ipfix/netflow/sflow generator for Linux

fprobe doesn't work properly because it has the input and output interface
IDs as both 0.

fprobe-ulog fixes this. From the http://fprobe.sourceforge.net/ front page:

  fprobe-ulog - libipulog-based fork of fprobe. It obtains packets
  through linux netfilter code (iptables ULOG target). The main
  advantages of this version are native input/output interface
  SNMP-index support and significant performance benefit. Of course,
  this version work on linux only.

We have used it here for a few years and have been quite happy with it.