Snap not seeing serial port passthrough in LXD container

LXD
1

Hi there,

I have a problem with a snap running within an LXD container, the home-assistant-snap to be precise. The host system is Ubuntu 20.04, as is the container (`lxc launch ubuntu:20.04).

I am trying to pass through a SkyConnect USB dongle to the container. The dongle is actually a USB-UART device, and on the host machine appears as /dev/ttyUSB0. Accordingly, I pass through the device using

$ lxc config device add homeassistant ttyUSB0 unix-char source=/dev/serial/by-id/usb-Nabu_Casa_SkyConnect_v1.0_9ccaf1470741ed11acce7ea7ccf2b06c-if00-port0 path=/dev/ttyUSB0 mode=666 gid=20

On the host machine, if I install the home-assistant-snap and follow the instructions to enable hotplug, I can see the serial device pop up as a slot

$ snap interface serial-port 
name:    serial-port
summary: allows accessing a specific serial port
plugs:
  - home-assistant-snap
slots:
  - snapd:skyconnectv10 (allows accessing a specific serial port)

However, within the LXD container although the /dev/ttyUSB0 device is present, there are no slots shown

# snap interface serial-port 
name:    serial-port
summary: allows accessing a specific serial port
plugs:
  - home-assistant-snap

thus any attempt to connect the Home Assistant to a serial-port slot fails. The result is that I cannot get Home Assistant to talk to the SkyConnect dongle.

Is this is restriction of the LXD container, or might I be missing a permissions thing?

Many thanks!

2

Most likely snapd relies on udevd to list the devices and udevd doesn’t know how to find those devices passed to containers.

There’s a good chance that using unix-hotplug would work as that will cause udev to be notified, but the annoying part is that for it to work, a hotplug event must be triggered, which means that you need to unplug and replug your USB device after starting the container.

3

Thanks Stéphane, your comments jogged my mind. It turns out that connecting the home-assistant-snap to raw-usb and hardware-observe does the trick – presumably as you say because of the udevd thing.

For others with the same issue, I found this very helpful post by the maintainer of the home-assistant-snap about a similar (though not identical!) problem.

Thanks again, Stéphane for the wonderful work on LXD!

4

Glad that did the trick!