Something

EXTIF="eth1"
INTIF="eth0"

echo 1 > /proc/sys/net/ipv4/ip_forward

iptables -F
iptables -X
iptables -t nat -F
iptables -t nat -X

iptables -t nat -A POSTROUTING -o $EXTIF -j MASQUERADE
iptables -A FORWARD -i $INTIF -o $EXTIF -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -A INPUT --protocol tcp --dport 21 -j ACCEPT
iptables -A INPUT --protocol tcp --dport 22 -j ACCEPT
iptables -A INPUT --protocol tcp --dport 80 -j ACCEPT
iptables -t nat -A PREROUTING -i eth1 -p tcp --dport 81 -j DNAT --to 192.168.0.8:80
iptables -t nat -A PREROUTING -d 142.227.61.21 -p tcp --dport 23 -j DNAT --to 192.168.0.8:21