Have you ever felt the need for a virtual machine in your NixOS server environment? Maybe the functionality you want is not available in NixOS? Maybe there’s a custom OS image for a service you want to provide? Maybe you just want to securely isolate your workload? NixOS promises reproducible and declarative systems, but for VMs it just wasn’t there yet. To close this gap we built the ctrl-os.vms NixOS module, a solution to define generic virtual machines directly in your NixOS configuration. It works just like virtualisation.oci-containers, but for VMs instead of containers. 20 lines of Nix make it possible to run any Linux distribution you want as a VM on your NixOS host. To put a cherry on top you can use cloud-init, from our NixOS configuration to configure your guests declaratively, too! ctrl-os.vms is part of the ctrl-os-modules repository.