nixpkgs/nixos/doc/manual/installation/installing-kexec.section.md

"Booting" into NixOS via kexec

In some cases, your system might already be booted into/preinstalled with another Linux distribution, and booting NixOS by attaching an installation image is quite a manual process.

This is particularly useful for (cloud) providers where you can't boot a custom image, but get some Debian or Ubuntu installation.

In these cases, it might be easier to use kexec to "jump into NixOS" from the running system, which only assumes bash and kexec to be installed on the machine.

Note that kexec may not work correctly on some hardware, as devices are not fully re-initialized in the process. In practice, this however is rarely the case.

To build the necessary files from your current version of nixpkgs, you can run:

nix-build -A kexec.x86_64-linux '<nixpkgs/nixos/release.nix>'

This will create a result directory containing the following:

• bzImage (the Linux kernel)
• initrd (the initrd file)
• kexec-boot (a shellscript invoking kexec)

These three files are meant to be copied over to the other already running Linux Distribution.

Note it's symlinks pointing elsewhere, so cd in, and use scp * root@$destination to copy it over, rather than rsync.

Once you finished copying, execute kexec-boot on the destination, and after some seconds, the machine should be booting into an (ephemeral) NixOS installation medium.

In case you want to describe your own system closure to kexec into, instead of the default installer image, you can build your own configuration.nix:

{ modulesPath, ... }: {
  imports = [
    (modulesPath + "/installer/netboot/netboot-minimal.nix")
  ];

  services.openssh.enable = true;
  users.users.root.openssh.authorizedKeys.keys = [
    "my-ssh-pubkey"
  ];
}

nix-build '<nixpkgs/nixos>' \
  --arg configuration ./configuration.nix
  --attr config.system.build.kexecTree

Make sure your configuration.nix does still import netboot-minimal.nix (or netboot-base.nix).