nixos-generators - one config, multiple formats

The nixos-generators project allows to take the same NixOS configuration, and generate outputs for different target formats.

Just put your stuff into the configuration.nix and then call one of the image builders.

For example:

nixos-generate -f iso

or

nixos-generate -f iso -c /etc/nixos/configuration.nix

it echoes the path to a iso image, which you then can flash onto an usb-stick or mount & boot in a virtual machine.

Installation

nixos-generators is part of nixpkgs and can be installed from there.

nixos-generators can be also installed from source into the user profile like this:

nix-env -f https://github.com/nix-community/nixos-generators/archive/master.tar.gz -i

or for flakes users like this:

nix profile install github:nix-community/nixos-generators

or run from the nix flake without installing:

nix run github:nix-community/nixos-generators -- --help

Supported formats

Usage

Run nixos-generate --help for detailed usage information.

select a specific nixpkgs channel

Adds ability to select a specific channel version.

Example:

nix-shell --command './nixos-generate -f iso -I nixpkgs=channel:nixos-19.09'

Using a particular nixpkgs

To use features found in a different nixpkgs (for instance the Digital Ocean image was recently merged in nixpkgs):

NIX_PATH=nixpkgs=../nixpkgs nixos-generate -f do

Setting the disk image size

To specify the size of the generated disk image, use the --disk-size argument, specifying the size in megabytes. This is currently supported by the following formats. If this argument is unspecified it defaults to automatic sizing based on the generated NixOS build.

• hyperv
• proxmox
• qcow
• raw-efi
• raw
• vm
• vm-nogui
• vmware

Example (20GB disk):

nixos-generate -c <your_config.nix> -f <format> --disk-size 20480

To set the disk size in flake.nix, set the virtualisation.diskSize module option.

{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.11";
    nixos-generators = {
      url = "github:nix-community/nixos-generators";
      inputs.nixpkgs.follows = "nixpkgs";
    };
    xc = {
      url = "github:joerdav/xc";
      inputs.nixpkgs.follows = "nixpkgs";
    };
  };

  outputs = { nixpkgs, nixos-generators, xc, ... }:
    let
      pkgsForSystem = system: import nixpkgs {
        inherit system;
        overlays = [
          (final: prev: { xc = xc.packages.${system}.xc; })
        ];
      };
      allVMs = [ "x86_64-linux" "aarch64-linux" ];
      forAllVMs = f: nixpkgs.lib.genAttrs allVMs (system: f {
        inherit system;
        pkgs = pkgsForSystem system;
      });
    in
    {
      packages = forAllVMs ({ system, pkgs }: {
        vm = nixos-generators.nixosGenerate {
          system = system;
          specialArgs = {
            pkgs = pkgs;
          };
          modules = [
            {
              # Pin nixpkgs to the flake input, so that the packages installed
              # come from the flake inputs.nixpkgs.url.
              nix.registry.nixpkgs.flake = nixpkgs;
              # set disk size to to 20G
              virtualisation.diskSize = 20 * 1024;
            }
            # Apply the rest of the config.
            ./configuration.nix
          ];
          format = "raw";
        };
      });
    };
}

Cross Compiling

To cross compile nixos images for other architectures you have to configure boot.binfmt.emulatedSystems or boot.binfmt.registrations on your host system.

In your system configuration.nix:

{
  # Enable binfmt emulation of aarch64-linux.
  boot.binfmt.emulatedSystems = [ "aarch64-linux" ];
}

Alternatively, if you want to target other architectures:

# Define qemu-arm-static source.
let qemu-arm-static = pkgs.stdenv.mkDerivation {
  name = "qemu-arm-static";
  src = builtins.fetchurl {
    url = "https://github.com/multiarch/qemu-user-static/releases/download/v6.1.0-8/qemu-arm-static";
    sha256 = "06344d77d4f08b3e1b26ff440cb115179c63ca8047afb978602d7922a51231e3";
  };
  dontUnpack = true;
  installPhase = "install -D -m 0755 $src $out/bin/qemu-arm-static";
};
in {
  # Enable binfmt emulation of extra binary formats (armv7l-linux, for exmaple).
  boot.binfmt.registrations.arm = {
    interpreter = "${qemu-arm-static}/bin/qemu-arm-static";
    magicOrExtension = ''\x7fELF\x01\x01\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x28\x00'';
    mask = ''\xff\xff\xff\xff\xff\xff\xff\x00\xff\xff\xff\xff\xff\xff\x00\xff\xfe\xff\xff\xff'';
  };

  # Define additional settings for nix.
  nix.extraOptions = ''
    extra-platforms = armv7l-linux
  '';
  nix.sandboxPaths = [ "/run/binfmt/arm=${qemu-arm-static}/bin/qemu-arm-static" ];
}

For more details on configuring binfmt, have a look at: binfmt options, binfmt.nix, this comment and clevers qemu-user.

