pretty simple

server with eth0 ip 1.1.1.1 needs to forward ALL Request to ip 2.2.2.2

iptables -N nat
iptables -t nat -A PREROUTING -i eth0 -d 1.1.1.1 -p ip -j DNAT --to 2.2.2.2
iptables -A INPUT -p tcp -j nat