README.nixwrt.md

What is it?

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

• nixwrt.nix contains the derivation which will eventually produce a firmware router image

• everything else is a lightly forked (I hope and expect that I can upstream it) nixpkgs with a few changes I've had to make for cross-compiling some packages

Milestones/initial use cases

• Milestone 0 ("what I came in for"): backup server on TL-WR842 with attached USB disk.

• Milestone 1: replace the wireless access point in the study (Trendnet something-or-other)

• Milestone 2: IP camera with motion detection on Raspberry Pi (note this is ARM not MIPS)

• Milestone 3: replace the router attached to my DSL modem ("GL-MT300N":https://www.gl-inet.com/mt300n/ which is a relink device not ar9xxx)

Status/TODO

Using QEMU

• builds a kernel
• builds a root filesystem
• mounts the root filesystem
• statically linked init (busybox) runs
• make shared libraries work
• convert to musl or uclibc
• reasonable user environment

On real hardware

• builds a kernel
• builds a root filesystem
• mounts the root filesystem
• ethernet driver
• bring the network up at boot
• wireless
• run some services
• route some packets

You can follow progress (or lack of) in my blog: start with https://ww.telent.net/2017/12/27/all_mipsy_were_the_borogroves and follow the 'next week' links at the bottom of each post.

How to run it

With QEMU

[ As of Mon Feb 5 23:35:51 2018 this is most likely broken. You need to create a malta.nix that looks similar but not identical to yun.nix and improvised on a theme of the instructions for "real hardware" ]

nix-build ./nixwrt.nix -A tftproot -o malta --argstr target malta
nix-shell  -p qemu --run "qemu-system-mips -M malta -m 64 -nographic -kernel malta/vmlinux   -append 'root=/dev/sr0 console=ttyS0 init=/bin/sh' -blockdev driver=file,node-name=squashed,read-only=on,filename=malta/rootfs.image -blockdev driver=raw,node-name=rootfs,file=squashed,read-only=on -device ide-cd,drive=rootfs -nographic" 

On real hardware

This is a little more complicated ...

Preparation

• Get an Arduino Yun: this is the initial target for no better reason than that I have one and the USB device interface on the Atmega side makes it easy to test with. The Yun is logically a traditional Arduino bolted onto an Atheros 9331 by means of a two-wire serial connection: we target the Atheros SoC and use the Arduino MCU as a USB/serial converter. The downside of this SoC is that mainstream Linux (4.14.x) has no support for its Ethernet device, but that seems to be true of most MIPS targets.

• In order to talk to the Atheros over a serial connection, upload https://www.arduino.cc/en/Tutorial/YunSerialTerminal to your Yun using the standard Arduino IDE. Once the sketch is running, rather than using the Arduino serial monitor as it suggests, I run minicom on /dev/ttyACM0

• install a TFTP server (most convenient if this is on your build machine itself)

• acquire a static IP address for your Yun, and find out the address of your TFTP server. In my case these are 192.168.0.251 and 192.168.0.2

Installation

Build the derivation and copy the result into your tftp server data directory:

nix-build yun.nix -A tftproot -o yun
rsync -cIa yun/ /tftp/ # rync should ignore timestamps when comparing

On a serial connection to the Yun, get into the U-Boot monitor (hit YUN RST button, then press RET a couple of times - or in newer U-Boot versions you need to type ard very quickly - https://www.arduino.cc/en/Tutorial/YunUBootReflash may help) Once you have the ar7240> prompt, run

setenv serverip 192.168.0.2 
setenv ipaddr 192.168.0.251 
setenv kernaddr 0x81000000
setenv rootaddr 1178000
setenv rootaddr_useg 0x$rootaddr
setenv rootaddr_ks0 0x8$rootaddr
setenv bootargs  console=ttyATH0,115200 panic=10 oops=panic init=/bin/init phram.phram=rootfs,$rootaddr_ks0,10Mi root=/dev/mtdblock0 memmap=11M\$$rootaddr_useg ath79-wdt.from_boot=n ath79-wdt.timeout=30 ethaddr=90:A2:DA:F9:07:5A machtype=AP121
setenv bootn " tftp $rootaddr_ks0 /tftp/rootfs.image; tftp $kernaddr /tftp/kernel.image ; bootm  $kernaddr"
run bootn

substituting your own IP addresses where appropriate. The constraints on memory addresses are as follows

• the kernel and root images don't overlap, nor does anything encroach on the area starting at 0x8006000 where the kernel will be uncompressed to
• the memmap parameter in bootargs should cover the whole rootfs image

If the output changes to gibberish partway through bootup, this is because the kernel serial driver is running at a different speed to U-Boot, and you need to change it (if using the YunSerialTerminal sketch, by pressing ~1 or something along those lines).

If it doesn't work, 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
• changing ath79-wdt.from_boot=n to ath79-wdt.from_boot=y: this will cause the board to reboot after 21 seconds, which is handy if it's wedging during the boot process - especially if you're not physically colocated with it.

Feedback

Is welcome. I'm subscribed to the nix-devel list and will see messages there. I'm @telent_net on Twitter if that suits better, and I occasionally show up as dan_b or some variant of that name on #nixos IRC.