The following is an extract from a very concise example of how to secure your Linux Computer using IPTables while allowing inbound SSH connections from only trusted hosts.

Okay, so this post will be brief but to the point. Today I needed to lock down a machine I administer so the only inbound connections which were allowed were SSH connections from trusted hosts. I’m using Debian so this will obviously work for other Debian based distros such as Ubuntu, Linux Mint etc.

Assuming you’re running a current version of Debian or a derivative then iptables will already be present on your system. One of the first things to take note is that iptables won’t hold its ruleset during a reboot so to start off this tutorial the first thing I ensured was that the ruleset will be restored when the machine is rebooted. So, as root I edited /etc/rc.local and before the exit line i added /etc/iptables-init.

The full article can be read on Jerry Walsh’s Blog located here:  http://jerrywalsh.org/2012/howto-secure-your-linux-box-with-iptables-0112.html