Realtek RTL8811CU/RTL8821CU USB wifi adapter driver version 5.4.1 for Linux 4.4.x up to 5.x

Before build this driver make sure make, gcc, linux-header/kernel-devel, bc and git have been installed.

First, clone this repository

sudo apt install -y raspberrypi-kernel-headers build-essential bc dkms git
git clone https://github.com/trysightdev/rtl8821CU.git
cd rtl8821CU
make clean
make -j4
sudo make install

Hostapd 5gh sample

/etc/hostapd/hostapd.conf

country_code=US
driver=nl80211
interface=wlan1

ssid=magnibot
hw_mode=a
ieee80211n=1
ieee80211h=1
ieee80211d=1

channel=36
vht_oper_centr_freq_seg0_idx=42
vht_oper_chwidth=2
ht_capab=[HT40+]

macaddr_acl=0
auth_algs=1
ignore_broadcast_ssid=0

Check the name of the interface

Check the interface name of your wifi adapter using ifconfig. Usually, it will be wlan0 by default, but it may vary depends on the kernel and your device. On Ubuntu, for example, it may be named as wlx + MAC address. (https://www.freedesktop.org/wiki/Software/systemd/PredictableNetworkInterfaceNames/)

If this is the case, you can either disable the feature following the link above, or replace the name used in the driver by

grep -lr . | xargs sed -i '' -e '/ifcfg-wlan0/!s/wlan0/<name of the device>/g'

Build and install with DKMS

DKMS is a system which will automatically recompile and install a kernel module when a new kernel gets installed or updated. To make use of DKMS, install the dkms package.

Debian/Ubuntu:

sudo apt-get install dkms

Arch Linux/Manjaro:

sudo pacman -S dkms

To make use of the DKMS feature with this project, just run:

./dkms-install.sh

If you later on want to remove it, run:

./dkms-remove.sh

Plug your USB-wifi-adapter into your PC

If wifi can be detected, congratulations. If not, maybe you need to switch your device usb mode by the following steps in terminal:

1. find your usb-wifi-adapter device ID, like "0bda:1a2b", by type:

lsusb

2. switch the mode by type: (the device ID must be yours.)

Need install usb_modeswitch (Archlinux: sudo pacman -S usb_modeswitch)

sudo usb_modeswitch -KW -v 0bda -p 1a2b
systemctl start bluetooth.service - starting Bluetooth service if it's in inactive state

It should work.

Make it permanent

If steps above worked fine and in order to avoid periodically having to make usb_modeswitch you can make it permanent (Working in Ubuntu 18.04 LTS):

1. Edit usb_modeswitch rules:

   sudo nano /lib/udev/rules.d/40-usb_modeswitch.rules

2. Append before the end line LABEL="modeswitch_rules_end" the following:

   # Realtek 8211CU Wifi AC USB
   ATTR{idVendor}=="0bda", ATTR{idProduct}=="1a2b", RUN+="/usr/sbin/usb_modeswitch -K -v 0bda -p 1a2b"
   

Make sure to set your ATTR{idVendor} and the -v argument to the left portion of the output of lsusb device ID, and your ATTR{idProduct} and -p argument to the right portion of the lsusb device ID. For example (for the Cudy AC600 usb wifi adapter) the output from lsusb command looks like this:

Bus 001 Device 016: ID 0bda:c811 Realtek Semiconductor Corp. 802.11ac NIC

then your configuration in /lib/udev/rules.d/40-usb_modeswitch.rules should be

# Realtek 8211CU Wifi AC USB
ATTR{idVendor}=="0bda", ATTR{idProduct}=="c811", RUN+="/usr/sbin/usb_modeswitch -K -v 0bda -p c811"

Build and install without DKMS

Use following commands:

cd ~/build/rtl8821CU
make
sudo make install

If you later on want to remove it, do the following:

cd ~/build/rtl8821CU
sudo make uninstall

Checking installed driver

If you successfully install the driver, the driver is installed on /lib/modules/<linux version>/kernel/drivers/net/wireless/realtek/rtl8821cu. Check the driver with the ls command:

ls /lib/modules/$(uname -r)/kernel/drivers/net/wireless/realtek/rtl8821cu

Make sure 8821cu.ko file present on that directory

Check with DKMS (if installing via DKMS):

sudo dkms status

Monitor mode

Use the tool 'iw', please don't use other tools like 'airmon-ng'

iw dev wlan0 set monitor none