What are the requierements to sniff UDP packets in a Wireless Ad-Hoc network?

In a wireless Ad-Hoc network, I have two computers which communicates in UDP together from 192.168.1.3 to 192.168.1.5 I have a third computer (192.168.1.6) which wants to listen to the packets which are not addressed to it. I can see the packets with tcpdump (which sets the wireless card in promiscuous mode). To redirect the packets, I am using : iptables -t nat -I PREROUTING -s 192.168.1.3 -j DNAT --to-destination 192.168.1.5 Is it the only rule I need to use? Why isn't it working? Did I miss other requirements?