Building Custom Raspberry Pi SD Card Images

After my post about using a Raspberry Pi 4 as a USB gadget got linked to by a YouTuber who worked out it also worked with the iPad Pro it has been getting a lot of traffic.

Along with the traffic came a number of comments from people wanting help setting things up. While the instructions are reasonably complete, they do assume a certain amount of existing knowledge and access to a monitor/keyboard/network to complete everything. Also given the majority of the readers were Apple users they couldn’t mount the main partition of the SDCard as it is a ext4 file system.

The quick and dirty solution is for me to modify a standard image and host it somewhere. This is OK, but it means I have to,

• Find somewhere to host a 500mb file
• Keep it up to date when new versions are released
• Provide some way for people to trust what changes I’ve made to the image

I can probably get round the first one pretty easily, bandwidth is a lot cheaper than it used to be. The second and third items are a little harder.

I started to look at a way to script the modifications to a standard Raspbian image, that way I could just host the script and people could provide their own starting image. On a Linux machine I could mount both partitions on the card and modify or add the required config files, but the problem was installing dnsmasq. This needs the image to actually be running for apt-get to run, which gets back to the chicken/egg problem of needing to boot the pi order to make the changes. That and it would only run on Linux, not a OSx or Windows machine.

What I need is a way to “run” a virtual Raspberry Pi and a way to run commands in that virtual machine. I know from working with services like Balena.io‘s build system that it is possible to emulate the ARM processor found in a Raspberry Pi on Intel based hardware. I found a couple of examples of people using Qemu to run virtual Pis and I was just about to set up the same when I came across dockerpi, which is a docker image with everything preconfigured. You can mount a SD card image

When started you end up with what is basically the virtual machine console as a command line app. You can interact with it just like any other console application. I logged in as pi and then used sudo to run apt-get update and apt-get install dnsmasq.

That works for doing it manually, but I need to script this, so it’s time to break out some old school Linux foo and use expect.

Expect is a scripting tool that reads from stdin, and outputs to stdout, but will wait for a known output before sending a reply. It was used in the early days of the Internet to script dial up internet.

#!/usr/bin/expect -f
set timeout -1
set imageName [lindex $argv 0]
if {[string trimleft $imageName] eq ""} {
  puts "No Image file provided"
  exit
}
set cwd [file normalize [file dirname $argv0]]
set imagePath [file join $cwd $imageName]
spawn docker run -i --rm -v $imagePath:/sdcard/filesystem.img lukechilds/dockerpi:vm
expect "login: "
send "pi\n"
expect "Password: "
send "raspberry\n"
interact

This expect script takes the name of the SD Card image as an argument, starts the Docker container and then logs in with the default pi/raspberry username & password.

With a bit more work we can get all all the changes done including creating the extra files.

...
proc slurp {file} {
    set fh [open $file r]
    set ret [read $fh]
    close $fh
    return $ret
}
...
set file [slurp "etc/network/interfaces.d/usb0"]
expect "# "
send "cat <<EOF >> /etc/network/interfaces.d/usb0\n"
send "$file\n"
send "EOF\n"
...

I’ve checked all the files into a github repository here. I’ve tested the output with a Pi Zero and things look good. To run it for yourself, clone the repo, copy the Raspbian Lite image (unzip it first) into the directory and run ./create-image <image file name>

There is a version of the output from here.

I’ve still got to get round to trying to the RNDIS Ethernet device support working so it will load the right drivers on Windows. And I need to extend the script to build the Personal CA Appliance from my last post.