GitHub - jonhermansen/darnix: Darwin system built with Nix

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Boot a fully open-source Darwin system using Nix. One command, no macOS install required (beyond the build host).

$ nix run github:jonhermansen/darnix

Boots XNU (macOS 26.4 / xnu-12377.101.15) in QEMU with an HFS+ root filesystem and serial console — all from a single Nix flake. The entire build runs under Nix's strict sandbox (sandbox = true) — no network access, no /dev access, no DMG mounting or hdiutil, no system side effects.

What this is

Darnix is a revival of the PureDarwin idea: run Apple's open-source XNU kernel with an entirely open userland managed by Nix. The long-term goal is a NixOS-style system on a Darwin kernel.

Current status: The kernel boots in QEMU (x86_64 emulated via TCG on aarch64-darwin), mounts an HFS+ ramdisk as root, and executes a statically linked init binary with serial console output. There is no networking, no kext loading, and no userland beyond init.

What we had to do

Nix build system (`darwin-xnu-build`)

Wrapped blacktop/darwin-xnu-build in a Nix flake that builds XNU in a fully sandboxed derivation
All Apple open-source dependencies (bootstrap_cmds, dtrace, AvailabilityVersions, Libsystem, libplatform, libdispatch) fetched as flake inputs
Xcode SDK and KDK extracted from Apple's downloads and wrapped as Nix derivations
nix run builds the kernel, assembles a GRUB EFI image with embedded ramdisk, and launches QEMU

XNU kernel patches (`xnu`)

QEMU platform support:

Built-in platform expert for QEMU (IOQEMUPlatform) — no IOKit kexts needed
PRNG fallback when RDRAND/RDSEED are unavailable (QEMU TCG doesn't emulate them)
ignore_msrs boot arg for unimplemented MSRs under TCG
TSC fallback when MSR_PLATFORM_INFO bus ratio reads as 0
EFI runtime mapping skip for QEMU's firmware
Default to 1 CPU when ACPI tables are missing

Bare boot (no kexts):

Skip networking, pthread shims, trust cache, corecrypto, AMFI
Guard subsystems that assume kexts/coalitions/entitlements are present
Allow PE_i_can_has_debugger on DEVELOPMENT builds
Skip code signing validation when enforcement is disabled

HFS+ as root filesystem:

Registered HFS as an in-kernel filesystem
Added VFC_VFSCANMOUNTROOT flag so vfs_mountroot() tries HFS
Fixed bsd_init console open: FREAD|FWRITE → O_RDWR, authfd=0 → AUTH_OPEN_NOAUTHFD

HFS+ in-kernel build (`hfs`)

Apple's HFS+ ships as a kext depending on IOKit and corecrypto. We ported it to compile directly into the kernel:

Stubbed IOKit helper functions
Called hfs_init_zones() from hfs_init() (previously only called from the kext's IOKit entry point)
Fixed hfs_getvoluuid() to use a fixed UUID (MD5 requires corecrypto which isn't available)
Stubbed AKS (Apple Key Services) for content protection code paths
Patched newfs_hfs to format regular files, not just block devices (runs inside Nix sandbox)

GRUB (`grub`)

Fixed xnu_kernel64 page fault loading 64-bit Mach-O kernels
Added Nix flake for building GRUB EFI on aarch64-darwin

HFS+ ramdisk image

Built with patched newfs_hfs + xpwn's hfsplus tool
8 MB image containing /sbin/launchd and /dev mountpoint
Created entirely inside the Nix sandbox — no root, no devices

Architecture

aarch64-darwin host
  └─ nix run
       └─ qemu-system-x86_64 (TCG, software emulation)
            └─ GRUB EFI (grub-mkstandalone)
                 ├─ xnu_kernel64 /boot/kernel
                 ├─ xnu_ramdisk /boot/rootfs-hfs.dmg
                 └─ boot
                      └─ XNU boots
                           ├─ HFS+ root on md0 (ramdisk)
                           ├─ devfs on /dev
                           ├─ serial console (fd 0/1/2)
                           └─ exec /sbin/launchd

Boot options

GRUB presents two menu entries:

Entry	Root filesystem	Description
Darnix (HFS+)	8 MB HFS+ ramdisk	Real filesystem with directory hierarchy
Darnix (mockfs)	Raw Mach-O binary	Minimal fake FS for debugging (the ramdisk is the binary)

Current limitations

x86_64 only — cross-compiled from aarch64-darwin, runs under TCG (software emulation)
No networking — network subsystem init is skipped
No crypto — corecrypto kext not loaded, volume UUID is hardcoded
No kext loading — HFS, devfs, and all filesystems are compiled into the kernel
No ACPI — fake platform expert, hardcoded to 1 CPU
Build host — currently requires aarch64-darwin (macOS on Apple Silicon) with Xcode SDK

Building

# Build everything (kernel + ESP + ramdisk)
nix build .#esp

# Build and boot in QEMU
nix run

# Build just the kernel
nix build .#xnu-x86_64

Repositories

Repo	Branch	Description
darnix	`nix`	Nix flake, build scripts, init binary, QEMU runner
xnu	`nix`	Patched XNU kernel
hfs	`nix`	HFS+ ported to in-kernel build
grub	`nix`	GRUB with Mach-O fix + Nix flake

What's next

Shell: get toybox running with serial I/O
Kext support: rebuild with proper kext loading infrastructure
Native x86_64 build: currently cross-compiles, should compile natively too
DMG installer: boot from a DMG on real hardware via development kernel

Credit

darwin-xnu-build — upstream build scripts that this project wraps
PureDarwin — the original vision
NixThePlanet — Nix-based OS virtualization
kernelshaman

Legal

Darnix is an independent project and is not affiliated with, endorsed by, or sponsored by Apple Inc. Apple, macOS, and related trademarks are the property of Apple Inc., registered in the U.S. and other countries. Darwin is licensed under the Apple Public Source License (APSL). All other trademarks are the property of their respective owners.