Getting a Raspberry Pi to Pass BCT June 17, 2020Over the past few weeks I have been working with Yocto, specifically to put together a layer containing PAPPL and other necessary bits to build headless Raspberry Pi-based print servers. And since I’m a standards guy I want this setup to more-or-less pass all of the network certification tests, including Apple’s Bonjour Conformance Tests. Unfortunately, this has taken a bit longer than I’d hoped…
I’ve been using a Raspberry Pi Zero W and a Rasberry Pi 3 Model B for my testing. These use the same wireless chip (a Broadcom BCM43438) but the ‘Pi 3 also offers 100baseT Ethernet for the wired network tests.
My iMac Pro serves as the build machine (running Parallels with an Ubuntu 18.04 VM), and my 16” MacBook Pro serves as the BCT test machine. Both the MacBook Pro and the Raspberry Pi under test are connected to a 4th generation Apple TimeCapsule (802.11a/b/g/n) base station for the tests.
I’m using Yocto to create custom Linux images for the Raspberry Pi’s. For those not familiar with it, Yocto is an embedded Linux development environment that can be configured to support a wide variety of embedded processing boards and/or virtual machine environments like QEMU.
Yocto is organized into “images” which are the final OS images you put on your board, “layers” which are logical pieces of the OS such as core, networking, multimedia, Board Support Packages (“BSPs”), etc., and “recipes” which represent individual packages or components for the OS. You can customize all of these things, and everything gets compiled from source code or (if you are very lucky) can be found in an official pre-compiled repository hosted by the Yocto project. Building a Yocto image for the first time takes several hours, even on my iMac Pro…
I’ve been working with the “Zeus” branch (which until recently was the newest) and the official meta-raspberrypi BSP layer. To that I’ve updated the Avahi, CUPS, and dhcpcd “recipes” to pull in the current versions of each.
The two public certification tests I’ve been focusing on have been the PWG’s IPP Everywhere™ Printer Self-Certification Tools and Apple’s Bonjour Conformance Test (“BCT”). PAPPL easily passes the IPP Everywhere tests, but getting the BCT to pass has been a major challenge thanks to some interesting bugs/features in the wireless drivers and firmware used for the Raspberry Pi.
The Wi-Fi chipset used in Raspberry Pi boards is a “FullMAC” implementation - the chipset handles all of the low-level Wi-Fi communications, power management, reading/writing of packets, etc. and just passes on the (processed) information to the kernel driver. So this means the Wi-Fi chip firmware needs to know how to handle low-level networking details and whether a given network packet that is received over Wi-Fi needs to be passed on to the kernel. This is where the problems begin.
The first part of the BCT is a test of IPv4 link-local address handling. With IPv4, link-local addresses (169.254.0.0/16) are typically only used when you don’t have any working Internet infrastructure (more specifically, when there isn’t a working DHCP server). Each network device will try a series of semi-random addresses when it first comes on the network to find an available address. The underlying protocol is defined in RFC 3927 and uses Address Resolution Protocol (“ARP”) packets - a device asks whether any other device on the network is using a particular IP address, and if nothing answers after three attempts it then claims the address by announcing it a few times.
This query process is where things break with the default wireless settings and the Raspberry Pi wireless firmware. ARP packets are not addressed to a particular network device and they really need to be passed on to the kernel for processing. The current firmware for the Raspberry Pi Wi-Fi chipset does not do this, however, preventing the link-local queries from working. In addition, the power saving mode puts the Wi-Fi chipset to sleep in the middle of the BCT tests which run for several hours!
After much research and experimentation, I ended up writing a small script to disable power management and put the Wi-Fi interface in “promiscuous” mode so that the Wi-Fi firmware sends all packets to the kernel and not just the ones with its local addresses:
"set-wireless-options.sh"
#!/bin/sh
iw dev wlan0 set power_save off
ifconfig wlan0 promisc
I call this script from the /etc/network/interfaces configuration file:
"/etc/network/interfaces" (snippet)
# Wireless interface
auto wlan0
iface wlan0 inet dhcp
wireless_mode managed
wireless_essid any
wpa-driver nl80211
wpa-conf /etc/wpa_supplicant.conf
# Replace "/PATH/TO" with the actual path to the script...
pre-up /PATH/TO/set-wireless-options.sh
With those changes I can get the Raspberry Pi Zero W to pass BCT.
At some point I will be making my Yocto layer public on Github, complete with the necessary recipes for PAPPL and other key components needed for a headless print server. I’m also hoping that I’ll be able to get some firmware and/or kernel driver bug fixes to make it unnecessary to use promiscuous mode or turn the power-saver features off - we’ll see…
