check if this is enabled or not by 

 sudo iptables -t nat -L

If There are any entries , delete them using:

 sudo iptables -F -t nat
Try without src dest flags:

 sudo iptables -t nat -A PREROUTING -p tcp --dport 30 -j REDIRECT --to-ports 1234

check again if this is enabled or not by 

 sudo iptables -t nat -L

 It is Important to remove chained entries queued by default to "OUTPUT" route.

Hope This helps!!