Security on the mail server using iptables

Example of a simple iptables configuration on the mail server using Centos 5.3.
Configuration file is located in /etc/sysconfig/iptables

*filter * filter
:INPUT DROP [0:0]
:FORWARD DROP [0:0]
:OUTPUT DROP [0:0]
:RH-Firewall-1-INPUT - [0:0]
-A INPUT -i lo -j ACCEPT
-A INPUT -p icmp -m icmp --icmp-type 8 -j ACCEPTICMP-type 8-j ACCEPT
-A INPUT -p tcp -m tcp --dport 25 --tcp-flags FIN,SYN,RST,ACK SYN -j ACCEPT #SMTP
-A INPUT -p udp -m udp --dport 50 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 51 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 110 --tcp-flags FIN,SYN,RST,ACK SYN -j ACCEPT #POP3
-A INPUT -p tcp -m tcp --dport 80 --tcp-flags FIN,SYN,RST,ACK SYN -j ACCEPT #webmail
-A INPUT -p tcp -m tcp --dport 443 --tcp-flags FIN,SYN,RST,ACK SYN -j ACCEPT
-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
-A INPUT -m state --state NEW -j REJECT --reject-with icmp-port-unreachable
-A OUTPUT -o lo -j ACCEPT -A OUTPUT-o lo-j ACCEPT
-A OUTPUT -p tcp -m state --state NEW,ESTABLISHED -j ACCEPT
-A OUTPUT -p udp -m state --state NEW,ESTABLISHED -j ACCEPT
-A OUTPUT -p icmp -m state --state NEW,RELATED,ESTABLISHED -j ACCEPT
COMMIT

After you have finished configuring the iptables, then start the service of iptables

# service iptables start
Or
# /etc/init.d/iptables start

End.