How to Configure Firewall in Linux

A Firewall is a network security system that controls the incoming and outgoing network traffic based on an applied rule set. A firewall controls access to the resources of a network through a positive control model.

Accept – Allow the connection.

Drop – Drop the connection, act like it never happened. This is best if you don’t want the source to realize your system exists.

Reject – Don’t allow the connection, but send back an error. This is best if you don’t want a particular source to connect to your system, but you want them to know that your firewall blocked them.

View Firewall

To view the current firewall configuration use the command.

# iptables -L

Firewall Configuration

Edit the iptables configuration file:

# vi /etc/sysconfig/iptables

-A INPUT -m state --state NEW -p tcp --dport 80 -j ACCEPT
-A INPUT -m state --state NEW -p tcp --dport 443 -j ACCEPT

OR
Execute the command on shell.

Firewall Rules:

Rule 1: Block all IP address

To block all connections from the specific IP address, define the rule

# iptables -A INPUT -s 192.168.0.100 -j DROP

Rule 2: Block network range of IP addresses

To block all of the IP addresses in specific network range then use the rule

# iptables -A INPUT -s 192.168.0.0/24 -j DROP

Rule 3: Enable Specific Ports

In order to check the well known port chart, use the link

# iptables -A input -p tcp -m state --state NEW -m tcp --dport 80 -j ACCEPT
# iptables -A INPUT -p tcp -m state --state NEW -m tcp --dport 443 -j ACCEPT
# iptables -A INPUT -p tcp -m state --state NEW -m tcp --dport 21 -j ACCEPT
# iptables -A INPUT -p tcp -m state --state NEW -m tcp --dport 25 -j ACCEPT
# iptables -A INPUT -p tcp -m state --state NEW -m tcp --dport 110 -j ACCEPT
# iptables -A INPUT -s 192.168.1.0/24 -p tcp -m tcp --dport 22 -j ACCEPT

Save Firewall Service

Save the iptable rules.

# /sbin/service iptables save