Enable monitoring mode for RTL8188CUS via USB on Raspbian

Question

I am trying to enable monitoring mode for a USB wifi dongle with the RTL8188CUS chipset on a raspberry pi model b+ (or any raspberry pi for that matter).

$ lsusb
Bus 001 Device 005: ID 0bda:8176 Realtek Semiconductor Corp. RTL8188CUS 802.11n WLAN Adapter
$ sudo iwconfig wlan0 mode monitor
Error for wireless request "Set Mode" (8B06) :
    SET failed on device wlan0 ; Invalid argument.

According to github/raspberrypi/linux/issues/369, you need to enable the rtlwifi/rtl8192cu kernel module that is included with the kernel distribution but not compiled. This requires minor modifications to some files as diff'ed below in 'STEP 2'.

The USB issue mentioned in that thread has been resolved as of 4.1.6+, so the rtlwifi driver should work.

Steps to recreate on a fresh raspberry pi (model B+)...

STEP 0: Update existing modules and kernel to latest

$ sudo apt-get update
$ sudo rpi-update
$ uname -a
Linux raspberrypi 4.1.7+ #815 PREEMPT Thu Sep 17 17:59:24 BST 2015 armv6l GNU/Linux

STEP 1: Get the raspbian kernel source and add missing dependencies

$ git clone --depth=1 https://github.com/raspberrypi/linux
$ sudo apt-get install bc lshw

STEP 2: Enable the rtlwifi (kernel) drivers for RTL8188CUS (RTL8192)

edit linux/drivers/net/wireless/Kconfig
-#source "drivers/net/wireless/rtlwifi/Kconfig"
-source "drivers/net/wireless/rtl8192cu/Kconfig"
+source "drivers/net/wireless/rtlwifi/Kconfig"
+#source "drivers/net/wireless/rtl8192cu/Kconfig"

(Wheezy) edit linux/drivers/net/wireless/Makefile
-#obj-$(CONFIG_RTLWIFI)         += rtlwifi/
+obj-$(CONFIG_RTLWIFI)          += rtlwifi/

(Jessie) edit linux/drivers/net/wireless/realtek/Makefile
-#obj-$(CONFIG_RTLWIFI)         += rtlwifi/
+obj-$(CONFIG_RTLWIFI)          += rtlwifi/

STEP 3: Compile and install kernel (took many hours)

Summarized from kernel building documentation .

$ cd linux
$ KERNEL=kernel
$ make bcmrpi_defconfig

$ make zImage modules dtbs
$ sudo make modules_install
$ sudo cp arch/arm/boot/dts/*.dtb /boot/
$ sudo cp arch/arm/boot/dts/overlays/*.dtb* /boot/overlays/
$ sudo cp arch/arm/boot/dts/overlays/README /boot/overlays/
$ sudo scripts/mkknlimg arch/arm/boot/zImage /boot/$KERNEL.img

STEP 4: Reboot

$ sudo reboot

STEP 5: Check that the rtlwifi/rtl8192cu module is loaded

$ lsmod | fgrep rtl8192cu
rtl8192cu             100806  0 
rtl_usb                14781  1 rtl8192cu
rtl8192c_common        72091  1 rtl8192cu
rtlwifi               101122  3 rtl_usb,rtl8192c_common,rtl8192cu
mac80211              623281  3 rtl_usb,rtlwifi,rtl8192cu
$
$ lshw
  *-network:0
       description: Ethernet interface
       physical id: 1
       bus info: usb@1:1.3
       logical name: wlan0
       serial: 00:0b:81:94:e9:a3
       capabilities: ethernet physical
       configuration: broadcast=yes driver=rtl8192cu driverversion=4.1.7+ firmware=N/A link=no multicast=yes

STEP 6: Try to activate monitoring mode

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

What did i miss?
Issue 369 seems to indicate that it can work with the rtlwifi driver?

Answer 1

Turns out the steps to recompile and load the rtlwifi module are correct. The problem is iwconfig not working to enable/determine monitoring mode in this situation.

Instead, I used iw as outlined by Steven Gordon - Capturing WiFi in Monitor mode with iw and it worked.

To summarize:

STEP 6b: List the physical network interfaces available

$ iw dev

STEP 7: Determine if the physical interface supports monitoring mode

$ iw phy phy0 info
... lots of stuff ...
Supported interface modes:
     * IBSS
     * managed
     * AP
     * AP/VLAN
     * monitor
     * mesh point
     * P2P-client
     * P2P-GO
... lots more stuff ...

STEP 8: Add a monitoring interface to that physical card

You need to explicitly add a 'monitoring' interface for the hardware you have.

$ sudo iw phy phy0 interface add mon0 type monitor

STEP 8: Start monitoring

In my case, I'm using tshark to facilitate monitoring, displaying a few useful fields rather than a lot of noise.

$ sudo apt-get install tshark
$ sudo tshark -i mon0 -f 'broadcast' -T fields -e frame.time_epoch -e wlan.sa -e radiotap.dbm_antsignal -e wlan.fc.type -e wlan.fc.subtype

Done.