Заменить foo_serverip на внутренний адрес
iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -m conntrack --ctstate NEW -j DNAT --to foo_serverip:80
iptables -t nat -A PREROUTING -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
iptables -A POSTROUTING -t nat -j MASQUERADE