from HypeTelecon on forum:
A while back I paid someone at rent-a-coder to update pfflowd so that it supported NetFlow v9 and properly indexed the interfaces with their corresponding SNMP index. The interface indexing allows pfflowd to work with picky NetFlow collectors (like the NTA module for SolarWinds Orion).
Here is a link to the result: (attached here)
It would be nice if someone could get the installable pfflowd package for pfSense updated to this this version of pfflowd.