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Generating Disk Images with Secrets Included using Disko

Using Disko on NixOS allows you to efficiently create .raw VM images from a system configuration. The generated image can be used as a VM or directly written to a physical drive to create a bootable disk. Follow the steps below to generate disk images:

Generating the .raw VM Image

  1. Create a NixOS configuration that includes the disko and the disk configuration of your choice

In the this example we create a flake containing a nixos configuration for myhost.

# save this as flake.nix
{
  description = "A disko images example";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
    disko.url = "github:nix-community/disko";
    disko.inputs.nixpkgs.follows = "nixpkgs";
  };

  outputs = { self, disko, nixpkgs }: {
    nixosConfigurations.myhost = nixpkgs.lib.nixosSystem {
      system = "x86_64-linux";
      modules = [
        # You can get this file from here: https://github.com/nix-community/disko/blob/master/example/simple-efi.nix
        ./simple-efi.nix
        disko.nixosModules.disko
        ({ config, ... }: {
          # shut up state version warning
          system.stateVersion = config.system.nixos.version;
          # Adjust this to your liking.
          # WARNING: if you set a too low value the image might be not big enough to contain the nixos installation
          disko.devices.disk.main.imageSize = "10G";
        })
      ];
    };
  };
}

  1. Build the disko image script: Replace myhost in the command below with your specific system configuration name:

    nix build .#nixosConfigurations.myhost.config.system.build.diskoImagesScript

  2. Execute the disko image script: Execute the generated disko image script. Running ./result --help will output the available options:

    ./result --help
Usage: $script [options]

Options:
* --pre-format-files <src> <dst>
  copies the src to the dst on the VM, before disko is run
  This is useful to provide secrets like LUKS keys, or other files you need for formatting
* --post-format-files <src> <dst>
  copies the src to the dst on the finished image
  These end up in the images later and is useful if you want to add some extra stateful files
  They will have the same permissions but will be owned by root:root
* --build-memory <amt>
  specify the amount of memory in MiB that gets allocated to the build VM
  This can be useful if you want to build images with a more involed NixOS config
  The default is 1024 MiB

    An example run may look like this:

    sudo ./result --build-memory 2048

    The script will generate the actual image outside of the nix store in the current working directory. The create image names depend on the names used in disko.devices.disk attrset in the NixOS configuration. In our code example it will produce the following image:

    $ ls -la main.raw
.rw-r--r-- root root 10 GB 2 minutes ago main.raw

Additional Configuration

  • For virtual drive use, define the image size in your Disko configuration:

    disko.devices.disk.<drive>.imageSize = "32G"; # Set your preferred size

  • If the .raw image size is not optimal, use --write-to-disk to write directly to a drive. This bypasses the .raw file generation, which saves on read/write operations and is suitable for single disk setups.

Understanding the Image Generation Process

  1. Files specified in --pre-format-files and --post-format-files are temporarily copied to /tmp.
  2. Files are then moved to their respective locations in the VM both before and after the Disko partitioning script runs.
  3. The NixOS installer is executed, having access only to --post-format-files.
  4. Upon installer completion, the VM is shutdown, and the .raw disk files are moved to the local directory.

Note: The auto-resizing feature is currently not available in Disko. Contributions for this feature are welcomed. Adjust the imageSize configuration to prevent issues related to file size and padding.

By following these instructions and understanding the process, you can smoothly generate disk images with Disko for your NixOS system configurations.