it is a purpose-built, immutable, container-optimised OS designed to run exclusively as a managed node in an OpenShift cluster. Its security posture is architecturally different from a hardened RHEL installation: rather than hardening a mutable system through configuration management, RHCOS makes the OS layer structurally resistant to modification by design. The root filesystem’s /usr tree is read-only (enforced at mount time by rpm-ostree and, in recent versions, by composefs over the OSTree object store), /etc and /var are writable but managed exclusively by the Machine Config Operator (MCO), and no package manager is available at runtime for ad-hoc software installation. An operator who wants to change any node-level configuration — kernel arguments, sysctl settings, systemd units, certificates, kubelet configuration — creates a MachineConfig object in the OpenShift API; the MCO renders it into an Ignition config, applies it to the target MachineConfigPool (master, worker, or custom), and drains and reboots the affected nodes in a rolling fashion. Direct SSH access to nodes for configuration changes is explicitly unsupported and actively discouraged — oc debug node/<name> is the supported emergency access path, dropping into a privileged container on the node’s host namespaces under audit.

bootc is a CNCF sandbox project, created by Colin Walters, that applies the OCI container model to operating system delivery. Where conventional container images package an application to run inside a host OS, a bootc image packages the entire OS — kernel (under /usr/lib/modules), initrd, systemd units, firmware, and all userspace — as a standard OCI image that can be built with podman build or buildah, stored in any OCI-conformant registry, signed with standard supply chain tools, and pulled to a machine where it becomes the running system. At runtime the base OS is not running inside a container; systemd is pid 1 as usual. The container image format is purely a transport and build model, not an execution model.