badger.foo writes "Have you ever wanted to know what's really going on in your network? Some free tools with surprising origins can help you to an almost frightening degree. Peter Hansteen shares some monitoring insights, anecdotes and practical advice in his latest column on how to really know your network. All of it with free software, of course."
From the article: "
The NetFlow protocol was invented at Cisco in the early 1990s. It's designed to collect traffic metadata, where the basic unit of reference is the flow, defined as the source and destination IP address pair, the matching source and destination port for protocols that use them, the protocol identifier, time started and ended, number of packets sent, number of bytes sent, and a few other fields that have varied somewhat over the NetFlow versions. ...
On OpenBSD, various netflow sensors and collectors had been available for a while when the new network pseudo device pflow
debuted in OpenBSD 4.5."