Paranoid Penguin - Building a Transparent Firewall with Linux, Part II

Use commodity to hardware to build a transparent firewall.
Hardware Considerations

Now that you understand how this setup will look, before and after firewalling, let's talk about firewall hardware. This article series isn't the first time I've tinkered with transparent Linux firewalls. Years ago when I started researching passive network monitoring, I set up several “white-box” PCs that each had multiple network interfaces and could monitor and restrict network traffic transparently.

When I began researching this new series, my first thought was to resuscitate one of those old systems or build a new one. That seemed like a waste of electricity, however. Why deal with case and CPU fans, hard drives and so forth, for something usually handled by optimized network appliances?

This line of thinking brought me to the idea of industrial/embedded platforms—small, diskless computers running an Atom or ARM processor. But the cost of these, especially models with multiple network interfaces, is similar to that of PCs, and I wanted to spend as little as possible.

Then it dawned on me that this is exactly what OpenWrt was designed for! In case you're unfamiliar with it, OpenWrt is a free Linux distribution designed to run on commodity WLAN gateways and broadband routers, such as Linksys' venerable WRT54G series. On the one hand, I'm not much interested in covering WLAN firewalling in this series (although once it's configured properly, a firewall with a WLAN interface can treat it just the same as any other network interface). But on the other hand, the WRT54G is basically a small computer with five network interfaces plus WLAN. Small memory and slow CPU aside, it should make an ideal Linux firewall platform.

This is how I settled on the Linksys WRT54GL wireless-G broadband router, which cost me only $58, as the test platform for my transparent Linux firewall experiments. How well does it perform and scale, and how stable is it? Time will tell. I would guess the short answer is “good enough for home use, but not quite Fortune-500-ready”. Besides, it's bright blue, cheap and cool.

If this sort of hardware hacking isn't quite your cup of tea, I hope you'll stay with me through the series anyhow, because most of the real iptables magic we'll be working in building our transparent firewalling examples is applicable to any Linux system with multiple network interfaces.

One last note on hardware selection. As a Linux firewall platform, a laptop computer makes a nice middle ground between broadband routers and desktop PCs with respect to cost and power consumption, and you easily can add network interfaces to one via USB. Although even a used laptop will cost more than an OpenWrt-compatible broadband router, it will be able to run practically any mainstream Linux distribution, giving you access to a much wider range of software than you can run on OpenWrt.

If you opt for the laptop approach, be sure to select USB Ethernet interfaces that support USB 2.0 (which is necessary for anything approaching acceptable performance—USB 2.0 operates at 480Mbps, but USB 1.1 is only 12Mbps, and 1.0 is a tiny 1.5Mbps!) and, of course, that are Linux-compatible!

I've had good luck with the D-Link DUB-E100, a USB 2.0, Fast Ethernet (100Mbps) interface. It's supported under Linux by the usbnet and asix kernel modules. (My Ubuntu system automatically detects my DUB-E100 interfaces and loads both modules.)

Installing OpenWrt on a Linksys WRT54GL

Back to my OpenWrt adventure, indulge me for a few more paragraphs (plus a few more next month) before we tackle firewall configuration proper. The first step in choosing hardware to use with OpenWrt is consulting the OpenWrt Web site to see what's supported by current versions of OpenWrt (see Resources).

If you choose a Linksys device, which probably is a good choice given that the OpenWrt Project began around the Linksys WRT54G product line, be sure to choose a model whose number ends in L, which indicates “Linux-compatible”. As I mentioned earlier, I chose the Linksys WRT54GL, still available at the time of this writing from various on-line retailers.

The OpenWrt Table of Hardware provides links to other OpenWrt pages giving detailed instructions on installing and configuring OpenWrt on each supported device. In the case of my Linksys WRT54GL, I followed these steps to install OpenWrt:

1. I downloaded the image file openwrt-wrt54g-squashfs.bin from the OpenWrt Web site (

2. I powered on the WRT54GL with its factory-installed firmware.

3. I connected to the WRT54GL by typing the URL in the browser of a laptop connected to one of the WRT54GL's Ethernet ports, not its “Internet” port. Note that my laptop's network interface was configured to use DHCP and actually pulled its IP address via DHCP from the WRT54GL. Hence, it was assigned an IP in the subnet, which the WRT54GL uses by default.

