vnstat is a network bandwidth monitoring application. it uses data collected by the kernel and so is quite light on the system.
the installation is dependent on your OS but once done, the program requires a little bit more to complete the setup:
this command will create databases to record the bandwidth for all interfaces on the system
for int in `vnstat --iflist| sed 's/Available interfaces://'`
do
vnstat -u -i $int
done
this next command is needed to allow vnstat to write to the db that was created as root