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. [TODO mention dependencies]

Using ImageBuilder in nix-shell

{ pkgs ? import <nixpkgs> {} }:

let
  fixWrapper = pkgs.runCommand "fix-wrapper" {} ''
    mkdir -p $out/bin
    for i in ${pkgs.gcc.cc}/bin/*-gnu-gcc*; do
      ln -s ${pkgs.gcc}/bin/gcc $out/bin/$(basename "$i")
    done
    for i in ${pkgs.gcc.cc}/bin/*-gnu-{g++,c++}*; do
      ln -s ${pkgs.gcc}/bin/g++ $out/bin/$(basename "$i")
    done
  '';

  fhs = pkgs.buildFHSUserEnv {
    name = "openwrt-env";
    targetPkgs = pkgs: with pkgs;
      [ git
        perl
        gnumake
        gcc
        unzip
        utillinux
        python3
        patch
        wget
        file
        subversion
        which
        pkgconfig
        openssl
        fixWrapper
        systemd
        binutils

        ncurses
        zlib
        zlib.static
        glibc.static
      ];
    multiPkgs = null;
    extraOutputsToInstall = [ "dev" ];
    profile = ''
      export hardeningDisable=all
    '';
  };
in fhs.env

Source: https://gist.github.com/Mic92/b59054188c595e5652cacf50485583e0 (Updated python to python3 because build of image failed without)

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 = "bee172501de3b4496ff81cc1621d307f167e9382";      
    sha256 = "<revision>";                                                   
  }) {};
...