Launch CoreOS instances with iPXE and cloud-config

While still in its infancy, CoreOS is getting a lot of attention lately as a fast and lightweight Linux distribution geared towards large-scale server deployments. It’s bundled with software like Docker, Fleet, and etcd to allow for quick cluster deployments and manageable app containers.

One particular point that helps ease its adoption is that it supports iPXE booting with a cloud-config parameter. For providers that support iPXE booting (like Vultr.com), launching CoreOS instances doesn’t get much easier. A simple script.txt file placed in a publicly accessible location (we use Amazon S3 with the resource set as public) will give you a CoreOS instance on Vultr.com in about 60 seconds.

Upon logging in, you’ll have a fresh CoreOS running in RAM. While this may be ideal in many setups that have RAM to spare, memory constrained systems such as Vultr’s smallest 768MB instance can be relieved by installing CoreOS to disk using the install script provided by CoreOS. This works well and gives you an extra 50 – 100MB after rebooting, but we wanted our deployments to be automatic and not require any manual intervention.

We’re going to create three files:

1. iPXE boot script – this is used to give the iPXE boot process commands to automatically execute and provide CoreOS with a shell script to execute after booting completes.
2. cloud-config-bootstrap.sh – provided to the iPXE boot process in the boot script, this script downloads our actual cloud-config.yaml file, installs CoreOS to disk, and reboots the system.
3. cloud-config.yaml – provided to the coreos-install program in the bootstrap script, this file contains options that start services like etcd and fleet, configure reboot settings, setup our public SSH key, and so on.

Because CoreOS supports a cloud-config-url parameter in the iPXE boot process, we can provide a URL (it can’t be HTTPS) to either a valid cloud-config.yaml file or a shell script. Unfortunately, CoreOS doesn’t yet support all of the cloud-config options such as runcmd and bootcmd, so we’ll need to resort to a shell script. But first, let’s add a cloud-config-url to our iPXE boot script:

#!ipxe

set base-url http://stable.release.core-os.net/amd64-usr/current
kernel ${base-url}/coreos_production_pxe.vmlinuz cloud-config-url=http://s3.amazonaws.com/[YOUR S3 BUCKET]/coreos/cloud-config-bootstrap.sh
initrd ${base-url}/coreos_production_pxe_image.cpio.gz
boot

Do note, you won’t be able to SSH into the instance if anything goes wrong because we removed the sshkey parameter. You can add this back into the script if you’d like.

Now, let’s create the file cloud-config-bootstrap.sh. Open up your favorite editor and create the following file:

#!/bin/bash

curl -O http://s3.amazonaws.com/[YOUR S3 BUCKET]/coreos/cloud-config.yaml
sudo coreos-install -d /dev/vda -c cloud-config.yaml
sudo reboot

(Replace /dev/vda with the disk you want CoreOS installed on. *** The install script will delete everything on that device ***)

Put this file in a publicly accessible location (it can go right next to your iPXE boot script).

Here we download our actual cloud-config file that we’ll want to use when our installed version of CoreOS boots up. By downloading the file we can supply it to the coreos-install program that will bootstrap it during installation. A final reboot will free up the extra RAM that was being used by CoreOS and boot from disk instead.

Once that file is created, we can make our cloud-config.yaml file that each instance will receive upon booting:

#cloud-config

coreos:
    update:
        reboot-strategy: best-effort
    units:
        - name: etcd.service
          command: start
        - name: fleet.service
          command: start

ssh_authorized_keys:
    - ssh-rsa [PUBLIC KEY]

This file should be tweaked to your needs (including the reboot-strategy) and include your SSH public key so you’ll be able to SSH into the instance. Remote Fleet management also requires this, so don’t forget to include it. You can see all of the cloud-config options supported by CoreOS here.

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