What is this?

An experiment, currently, to see if Nixpkgs is a good way to build an OS for a domestic wifi router or IoT device, of the kind that OpenWrt or DD-WRT or Tomato run on.

This is not NixOS-on-your-router. This is an immutable "turnkey" image that can be flashed onto a router (or other IoT device), built using the Nix language and the Nix package collection.

"Supported" hardware

We use the OpenWrt kernel sources (approximately), so it should be not impossible to get anything working that they have already ported to. I use

• devices based on Mediatek MT7620 and MT7628 (GL-MT300A and GL-MT300N-v2)
• devices based on Atheros ath79 (as of Oct 2020, GL-AR750)
• Qemu, for quick and easy testing of userland changes without real hardware

Previously we built on some ar71xx devices as well (Trendnet TEW-731BR/Atheros AR9341 and Arduino Yun/AR9331) but support for those has not been brought forwards to kernel 5.x as I don't have hardware (or inclination) to test them. Anything that was previously supported by ar71xx should be buildable with ath79, but may require more or less faff to port depending on whether someone else has written the device tree for it already.

Applications and use cases (former, current and prospective)

• Working: Rsync backup server (see examples/arhcive/)

• Working: Wireless extender (see examples/{extensino,upstaisr}

• WIP: PPPoE router/access point (examples/defalutroute)

What's it made of?

The Nix Package Collection

As of October 2021 it has most recently been built nixpkgs master git rev 2deb07f3ac4, which at that time was the latest commit on nixos-unstable to have built on Hydra. If you're using a later or earlier version, your mileage may vary.

To upgrade/downgrade the revision of nixpkgs that we build against, edit the definition of nixpkgsTarball in nixwrt/default.nix

a Nixpkgs overlay

In nixwrt/overlay.nix we create a couple of new derivations that don't exist in nixpkgs, and customize a few others for smaller size or to fix cross-compilation bugs. Where possible and when time is available these changes will be pushed upstream.

a module-based configuration system

A NixWRT image is created by passing a configuration attrset to a derivation that creates a root filesystem image based on that configuration, then does further work to build a suitable kernel and glue it all together into a binary that you can flash onto your router.

Conventionally, you create that configuration by starting with a fairly bare bones attrset that defines a few files and network interface names, then pass it in turn to a number of modules which augment it with the features your application needs. There are a number of pre-written modules to add support for things like ntpd and ssh server, more will be added over time (that's not a promise, but somewhere between a prediction and a prophecy), and you can write your own (details later). If you write your own and then send pull requests, you will have helped fulfill the prophecy.

How to build it

One-time setup

You will need

• a target device (unless you only want to run it on Qemu). I have used various of the cheap "travel routers" from GL.iNet because they're cheap and hobbyist-friendly and they have adequate RAM.

• some kind of PC or other reasonably well-powered machine to build everything on. This is entirely cross-compiled, there is no development on the target. I do it all under NixOS, but any system with Nix installed should work - I tried it successfully in a Docker container, for example

• an ethernet connecting your build machine to your target device. Perhaps you can put them on a LAN together, perhaps you can connect them directly to each other with a patch cable. (The latter is a good idea if you plan to test things like DHCP servers on the target, otherwise they may start answering IP address requests for other hosts on your LAN). Ideally you want statically allocated IP addresses for the build machine and target, because U-Boot probably won't work with DHCP.

  • Provided without warranty is nol.nix, a script I use on my build machine to provide better isolation between my real LAN and my test network. It generates a QEMU VM which I run with PCI passthru so that it has exclusive access to my second network card. It may or may not work for you, but feel free to adapt or use it for inspiration

• access to your target devices's boot monitor (usually U-Boot). This will very often involve opening it up and attaching a USB serial convertor to some header pins: sometimes it involves soldering those pins into holes. On other devices it's not nearly as complicated as you can access U-Boot across the network. The OpenWrt wiki is often very helpful here.

Clone the nixwrt repo:

$ git clone git@github.com:telent/nixwrt
$ cd nixwrt

The best way to get started is to look at one of the examples in examples/ and choose the one which has most similar hardware to the device you want to use and ideally which has most recently been updated. There should be advisory and/or warning comments at the top of each.

Each example consists of two files:

• config.nix contains a base configuration and an array of "modules" which are merged into it

• device.nix names a device (router make/model) which should correspond to one of the keys in nixwrt/devices/default.nix

Build it

There is a Makefile to help you get started on building any of the examples. To build the extensino example, run

$ cp secrets.example secrets
$ $EDITOR secrets # update SSID and other details
$ make extensino image=phramware

This should create a file out/extensino/firmware.bin which you need to copy to your TFTP server.

Caveat: the makefile is a convenience thing for hacking/testing and not intended as the nucleus of any kind of production build pipeline. If you want something to build on for large-scale deploys, write something that invokes nix-build directly.

Running it from RAM

The image you just built is configured to run from RAM without needing to write to the router flash memory. This is great when you're testing things and don't want to keep erasing the flash (because it takes a long time and because it has limited write cycles). It's not great when you want to do a permanent installation, because the router RAM contents don't survive a reset. It uses the phram driver to emulate flash using system RAM.

