udp data redirecting

Hi

im using: iptables -t nat -A PREROUTING -p -tcp --dport 544 -j REDIRECT
--to-port 9999

to redirect tcp packets to an loaclhost port on a proxy, which normaly
would be routed to a server. Now this works fine. When im sending a tcp
packet back to the client over the registrated socket the proxy has the
identity of the server, so i set the proxy transparent for the client.

now i want to do the same thing with UDP-Data. But im not sure if it
works. Normally i just would use the command:
iptables -t nat -A PREROUTING -p -udp --dport 12000 -j REDIRECT
--to-port 12999

but the difference is, that the tcp-data stream is init for the client
and the udp-data is not.

so, is it posible to use iptables, to send data from a udp-sockt of the
proxy with the identity of a server to a client?

thanks