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.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