Configuring IPtables on ubuntu server
Please see the article in the Linode Library on Configuring a Basic Firewall in Debian Lenny for more information and additional resources.
Whenever you setup any internet facing server firewalling is very important. so you can have a more secure installation. To start with, we’re going to have three ports open: ssh, http and https.
Note: If for some reason iptables is not installed, run the command
apt-get install iptables
We’re going to create two files, /etc/iptables.test.rules and /etc/iptables.up.rules. The first is a temporary (test) set of rules and the second the ‘permanent’ set of rules (this is the one iptables will use when starting up after a reboot for example).
Note: that we are logged in as the root user. This is the only time we will log in as the root user. As such, if you are completing this step at a later date using the admin user, you will need to put a ’sudo’ in front of the commands.
Now let’s see what’s running at the moment:
You will see something similar to this:
Chain INPUT (policy ACCEPT) target prot opt source destination Chain FORWARD (policy ACCEPT) target prot opt source destination Chain OUTPUT (policy ACCEPT) target prot opt source destination
As you can see, we are accepting anything from anyone on any port and allowing anything to happen.
One theory is that if there are no services running then it doesn’t matter. I disagree. If connections to unused (and popular) ports are blocked or dropped, then the vast majority of script kiddies will move on to another machine where ports are accepting connections. It takes two minutes to set up a firewall - is it really worth not doing?
Let’s assume you’ve decided that you want a firewall. Create the file /etc/iptables.test.rules and add some rules to it; if you or another admin user for this slice has worked through these or similar steps previously, this file may not be empty:
The rules are very simple and it is not designed to give you the ultimate firewall. It is a simple beginning.
Hopefully you will begin to see the pattern of the configuration file. It isn’t complicated and is very flexible. You can change and add ports as you see fit. Don’t forget to change the port number ‘30000′ to whatever you specified in your sshd_config.
Good. Defined your rules? Then lets apply those rules to our server:
iptables-restore < /etc/iptables.test.rules
Let’s see if there is any difference:
Notice the change? (If there is no change in the output, you did something wrong. Try again from the start).
Have a look at the rules and see exactly what is being accepted, rejected and dropped. Once you are happy with the rules, it’s time to save our rules permanently:
iptables-save > /etc/iptables.up.rules
Now we need to ensure that the iptables rules are applied when we reboot the server. At the moment, the changes will be lost and it will go back to allowing everything from everywhere.
Open the file /etc/network/interfaces
Add a single line (shown below) just after ‘iface lo inet loopback’:
... auto lo iface lo inet loopback pre-up iptables-restore < /etc/iptables.up.rules # The primary network interface ...
As you can see, this line will restore the iptables rules from the /etc/iptables.up.rules file.
Simple but effective firewall.
Sources: Router Mods I am the writer.