Building an x86 NixOS ISO on an Apple Silicon Mac

Introduction

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I’ve recently taken the dive into the NixOS system, and for the most part, it’s been great. It’s a fully declarative and reproducible OS, and I highly recommend you check it out. However, as expected when learning new things, there’s been some growing pains. And unfortunately, the first one came before I even had the it installed.

One of the cool features of NixOS is that you can build your own ISO using an ISO generator, with predefined configuration such as your SSH keys, or a few useful packages. However, when I tried to build the ISO on my Silicon Mac, I was greeted with the error message above. This post will show how I solved the issue and was able to build the ISO on my Apple Silicon based Mac.

The Plan

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While I thought that perhaps I could get NixOS to use the Apple Rosetta translation layer to build the ISO, I was unable to figure that out (Is it possible?). Instead, I opted to use a forced x86_64 environment using Docker. Fortunately, NixOS has an official Docker image that I could use to build the system.

Inside a folder aptly named iso, I created a flake.nix file to define my custom installation:

{
  description = "Minimal NixOS ISO";
  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-24.05";

  outputs = { self, nixpkgs }: {
    nixosConfigurations = {
      exampleIso = nixpkgs.lib.nixosSystem {
        system = "x86_64-linux";
        modules = [
          ({ pkgs, modulesPath, ... }: {
            imports = [ (modulesPath + "/installer/cd-dvd/installation-cd-minimal.nix") ];
            environment.systemPackages = with pkgs; [
              vim
              git
            ];
            systemd.services.openssh.enable = true;
            systemd.services.sshd.wantedBy = [ "multi-user.target" ];
            users.users.root.openssh.authorizedKeys.keys = [
              "ssh-ed25519 <MY_PUBLIC_SSH_KEY>"
            ];
          })
        ];
      };
    };
  };
}

Without going into too much detail, this flake defines a base NixOS install, along with enabling SSH from my public key, and installs the wonderful packages vim and git. I then ran the following command to build the ISO:

nix build .#nixosConfigurations.exampleIso.config.system.build.isoImage

On an x86_64 based system, this would build the ISO without issue and you’d be done! However, on my Apple Silicon Mac, I was greeted with the error message below. Part of the Apple tax I guess.

error: a 'x86_64-linux' with features {} is required to build '/nix/store/pb15l48pzy7mkl7zw7fjlv2m94jajkxj-loopback.cfg.drv', but I am a 'aarch64-darwin' with features {apple-virt, benchmark, big-parallel, nixos-test}

Building the ISO in a Docker Container

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Thankfully, thanks to Rosetta, Docker can run x86_64 containers on Apple Silicon Macs. Still inside the folder containing my flake.nix above, I created an ephemeral Docker container to build the ISO using the following command:

docker run --platform=linux/amd64 --rm -it -v $PWD:/out -w=/out nixos/nix

This command creates a temporary Docker container (the --rm flag) using the x86_64 architecture (the --platform=linux/amd64 flag), and maps the current directory ($PWD) to /out in the container. Then it drops you into a command line inside the container (-it). Once inside, I found that some experimental Nix features were required to build the ISO:

export NIX_CONFIG=$'filter-syscalls = false\nexperimental-features = nix-command flakes'

While I could create a Dockerfile and host my own Docker image with these features enabled, I don’t think that I’m going to be building enough ISOs to warrant that. So I’ll just keep this command handy for when I need to build another.

Now that I’m set to build the ISO, I ran the following command from before (This will take a while):

nix build .#nixosConfigurations.exampleIso.config.system.build.isoImage

Once it finishes building, you’ll find the fresh ISO in the result folder. To copy it to your host machine (the /out bindmount), you can use the following command:

mv result/iso/nixos-*.iso /out

Conclusion

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And that’s it! You can now use your new custom ISO to install NixOS on any number of nodes and automate the installation using SSH. While I know this won’t be the last issue I face with NixOS, at least they’ll be related to actually using the system… right?

Credits

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• ferdy.me for the ephemeral Docker command.
• maulana.id for the experimental Nix features.