Bash file to quickly install CentOS firewall rules

firewall.sh

# flush all rules
iptables -F

# block null packets
iptables -A INPUT -p tcp --tcp-flags ALL NONE -j DROP

# block syn-flood attacks
iptables -A INPUT -p tcp ! --syn -m state --state NEW -j DROP

# block XMAS packets
iptables -A INPUT -p tcp --tcp-flags ALL ALL -j DROP

# allow localhost
iptables -A INPUT -i lo -j ACCEPT

# allow 80 and 443 for webserver
iptables -A INPUT -p tcp -m tcp --dport 80 -j ACCEPT
iptables -A INPUT -p tcp -m tcp --dport 443 -j ACCEPT

# allow SMTP on 465
iptables -A INPUT -p tcp -m tcp --dport 465 -j ACCEPT

# allow SSH access
iptables -A INPUT -p tcp -m tcp --dport 22 -j ACCEPT

# allow outgoing connections block everything else
iptables -I INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -P OUTPUT ACCEPT
iptables -P INPUT DROP

# save rules
iptables -L -n
iptables-save > /etc/sysconfig/iptables

# restart iptables
service iptables restart