We have one central server which functions as an internet gateway. This server is connected to the internet, and using iptables we forward traffic and share the internet connection among all computers in the network. This works just fine.
However, sometimes internet gets really slow. Most likely one of the users is downloading videos or other large files. I want to pinpoint the culprit. I'm thinking of installing a tool that can monitor the network traffic that passes through the server, by IP. Preferably in real time as well as an accumulated total (again by IP). Any tool that is recommended for this? Preferably something in the Ubuntu repositories.