vnStat is a network traffic monitor for Linux that keeps a log of daily 
network traffic for the selected interface(s). vnStat isn't a packet 
sniffer. The traffic information is analyzed from the /proc filesystem, 
so vnStat can be used without root permissions. 

This program is open source (GPL) and can be installed in 'single' and 
'root' mode. 

Add the following line to your crontab:
0-55/5 * * * * if [[ -x /usr/bin/vnstat && $(ls /var/lib/vnstat/ | wc -l) -ge 1 ]]; then /usr/bin/vnstat -u; fi