How I put Gentoo on my ASUS router

[Zeault]

I created this post asking some questions about putting Gentoo on my router a while back. I have since found a solution and I thought I would share what I did with the forums.

I was preparing to overwrite the onboard flash with my new image, but I was growing apprehensive. I had access to the system mtd-write tool through ssh, but it wasn't working and was compiled without any error messages. I then came up with the idea of building an image similar to those provided by ASUS that I could feed into the regular firmware update utility. I didn't really want to do this so I checked around the community one more time for alternative ideas.

For some background: I used the router for a good long time, but I was outgrowing it a little, and I wanted some additional capabilities like split DNS and VLANs which the stock firmware cannot do. I would have flashed the router with one of the open source firmwares like OpenWRT, AsusWRT-Merlin, or DD-WRT but somehow I accidentally purchased one of the few models not supported by these firmwares because of its proprietary Broadcom kernel and modules. So here we are.

I found a comment on the small net builder forums that there is an nvram variable on most asus routers called script_usbmount. Setting it to a path tells the router to execute the script located at the path when a USB drive is inserted. Now I had a real plan.

I built a crossdev prefix root for the architecture of the router: armv7l-unknown-linux-musleabi. I cross built all the tools I wanted, and then a few more I wanted to try out. I made sure chroot was built as a statically-linked executable. I fleshed out the prefix a little more with the FHS directories like /run, /tmp, /mnt, ... I even installed openrc despite it complaining a little bit. Finally I ran "rsync -a --del . /mnt/gentoo" which copied all of the file onto a USB flash drive formatted with ext3 because that was one of the filesystems the router had builtin.

I ssh'd back into the router and saw that the usb device was mounted automatically and my script_usbmount file executed successfully. After some fiddling around I was able to use the static chroot executable to run the dynamically linked /bin/bash using musl libc. From there I fiddled around some more and tried to get OpenRC working.

OpenRC makes it clear that you are not really supposed to use it on a system that wasn't booted by OpenRC, however if you create the file /run/openrc/softlevel it will stop complaining and go ahead anyways. I had to remove almost every single service file from the runlevels because they would conflict with the router which was already booted. I left swap in the sysinit runlevel and that was it. I wanted to run dnsmasq and metalog but they depended on other virtual services which I had removed.

There was also another problem. The ASUS router already runs its own version of dnsmasq and other daemons which will block the operation of my daemons. I simply created a service called /etc/init.d/usurp which killed these old services and also "provided" the virtual services like net and localmount which I had removed. I added the usurp service to the boot runlevel. Here it is:

Code: Select all

#!/usr/sbin/openrc-run
# Copyright (c) 2007-2015 The OpenRC Authors.
# See the Authors file at the top-level directory of this distribution and
# https://github.com/OpenRC/openrc/blob/HEAD/AUTHORS
#
# This file is part of OpenRC. It is subject to the license terms in
# the LICENSE file found in the top-level directory of this
# distribution and at https://github.com/OpenRC/openrc/blob/HEAD/LICENSE
# This file may not be copied, modified, propagated, or distributed
# except according to the terms contained in the LICENSE file.

description="Finalizes setup of chroot and stuff"

depend()
{
	provide localmount net netmount
}

start()
{
	# Kill processes we are going to replace from the old operating environment
	if [ -f "/run/dnsmasq.pid" ] ; then
		kill $(cat "/run/dnsmasq.pid")
		rm "/run/dnsmasq.pid"
	fi
	if [ -f "/run/syslogd.pid" ] ; then
		kill $(cat "/run/syslogd.pid")
		rm "/run/syslogd.pid"
	fi
}

stop()
{
  return 0;
}

This worked if I started my daemons right away, but the ASUS router comes with a watchdog process. I found out through some trial and error that the watchdog just scans the /proc directory to see if a process with a similar name is still alive and restarts it if it isn't. This means my replacement daemons would run just fine as long as they won the race between the killing of the old daemon and their own startup. I added another few lines to /etc/init.d/dnsmasq just to double check that the old daemon was dead and kill it again if it isn't.

Finally I backed out of the chroot and changed script_usbmount to this:

Code: Select all

#!/bin/sh

PREFIX="/tmp/mnt/gentoo"

# Wait if the mount point isn't actually ready yet
if [ ! -d "${PREFIX}" ] ; then
	sleep 5
fi

if [ ! -d "${PREFIX}" ] ; then
	echo "Error: prefix not available." >> /jffs/error.txt
fi

# Busybox mount syntax
mount -t proc /proc "${PREFIX}/proc/"
mount -o rbind,rslave /sys "${PREFIX}/sys"
mount -o rbind,rslave /dev "${PREFIX}/dev"
mount -o bind /tmp "${PREFIX}/mnt/tmproot"
mount -o bind /var/run "${PREFIX}/run"

# Tell OpenRC it is not running in a sane environment
mkdir -p "${PREFIX}/run/openrc/"
touch "${PREFIX}/run/openrc/softlevel"

"${PREFIX}/bin/chroot" "${PREFIX}" /usr/sbin/openrc default

I saved it on the small jffs partition of the router which persists between reboots.

All that done and I'm surprised but it actually works. Of course it didn't work on the first try but its running now. And anytime I want to go back to stock configuration I can just umount and remove the usb drive.