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nixos/modules/system/boot/loader/external/external.md

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+# External Bootloader Backends {#sec-bootloader-external}
+
+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 package called FooBoot.
+FooBoot provides a program at `${pkgs.fooboot}/bin/fooboot-install` which takes the system closure's path as its only argument and configures the system's bootloader.
+
+You can enable FooBoot like this:
+
+```nix
+{ pkgs, ... }:
+{
+  boot.loader.external = {
+    enable = true;
+    installHook = "${pkgs.fooboot}/bin/fooboot-install";
+  };
+}
+```
+
+## Developing Custom Bootloader Backends {#sec-bootloader-external-developing}
+
+Bootloaders should use [RFC-0125](https://github.com/NixOS/rfcs/pull/125)'s Bootspec format and synthesis tools to identify the key properties for bootable system generations.
+