nw.nix

let
  p = builtins.fetchTarball "https://github.com/NixOS/nixpkgs/archive/nixos-20.09.tar.gz";
  common = { pkgs, ... }: {
    fileSystems."/" = { device = "/dev/null"; fsType = "ext4"; };
    boot.loader.grub.devices = ["/dev/null"];
    environment.systemPackages = [pkgs.ngrok];
  };
in
{

  host1 = { ... }: {  # this host will refuse to eval, because ngrok
    imports = [ common ];
    nixpkgs.pkgs = import p { # overrides "network.pkgs"
      config.allowUnfree = false;
    };
  };

  host2 = { ... }: {
    imports = [ common ];
  };

  host3 = { ... }: {
    imports = [ common ];
  };

  network.pkgs = import p {
    config.allowUnfree = true;
  };

}