Дальше уж думать будем где не то нарисовал.
iptables -L
Chain INPUT (policy ACCEPT)
target prot opt source destination
Chain FORWARD (policy ACCEPT)
target prot opt source destination
ACCEPT tcp -- 192.168.0.0/24 anywhere tcp dpt:smtp
ACCEPT tcp -- 192.168.0.0/24 anywhere tcp spt:smtp
ACCEPT tcp -- 192.168.0.0/24 anywhere tcp dpt:domain
ACCEPT tcp -- 192.168.0.0/24 anywhere tcp spt:domain
ACCEPT tcp -- 192.168.0.0/24 anywhere tcp dpt:pop3
ACCEPT tcp -- 192.168.0.0/24 anywhere tcp spt:pop3
ACCEPT all -- anywhere anywhere state RELATED,ESTABLISHED
DROP tcp -- anywhere anywhere tcp
DROP udp -- anywhere anywhere udp
Chain OUTPUT (policy ACCEPT)
target prot opt source destination