Take a look at Cacti, which is available in the EPEL repo: https://www.cacti.net/ It's not just for network accounting. It polls multiple hosts for all kinds of data and keeps RRD tables for display. Cacti provides a web interface that can display the data in charts. You'll need to install plugins for iptables to do the actual data collection. I've used this to track per-host Internet usage on my LAN by adding an iptables chain with one do-nothing rule per LAN host, just to maintain a counter for Cacti to poll.