Introduction

Fedora CoreOS is an awesome Linux operating system. Fedora CoreOS is an immutable OS excluding the /var and /etc paths, both of which can be modified. While /home exists, it's a symlink that point to /var/home. To configure Fedora CoreOS the way we want it, we can use the ignition Config File.

Pre-requisites

To follow along with this walk through you need

• The KVM image.

Ignition Config

• The Ignition Config can be used to provision a Fedora CoreOS system.
• It is run only once at boot
• It can be used to configure things like storage and filesystems, systemd units, and users. This means that we can
  • run podman systemd units on startup to run containers. This alone gives us almost endless possibilities!
  • we can setup files and folders on the system. For example, our application can be configured through a config file config.yaml which needs to be placed in /etc/<app>/config.yaml, we can defiine this in the ignition config.

Learn more about the ignition config here

Setup

For a quick setup to see the workflow of the deployment, let's create a directory and write a butane config to acheive a simple task.

Project Setup

first let's create a directory using mkdir -p /home/Documents/fcos

Butane Config

we can now create a butane file at /home/Documents/fcos/config.bu. For now, let's copy the sample config file provided at butane config

variant: fcos
version: 1.5.0
passwd:
  users:
    - name: core
      ssh_authorized_keys:
        - ssh-rsa <your-public-ssh-key>

This config file creates a user core and sets the public key for who is allowed to login as this user using ssh.

Simple deployment

Steps needed for a simple deployment include

1. creating a butane config file (a more human readable ignition config)
2. convert the butane file into an ignition config file
3. launch a Fedora CoreOS system with the defined config.

we can create a short bash script to automatically convert the butane.bu file to an ignition config file and then run the system.

#!/bin/bash

# path to butane config
BUTANE_CONFIG="/home/Documents/fcos/config.bu"

podman run --interactive --rm quay.io/coreos/butane:release \
       --pretty --strict < ${BUTANE_CONFIG} > /home/Documents/fcos/transpiled_config.ign

IGNITION_CONFIG="/home/Documents/fcos/transpiled_config.ign"
IMAGE="path/to/qcow2/image.qcow2"

# for x86/aarch64:
IGNITION_DEVICE_ARG="-fw_cfg name=opt/com.coreos/config,file=${IGNITION_CONFIG}"

qemu-kvm -m 2048 -cpu host -nographic -snapshot \
  -drive if=virtio,file=${IMAGE} ${IGNITION_DEVICE_ARG} \
  -nic user,model=virtio,hostfwd=tcp::2222-:22

save the file to /home/Documents/fcos/run.sh. run the script to run Fedora CoreOS. we can then ssh into the instance using the command ssh core@localhost -p 2222.