Building a Transparent Firewall with Linux, Part V

Build a transparent firewall using an ordinary PC.
Enabling the Firewall Script

As you may be aware, Ubuntu has adopted a new startup script system. The old one, the init system, still works, and if you prefer, you can enable the script in Listing 2 the old-school way by making it executable and creating rc.d links by running this command:

bash-$ sudo update-rc.d iptables_custom start 36 2 3 4 5 . 
 ↪stop 98 0 1 6

However, I recommend you take the plunge into the world of the newer “upstart” system by skipping update-rc.d and instead adding the following script, iptables_custom.conf (Listing 3), to /etc/init (not /etc/init.d).

Rather than requiring you to figure out which start/stop number to assign to your “rc.” links, upstart lets you just specify what needs to start beforehand (in this example: network-interface, network-manager or networking). As you can see, this iptables_custom.conf file then invokes /etc/init.d/iptables_custom, as listed in Listing 2, to do the actual work of loading or unloading rules. For that reason, /etc/init.d/iptables_custom must be executable whether you use it as an init script or an upstart job.

After saving your /etc/init/iptables_custom.conf file, you must enable it with this command:

bash-$ sudo initctl reload-configuration

Now you either can reboot or enter this command to load the firewall rules:

bash-$ sudo initctl start iptables_custom


And that, in one easy procedure, is how to create a bridging firewall using a Linux PC! I hope I've explained all of this clearly enough for you to figure out how to make it meet your specific needs. I also hope you found the previous few months' foray into OpenWrt to be worthwhile.

The Paranoid Penguin will return in a couple months, after I've had a short break. In the meantime, go forth and secure things!

Mick Bauer ( is Network Security Architect for one of the US's largest banks. He is the author of the O'Reilly book Linux Server Security, 2nd edition (formerly called Building Secure Servers With Linux), an occasional presenter at information security conferences and composer of the “Network Engineering Polka”.