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Deploy Fedora CoreOS in Hetzner cloud

·1020 words·5 mins·

Hetzner has always been a reliable and cost-effective hosting company for me for several years. I’ve run on their dedicated servers and I run several small applications in their cloud.

When I run containers, I love using Fedora CoreOS for its easy updates and very small server footprint. Almost everything you need for hosting containers is provided right out of the box, but you can add extra packages via rpm-ostree layers and reboot to use them.

This post shows you how to deploy Fedora CoreOS in Hetzner’s cloud as an inexpensive and efficient method for hosting your container workloads.

Roadblocks #

First off, Hetzner does not offer Fedora CoreOS as one of its cloud base images. That normally wouldn’t be a problem, but they don’t allow you to upload snapshots or base images, either. Don’t worry! We can think creatively and deploy the image via the cloud rescue environment.

Another challenge is that ignition, the CoreOS first boot configuration tool, does not support Hetzner’s metadata service at this time. I’m working on a pull request to add this support. We can get creative here and embed our ignition configuration inside the instance itself. That’s a bit annoying since all of your instances will get the same configuration, but that works out fine for my needs since I mainly care about ssh keys being present.

Generating ignition configuration #

The ignition configuration allows us to specify ssh keys (and many other possible configurations) for the first boot. First, we start with a really basic butane configuration:

# hetzner-coreos.butane
variant: fcos
version: 1.4.0
    - name: core
        - wheel
        - ssh-rsa AAAAB3NzaC1y...

You can use any username here that you prefer and add your public ssh key. Compile the configuration into an ignition file with butane:

butane hetzner-coreos.butane > config.ign

Keep this config.ign file handy because we need it before we snapshot our Fedora CoreOS image later.

Prepare the CoreOS snapshot #

Although Hetzner doesn’t allow for uploading your own snapshot, you can replace the entire root disk of an instance from the rescue system. From there, we can snapshot the root disk storage and use that as our new base image.

Download and install the hcloud tool first. Follow the installation instructions to generate an API key and store it on your system.

Start by building a basic Fedora 34 instance where we can deploy the Fedora CoreOS image:

$ hcloud server create --datacenter nbg1-dc3 --image fedora-34 \
    --type cpx11 --name coreos-deployer

Activate the rescue system for the instance you just created and reboot into the rescue environment:

$ hcloud server enable-rescue coreos-deployer
1.148s [=================================] 100.00%
Rescue enabled for server 14168423 with root password: xxxxx
$ hcloud server reboot coreos-deployer
656ms [==================================] 100.00%
Server 14168423 rebooted

Use ssh to log into the rescue environment (user: root, password provided in the enable-rescue step). The rescue environment is in a ramdisk and we don’t have enough space to build the coreos-installer or download a raw disk, but we can get creative and stream the filesystem directly from a Fedora CoreOS image download.

I prefer to live dangerously and I run the testing release, but there are three releases available for download. Download, decompress, and write the image to the root disk all at once:

export COREOS_DISK=""
curl -sL $COREOS_DISK | xz -d | dd of=/dev/sda status=progress

This process should take one or two minutes to complete.

Embed the ignition configuration #

We could hop out of rescue now and reboot right into Fedora CoreOS, but we need to provide SSH keys for our instance. The config.ign file we generated in the first section of this post will be deployed as /ignition/config.ign in the image.

Let’s find the boot partition:

# fdisk -l /dev/sda
Disk /dev/sda: 38.2 GiB, 40961572864 bytes, 80003072 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: 00000000-0000-4000-A000-000000000001

Device       Start     End Sectors  Size Type
/dev/sda1     2048    4095    2048    1M BIOS boot
/dev/sda2     4096  264191  260096  127M EFI System
/dev/sda3   264192 1050623  786432  384M Linux filesystem   Β« boot
/dev/sda4  1050624 5003230 3952607  1.9G Linux filesystem   Β« root

Mount the boot partition and deploy the config.ign:

mount /dev/sda3 /mnt
mkdir /mnt/ignition
vi /mnt/ignition/config.ign      Β« copy/paste your config.ign from earlier
umount /mnt

Let’s power off the instance to avoid booting it. We want all subsequent boots to be as clean as possible.

root@rescue ~ # poweroff

Make the snapshot #

Create a snapshot from our powered off server:

hcloud server create-image --description fedora-34-coreos \
    --type snapshot coreos-deployer

This usually takes 1-2 minutes. Let’s get our image ID:

$ hcloud image list | grep fedora-34-coreos
46874212   snapshot   -                 fedora-34-coreos   0.96 GB      40 GB       Wed Sep  1 10:07:04 CDT 2021   -

Boot the instance #

Now that we have a snapshot with our ignition configuration embedded in it, let’s make a new instance!

hcloud server create --datacenter nbg1-dc3 --image 46874212 --type cpx11 \
    --ssh-key personal_servers --name first-coreos-instance

Hetzner normally boots cloud images really quickly, but it takes a bit longer when booting from snapshots. I assume that they have the common base images cached on most hypervisors so they can provision them really quickly. The delay isn’t too bad here: the instances usually take about 90 seconds to boot.

Once it boots, you should see notes from ignition on bootup about your configuration:

Fedora CoreOS console

Use ssh to login as the core user:

Fedora CoreOS 34.20210821.2.0

[core@localhost ~]$ podman --version
podman version 3.3.0

Wrapping up #

Now that you have a snapshot made, you can delete your original instance (I called it coreos-deployer above) and just build off that snapshot whenever you need to. That should save you a few Euros per month. πŸ’Έ

Once ignition has support for Hetzner’s metadata service, the extra step of embedding your configuration won’t be needed.

Also, if anyone from Hetzner is reading this post, I’d love to get Fedora CoreOS as one of the options for base images in your cloud! πŸ€—

Photo credit: Daniel Seßler on Unsplash