firmware: failed to load rtl_nic/rtl8168g-3.fw

firmware: failed to load rtl_nic/rtl8168g-3.fw

nano /etc/apt/sources.list

deb http://ftp.pt.debian.org/debian stretch main
# change from above to bellow
deb http://ftp.pt.debian.org/debian stretch main non-free

sudo apt-get update
sudo apt-get install firmware-realtek