Install vnstat using
sudo apt-get install vnstatthen configure it to monitor a network interface
sudo vnstat -u -iyou can find your network interface name using ifconfig or ip addr command. In my case its eth0
Now to see your network usage issue vnstat command
to view daily, weekly or monthly stats you can use -d, -w or -m switches with vnstat respectively i.e vnstat -m
To reset all the counters to zero you will need to delete the interface's database with vnstat and then recreate it. Use the following command for this purpose
rm /var/lib/vnstat/<interface>; vnstat -u -i <interface>
Please note that vnstat is not a packet sniffer, it simply uses the network stats exposed by kernel. This means there is non-internet network traffic on your interface e.g file sharing, network gaming or etc than it will also be included in the stats. You can use vnstat as bandwidth monitor if you use a dedicated interface for internet usage like 3g usb modem.