Firewall:

Definition:
Firewall means that to protect from the several processing into a server.
Here firewall as a IPTABLES.
IPTABLES-Firewall:
iptables -L --> list out the entries
iptables -L --> flash the entries
iptables -L --> adding the new role
service ip tables save
-p --> specify protocols
-s --> source ip address
-d --> designation ip address
--sport --> source port address
--dport --> designation port address
--j --> applying the policy

ACCEPT --> allow
REJECT --> deny
DROP --> Temporary stop.

Example:
#iptables -A INPUT -P ICMP -s 192.168.0.0/24 -d 192.168.0.1 --j drop
- do not to ping the server ip to any system in a network.




Note:
vi /etc/sysctl.conf
line no 7 and 13 --> to change 0 to 1. both of it.
Don't to forgot to change the configuration file.
once time when am i used that method, that time i forgot to configured.