4. I “upgraded” the WRT54GL's firmware with the file openwrt-54g-squashfs.bin and waited a few minutes for the upload to complete and for the WRT54GL to reboot with the new firmware.

5. Finally, from my laptop, I ran the command telnet to connect to the WRT54GL, and I was greeted with this message and prompt:

=== IMPORTANT ============================
 Use 'passwd' to set your login password
 this will disable telnet and enable SSH

BusyBox v1.11.2 (2009-12-02 06:19:32 UTC) built-in shell (ash)
Enter 'help' for a list of built-in commands.

  _______                     ________        __
 |       |.-----.-----.-----.|  |  |  |.----.|  |_
 |   -   ||  _  |  -__|     ||  |  |  ||   _||   _|
 |_______||   __|_____|__|__||________||__|  |____|
          |__| W I R E L E S S   F R E E D O M
 KAMIKAZE (8.09.2, r18961) -------------------------
  * 10 oz Vodka       Shake well with ice and strain
  * 10 oz Triple sec  mixture into 10 shot glasses.
  * 10 oz lime juice  Salute!

Success! Not only had I successfully turned my inexpensive broadband router into a five-Ethernet-interfaced Linux computer, I'd also learned the recipe for a refreshing cocktail, the Kamikaze. Looking around, I was pleased to discover a fairly ordinary Linux environment.

The only thing missing was a Linux 2.6 kernel. I had one more task before proceeding to turning this blue beastie into a firewall—upgrading its kernel. According to the OpenWrt Wiki, you can do so only after first installing a 2.4 kernel (which I'd just done) and changing some NVRAM settings like so via telnet:

root@OpenWrt:/# nvram set boot_wait=on
root@OpenWrt:/# nvram set boot_time=10
root@OpenWrt:/# nvram commit && reboot

This done, on my laptop, I downloaded the latest Backfire version of OpenWrt (v. 10.03 at the time of this writing) from The file I downloaded for my WRT54GL was openwrt-wrt54g-squashfs.bin.

To flash it to my WRT54GL, I opened a command window on my Windows laptop, navigated to the directory to which I'd just downloaded my new firmware image, and without pressing Enter just yet, typed the following command:

tftp -i PUT openwrt-wrt54g-squashfs.bin

Before pressing Enter, I unplugged my WRT54GL, waited a few seconds, plugged it back in, and immediately pressed Enter in my Windows laptop's command window to execute that tftp command. After a few seconds, I got a “Transfer successful” message. The router decompressed the new firmware image, and it rebooted itself to Backfire. When I telneted back in to the router, I was greeted with a new banner:

BusyBox v1.15.3 (2010-04-06 04:08:20 CEST) built-in shell (ash)
Enter 'help' for a list of built-in commands.

  _______                     ________        __
 |       |.-----.-----.-----.|  |  |  |.----.|  |_
 |   -   ||  _  |  -__|     ||  |  |  ||   _||   _|
 |_______||   __|_____|__|__||________||__|  |____|
          |__| W I R E L E S S   F R E E D O M
 Backfire (10.03, r20728) --------------------------
  * 1/3 shot Kahlua    In a shot glass, layer Kahlua
  * 1/3 shot Bailey's  on the bottom, then Bailey's,
  * 1/3 shot Vodka     then Vodka.
root@OpenWrt:/# which tftp

Again, success! Now, not only is my WRT54GL broadband router running Linux, it's also running a reasonably current 2.6 kernel. I'm definitely ready to start configuring this machine for its new stealth firewall duties.



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Transparent firewall

keeper.oregon's picture

Glad to see someone else is trying to tackle this issue. Over the winter break/holidays, I resdesigned my home/remote-office/spouses-business networks from three seperate layer-3 firewalls to a single layer-3 network. I was hoping to utilize the fifth port on two of the wireless routers. I have been trying to get my Cisco LinkSys WRT320N to keep broadcast/multi-cast media traffic local. I will look into the OpenWRT to attempt layer-2 firewall the WRT320N.