I have two network servers I'd like to test in parallel using live data. I'd like to have something listen on port A and forward the traffic to both port B and port C. The data stream in inbound only (i.e. the client just sends data via UDP and never receives a response). I read about netcat but didn't see a way to have it forward to multiple destinations. I could write some code to listen and forward to multiple destinations, but if there is an existing tool that can do this I'd like to learn about it.
EDIT: It looks like the iptables approach is a solution to this problem. I ended up doing this on a FreeBSD box running PF rather than iptables, so I used the dup-to directive.