16. Allow outbound DNS
The following rules allow outgoing DNS connections.
iptables -A OUTPUT -p udp -o eth0 --dport 53 -j ACCEPT
iptables -A INPUT -p udp -i eth0 --sport 53 -j ACCEPT
15. Allow Internal Network to External network.
On the firewall server where one ethernet card is connected to the external, and another ethernet card connected to the internal servers, use the following rules to allow internal network talk to external network.
In this example, eth1 is connected to external network (internet), and eth0 is connected to internal network (For example: 192.168.1.x).
iptables -A FORWARD -i eth0 -o eth1 -j ACCEPT