OpenWRT

is an open-source project for embedded operating systems based on Linux, primarily used on embedded devices to route network traffic. The main components are Linux, util-linux, musl, and BusyBox. All components have been optimized to be small enough to fit into the limited storage and memory available in home routers. (Source: https://en.wikipedia.org/wiki/OpenWrt)

OpenWRT provides different possibilities (https://openwrt.org/docs/guide-developer/imagebuilder_frontends) to build custom images for your devices. There is also a little helper tool suggested on IRC called "autobuild" (https://johannes.truschnigg.info/code/openwrt_autobuild/) which helps to manage/keep track of image-configurations for your devices. It's also working with the nix-shell below - please note that there is currently some misbehavior when "make_clean" is set to "True" the system says build is triggered but there are no files/logs/processes running then.

Using ImageBuilder in nix-shell

Get a copy of: https://github.com/nix-community/nix-environments/blob/master/envs/openwrt/shell.nix

Currently (01/2022) I've tested builds using nixpkgs-21.11 and unstable, and both worked. This could possibly break/behave different in the future so you can e.g. pin to an older nixpkgs-revision by replacing the first lines of the above code with this:

{ opkgs ? import <nixpkgs> {} }:                           
                                                           
let                                                        
  pkgs = import (opkgs.fetchFromGitHub {                   
    owner = "NixOS";
    repo = "nixpkgs";            
    rev = "<revision>";      
    sha256 = "<hash>";                                                   
  }) {};
...

Using ImageBuilder in a Nix Derivation

See https://github.com/astro/nix-openwrt-imagebuilder/