Since I often work remotely (coffices and stuff), I use 3G/4G modems with me to provide some connectivity while I'm out. Adding the VPN services in-between I need to know how much traffic is passing through my network, inbound and outbound.
So iftop was the easiest and lightest thing I found. It's in Fedora's repos (I believe Ubuntu's too) so a simple `yum install iftop` resolved the deps for me with everything.
To run that, a:
iftop -i wlan0
(or whatever your interface is) would start that monitor. The listening would probably require you to sudo before that.