As easy it is to setup and run a light VM using nix, I think there is no shortage of simple and yet concrete examples of what you can do with it to get started. This is what I propose here today: to build up the most simple VM with nix for just any purpose. Note that this works on NixOS but also on any other host supporting nixpkgs.

As for me, I mainly use this kind of VM for two reasons:

• to connect to my work VPN without messing up with the network configuration of my computer
• to test software in isolation of my current system

The code is available here. It uses flakes, so it should be fairly reproducible.

One can build the VM with:

$ nix build ./#nixosConfigurations.vm.config.system.build.vm

The build is usually pretty fast as the nix store is shared between the VM and the host.

It is then possible to run the VM with:

$ ./result/bin/run-nixos-vm

This starts a qemu window with the xfce desktop environment. xfce is lightweight enough to make it able to run even on computers with limited amount of CPU power.

Nix will create a file for the disk of the VM in the current directory. It is called nixos.qcow2. Thanks to this file, if you put data into the home directory of the VM user, it will be persistent between reboots.

Nothing fancy here, just plain simple nix code, but as usual with nix still very hard to guess if you don’t have any example around.

Let’s go through interesting parts of the code. The VM resolution can be changed here for something suitable for your screen:

  # Options for the screen
  virtualisation.vmVariant = {
    virtualisation.resolution = {
      x = 1280;
      y = 1024;
    };
    virtualisation.qemu.options = [
      # Better display option
      "-vga virtio"
      "-display gtk,zoom-to-fit=false"
      # Enable copy/paste
      # https://www.kraxel.org/blog/2021/05/qemu-cut-paste/
      "-chardev qemu-vdagent,id=ch1,name=vdagent,clipboard=on"
      "-device virtio-serial-pci"
      "-device virtserialport,chardev=ch1,id=ch1,name=com.redhat.spice.0"
    ];
  };

The user configuration now. Being part of the wheel group makes it possible to use sudo if needed. You might want to change the password here.

  # A default user able to use sudo
  users.users.guest = {
    isNormalUser = true;
    home = "/home/guest";
    extraGroups = [ "wheel" ];
    initialPassword = "guest";
  };

  security.sudo.wheelNeedsPassword = false;

The internationalization options:

  # Internationalisation options
  i18n.defaultLocale = "en_US.UTF-8";
  console.keyMap = "fr";

The xserver options:

  # X configuration
  services.xserver.enable = true;
  services.xserver.layout = "fr";

  services.xserver.displayManager.autoLogin.user = "guest";
  services.xserver.desktopManager.xfce.enable = true;
  services.xserver.desktopManager.xfce.enableScreensaver = false;

  services.xserver.videoDrivers = [ "qxl" ];

  # For copy/paste to work
  services.spice-vdagentd.enable = true;

That last line along the qemu options is especially important. This allows to copy/paste between the host computer and the VM. Not having this makes the experience incredibly frustrating. More details about this is available here.

One can tweak the packages available:

  # Included packages here
  nixpkgs.config.allowUnfree = true;
  environment.systemPackages = with pkgs; [
    dig
    hey
    httpie
    google-chrome
    neovim
    wget
    wrk
  ];

Checkout the huge list of available packages in nixpkgs for inspiration.

You also might want to ssh to the VM from the host, so just enable the ssh server on the VM:

  # Enable ssh
  services.sshd.enable = true;

To connect to the VM, one must forward the 22 port to the host. That can be done with an environment variable before starting the VM:

$ QEMU_NET_OPTS="hostfwd=tcp::2222-:22" ./result/bin/run-nixos-vm

And that’s it for today ! Give it a try and let me know if you have any question.