iptables -t nat -A OUTPUT -m addrtype --src-type LOCAL --dst-type LOCAL -p tcp --dport 3306 -j DNAT --to-destination ip.ip.ip.ip
iptables -t nat -A POSTROUTING -m addrtype --src-type LOCAL --dst-type UNICAST -j MASQUERADE
sysctl -w net.ipv4.conf.all.route_localnet=1
Make sure you substitute "ip.ip.ip.ip" for your real public IP and also the "--dport 3306" for the port you want to forward.
Finally run the sysctl command and........
iptables -t nat -A PREROUTING -p tcp -m multiport --dports 80,443,2068,8192 -j DNAT --to-destination 192.168.1.175
Just adjust the "--dports" to the ports you need and the --to-destination to the destination IP (note it must be on the same network as the server running iptables........