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How to configure iptables on CentOS

CentOS is an enterprise-class Linux operating system that's supported by a community of users and developers. It's intended to be completely compatible with Red Hat Enterprise Linux, which is CentOS's upstream source. CentOS has a powerful firewall built into it that uses a set of rules to determine the traffic that will be allowed to enter and exit a network. System administrators can configure a CentOS firewall with the iptables userspace module. Iptables allows administrators to enter rules for the firewall into existing tables from the command line.

DIFFICULTY Basic - 1 | Medium - 2 | Advanced - 3
RELATED PRODUCTS CentOS-based VPS or dedicated servers

Here is a quick tutorial on how to setup a firewall on CentOS. This tutorial creates a simple rule set that blocks some incoming connections, while allowing all outgoing connections.

Start the firewall

Sign on to your server as root and open a command window. Ensure that iptables is running with the following command:

# iptables -L

The above command should produce output similar to the following:

Start iptables with the following command if it isn't already running:

# /etc/init.d/iptables start

Write the rule set

Flush the existing rules with this command:

# iptables -F

This command drops an incoming packet if it doesn't match any rules:

# iptables -P INPUT DROP

The computer in this example isn't being used as a router, so the following command drops a forwarded packet if it doesn't match any rules:

# iptables -P FORWARD DROP

Users on this computer are trusted, so outgoing packets will be allowed unless a rule specifically prohibits it:

# iptables -P OUTPUT ACCEPT

Many applications must communicate with the localhost interface, so this rule will allow incoming packets that are destined for localhost:

# iptables -A INPUT -i lo -j ACCEPT

This rule loads the state module which examines the incoming packets, and accepts those that are part of an established connection or related to such a connection.

# iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

Save the rules with the following command so that they'll be automatically reloaded when you reboot your computer:

# /etc/init.d/iptables save

Display the new settings

Use the following command to verify that the rules you've just added have been loaded correctly:

# iptables -L -v

The above command will produce output similar to the following screenshot:

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