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nixpkgs/nixos/doc/manual/installation/building-nixos.chapter.md
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Building a NixOS (Live) ISO

Default live installer configurations are available inside nixos/modules/installer/cd-dvd. For building other system images, nixos-generators is a good place to start looking at.

You have two options:

  • Use any of those default configurations as is
  • Combine them with (any of) your host config(s)

System images, such as the live installer ones, know how to enforce configuration settings on which they immediately depend in order to work correctly.

However, if you are confident, you can opt to override those enforced values with mkForce.

Practical Instructions

To build an ISO image for the channel nixos-unstable:

$ git clone https://github.com/NixOS/nixpkgs.git
$ cd nixpkgs/nixos
$ git switch nixos-unstable
$ nix-build -A config.system.build.isoImage -I nixos-config=modules/installer/cd-dvd/installation-cd-minimal.nix default.nix

To check the content of an ISO image, mount it like so:

# mount -o loop -t iso9660 ./result/iso/cd.iso /mnt/iso

Additional drivers or firmware

If you need additional (non-distributable) drivers or firmware in the installer, you might want to extend these configurations.

For example, to build the GNOME graphical installer ISO, but with support for certain WiFi adapters present in some MacBooks, you can create the following file at modules/installer/cd-dvd/installation-cd-graphical-gnome-macbook.nix:

{ config, ... }:

{
  imports = [ ./installation-cd-graphical-gnome.nix ];

  boot.initrd.kernelModules = [ "wl" ];

  boot.kernelModules = [ "kvm-intel" "wl" ];
  boot.extraModulePackages = [ config.boot.kernelPackages.broadcom_sta ];
}

Then build it like in the example above:

$ git clone https://github.com/NixOS/nixpkgs.git
$ cd nixpkgs/nixos
$ export NIXPKGS_ALLOW_UNFREE=1
$ nix-build -A config.system.build.isoImage -I nixos-config=modules/installer/cd-dvd/installation-cd-graphical-gnome-macbook.nix default.nix

Technical Notes

The config value enforcement is implemented via mkImageMediaOverride = mkOverride 60; and therefore primes over simple value assignments, but also yields to mkForce.

This property allows image designers to implement in semantically correct ways those configuration values upon which the correct functioning of the image depends.

For example, the iso base image overrides those file systems which it needs at a minimum for correct functioning, while the installer base image overrides the entire file system layout because there can't be any other guarantees on a live medium than those given by the live medium itself. The latter is especially true before formatting the target block device(s). On the other hand, the netboot iso only overrides its minimum dependencies since netboot images are always made-to-target.