Once you've run nixos-rebuild with these options, you can use the --system option to create images for other architectures.

Using as a nixos-module

nixos-generators can be included as a NixOS module into your existing configuration.nix making all available formats available through config.formats and configurable through config.formatConfigs. New formats can be defined by adding a new entry like config.formatConfigs.my-new-format = {config, ...}: {}.

An example flake.nix demonstrating this approach is below.

Images can be built from that flake by running:

• nix build .#nixosConfigurations.my-machine.config.formats.vmware or
• nix build .#nixosConfigurations.my-machine.config.formats.my-custom-format or
• nix build .#nixosConfigurations.my-machine.config.formats.<any-other-format>

{
  inputs = {
    nixpkgs.url = "nixpkgs/nixos-unstable";
    nixos-generators = {
      url = "github:nix-community/nixos-generators";
      inputs.nixpkgs.follows = "nixpkgs";
    };
  };
  outputs = { self, nixpkgs, nixos-generators, ... }: {

    # A single nixos config outputting multiple formats.
    # Alternatively put this in a configuration.nix.
    nixosModules.myFormats = { config, ... }: {
      imports = [
        nixos-generators.nixosModules.all-formats
      ];

      nixpkgs.hostPlatform = "x86_64-linux";

      # customize an existing format
      formatConfigs.vmware = { config, ... }: {
        services.openssh.enable = true;
      };

      # define a new format
      formatConfigs.my-custom-format = { config, modulesPath, ... }: {
        imports = [ "${toString modulesPath}/installer/cd-dvd/installation-cd-base.nix" ];
        formatAttr = "isoImage";
        fileExtension = ".iso";
        networking.wireless.networks = {
          # ...
        };
      };
    };

    # a machine consuming the module
    nixosConfigurations.my-machine = nixpkgs.lib.nixosSystem {
      modules = [ self.nixosModules.myFormats ];
    };
  };
}

Using in a Flake

nixos-generators can be included as a Flake input and provides a nixosGenerate function for building images as Flake outputs. This approach pins all dependencies and allows for conveniently defining multiple output types based on one config.

An example flake.nix demonstrating this approach is below. vmware or virtualbox images can be built from the same configuration.nix by running nix build .#vmware or nix build .#virtualbox

Custom formats can be defined by building a format module (see the formats directory for examples) and passing it to nixosGenerate via an the customFormats argument. customFormats should be in the form of an attribute sets of the form <format name> = <format module> and can define multiple custom formats. nixosGenerate will then match against these custom formats as well as the built in ones.

{
  inputs = {
    nixpkgs.url = "nixpkgs/nixos-unstable";
    nixos-generators = {
      url = "github:nix-community/nixos-generators";
      inputs.nixpkgs.follows = "nixpkgs";
    };
  };
  outputs = { self, nixpkgs, nixos-generators, ... }: {
    packages.x86_64-linux = {
      vmware = nixos-generators.nixosGenerate {
        system = "x86_64-linux";
        modules = [
          # you can include your own nixos configuration here, i.e.
          # ./configuration.nix
        ];
        format = "vmware";

        # optional arguments:
        # explicit nixpkgs and lib:
        # pkgs = nixpkgs.legacyPackages.x86_64-linux;
        # lib = nixpkgs.legacyPackages.x86_64-linux.lib;
        # additional arguments to pass to modules:
        # specialArgs = { myExtraArg = "foobar"; };

        # you can also define your own custom formats
        # customFormats = { "myFormat" = <myFormatModule>; ... };
        # format = "myFormat";
      };
      vbox = nixos-generators.nixosGenerate {
        system = "x86_64-linux";
        format = "virtualbox";
      };
    };
  };
}

Format-specific notes

raw and raw-efi

raw and raw-efi images can be used on physical hardware, but benefit from some tweaks.

• These images are configured to log to the serial console, and not to your display. One workaround for this is to add boot.kernelParams = [ "console=tty0" ]; to your configuration, which will override the image's default console=ttyS0.
• By default, grub will timeout after 1 second. To extend this, set boot.loader.timeout = 5; (or longer)
• If boot fails for some reason, you will not get a recovery shell unless the root user is enabled, which you can do by setting a password for them (users.users.root.password = "something";, possibly users.mutableUsers = true; so you can interactively change the passwords after boot)
• After booting, if you intend to use nixos-switch, consider using nixos-generate-config.

License

This project is licensed under the MIT License.

FAQ

No space left on device

This means either /tmp, /run/user/$UID or your TMPFS runs full. Sometimes setting TMPDIR to some other location can help, sometimes /tmp needs to be on a bigger partition (not a tmpfs).