Fiz o seguinte:
iptables -t nat -A PREROUNTING -d ! x.x.x.x/24 -p tcp --dport 21 -j REDIRECT --to-port 3128
iptables -t nat -A PREROUNTING -d ! x.x.x.x/24 -p tcp --dport 4226 -j REDIRECT --to-port 3128
iptables -t nat -A PREROUNTING -d ! x.x.x.x/24 -p tcp --dport 21 -j REDIRECT --to-port 3128
iptables -t nat -A PREROUNTING -d ! x.x.x.x/24 -p tcp --dport 4226 -j REDIRECT --to-port 3128
iptables -t nat -A PREROUNTING -d ! x.x.x.x/24 -p tcp --dport 80 -j REDIRECT --to-port 3128
iptables -t nat -A PREROUNTING -d ! x.x.x.x/24 -p tcp --dport 80 -j REDIRECT --to-port 3128
iptables -A FORWARD -j ACCEPT -p tcp --dport 21
iptables -t nat -A PREROUTING -i eth1 -p tcp --dport 21 -j DNAT --to 192.168.0.118:21
iptables -t nat -A PREROUTING -i eth1 -p tcp --dport 4226 -j DNAT --to 192.168.0.118:4226
iptables -t nat -A PREROUTING -i eth1 -p tcp --dport 21 -j DNAT --to 192.168.0.114:21
iptables -t nat -A PREROUTING -i eth1 -p tcp --dport 4226 -j DNAT --to 192.168.0.114:4226