Firewalls are usually designed to prevent outsiders from accessing services on our network, they're also used to prevent DoS (Denial of Service) and DDoS (Distributed Denial of Service) attacks.

FreeBSD comes with a superb firewalling tool called ipfw. It's very flexible has tons of options yet it's pretty easy to use & set up. Now we're going to discuss how to set up a firewall for a workstation connected to the internet trough a dial-up connection (that's what I have at home), however you can use this document for other connection types too.

First of all you have to reconfigure your kernel. If you don't know how to do that, check in the FreeBSD handbook.

Edit your own kernel and add the following lines.

Now all you have to do is recompile the kernel. But don't reboot yet. We're gonna still need to do some changes in /etc/rc.conf.

Open /etc/rc.conf in your favorite text editor and add these lines.

Now, if you are a dial-up user you might have natd enabled either in your rc.conf file either in your ppp.conf file. Disable them by adding/modifying this line in rc.conf:

ppp_nat="NO"

Now let's get down to the firewall rules. Create the /etc/firewall/ directory and start editing the ipfw.rules file (or whatever you added to rc./conf) - example.

That's all. Now reboot your machine. In order to check if the firewall was loaded type:

ipfw show


You should see the firewall rules you just made. Now your firewall is up and running protecting you from the outside world. If you want to disable your firewall do:

ipfw -f flush

The great thing about ipfw is that you can add commands on the way. All you have to do is type:

ipfw add Rule_Number Rule

The Rule_Number is important because ipfw interprets the commands in the order you specify them. For more advanced ipfw rules read the man.

Testing your security

OK, now you have a pretty secure workstation that you can safely connect to the internet. But how can you know that for sure ? Test it.

Testing the firewall

For this operation you'll probably need a friend's help (because your firewall was configured to allow localhost connections. Of course you could add something like:



ipfw del 120

ipfw del 130

ipfw del 140



Or whatever the numbers of the rules that allow localhost connections are. Either way you have to find a way to be "outside" the firewall.

Now, ask a friend, or by yourself do a nmap scan (you can install it in freeBSD from /usr/ports/security/nmap). The command should be something like:

nmap -v -O -sS your.host.com