NixOS has support for several bootloader backends by default: systemd-boot, grub, uboot, etc. The built-in bootloader backend support is generic and supports most use cases. Some users may prefer to create advanced workflows around managing the bootloader and bootable entries. You can replace the built-in bootloader support with your own tooling using the external bootloader option. Imagine you have created a new packaged called FooBoot. FooBoot provides a program at ${pkgs.fooboot}/bin/fooboot-install which takes the system closure’s path as its only argument and configures the system’s bootloader. You can enable FooBoot like this: { pkgs, ... }: { boot.loader.external = { enable = true; installHook = "${pkgs.fooboot}/bin/fooboot-install"; }; }

Bootloaders should use RFC-0125’s Bootspec format and synthesis tools to identify the key properties for bootable system generations.