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 |
| TIME REQUIRED | 15 min |
| 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:
The above command should produce output similar to the following:
Start iptables with the following command if it isn't already running:
Write the rule set
Flush the existing rules with this command:
This command drops an incoming packet if it doesn't match any rules:
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:
Users on this computer are trusted, so outgoing packets will be allowed unless a rule specifically prohibits it:
Many applications must communicate with the localhost interface, so this rule will allow incoming packets that are destined for localhost:
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.
Save the rules with the following command so that they'll be automatically reloaded when you reboot your computer:
Display the new settings
Use the following command to verify that the rules you've just added have been loaded correctly:
The above command will produce output similar to the following screenshot:
