Forwarding port 3000 to 80 on ubuntu.

sudo iptables -A INPUT -i eth0 -p tcp --dport 80 -j ACCEPT
sudo iptables -A INPUT -i eth0 -p tcp --dport 3000 -j ACCEPT
sudo iptables -A PREROUTING -t nat -i eth0 -p tcp --dport 80 -j REDIRECT --to-port 3000

That works.