Nifmon is a curses-based tool for FreeBSD which can be used to monitor network
interfaces (also tun devices).

It displays continously updated statistics for the following quantities:
 - total amount of bytes/packets transferred in/out
 - bytes/packets transferred in/out during the last second/minute/hour
 - a stripped down tcpdump output

You will need super-user permissions in order to use nifmon.