initiate a broadcast with dnat

hello,

i want to initiate a broadcast if a udp packet with destination port 9999 reaches my router.
i have two devices: ppp0 (wan), eth0 (lan). broadcast of eth0 is 192.168.0.255.

when i use
iptables -t nat -A PREROUTING -i ppp0 -p upd --dport 9999 -j DNAT --to-destination 192.168.0.255:9

the packet gets into the INPUT chain of ppp0 instead of the FORWARD chain. if i use a non broadcast destination ip, the packet gets into FORWARD chain.

is there a way to initiate a broadcast with iptables?

regards, marcel