Getting started with Fedora CoreOS

Getting started with Fedora CoreOS

Photo by Paweł Czerwiński on Unsplash

This has been called the age of DevOps, and operating systems seem to be getting a little bit less attention than tools are. However, this doesn’t mean that there has been no innovation in operating systems. [Edit: The diversity of offerings from the plethora of distributions based on the Linux kernel is a fine example of this.] Fedora CoreOS has a specific philosophy of what an operating system should be in this age of DevOps.

Fedora CoreOS’ philosophy

Fedora CoreOS (FCOS) came from the merging of CoreOS Container Linux and Fedora Atomic Host. It is a minimal and monolithic OS focused on running containerized applications. Security being a first class citizen, FCOS provides automatic updates and comes with SELinux hardening.

For automatic updates to work well they need to be very robust. The goal being that servers running FCOS won’t break after an update. This is achieved by using different release streams (stable, testing and next). Each stream is released every 2 weeks and content is promoted from one stream to the other (next -> testing -> stable). That way updates landing in the stable stream have had the opportunity to be tested over a long period of time.

Getting Started

For this example let’s use the stable stream and a QEMU base image that we can run as a virtual machine. You can use coreos-installer to download that image.

From your (Workstation) terminal, run the following commands after updating the link to the image. [Edit: On Silverblue the container based coreos tools are the simplest method to try. Instructions can be found at , in particular “Setup with Podman or Docker”.]

$ sudo dnf install coreos-installer
$ coreos-installer download --image-url
$ xz -d fedora-coreos-32.20200907.3.0-qemu.x86_64.qcow2.xz
$ ls

Create a configuration

To customize a FCOS system, you need to provide a configuration file that will be used by Ignition to provision the system. You may use this file to configure things like creating a user, adding a trusted SSH key, enabling systemd services, and more.

The following configuration creates a ‘core’ user and adds an SSH key to the authorized_keys file. It is also creating a systemd service that uses podman to run a simple hello world container.

version: "1.0.0"
variant: fcos
    - name: core
        - ssh-ed25519 my_public_ssh_key_hash fcos_key
      contents: |
          Description=Run a hello world web service

          ExecStart=/bin/podman run --pull=always   --name=hello --net=host -p 8080:8080
          ExecStop=/bin/podman rm -f hello

      enabled: true
      name: hello.service

After adding your SSH key in the configuration save it as config.yaml. Next use the Fedora CoreOS Config Transpiler (fcct) tool to convert this YAML configuration into a valid Ignition configuration (JSON format).

Install fcct directly from Fedora’s repositories or get the binary from GitHub.

$ sudo dnf install fcct
$ fcct -output config.ign config.yaml

Install and run Fedora CoreOS

To run the image, you can use the libvirt stack. To install it on a Fedora system using the dnf package manager

$ sudo dnf install @virtualization

Now let’s create and run a Fedora CoreOS virtual machine

$ chcon --verbose unconfined_u:object_r:svirt_home_t:s0 config.ign
$ virt-install --name=fcos \
--vcpus=2 \
--ram=2048 \
--import \
--network=bridge=virbr0 \
--graphics=none \
--qemu-commandline="-fw_cfg name=opt/com.coreos/config,file=${PWD}/config.ign" \

Once the installation is successful, some information is displayed and a login prompt is provided.

Fedora CoreOS 32.20200907.3.0
Kernel 5.8.10-200.fc32.x86_64 on an x86_64 (ttyS0)
SSH host key: SHA256:BJYN7AQZrwKZ7ZF8fWSI9YRhI++KMyeJeDVOE6rQ27U (ED25519)
SSH host key: SHA256:W3wfZp7EGkLuM3z4cy1ZJSMFLntYyW1kqAqKkxyuZrE (ECDSA)
SSH host key: SHA256:gb7/4Qo5aYhEjgoDZbrm8t1D0msgGYsQ0xhW5BAuZz0 (RSA)
ens2: fe80::5054:ff:fef7:1a73
Ignition: user provided config was applied
Ignition: wrote ssh authorized keys file for user: core

The Ignition configuration file did not provide any password for the core user, therefore it is not possible to login directly via the console. (Though, it is possible to configure a password for users via Ignition configuration.)

Use Ctrl + ] key combination to exit the virtual machine’s console. Then check if the hello.service is running.

$ curl
Hello from Fedora CoreOS!

Using the preconfigured SSH key, you can also access the VM and inspect the services running on it.

$ ssh core@
$ systemctl status hello
● hello.service - Run a hello world web service
Loaded: loaded (/etc/systemd/system/hello.service; enabled; vendor preset: enabled)
Active: active (running) since Wed 2020-10-28 10:10:26 UTC; 42s ago

zincati, rpm-ostree and automatic updates

The zincati service drives rpm-ostreed with automatic updates.
Check which version of Fedora CoreOS is currently running on the VM, and check if Zincati has found an update.

