Introduction

I'm trying to set-up a wireless packet monitor (it's something I've long thought might be an interesting source of data, and now I need it for work too). My thought was that I'd set up a raspberry pi to experiment with - I don't think it is powerful enough to work, but it should work just to mess with code, and a distributed system might get some interesting results, but anyway - but when I tried to put my raspberry pi's wireless interface into monitor mode I got an error.

iwconfig wlan0 mode monitor
Error for wireless request "Set Mode" (8B06) :
    SET failed on device wlan0 ; Operation not supported.

Looking around on the web I found this reddit post as well as some Stack Overflow posts that said that monitor mode isn't supported on the Raspberry Pi. There is a project called nexmon that apparently lets up add a firmware patch to enable it, which I'll probably try later, but before I tried that I remembered that I have a Realtek 8812AU USB WiFi adapter that I bought a while ago for an old desktop I had that I wasn't using so I decided to try it.

What I tried

Plug-and-Pray

The first thing I did was to see if it would just work. I plugged the Realtek into the USB port and although lsusb showed it, iwconfig didn't show it as an interface. Back to the internet.

Github

Next I found a repository on github that has the driver for the Realtek set up for linux machines. I downloaded it and followed the instructions to build it - the main thing is to set:

CONFIG_PLATFORM_I386_PC = n
CONFIG_PLATFORM_ARM_RPI = y

in the Makefile - but when I tried to build it I got this error.

sudo dkms install -m $DRV_NAME -v $DRV_VERSION

'make' KVER=4.4.38-v7+....(bad exit status: 2)
ERROR (dkms apport): binary package for rtl8812AU: 4.3.20 not found
Error! Bad return status for module build on kernel: 4.4.38-v7+ (armv7l)
Consult /var/lib/dkms/rtl8812AU/4.3.20/build/make.log for more information.

There was also a message in the make.log file but I didn't remember to copy it.

What fixed it

The solution was in this StackOverflow post - the make program is being pointed to a folder named arm7l (thats 'arm seven ell') but it should actually be pointed to one named arm. The simple solution is to create an alias with the correct name.

sudo ln -s /usr/src/linux-headers-4.4.38-v7+/arch/arm/ /usr/src/linux-headers-4.4.38-v7+/arch/armv7l

This turns out to fix the build problem and after a reboot the network interface showed up.

Conclusion

The Raspberry Pi 3 doesn't support monitor mode for its wireless interface out of the box, and while there is a firmware patch to enable it, I chose to use a Realtek RTL 8812AU USB WiFi adapter instead. You need a little bit of extra work to get it going, but it does seem to work. One thing I noticed is that iwconfig will put it in monitor mode but airmon-ng doesn't (I haven't figured out why yet). It doesn't report an error, it just doesn't seem to work. Also, iw always reports the interface as managed, even when it isn't… maybe I'll try the firmware patch after all.