Packets don't flow from ng_netflow

[Matthew Reimer]

I'm trying to use ng_netflow to monitor our network traffic but for some 
reason NetFlow packets aren't emitted unless tcpdump is running on the 
interface configured with ng_netflow.

The box is running FreeBSD 4.11-STABLE and the latest ng_netflow from ports. 
It has two NICs: the main NIC fxp0 which is configured for IP, and a second 
NIC dc0 which is up but with no IP configuration. I've configured port 
mirroring on our Cisco switch to tee all traffic going through our upstream 
port to dc0:

# ifconfig dc0
dc0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
        ether 00:04:5a:79:72:f7
        media: Ethernet autoselect (100baseTX <full-duplex>)
        status: active

netgraph config:

+ mkpeer dc0: netflow lower iface0
+ name dc0:lower netflow
+ mkpeer netflow: ksocket export inet/dgram/udp
+ msg netflow:export connect inet/192.168.1.2:1234


The problem is that no NetFlow packets are emitted unless I run tcpdump on 
dc0. Is this not a valid configuration? Or is there a bug in 
netgraph/ng_netflow?

Thanks for any help you can give.

Matt