Example of Iptable rules to keep your Linux server safe

iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A INPUT -p tcp --dport 21 -j ACCEPT
iptables -A INPUT -p tcp --dport 22 -j ACCEPT
iptables -A INPUT -p tcp --dport 23 -j ACCEPT
iptables -A INPUT -p tcp --dport 25 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j ACCEPT
iptables -A INPUT -p tcp --dport 81 -j ACCEPT
iptables -A INPUT -p tcp --dport 110 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j ACCEPT
iptables -A INPUT -p tcp --dport 587 -j ACCEPT
iptables -A INPUT -p tcp --dport 3306 -j ACCEPT
iptables -A INPUT -p tcp --dport 8080 -j ACCEPT
iptables -A INPUT -p tcp --dport 8009 -j ACCEPT
iptables -A INPUT -p tcp --dport 8089 -j ACCEPT
iptables -A INPUT -p tcp --dport 8443 -j ACCEPT
iptables -A INPUT -p tcp --dport 8081 -j ACCEPT
iptables -A INPUT -p tcp --dport 81 -j ACCEPT
iptables -A INPUT -p tcp --dport 11211 -j ACCEPT
iptables -A INPUT -p udp --dport 11211 -j ACCEPT
iptables -A INPUT -p tcp --dport 50000 -j ACCEPT
iptables -A INPUT -p tcp --dport 50001 -j ACCEPT
iptables -A INPUT -p tcp --dport 60000 -j ACCEPT
iptables -A INPUT -p icmp --icmp-type echo-request -j ACCEPT
iptables -A INPUT -p tcp --syn -m limit --limit 1/s --limit-burst 3 -j RETURN

You can use -I and with a number to add a rule to the top of the list (highest priority). For example:

iptables -I INPUT 1 -p tcp --sport 80 -j ACCEPT
iptables -I INPUT 1 -p tcp --sport 8080 -j ACCEPT
iptables -I OUTPUT 1 -p tcp --dport 8080 -j ACCEPT
iptables -I OUTPUT 1 -p tcp --dport 80 -j ACCEPT
iptables -I OUTPUT 1 -p tcp --sport 8080 -j ACCEPT
iptables -I OUTPUT 1 -p tcp --sport 80 -j ACCEPT

Refuse all other requests depends on your network settings

iptables -A INPUT -i eth0 -j REJECT
iptables -A INPUT -i venet0 -j REJECT

Save and restart Iptables to active the firewall.

/etc/init.d/iptables save
/etc/init.d/iptables restart

Simple! Isnt it?