Docs
Guide
Flatcar Container Linux

Flatcar Container Linux

“Flatcar Container Linux is a container optimized OS that ships a minimal OS image, which includes only the tools needed to run containers. The OS is shipped through an immutable filesystem and includes automatic atomic updates.”

Ignition

For bootstrapping Flatcar Container Linux a ignition.json can be used. It is possible to create it directly in json but for ease of use the project provides a config transpiler called butane where you can create a config in yaml. A butane config for running technitium dns server might look like this:

variant: flatcar
version: 1.0.0
passwd:
  users:
    - name: core
      shell: /sbin/nologin
    - name: thorados
      ssh_authorized_keys:
        - ssh-ed25519 AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA Thorados
      groups:
        - wheel
        - docker
systemd:
  units:
    - name: technitium.service
      enabled: true
      contents: |
        [Unit]
        Description=Technitium DNS Server
        [Service]
        TimeoutStartSec=0
        ExecStartPre=-/usr/bin/docker rm --force technitium
        ExecStart=/usr/bin/docker run --name technitium --env "DNS_SERVER_DOMAIN=technitium" -p "5380:5380/tcp" -p "53:53/udp" -p "53:53/tcp" --volume "config:/etc/dns" --restart unless-stopped --pull always --log-driver=journald --sysctl "net.ipv4.ip_local_port_range=1024 65535" technitium/dns-server:latest
        ExecStop=/usr/bin/docker stop technitium
        Restart=always
        RestartSec=5s
        [Install]
        WantedBy=multi-user.target
storage:
  directories:
    - path: /etc/systemd/resolved.conf.d
      overwrite: true
      mode: 0644
      user:
        id: 0
      group:
        id: 0
  files:
    - path: /etc/ssh/sshd_config.d/custom.conf
      overwrite: true
      mode: 0600
      contents:
        inline: |
          PermitRootLogin no
          AllowUsers thorados
    - path: /etc/sudoers.d/core-passwd
      mode: 0644
      contents:
        inline: |
          core	ALL=(ALL) 	ALL
    - path: /etc/sudoers.d/thorados-passwd
      mode: 0644
      contents:
        inline: |
          thorados	ALL=(ALL) 	NOPASSWD: ALL
    - path: /etc/systemd/resolved.conf #.d/00-technitium.conf
      mode: 0644
      append:
        - inline: |
            #[Resolve]
            DNS=192.168.20.1
            DNSStubListener=no

Butane

The ignition.yaml can then be transpiled with the docker image as follows:

docker run --rm -i quay.io/coreos/butane:latest < butane.yaml > ignition.json

Installing Flatcar Container Linux on Proxmox

To install Flatcar Container Linux on Proxmox first it is required to pull the image via wget and then create the vm via with its ignition config. To do so one can use terraform or the provided shell script from the docs like shown below.
For the ignition file the shell script uses the cloudinit CD-ROM drive. Thefore it is not possible to use both at the same time. Forthermore flatcar does not come with python preinstalled which means that it is necessary to install it via e.g. systemd-sysex.
The transpiled ignition.json needs to be safed in /var/lib/vz/snippets/user-data.

export VM_ID=123

# create the vm and import the image to it's disk
qm create $VM_ID --cores 2 --memory 4096 --net0 "virtio,bridge=vmbr0" --ipconfig0 "ip=dhcp"
qm disk import $VM_ID flatcar_production_proxmoxve_image.img local-lvm

# tell the vm to boot from the imported image
qm set $VM_ID --scsi0 local-lvm:vm-$VM_ID-disk-0
qm set $VM_ID --boot order=scsi0

# Create the cloud-init CD-ROM drive which activates the cloud-init options for the VM.
# This is required for using ignition config as well.
qm set $VM_ID --ide2 local-lvm:cloudinit

# Providing a ignition.json

qm set $VM_ID --cicustom "user=local:snippets/user-data"

Resetting Flatcar

It is also possible to reset Flatcar to reapply the ignition.json and its state. To do so the flatcar-reset tool is available which can be run via ssh as sudo. With the next reboot flatcar will be reset.

Sources

Azerothcore