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

Create a file iso.nix:

# This module defines a small NixOS installation CD.  It does not
# contain any graphical stuff.
{ 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>
  ];
}

Build the image via:

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

The resulting image can be found in result:

$ ls result/iso/
nixos-17.09.git.158ec57-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.ip4 = [{
      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%

See also: mksquashfs benchmarks

If you don't care about file size, you can use a faster compression by adding this to your iso.nix:

{
  isoImage.squashfsCompression = "gzip -Xcompression-level 1";
}

See also

• NixOS Manual: Building a NixOS (Live) ISO.