Just use the following command:

iptables -A INPUT -p tcp --syn --dport 22 -m connlimit --connlimit-above 5 -j REJECT

In this example our connections are limited to 5.

Linux machines are known to be pretty secure.

Studies have shown that Linux has been designed in a secure manner. Yet, despite all the security features that come bundled with a Linux installation, you need to configure these features correctly to make them work for you. I’ll guide you through the process of setting up of one of the tools that help secure your machine – the firewall. We will use the iptables firewall for this exercise. I am assuming that you are using a server running Red Hat Enterprise Linux 4 or similar. However, most of the steps should work fine on other Linux distributions as well. In this article we will setup a firewall on a Linux server running the Apache Web Server, FTP, and SSH.

Let us first see what ports these applications use and which of them need to have a port open on the firewall.

The Apache web server runs on port 80 by default. Apache is going to server all our web content on this port, therefore we need to keep this port open on the firewall. The SSH service runs on port 22. We need to be able to remotely connect to our server to work, so we keep it open. FTP runs on port 21 and it too needs the port to be open to communication.

Next, make sure you have iptables installed. Run this command as the root user:

apt-cache search iptables Continue Reading…

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