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. 

Note that you can run vnstat either from cron or as a daemon from /etc/rc.d.

To run vnstat from cron:
  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

To run vnstat from /etc/rc.d:
  chmod +x /etc/rc.d/rc.vnstat
  
Note that to have vnstat start automatically this way it should be called from
rc.local (and optionally rc.local_shutdown) like so:

  In rc.local:
  # Start vnstat
  if [ -x /etc/rc.d/rc.vnstat ]; then
    /etc/rc.d/rc.vnstat start
  fi

  In rc.local_shutdown:
  # Stop vnstat
  if [ -x /etc/rc.d/rc.vnstat ]; then
    /etc/rc.d/rc.vnstat stop
  fi