Moving from Fedora Atomic to Fedora CoreOS

Recently the merging of Fedora Atomic and CoreOS Container Linux into what is now known as Fedora CoreOS has moved out of preview and is ready for prime time use. I’ve been running a few small container apps on a Fedora Atomic 28 VM, which is now ripe to be moved into the new replacement Fedora CoreOS. Here’s how I did this.

I’ll be deploying on a KVM environment, so I downloaded the new QCOW2 image (You can also get an AMI for AWS). The Fedora documentation has instructions on how to migrate from Fedora Atomic Host, so this is what I followed. Essentially we need to move from using cloud-init and cloud-config scripts to ignition which uses a JSON formatted config to provision a Fedora CoreOS system.

First we create a YAML formatted configuration file, which we’ll then convert to JSON with the fcct tool. I only have some basic configuration items I need on my CoreOS system:

Here is what my config file looks like. You can use the FCCT spec docs to add in config for storage, files, disks, etc.

$ cat coreos.fcc
variant: fcos
version: 1.0.0
    - path: /etc/NetworkManager/system-connections/eth0.nmconnection
      mode: 0600
      overwrite: true
        inline: |


    - path: /etc/hostname
      mode: 0600
      overwrite: true
        inline: |

    - name: core
        - "[ public SSH key hash ]"
    - name: aludwar
      password_hash: "[ password hash, salted ]"
        - "[ public SSH key hash ]"
      groups: [ sudo, docker ]

I don’t have the fcct tool local on my Fedora 30 workstation, so I’ll use the podman container to convert the YAML to JSON:

$ podman pull
$ podman run -i --rm --pretty --strict < coreos.fcc > coreos.ign

Then, with virt-install, I create a new VM referencing the QCOW2 image, and provide the coreos.ign file in the qemu command line parameter:

$ virt-install --connect qemu:///system -n --memory 1024 --vcpus 1 --network default --os-variant=fedora31 --import --graphics=none \
--disk=/home/aludwar/VMs/fedora-coreos-31-qemu.qcow2 --qemu-commandline="-fw_cfg name=opt/com.coreos/config,file=/home/aludwar/VMs/coreos.ign"

At first boot, the VM will use DHCP for its address, but will also have the ignition configuration applied to the host. Reboot the host for the config to be applied:


[ 1.573639] systemd[1]: systemd v243.5-1.fc31 running in system mode. (+PAM +AUDIT +SELINUX +IMA -APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ +LZ4 +SECCOMP +BLKID +ELFUTILS +KMOD +IDN2 -IDN +PCRE2 default-h)
[ 1.576925] systemd[1]: Detected virtualization kvm.
[ 1.577759] systemd[1]: Detected architecture x86-64.
[ 1.578555] systemd[1]: Running in initial RAM disk.

Welcome to Fedora CoreOS 31.20200127.3.0 dracut-049-27.git20181204.fc31.1 (Initramfs)!

[ 1.581667] systemd[1]: No hostname configured.
[ 1.582401] systemd[1]: Set hostname to <localhost>.
[ 1.583229] systemd[1]: Initializing machine ID from KVM UUID.
[ 1.650595] systemd[1]: Started Dispatch Password Requests to Console Directory Watch.


[ 3.904515] systemd[1]: Successfully loaded SELinux policy in 389.023ms.
[ 3.943381] systemd[1]: Relabelled /dev, /dev/shm, /run, /sys/fs/cgroup in 23.715ms.
[ 3.946100] systemd[1]: systemd v243.5-1.fc31 running in system mode. (+PAM +AUDIT +SELINUX +IMA -APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ +LZ4 +SECCOMP +BLKID +ELFUTILS +KMOD +IDN2 -IDN +PCRE2 default-h)
[ 3.949186] systemd[1]: Detected virtualization kvm.
[ 3.949897] systemd[1]: Detected architecture x86-64.

Welcome to Fedora CoreOS 31.20200127.3.0!

[ 3.952521] systemd[1]: Set hostname to <>.
[ 4.148591] systemd[1]: initrd-switch-root.service: Succeeded.

[ OK ] Reached target Multi-User System.
Starting Update UTMP about System Runlevel Changes...
[ OK ] Started Update UTMP about System Runlevel Changes.
[ OK ] Started RPM-OSTree System Management Daemon.

Fedora CoreOS 31.20200127.3.0
Kernel 5.4.13-201.fc31.x86_64 on an x86_64 (ttyS0)

eth0: fe80::605c:b122:5e5d:6a5f
coreos login:

And we’re done! We now have a container specific OS that will get updates beyond Fedora Atomic & CoreOS Container Linux EOL.