Flatcar Container Linux

A minimal, immutable Linux distribution for running containers, with an A/B partition layout and automatic atomic updates.

• Category: Runtime
• CNCF maturity: Incubating
• Language: Shell and Python
• License: BSD-3-Clause (the scripts repository)
• Repository: flatcar/scripts
• Documented at commit: d2c217c (branch main, June 2026)

What it is

Flatcar Container Linux is an operating system built to do one job: run containers. There is no package manager on the running host. The /usr filesystem is mounted read-only and protected by dm-verity, and the OS ships as a whole image rather than a set of installable packages.

Updates are atomic. A new image is written to a second /usr partition while the system runs from the first, and a reboot swaps them. If the new image fails to boot, the bootloader falls back to the previous one. This is the same model CoreOS Container Linux introduced; Flatcar is the maintained continuation of that lineage.

The implementation lives in flatcar/scripts, a Gentoo/Portage-based build system that emerges pre-built binary packages into a root filesystem and bakes it into a GPT image. The umbrella repository flatcar/Flatcar holds documentation, governance, and the issue tracker.

When to use it

• You run Kubernetes or plain container workloads and want the host OS to be a managed, replaceable unit rather than a server you patch in place.
• You need the same OS across multiple clouds and bare metal, configured declaratively at first boot with Ignition.
• You want automatic updates with rollback, and a conservative release cadence that avoids breaking running containers.
• You want to keep SSH and Docker compatibility for debugging, rather than a fully API-only host.

It is a poor fit when you need to install arbitrary system packages on the running host, or when a single-cloud appliance OS already covers your whole fleet.

In this deep-dive

• History: origin, milestones, and why it exists.
• Architecture: components and how an image is built.
• Adoption & Ecosystem: who runs it and what surrounds it.
• Internals: the code paths that matter, read from source.
• Getting Started: install and a first working setup.

Sources

1. flatcar/scripts, the image build and composition scripts (pinned commit d2c217c).
2. Flatcar brings Container Linux to the CNCF Incubator, CNCF blog.
3. Flatcar Container Linux project page, CNCF.
4. Flatcar accepted into CNCF at incubating level, Microsoft Open Source.
5. Propose Flatcar for Incubation, cncf/toc PR #991.
6. flatcar/Flatcar ADOPTERS.md.
7. Flatcar Container Linux enters new era after CoreOS End-of-Life, Kinvolk.
8. Microsoft acquires Kinvolk, Azure blog.