$ ssh core@
$ rpm-ostree status
State: idle
● ostree://fedora:fedora/x86_64/coreos/stable
Version: 32.20200907.3.0 (2020-09-23T08:16:31Z)
Commit: b53de8b03134c5e6b683b5ea471888e9e1b193781794f01b9ed5865b57f35d57
GPGSignature: Valid signature by 97A1AE57C3A2372CCA3A4ABA6C13026D12C944D0
$ systemctl status zincati
● zincati.service - Zincati Update Agent
Loaded: loaded (/usr/lib/systemd/system/zincati.service; enabled; vendor preset: enabled)
Active: active (running) since Wed 2020-10-28 13:36:23 UTC; 7s ago
Oct 28 13:36:24 cosa-devsh zincati[1013]: [INFO ] initialization complete, auto-updates logic enabled
Oct 28 13:36:25 cosa-devsh zincati[1013]: [INFO ] target release '32.20201004.3.0' selected, proceeding to stage it

... zincati reboot ...

After the restart, let’s remote login once more to check the new version of Fedora CoreOS.

$ ssh core@
$ rpm-ostree status
State: idle
● ostree://fedora:fedora/x86_64/coreos/stable
Version: 32.20201004.3.0 (2020-10-19T17:12:33Z)
Commit: 64bb377ae7e6949c26cfe819f3f0bd517596d461e437f2f6e9f1f3c24376fd30
GPGSignature: Valid signature by 97A1AE57C3A2372CCA3A4ABA6C13026D12C944D0
Version: 32.20200907.3.0 (2020-09-23T08:16:31Z)
Commit: b53de8b03134c5e6b683b5ea471888e9e1b193781794f01b9ed5865b57f35d57
GPGSignature: Valid signature by 97A1AE57C3A2372CCA3A4ABA6C13026D12C944D0

rpm-ostree status now shows 2 versions of Fedora CoreOS, the one that came in the QEMU image, and the latest one received from the update. By having these 2 versions available, it is possible to rollback to the previous version using the rpm-ostree rollback command.

Finally, you can make sure that the hello service is still running and serving content.

$ curl
Hello from Fedora CoreOS!

More information: Fedora CoreOS updates

Deleting the Virtual Machine

To clean up afterwards, the following commands will delete the VM and associated storage.

$ virsh destroy fcos
$ virsh undefine --remove-all-storage fcos


Fedora CoreOS provides a solid and secure operating system tailored to run applications in containers. It excels in a DevOps environment which encourages the hosts to be provisioned using declarative configuration files. Automatic updates and the ability to rollback to a previous version of the OS, bring a peace of mind during the operation of a service.

Learn more about Fedora CoreOS by following the tutorials available in the project’s documentation.

For Developers For System Administrators


  1. Sergey

    Thank you for the article.
    And where can I read about CoreOS + Kubernetes ?
    Or maybe I don’t fully understand the purpose of CoreOS ?

  2. Sergey

    Excuse me one more question.
    Is the information in this article relevant ? Why can’t you use ordinary images from the site ?


    • You can and should use images from, this article is using an old image so that you can experience the auto-update feature.

      Does that makes sense ?

  3. husimo

    Hello, using Fedora 33 when I try

    fcct -input config.yaml -output config.ign

    i have

    unknown shorthand flag: 'i' in -input

    . Seems the proper command is

    fcct config.yaml -o config.ign


    Then I have an error using the yaml provided :

    Error translating config: Error unmarshaling yaml: yaml: line 13: could not find expected ':'
    • Thanks for pointing it out. It should be fixed now (fcct got a new release that changes the flags and the yaml formating was lost when adding the file in wordpress 🙂 )

    • Anders Jackson

      I guessing now, but usually there are Short and Long switches. Long are usually marked with two “-” characters and Short with one “-” characters prefix.
      Short switches can be combined, so Short switch -i and -n could be combined into -in. Which isn’t a Long switch,.
      So, long argument –input have two – prefix. characters and Short prefix version is just -i.

      Markup languages usually change two – to a dash character, which isn’t a minus character… So that is why it can be wrong of you just cutand paste Commands into articles like this.

      I would guess that -input is interpreted as the five Short switches -i -n -p -u -t and not the Long switch –input (with two -).

  4. Ignorante

    What are the differences with Fedora Silverblue and Fedora IoT?

    • Sergey

      As I understand it :
      Silverblue – desktop
      IOT – rasperberry, router
      CoreOS – container launching.

      Maybe I am wrong.

  5. Dennis

    Thanks! It’s very useful info for me!

  6. Sergey

    Maybe not quite in the subject, but what are the mechanisms of local update ?

    For example, we rock a new image, place it somewhere locally in the network and it is already being updated from …

    Recently, using flatpak, SilverBlue and now CoreOS I have difficulties with the organization of a local mirror …

    • Many users rely on Pulp ( to create local mirrors of repositories. While Pulp does not support creating mirrors for Flatpak, SilverBlue or CoreOS, we would like to add support for all three.

      Would you be interested in helping us understand what kind of workflows you are looking for?

      Please file an issue at

      Thank you!

  7. Artur

    You don’t need to know download address for coreos-installer, you can use in example:

    coreos-installer download -s testing -p qemu -f qcow2.xz -d

    it will download the newest testing image to current directory and decompress it for you.

  8. I really like Fedora CoreOS. However, to be able to manage it using Ansible, one needs to install some python stuff in it.

    So how can we install software using fcct and rpm-ostree, or precisly, how to manage it via Ansible?

    • To answer my self, one can add a one-shot systemd unit that hooks after system install and run an rpm-ostree install command.

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