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. 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 RHEL CoreOS. While Fedora CoreOS embraces a variety of containerized use cases, RHEL CoreOS provides a focused OS for OpenShift, released and life-cycled in tandem with the platform.

Fedora Bootc is a Fedora project aimed at building Fedora and CentOS-based bootable containers. The goal is to eventually have Fedora CoreOS build on top of Fedora Bootc both literally and in the broader sense of being part of the same ecosystem. The roadmap for this can be found on GitHub.

Fedora CoreOS adds a stream model, platform-specific integration and disk images, provisioning via Ignition, and more tooling around automatic updates. The goal is also to have Fedora CoreOS integrate more strongly with the Kubernetes ecosystem.

If you find yourself needing to heavily customize Fedora CoreOS beyond what OS extensions provide, it might make more sense to instead build on top of Fedora Bootc directly. It will eventually be possible to layer Ignition and e.g. Zincati on top if desired. These tools are intended to be used generically and are not strictly tied to Fedora CoreOS.

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.

Fedora CoreOS artifacts are available at fedoraproject.org.

Yes, Fedora CoreOS comes with automatic updates and regular releases. Multiple update channels are provided catering to different users' needs. Fedora CoreOS provides a node-update service based on rpm-ostree technologies, with a server component that can be optionally self-hosted.

Fedora CoreOS is provisioned with Ignition. Existing cloud-init configurations are not supported and will need to be migrated into their Ignition equivalent.

The directories /etc and /var are mounted as read-write which lets users write and modify files.

The directory /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 Mounted Filesystems section.

Fedora CoreOS includes Docker and podman by default. Based on community engagement and support this list could change over time.

Yes. However, Fedora CoreOS does not include a specific container orchestrator (or version of Kubernetes) by default.

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 toolbox utility.

Debugging with Toolbx.

The Zincati update manager includes a lock-based updates strategy that supports multiple backends.

OKD’s Machine Config Operator (MCO) automatically handles updates of Fedora CoreOS in OKD clusters. 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.

To learn more about these APIs, see the AWS documentation for importing snapshots and creating EBS-backed AMIs.

No. Running containers via docker and 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 docker.service disabled by default but it is easily started if anything communicates with the /var/run/docker.sock because 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.

In coreos/fedora-coreos-tracker#408 it was pointed out that because of socket activation users who are using 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 masking the docker.service systemd unit.

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, such as terraform, matchbox, openshift-installer, and Butane.

Butane is one such high-level tool. 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 Butane 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. Butane 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 Butane, 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 X.Y.Z.A

The version numbering scheme is subject to change and is not intended to be parsed by machine.

We have found that the dnsmasq binary can be used for several host applications, including podman and NetworkManager. For this reason we include the dnsmasq package in the base OSTree layer, but we discourage the use of the dnsmasq.service in the host by masking it with systemctl mask dnsmasq.service.

"Why do you mask the service?"

dnsmasq is useful for running a DHCP/DNS/TFTP server for external clients (i.e. not local to the host), too, but that is something we’d prefer users to do in a container. Putting the service in a container insulates the hosted service from breakage as a result of host layer changes. For example, if NetworkManager and podman stopped using dnsmasq, we would remove it from the host and the service you depend on would cease to work.

"But, I really want to use it!"

We don’t recommend it, but if you really want to use it you can just unmask and enable it:

For more information see the tracker issue discussion.

Currently, the OSTree and SELinux tooling conflict a bit. If you have permanently applied local policy modifications then policy updates delivered by the OS will no longer apply; your policy stays frozen. This means any policy "fixes" needed to enable new functionality will not get applied. See coreos/fedora-coreos-tracker#701 for more details.

This means you may see denials like the following, which can take down critical parts of a system like in coreos/fedora-coreos-tracker#700:

To see if your system currently has local policy modifications you can run ostree admin config-diff. The following system has a modified policy:

To work around this incompatibility, please attempt to apply policy modifications dynamically. For example, for an SELinux boolean you can use the following systemd unit that executes on every boot:

If your system’s basic functionality has stopped working because of SELinux denials check to see if your system currently has local policy modifications. You can check with ostree admin config-diff:

If your system is in this state you have two options:

system-repart is a tool to grow and add partitions to a partition table. On Fedora CoreOS, we only support using Ignition to create partitions, filesystems and mount points, thus systemd-repart is masked by default.

Ignition runs on first boot in the initramfs and is aware of Fedora CoreOS specific disk layout. It is also capable of reconfiguring the root filesystem (from xfs to ext4 for example), setting up LUKS, etc…​ See the Configuring Storage page for examples.

See the Why is the dnsmasq.service systemd unit masked entry for an example config to unmask this unit.

Some Wi-Fi firmwares were split into subpackages in Fedora 39 and Fedora 40. Fedora CoresOS will keep them in until Fedora 41, but display a warning message in the console if NetworkManager-wifi is layered without any other Wi-Fi firmware packages layered.

To request the Wi-Fi firmware stay installed even when Fedora CoreOS drops these packages please follow the steps to perform Wi-Fi enablement on an existing system.

Once the packages are requested you can now disable the warning so it won’t be checked on subsequent boots.