What

It's a script to build a customized OpenWrt firmware image using ImageBuilder.

If the generated image is flashed on a router, then during its boot process it will try to set up extroot on any (!) storage device plugged into the USB port. Put another way, it unconditionally reformats /dev/sda if it fails to mount an extroot early in the boot process.

Why

So that e.g. customers can buy a router on their own, download and flash our custom firmware, plug in a pendrive, and manage their SIP (telephony) node from our webapp.

I've extracted the generic parts from the above mentioned auto-provision project because I thought it's useful enough for making it public.

It also serves me well on my own routers ever since then.

How

You can read more about the underlying technology on the OpenWrt wiki: see e.g. the ImageBuilder page, or the page that lists some other ImageBuilder frontends.

As for the actual mechanism: custom scripts are baked into the boot process of the flashed firmware. If the extroot overlay is properly set up, then these scripts get hidden by it; i.e. they will only be run when the extroot has failed to mount early in the boot process.

Keep in mind that this will automatically erase/format any inserted storage device while the router is in the initial setup phase! Unfortunately there's little that can be done at that point to ask the user for confirmation.

Building

OpenWrt's ImageBuilder only works on Linux x86_64. To build a firmware, issue the following command: ./build.sh architecture variant device-profile, e.g.:

• ./build.sh ath79 generic tplink_tl-wr1043nd-v1
• ./build.sh ath79 generic tplink_archer-c6-v2
• ./build.sh ath79 generic tplink_tl-wdr4300-v1
• ./build.sh bcm53xx generic dlink_dir-885l

Results will be under build/openwrt-imagebuilder-${release}-${architecture}-${variant}.Linux-x86_64/bin/.

To see a list of available targets, run make info in the ImageBuilder dir.

If you want to change which OpenWrt version is used, then try editing the relevant variable(s) in build.sh. It's not guaranteed to work across OpenWrt releases, therefore we keep git branches for the past releases.

Setup stages

Blinking leds show which phase the extroot setup scripts are in. Consult the sources for details: autoprovision-functions.sh.

Stage 1: setup extroot

When the custom firmware first boots, the autoprovision script will wait for anything (!) in /dev/sda to show up (that is >= 512M), then erase it and set up a swap, an extroot, and a datafilesystem (for the remaining space), and then reboot.

Stage 2: download and install some packages from the internet

Once it rebooted into the new extroot, it will continuously keep trying to install some OpenWrt packages until an internet connection is set up on the router. You need to do that manually either by using ssh or the web UI (LuCI).

Stage 3, optional

We also have a 3rd stage, written in Python, but it's commented out here. Search for autoprovision-stage3.py to see how it's done.

Login

After flashing the firmware the router will have the standard 192.168.1.1 IP address.

By default the root passwd is not set, so the router will start telnet with no password. If you want to set up a password, then edit the stage 2 script: autoprovision-stage2.sh.

If a password is set, then telnet is disabled by OpenWrt and SSH will listen using the keys specified in authorized_keys.

Once connected, you can read the log with logread -f.

Status

This is more of a template than something standalone, but I use it for my home routers as is for years now. For more specific applications you most probably want to customize this script here and there; search for CUSTOMIZE for places of interest.

Most importantly, set up a password and maybe add your ssh key by adding it to image-extras/common/etc/dropbear/authorized_keys.

None of this script is hardware specific except setLedAttribute, which is used to provide feedback about the progress of the initial setup phase. At the time of writing it only works on a few routers (mostly ath79 ones), but without this everything should work fine, if only a bit less convenient.

Troubleshooting

Which file should I flash?

You should consult the OpenWrt documentation. The produced firmware files should be somewhere around ./build/openwrt-imagebuilder-21.02.0-ath79-generic.Linux-x86_64/bin/targets/ath79/generic/.

In short:

• You need a file with the name -factory.bin or -sysupgrade.bin. The former is to be used when you first install OpenWrt, the latter is when you upgrade an already installed OpenWrt.

• You must carefully pick the proper firmware file for your hardware version! I advise you to look up the wiki page for your hardware on the OpenWrt wiki, because most of them have a table of the released hardware versions with comments on their status (sometimes new hardware revisions are only supported by the latest OpenWrt, which is not released yet).

Help! The build has finished but there's no firmware file!

If the build doesn't yield a firmware file (*-factory.bin and/or *-sysupgrade.bin): when there's not enough space in the flash memory of the target device to install everything then the OpenWrt ImageBuilder prints a hardly visible error into its flow of output and silently continues. Look into build.sh and try to remove some packages that you can live without.

Extroot is not mounted after a sysupgrade

In short, this is an OpenWrt issue, and the solution is to mount the extroot somewhere, and delete /etc/.extroot-uuid. More details are available in this issue, and a way to deal with it can be found in this blog post. You may also want to check out the official OpenWrt wiki on this topic.