Fedora CoreOS Frequently Asked Questions
Fedora CoreOS is an automatically updating, minimal, monolithic, container-focused operating system, designed for clusters but also operable standalone, optimized for Kubernetes but also great without it. It aims to combine the best of both CoreOS Container Linux and Fedora Atomic Host, integrating technology like Ignition from Container Linux with rpm-ostree and SELinux hardening from Project Atomic. Its goal is to provide the best container host to run containerized workloads securely and at scale.
Fedora CoreOS is a freely available, community distribution that is the upstream basis for Red Hat CoreOS. While Fedora CoreOS embraces a variety of containerized use cases, Red Hat CoreOS provides a focused OS for OpenShift, released and life-cycled in tandem with the platform.
Fedora CoreOS is the official successor to CoreOS Container Linux, but it is not a drop-in replacement. CoreOS Container Linux will reach its end of life on May 26, 2020, and will no longer receive updates after that date. For notes on migrating from Container Linux to Fedora CoreOS, see the migration guide.
Does Fedora CoreOS replace Fedora Atomic Host? What happens to Fedora Atomic Host and CentOS Atomic Host?
Fedora CoreOS is the official successor to Fedora Atomic Host. The last Fedora Atomic Host release was version 29, which has now reached end-of-life.
CentOS Atomic Host will continue producing downstream rebuilds of RHEL Atomic Host and will align with the end-of-life. The Fedora CoreOS project will be the consolidation point for the community distributions. Users are encouraged to move there in the future.
Project Atomic is an umbrella project consisting of two flavors of Atomic Host (Fedora and CentOS) as well as various other container-related projects. Project Atomic as a project name will be sunset by the end of 2018 with a stronger individual focus on its successful projects such as Buildah and Cockpit. This merges the community side of the operating system more effectively with Fedora and allows for a clearer communication for other community-supported projects, specifically the well-adopted #nobigfatdaemons approach of Buildah and the versatile GUI server manager Cockpit.
We have the following new communication channels around Fedora CoreOS:
There is a community meeting that happens every week. See the Fedora CoreOS fedocal for the most up-to-date information.
If you think you have found a problem with Fedora CoreOS, file an issue in our issue tracker.
Yes, Fedora CoreOS comes with automatic updates and regular releases. Multiple update channels are provided catering to different users' needs. It introduces a new node-update service based on rpm-ostree technologies, with a server component that can be optionally self-hosted. Failures that prevent an update from booting will automatically be reverted.
Fedora CoreOS is provisioned with Ignition. However, existing Ignition configurations will require changes, as the OS configuration will be different from Container Linux. Existing cloud-init configurations are not supported and will need to be migrated into their Ignition equivalent.
/var are mounted as read-write which lets users
write and modify files.
/etc may be changed by deployments, but will not override user
made changes. The content under
/var is left untouched by rpm-ostree when
applying upgrades or rollbacks. For more information, refer to the
As with Container Linux, the best practice is to re-provision the node, due to the cloud-init/Ignition transition at least. Since Fedora CoreOS uses rpm-ostree technology, it may be possible to rebase from Fedora Atomic Host to Fedora CoreOS, but it is not recommended. It’s preferable to gain experience deploying systems using Ignition so that they can be re-provisioned easily if needed. For notes on migrating from Atomic Host to Fedora CoreOS, see the migration guide.
Fedora CoreOS includes Docker and podman by default. Based on community engagement and support this list could change over time.
Fedora CoreOS runs on at least
and bare-metal systems if installed to disk or network-booted.
Yes. However, we envision Fedora CoreOS as not including a specific container orchestrator (or version of Kubernetes) by default — just like Container Linux and Atomic Host. We will work with the upstream Kubernetes community on tools (e.g. kubeadm) and best practices for installing Kubernetes on Fedora CoreOS.
On Fedora CoreOS, containers are the way to install and configure any software not provided by the base operating system. The package layering mechanism provided by rpm-ostree will continue to exist for use in debugging a Fedora CoreOS machine, but we strongly discourage its use. For more about this, please refer to documentation.
The FCOS image is kept minimal by design. Not every troubleshooting tool are
included by default. Instead, it is recommended to use the
How do I coordinate cluster-wide OS updates? Is locksmith or the Container Linux Update Operator available for Fedora CoreOS?
etcd-lock feature from locksmith has
been directly ported to Zincati, as a lock-based updates strategy.
It has also been augmented to support multiple backends, not being anymore
constrained to etcd2 only.
The capabilities of Container Linux Update Operator (CLUO) have been embedded into the Machine Config Operator (MCO), which is a core component of OKD. The MCO additionally covers reconciliation of machine configuration changes.
Fedora CoreOS today is only uploaded to the standard AWS regions. For regions in other AWS partitions like GovCloud and AWS China, you must upload the images yourself.
Note that Fedora CoreOS uses a unified BIOS/UEFI partition layout. As such, it
is not compatible with the
aws ec2 import-image API (for more information,
see related discussions). Instead,
you must use
aws ec2 import-snapshot combined with
aws ec2 register-image.
No. Running containers via
podman at the same time can cause
issues and unexpected behavior. We highly recommend against trying to use them
both at the same time.
It is worth noting that in Fedora CoreOS we have
disabled by default but it is easily started if anything communicates
docker.socket is enabled by
default. This means that if a user runs any
docker command (via
sudo docker) then the daemon will be activated. We did this to make
the transition easier for users of Container Linux.
it was pointed out that because of socket activation users who are
podman for containers could unintentionally start the docker
daemon. This could weaken the security of the system because of the
interaction of both container runtimes with the firewall on the system.
To prevent making this mistake you can disable
docker completely by
docker.service systemd unit.
variant: fcos version: 1.1.0 systemd: units: - name: docker.service mask: true
The x86_64 images we provide can be used for either BIOS (legacy) boot or UEFI boot. They contain a hybrid BIOS/UEFI partition setup that allows them to be used for either. The exception to that is the
metal4k 4k native image, which is targeted at disks with 4k sectors and does not have a BIOS boot partition because 4k native disks are only supported with UEFI.
Ignition configuration is a low-level interface used to define the whole set of customizations for an instance. It is primarily meant as a machine-friendly interface, with content encoded as JSON and a fixed structure defined via JSON-Schema. This JSON configuration is processed by each FCOS instance upon first boot.
Many high-level tools exist that can produce an Ignition configuration starting from their own specific input formats,
Fedora CoreOS Configuration Transpiler (
fcct) is one of such high-level tools.
It is primarily meant as a human-friendly interface, thus defining its own richer configuration entries and using YAML documents as input.
This YAML configuration is never directly processed by FCOS instances (only the resulting Ignition configuration is).
Although similar, Ignition configurations and FCCT ones do not have the same structure; thus, converting between them is not just a direct YAML-to-JSON translation, but it involves additional logic. FCCT exposes several customization helpers (e.g. distribution specific entries and common abstractions) that are not present in Ignition and make the formats not interchangeable. Additionally, the different formats (YAML for FCCT, JSON for Ignition) help to avoid mixing up inputs by mistake.
This is covered in detail in the design docs.
The summary is that Fedora CoreOS uses the format
Xis the Fedora major version (i.e.
Yis the datestamp that the package set was snapshotted from Fedora (i.e.
Zis a code number used by official builds
Ais a revision number that is incremented for each new build with the same
The version numbering scheme is subject to change and is not intended to be parsed by machine.