IP/Process based bandwidth usage stats tool for linux

If you already know WHICH server is "bleeding" bandwidth you might just want to try something as simple as installing your IPTraf package.

IPAudit

We've used "IPAudit" (http://ipaudit.sourceforge.net/) for monitoring/reporting bandwidth usage by host (incoming and outgoing). It gives you pretty granular info about outgoing/incoming ports/protocol and bandwidth - with also granular reporting by traffic type and time (as configured). It's fairly simple to use and easy to setup. Other comparable solutions to this are ntop and Cacti.

Here are some screenshots:

From: Monitor Internet bandwidth