flowd is a small, fast and secure NetFlow collector. It offers the following
features:

    * Understands NetFlow protocol v.1, v.5, v.7 and v.9 (including IPv6 flows)
    * Supports both IPv4 and IPv6 transport of flows
    * Secure: flowd is privilege separated to limit the impact of any compromise
    * Supports filtering and tagging of flows, using a packet filter-like syntax
    * Stores recorded flow data in a compact binary format which supports
      run-time choice over which flow fields are stored
    * Ships with Perl interface for reading and parsing the
      on-disk record format
    * Is licensed under a liberal BSD-like license

WWW: https://www.mindrot.org/projects/flowd/