Updated Pi Zero Gadgets

Following on from my last post I’ve continued to work on improving my instructions for a USB connectable gadget based on a Raspberry Pi Zero.

Firstly I’ve got a slight improvement to the dnsmasq config I mentioned last time. This removes the dnsmasq.leases file which can cause issues if you plug the Zero into multiple computers. This can be a problem because while I had managed to fix the mac address for Host computer end of the connection the OS still pushes down the host name and it’s own unique id when it requests a DHCP address from dnsmasq, this causes dnsmasq to cycle through it’s small pool of addresses. This combined with the fact the clock on Zero is not battery backed up so only gets set correctly when it can reach internet can cause strangeness with addresses getting expired in strange ways. Anyway there is a pretty simple fix.

Adding leasefile-ro to the dnsmasq config causes it to not write lease information to disk, but rely on the dhcp-script to keep track of things. To do this I needed to add handling for a new option to the script to deal with dnsmasq wanting to read the current lease state at startup.


if [[ $op == "init" ]]; then
  exit 0

if [[ $op == "add" ]] || [[ $op == "old" ]]; then
  route add default gw $ip usb0

Now on to getting things working better with Windows machines.

To do this properly we need to move from the g_ether to the g_multi kernel module, this lets the Zero be a USB Mass Storage device, a network device (and a serial device) at the same time. This is useful because it lets me bundle .inf files that tell Windows which drivers to use on the device it’s self so it they can be installed just by plugging it in.

The first order of business is to fix the cmdline.txt to load the right module, after making the changes in the last post it looks like this:

dwc_otg.lpm_enable=0 console=serial0,115200 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=deadline fsck.repair=yes rootwait modules-load=dwc2,g_ether

The g_ether needs replacing with g_multi so it looks like this:

dwc_otg.lpm_enable=0 console=serial0,115200 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=deadline fsck.repair=yes rootwait modules-load=dwc2,g_multi

next we need to fix the options passed to the module, these are in the /etc/modprobe.d directory probably in a file called g_ether.conf. We should don’t need to, but to make it obvious when we come back to this a lot later on we’ll rename it to g_multi.conf. Now we’ve renamed it we need to add a few more options.

It currently looks like this:

options g_ether host_addr=00:dc:c8:f7:75:15 dev_addr=00:dd:dc:eb:6d:a1

It needs the g_ether needs changing to e_multi and some options adding to point to a disk image.

options g_multi file=/opt/disk.iso cdrom=y ro=y host_addr=00:dc:c8:f7:75:15 dev_addr=00:dd:dc:eb:6d:a1

Now we have the config all sorted we just need to build the disk image. First we need to create a directory called disk-image to use as the root of the file system we will share from the Zero. Then we need to get hold of the 2 .inf files, they ship with the Linux Kernel doc, but can be found online (serial port, RNDIS Ethernet).

Once we have the files place them in a directory called disk-image/drivers. We should also create a README.txt to explain what’s going on, put that in the root of disk-image. Along side that we want to create a file called Autorun.inf, this tell Windows about what’s on the “cd” when it’s inserted and where it should search for the driver definitions.




Full details of what can go in the Autorun.inf file can be found here, but the interesting bits are the DriverPath=drivers which points to the directory that we put the .inf files in earlier. Also the open=”documentation\index.html” which should open documentation/index.html when the device is plugged in which explains how to install the drivers. I also added an icon file so the drive shows up looking like a clock in the file manager.

That should be the bare minimum that needs to be on the image, but I ran into an issue with the g_multi module complaining the disk image being too small, to get round this I just stuck a 4mb image in the directory as well. To build the iso image run the following command:

mkisofs -o disk.iso -r -J -V "Zero Clock" disk-image

This will output a file called disk.iso that you should copy to /opt/disk.iso on the Zero (I built the image on my laptop as it’s easier to edit files and mkisofs is not installed in the default raspbian image).

This is working well on Linux and Windows, but I’m still getting errors on OSx to do with the file system. It’s not helped by the fact I don’t have a Mac to test on so I end up having to find friends that will let me stick a random but of hardware in to the side of their MacBook.

Once I’ve got the OSx bits sorted out I’ll put together script to build images for anything you want.

So now we have a Pi Zero that should look like a CD-ROM drive and a Network adapter when plugged into pretty much any machine going, it brings the driver information with it for windows, sets up a network connection with a static IP address and a Avahi/Bonjour/mDNS address to access it. I’m planning on using this to set up my Linear Clock on the local WiFi but there are all manner of interesting things that could be done with a device like this. e.g. an offline Certificate Authority, a 2FA token, a Hardware VPN solution or a Web Controllable display device.