Netflow Source Interface

[xer0sis]

I am trying to setup my opnsense firewall running version OPNsense 20.1.3-amd64 to send netflow data to a netflow collector that is across an IPSec VPN tunnel. I confirmed that hosts on my LAN are able to access this device across the VPN tunnel.

From the testing I have done I see that the firewall itself is trying to send the netflow data straight out the WAN interface rather than across the IPSec VPN tunnel. I've confirmed this via packet captures on the LAN, WAN, and IPSec interfaces.

I also noticed that the OPNsense firewall is unable to ping the netflow collector unless I specify a source interface of "LAN". If I leave this as "Default" the ping fails.

Currently I have Netflow configured as follows:
Listening interfaces: WAN
WAN interfaces: WAN
Capture local: unchecked
Version: v9
Destinations: 172.16.0.81:2055

Does anyone know what I need to do in order to specify a source interface for sending of the netflow traffic?