Iptables

Here's a great tutorial on CentOS / Redhat Iptables Firewall Configuration, http://www.cyberciti.biz/faq/rhel-fedorta-linux-iptables-firewall-configuration-tutorial/.


Turn On Firewall

chkconfig iptables on
service iptables start
# restart the firewall
service iptables restart
# stop the firewall
service iptables stop



Display Default Rules
iptables --line-numbers -n -L




Understanding Firewall
There are total 4 chains:




INPUT - The default chain is used for packets addressed to the system. Use this to open or close incoming ports (such as 80,25, and 110 etc) and ip addresses / subnet (such as 202.54.1.20/29).
OUTPUT - The default chain is used when packets are generating from the system. Use this open or close outgoing ports and ip addresses / subnets.
FORWARD - The default chains is used when packets send through another interface. Usually used when you setup Linux as router. For example, eth0 connected to ADSL/Cable modem and eth1 is connected to local LAN. Use FORWARD chain to send and receive traffic from LAN to the Internet.
RH-Firewall-1-INPUT - This is a user-defined custom chain. It is used by the INPUT, OUTPUT and FORWARD chains.


Learn more about iptables here.