iptables examples

iptables has three chains - INPUT, OUTPUT, FORWARD

Set default policy using -P flag

iptables -P INPUT DROP
iptables -P OUTPUT DROP
iptables -P FORWARD DROP

Allow TCP traffic on port 80

iptables -A INPUT -p TCP --dport 80 -j ACCEPT

Allow TCP traffic on port 443

iptables -A INPUT -p TCP --dport 443 -j ACCEPT

Allow UDP traffic on port 53

iptables -A INPUT -p udp --dport 53 -j ACCEPT

Allow TCP traffic on port 3306 only from 192.168.0.2

iptables -A INPUT -p tcp --dport 3306 -s 192.168.0.2 -j ACCEPT

Allow outgoing TCP and UDP traffic on any port but only as part of existing connection state. DROP everything else

iptables -P OUTPUT -p tcp -m state --state ESTABLISHED -j ACCEPT
iptables -P OUTPUT -p udp -m state --state ESTABLISHED -j ACCEPT
iptables -P OUTPUT DROP

Configure iptables to
(1) ACCEPT all TCP traffic on port 80 in the FORWARD chain
(2) ACCEPT all UDP traffic on port 53 if it comes from the IP 192.168.0.1 in the FORWARD chain
(3) Set the default FORWARD policy to DROP.

iptables -A FORWARD -p TCP --dport 80 -j ACCEPT
iptables -A FORWARD -p udp --dport 53 -j ACCEPT
iptables -P FORWARD DROP