nix vm using flake.md

Setting up qemu VM using nix flakes

Did you know that it is rather easy to setup a VM to test your NixOs configuration?

Create simple flake:

# flake.nix
{
  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";

  outputs = { self, nixpkgs, ... }:
    let
      system = "x86_64-linux";
    in
    {
      # test is a hostname for our machine
      nixosConfigurations.test = nixpkgs.lib.nixosSystem {
        inherit system;
        modules = [
          ./configuration.nix
        ];
      };
    };
}

Add your configuration

# configuration.nix
{ config, lib, pkgs, ... }: {
  # customize kernel version
  boot.kernelPackages = pkgs.linuxPackages_5_15;
  
  users.groups.admin = {};
  users.users = {
    admin = {
      isNormalUser = true;
      extraGroups = [ "wheel" ];
      password = "admin";
      group = "admin";
    };
  };

  virtualisation.vmVariant = {
    # following configuration is added only when building VM with build-vm
    virtualisation = {
      memorySize = 2048; # Use 2048MiB memory.
      cores = 3;
      graphics = false;
    };
  };

  services.openssh = {
    enable = true;
    settings.PasswordAuthentication = true;
  };

  networking.firewall.allowedTCPPorts = [ 22 ];
  environment.systemPackages = with pkgs; [
    htop
  ];

  system.stateVersion = "23.05";
}

Run a VM

git init # skip this step if you are inside already tracked repository
git add . # flakes requires at least tracking the files
nixos-rebuild build-vm --flake .#test
# expose port 22 from guest
export QEMU_NET_OPTS="hostfwd=tcp::2221-:22"
result/bin/run-nixos-vm

Profit!

# ssh onto the machine
ssh -oUserKnownHostsFile=/dev/null -oStrictHostKeyChecking=no admin@localhost -p 2221

Thanks @eljamm for the exaustive explanation!

program = "${inputs.self.nixosConfigurations.${name}.config.system.build.vm}/bin/run-nixos-vm";

inherit (import ./utils.nix { inherit inputs system; }) mkAppVM mkSystem;

Not what I'd call "cleaner" and "more readable" if you ask me 😅
Of course this is me trying to leverage the wonders of NixOS (declarativeness, reproducibility) but being too lazy to learn yet another programming language 🤣

But seriously, thanks again, I'll make treasure of your lesson!

I just wanted to leave a comment that this gist helped me a lot so I wanted to thank you for writing this, the comments on here are also very insightful and gives me plenty to dive in.

Thank you all!!

To use without nixos-rebuild:

nix build .#nixosConfigurations.test.config.system.build.vm

Or just nix run .#nixosConfigurations.test.config.system.build.vm

Hate to Necropost but huge thanks to all of you! This was exactly what i was searching for.

May you always have the right amount of milk for your cereal and coffee :)

thank you guys, this helped me a lot!

Note: The rec keyword allows us to refer to vm-test from within the same attrset

this was the best explanation about rec I ever read! not overcomplicated, to the point. 😄

I found that the VM executable is named after the hostname. so if the hostname is different from the config, like setting it explicitly in config.networking.hostName this should work:

  mkVM = name: {
    type = "app";
    program = "${inputs.self.nixosConfigurations.${name}.config.system.build.vm}/bin/run-${
        inputs.self.nixosConfigurations.${name}.config.networking.hostName
      }-vm";
  };

(linebreaks don't matter in the .nix file, but better for readability)

You could also use lib.getExe instead of typing out self.nixosConfigurations.${name}... twice and lib.genAttrs instead of flake-utils (see nixpkgs' flake.nix).

Mostly pet peeves, since these functions do exactly what everything else before does.

{
  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";

  outputs =
    inputs@{
      self,
      nixpkgs,
      ...
    }:
    let
      inherit (nixpkgs) lib;
      forSystems = lib.genAttrs lib.systems.flakeExposed;

      mkVM = name: {
        type = "app";
        program = lib.getExe self.nixosConfigurations.${name}.config.system.build.vm;
      };
    in
    {
      nixosConfigurations.host1 = lib.nixosSystem {
        # You don't need to specify `system` here, since the `nixpkgs.hostPlatform` option does already.
        specialArgs = { inherit inputs; };
        modules = [ ./hosts/host1 ];
      };

      apps = forSystems (system: {
        default = self.apps.${system}.vm-host1;
        vm-host1 = mkVM "host1";
      });
    };
}

Does anyone know how which config to add to pass through an usb device?