tcpdump of UDP traffic

I'm running 9.3.1HF3. I've got a couple of proxy servers who stream RTSP over UDP from the Internet back to the clients. I'm trying to determine if I can capture this traffic with tcpdump.

 

 

What I'm struggling with is that when I typically do a tcpdump, I'm looking for traffic inbound on the external interface. And in that case, I understand if there is no Virtual Server for this traffic, it is routed through the forwarding virtual server and I can't capture it.

 

 

However in the case of this RTSP traffic, the source is our internal nodes. What I can't wrap my mind around is if I can capture that data when there is no Virtual Server listening for inbound traffic on the Internal interface. Can you even set up a Virtual Server listening for inbound traffic on the Internal interface - does that even make sense?