This tool is really a collection of two parts. One part ISO to image generator and another part with the ability to parse the same metadata that cloud-init consumes to setup an image.
VMware ESXi does not provide OS images that can be used in the cloud like some other OSes do. You are also unable to install ESXi and then image that disk and reuse it for a different machine. Due to these limitations, and with the kind suggestions from Jens Sandmann at SAP Converged Cloud, we've packaged the installer into an image which can be deployed like a cloud image onto a system. The installer is then configured to perform a non-interactive installation and reboot the machine giving the user a working VMware ESXi machine.
VMware ESXi uses an installer based on RedHat's Anaconda installer, that they've called weasel. It can be programmatically driven using kickstart files which have slight differences from RedHat's kickstart files. This is what is used to perform a non-interactive installation. To facilitate the data that cloud-init would use to configure an OS image we must add some extra steps to the process. The installer allows for user supplied scripts that will run with either busybox ash or Python. They can run either before or after the installer or the first time the machine is booted if it was not configured with Secure Boot.
To make the installation work we've split the scripts up into two parts. The ones that run inside of the installer and the ones that run on the firstboot.
The installer based pieces involve setting the root password and getting our firstboot scripts and data copied over to the installed system so that our firstboot scripts can run.
The firstboot scripts involve configuring the hostname, configuring the network, enabling the console, enabling SSH and setting the SSH key.
The VMware ESXi installer uses a boot protocol called multiboot which involves loading multiple modules. Modules that are specifically crafted tar files are extracted into the installer OS and made available to it. We utilize this method to get our data and scripts into the environment.
To create the image we craft a whole disk image with a bootable partition. We then copy the contents of the ISO into this bootable partition and configure our tar file to be included as a module as well as reference a kickstart file which makes the installation non-interactive.
# using uvx
uvx esxi-image --output esxi.img path/to/esxi.iso
# using pip
python -m venv .venv
source .venv/bin/activate
pip install esxi-img
esxi-img --output esxi.img path/to/esxi.iso