Instructions vary depending on your device, but on my GL-Inet MT300N v2, I reset the router, hit RETURN when it says

Hit any key to stop autoboot: 2

and then type the following commands at the uboot gl-mt300an> prompt:

setenv serverip 192.168.0.2
setenv ipaddr 192.168.0.251
setenv startaddr a00000
setenv startaddr_useg 0x${startaddr}
setenv startaddr_ks0 0x8${startaddr}
setenv dir /tftp/extensino
tftp ${startaddr_ks0} ${dir}/firmware.bin ; bootm ${startaddr_useg}

Depending on your network and tftp configuration, you probably need to change IP addresses and paths here. The startaddr must be some location in ordinary RAM (i.e. not flash) that doesn't conflict with the area starting at 0x6000 to which the kernel is uncompressed. 0xa00000 (and 0x8a00000 which is the same physical RAM but differently mapped) seems to do the job.

Making it permanent (flashable image)

If you're sure you want to toast a perfectly good OpenWrt installation ... read on. I accept no responsibility for anything bad that might happen as a result of following these instructions.

The moderately straightforward way

If you have a working NixWRT with a running ssh daemon (usually by including the sshd module) and the flashcp busybox app (currently this is installed by default) you can install a new image from inside the running system without recourse to any U-Boot/serial connection shenanigans. This is a win when you've deployed the device and don't wish to pop the top off.

Step 1: Build the regular (non-phram) firmware

$ make extensino SSID=mysid PSK=db6c814b6a96464e1fa02efabb240ce8ceb490ddce54e6dbd4fac2f35e8184ae image=firmware

Step 2: copy it onto the device

$ cat extensino/firmware.bin | ssh root@extensino.lan 'cat > /tmp/nixwrt.bin'

Step 3: ssh into the device and write it to the firmware mtd partition. Note: the real firmware partition, not the emulated phram one.

# cat /proc/mtd
dev:    size   erasesize  name
mtd0: 00030000 00001000 "u-boot"
mtd1: 00010000 00001000 "u-boot-env"
mtd2: 00010000 00001000 "factory"
mtd3: 00f80000 00001000 "firmware"
mtd4: 00220000 00001000 "kernel"
mtd5: 00d60000 00001000 "rootfs"
mtd6: 00b57000 00001000 "rootfs_data"
mtd7: 00010000 00001000 "art"

# flashcp -v /tmp/nixwrt.bin /dev/mtd3

Step 4: reboot the device

# reboot

The complicated way

This procedure is a good way to brick your router if you get it wrong. Do not follow it blindly without making some attempt to understand if it'll work for you. There are a number of magic numbers which are most likely correct if you have the same hardware as I have and almost certainly incorrect if you don't.

Find the flash address

You will need to find the address of your flash chip. If you don't know you can probably make a reasonable guess: either use the U-boot flinfo command if your router has it, or otherwise my suggestion is to look at the boot log for a line of the form Booting image at 9f070000 and then double check by looking at the output of cat /proc/mtd in OpenWrt and see if there's a partition starting at 0x70000. If you get this wrong you may brick your device, of course.

Build the regular (non-phram) firmware

$ make extensino SSID=mysid PSK=db6c814b6a96464e1fa02efabb240ce8ceb490ddce54e6dbd4fac2f35e8184ae image=firmware

Now do whatever you need to make it available to the TFTP server.

Flash it

Get into u-boot, then do something like this

setenv serverip 192.168.0.2
setenv ipaddr 192.168.0.251
erase 0xbc050000 0xbcfd0000
setenv dir /tftp/extensino
tftp 0x80060000 ${dir}/firmware.bin
cp.b 0x80060000 0xbc050000 ${filesize}

The magic numbers here are

• 0x80060000 : somewhere in RAM, not critical
• 0xbc050000 : flash memory address for "firmware" partition (as per nixwrt/devices.nix)
• 0xbcfd0000 : end of flash firmware partition image

If that looked like it worked, type reset to find out if you were right.

Running it in Qemu

The build process for Qemu is subtly different because Qemu wants an ELF kernel image and a root filesystem instead of a combined firmware image, and also because Qemu doesn't appear to support device trees.

$ make emu SSID=mysid PSK=whatevs  LOGHOST=loghost.lan image=emulator
$ ./out/emu

Troubleshooting

• if the kernel boots but gets stuck where the userland should be starting, you could try changing init=/bin/init to init=/bin/sh. Sometimes the ersatz edifice of string glommeration that creates the contents of /etc goes wrong and generates broken files or empty files or no files. This will give you a root shell on the console with which you can poke around

• or use Binwalk to unpack the image on the host

• There is a syslog module: if it seems to work mostly but services are failing and you think they may be generating error messages, add the syslog module to your config and point it at a syslog server. Configuring the syslog server is outside the scope of this README, but essentially it needs to be able to receive UDP on port 514. I use RSYSLOG: other choices are available.

• I find a remote-controlled power switch is invaluable. You might too. See here or here

Feedback

Is very welcome. Please open an issue on Github for anything that involves more than a line of text, or find me in the "Fediverse" @dan@terse.telent.net (preferred) or on Twitter @telent_net (less preferred) if not.

I do occasionally hang out on #nixos IRC as dan_b or as telent but not often enough to make it a good way of getting in touch.