darwin.linux-builder provides a way to bootstrap a Linux builder on a macOS machine.

This requires macOS version 12.4 or later.

The builder runs on host port 31022 by default. You can change it by overriding virtualisation.darwin-builder.hostPort. See the example.

You will also need to be a trusted user for your Nix installation. In other words, your /etc/nix/nix.conf should have something like:

extra-trusted-users = <your username goes here>

To launch the builder, run the following flake:

$ nix run nixpkgs#darwin.linux-builder

That will prompt you to enter your sudo password:

+ sudo --reset-timestamp /nix/store/…-install-credentials.sh ./keys

… so that it can install a private key used to ssh into the build server. After that the script will launch the virtual machine and automatically log you in as the builder user:

<<< Welcome to NixOS 22.11.20220901.1bd8d11 (aarch64) - ttyAMA0 >>>

Run 'nixos-help' for the NixOS manual.

nixos login: builder (automatic login)


Note: When you need to stop the VM, run shutdown now as the builder user.

To delegate builds to the remote builder, add the following options to your nix.conf file:

# - Replace ${ARCH} with either aarch64 or x86_64 to match your host machine
# - Replace ${MAX_JOBS} with the maximum number of builds (pick 4 if you're not sure)
builders = ssh-ng://builder@linux-builder ${ARCH}-linux /etc/nix/builder_ed25519 ${MAX_JOBS} - - - c3NoLWVkMjU1MTkgQUFBQUMzTnphQzFsWkRJMU5URTVBQUFBSUpCV2N4Yi9CbGFxdDFhdU90RStGOFFVV3JVb3RpQzVxQkorVXVFV2RWQ2Igcm9vdEBuaXhvcwo=

# Not strictly necessary, but this will reduce your disk utilization
builders-use-substitutes = true

To allow Nix to connect to a builder not running on port 22, you will also need to create a new file at /etc/ssh/ssh_config.d/100-linux-builder.conf:

Host linux-builder
  Hostname localhost
  HostKeyAlias linux-builder
  Port 31022

… and then restart your Nix daemon to apply the change:

$ sudo launchctl kickstart -k system/org.nixos.nix-daemon

Example flake usage

  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-22.11-darwin";
    darwin.url = "github:lnl7/nix-darwin/master";
    darwin.inputs.nixpkgs.follows = "nixpkgs";

  outputs = { self, darwin, nixpkgs, ... }@inputs:

    inherit (darwin.lib) darwinSystem;
    system = "aarch64-darwin";
    pkgs = nixpkgs.legacyPackages."${system}";
    linuxSystem = builtins.replaceStrings [ "darwin" ] [ "linux" ] system;

    darwin-builder = nixpkgs.lib.nixosSystem {
      system = linuxSystem;
      modules = [
        { virtualisation.host.pkgs = pkgs; }
  in {

    darwinConfigurations = {
      machine1 = darwinSystem {
        inherit system;
        modules = [
            nix.distributedBuilds = true;
            nix.buildMachines = [{
              hostName = "ssh://builder@localhost";
              system = linuxSystem;
              maxJobs = 4;
              supportedFeatures = [ "kvm" "benchmark" "big-parallel" ];

            launchd.daemons.darwin-builder = {
              command = "${darwin-builder.config.system.build.macos-builder-installer}/bin/create-builder";
              serviceConfig = {
                KeepAlive = true;
                RunAtLoad = true;
                StandardOutPath = "/var/log/darwin-builder.log";
                StandardErrorPath = "/var/log/darwin-builder.log";


Reconfiguring the builder

Initially you should not change the builder configuration else you will not be able to use the binary cache. However, after you have the builder running locally you may use it to build a modified builder with additional storage or memory.

To do this, you just need to set the virtualisation.darwin-builder.* parameters as in the example below and rebuild.

    darwin-builder = nixpkgs.lib.nixosSystem {
      system = linuxSystem;
      modules = [
          virtualisation.host.pkgs = pkgs;
          virtualisation.darwin-builder.diskSize = 5120;
          virtualisation.darwin-builder.memorySize = 1024;
          virtualisation.darwin-builder.hostPort = 33022;
          virtualisation.darwin-builder.workingDirectory = "/var/lib/darwin-builder";

You may make any other changes to your VM in this attribute set. For example, you could enable Docker or X11 forwarding to your Darwin host.