NetFlow v9 Exporting from FreeBSD routers/firewalls

Sometimes it’s useful to have netflow reporting from a FreeBSD router or firewall.

For this example, we’ll assume you want to export flows on the igb0 network interface with netflow v9 packets to the flow collector at port 1234 (UDP) – v9 packets are useful as they can contain IPv6 flows.

First, we need to load some kernel modules at boot time – to do this we need to add the following lines to your /boot/loader.conf file:


You can load these now without having to reboot by typing:

kldload netgraph ng_ether ng_socket ng_ksocket ng_tee ng_netflow

Next we need a startup script to configure netflow.  Create a file with the following contents called /usr/local/etc/rc.d/

case “$1” in
/usr/sbin/ngctl -f- <<-SEQ
mkpeer igb0: netflow lower iface0
name igb0:lower netflow
connect igb0: netflow: upper out0
mkpeer netflow: ksocket export9 inet/dgram/udp
msg netflow:export9 connect inet/
echo “Please specify ‘start’ or ‘stop'”

And ensure it runs at boot time with:

chmod a+x /usr/local/etc/rc.d/

and now you can start it:

/usr/local/etc/rc.d/ start

You should start seeing netflow packets arrive at your collector.

FreeBSD doesn’t have ASN information in its kernel, so the netflow packets will only report flows with IPs and port numbers present.

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