Creating a NixOS live CD

Motivation

Creating a modified NixOS LiveCD out of an existing working NixOS installation has a number of benefits:

Ensures authenticity.
No need for internet access.
It is easy to add your own packages and configuration changes to the image.

Building

Building minimal NixOS installation CD with the nix-build command by creating this iso.nix-file. In this example with Neovim preinstalled.

{ config, pkgs, ... }:
{
  imports = [
    <nixpkgs/nixos/modules/installer/cd-dvd/installation-cd-minimal.nix>

    # Provide an initial copy of the NixOS channel so that the user
    # doesn't need to run "nix-channel --update" first.
    <nixpkgs/nixos/modules/installer/cd-dvd/channel.nix>
  ];
  environment.systemPackages = [ pkgs.neovim ];
}

Build the image via:

nix-build '<nixpkgs/nixos>' -A config.system.build.isoImage -I nixos-config=iso.nix

Alternatively, use Nix Flakes to generate a ISO installation image, using the nixos-24.05 branch as nixpkgs source:

flake.nix

{
  description = "Minimal NixOS installation media";
  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 = [ pkgs.neovim ];
          })
        ];
      };
    };
  };
}

The following commands will generate the iso-image

# git init
# git add flake.nix
# nix build .#nixosConfigurations.exampleIso.config.system.build.isoImage

The resulting image can be found in result:

$ ls result/iso/
nixos-24.05.20240721.63d37cc-x86_64-linux.iso

Testing the image

To inspect the contents of the ISO image:

$ mkdir mnt
$ sudo mount -o loop result/iso/nixos-*.iso mnt
$ ls mnt
boot  EFI  isolinux  nix-store.squashfs  version.txt
$ umount mnt

To boot the ISO image in an emulator:

$ nix-shell -p qemu
$ qemu-system-x86_64 -enable-kvm -m 256 -cdrom result/iso/nixos-*.iso

SSH

In your iso.nix:

{
  ...
  # Enable SSH in the boot process.
  systemd.services.sshd.wantedBy = pkgs.lib.mkForce [ "multi-user.target" ];
  users.users.root.openssh.authorizedKeys.keys = [
    "ssh-ed25519 AaAeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee username@host"
  ];
  ...
}

Static IP Address

Static IP addresses can be set in the image itself. This can be useful for VPS installation.

{
  ...
  networking = {
    usePredictableInterfaceNames = false;
    interfaces.eth0.ipv4.addresses = [{
      address = "64.137.201.46";
      prefixLength = 24;
    }];
    defaultGateway = "64.137.201.1";
    nameservers = [ "8.8.8.8" ];
  };
  ...
}

Building faster

The build process is slow because of compression.

Here are some timings for nix-build:

Compression results
squashfsCompression	Time	Size
`lz4`	100s	59%
`gzip -Xcompression-level 1`	105s	52%
`gzip`	210s	49%
`xz -Xdict-size 100%` (default)	450s	43%

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