# TIL: How to Create a Template VM Disk Image

While experimenting with [dqlite](https://dqlite.io/), I needed to create
multiple virtual machines from a custom template to manually test a few
scenarios. [`cloud-init`](https://cloud-init.io/) combined with one of the cloud
images from [here](https://cloud-images.ubuntu.com/noble/current/) would have
worked, but I wanted to avoid waiting for the VM to boot and configure itself
each time I spun up a new instance.

After some research, I discovered `virt-sysprep`, which turned out to be the
perfect tool for the task. From the official
[documentation](https://libguestfs.org/virt-sysprep.1.html):

> Virt-sysprep can reset or unconfigure a virtual machine so that clones can be
> made from it. Steps in this process include removing SSH host keys, removing
> persistent network MAC configuration, and removing user accounts. Virt-sysprep
> can also customize a virtual machine, for instance by adding SSH keys, users
> or logos. Each step can be enabled or disabled as required.

Below, I'll document the steps I followed to create a custom template VM disk
image.

## Prerequisites

1. A Linux host or VM
2. `virt-sysprep` (install `libguestfs-tools` on Ubuntu/Debian)
3. [Optional]
   [ovftool](https://developer.broadcom.com/tools/open-virtualization-format-ovf-tool/latest)
   (can also be installed with `brew` on macOS)

## Steps

1. Create a virtual machine, configure it as needed, and install the required
   packages (e.g. `open-vm-tools`).
2. Shutdown the VM.
3. [Optional] Use `ovftool` to Convert the VM into an OVF (e.g.
   `ovftool /path/to/template.vmx /output/path/to/template.ovf`)
4. Use `virt-sysprep` to "unconfigure" the disk image:

   ```bash
   ops=$(virt-sysprep --list-operations | egrep -v 'fs-uuids|lvm-uuids' | awk '{ printf "%s,", $1}' | sed 's/,$//')
   sudo virt-sysprep -a /path/to/disk.vmdk --hostname localhost --remove-user-accounts ansible --enable $ops --firstboot-command 'dpkg-reconfigure openssh-server' --root-password password:changeme
   ```

5. Create a new VM from the disk image.