Feature:
DIY Wireless - Base station not required?

Seamus Phan , 1-Feb-2002

Even with 802.11b wireless base stations being more widely deployed today, the base stations still cost US$300 and upwards. Functionality aside, it can be expensive to link up a larger workplace through a continuous wireless mesh.

There is an answer though, if you happen to be a Unix geek, and donÕt mind dirtying your hands trying to hack some code. All you need are FreeBSD or Linux workstations at your workplace that arenÕt used extensively. Some of these stations may serve as redundant file or print servers, or even workstations for temporary staff, and would be ideal for use as makeshift 802.11b base stations.

The Powered Base

There are technical advantages for using FreeBSD and Linux workstations as base stations - security, access control and bandwidth management. Since BSD and Linux come with the likes of packet filtering, user databases, logging, DNS, Squid Web caching and so on, turning these workstations into wireless base stations also means that you can customise these extra features.

For example, you may define rules in the firewall to prevent users not in your access list from entering your workgroup. You can also define ports that are closed (especially pertaining to common ports employed by Trojans and other malware) ports to be redirected, and protocols accepted to be used in and out of your workgroup.

Likewise, you may want to turn on local DNS so that common entries are recognised, and even block out common advertisement and spam host domains by mapping their domains to 127.0.0.1 (localhost).

You can also manually program pornographic, fanatical, political and even competitors' domains into this hosts file.

With Squid, often installed with default BSD and Linux implementations, you can also turn your workstation into a Web caching device so that commonly visited Web sites can be retrieved more quickly, and WAN bandwidth utilisation can be significantly reduced.

Another default option within BSD and Linux implementations is DHCP, which allows you to assign local IP addresses to your workgroup.

What You Need

First off, since most Linux communities seem to use the Lucent Orinoco or WaveLAN 802.11b PC Card adapters as the basis for their hacks, you may want to use that for your BSD or Linux workstation as well. There are also some specific hacks for the Intersil chipset, and you may want to experiment with some of the PC Cards you have first, since instructions for Unix kernel device names may differ.

The best way is to run your workstation with a PC Card slot built in. This is the most direct method for turning workstations into base stations. In lieu of that, you can purchase the ISA or PCI adapter which allows you to plug the 802.11b PC Card into the ISA or PCI adapter.

FreeBSD 4.4 Stable should be used to recognise PCI adapters while the lowest version for ISA adapters should be FreeBSD 4.1 Stable and above. Your workstation should also have two network interfaces, where one is connected to the Internet (through a hub, switch or even directly), and the other to your 802.11b interface.

Next, the Unix kernel has to be configured as an IP router. Essentially, 802.11b base stations are routers, and turning the workstation into a router will allow you to route wireless and wired traffic.

On Linux, your kernel can be configured by the CONFIG_IP_ROUTER set, and DHCP should be turned on.

Then create an /etc/pccard.conf file (to allow configuration of PC Cards) after you configured the "wi" (wireless interface) device in the kernel. For Linux, you are in luck, since there are ÒWireless ExtensionsÓ publicly available. Add "CONFIG_NET_RADIO=y" to your kernel configuration file, usually /usr/src/linux/.config, if you don't find the file /proc/net/wireless. Once all these are done, rebuild your kernel.

What's the catch?

Glad you asked. A true 802.11b base station needs to be able to run in infrastructure BSS (basic service set) mode, which means that it should be able to act as an Ethernet hub. Wireless clients should not communicate directly with each other, but send data packets to the base station to be relayed to other clients.

Linux workstations can be configured as independent or ad-hoc mode (peer-to-peer), which is not entirely practical or useful, or independent BSS mode (IBSS), which is not quite full infrastructure BSS.

However, Jouni Malinen of Finland has developed a Linux driver that enables PC Cards based on IntersilÕs Prism2 chipset to work in infrastructure BSS (or host) mode. Some of the retail adapters found to be compatible with his driver in infrastructure BSS mode include some from D-Link, Compaq and Linksys, specifically MAC HFA3841 and HFA3842.

And if you prefer to use the good old FreeBSD, version 4.3 onwards now is defaulted to infrastructure BSS mode. FreeBSD is a BSD Unix variant, and specifically used in x86 architectures (A good use for those old and obsolete x86 workstations).

If you dig Apples

Apple has conveniently left out the software base station functionality in its new OS X operating system. However, a software base station is basically a masquerading firewall, and it is not too difficult to turn an AirPort-capable Mac (AirPort is Apple's term for 802.11b WLAN) into a base station as well.

First, create a NAT script in a text editor such as BBedit or Pepper:

#!/bin/sh
/usr/sbin/natd -dynamic -interface ppp0
/sbin/ipfw -f flush
/sbin/ipfw add divert natd all from any to any via ppp0/sbin/ipfw add pass all from any to any

Open the Terminal application. Type "su" and press "RETURN".

Then type in your root password and press "RETURN" again. This will enable the root account. Then copy the text block above and type "vi NATd" in the terminal and paste the block into the new file. Press "ESC" and type "ZZ" to save and quit the vi editor.

While still in the Terminal application, type "chmod 700 NATd" to set the permission of the NATd file. Then click the Airport icon in the Dock and select "Create Computer to Computer Network...". Give the network a name and a password if necessary. Choose the System Preferences panel and select "Network." In the "configure" dropdown select "Airport".

Select the TCP/IP tab and set the card to manual configuration. Set the IP address to 10.0.1.1 and the netmask to 255.255.255.0 and leave the rest alone. Click the Airport tab and set the network selection to the network we just created.

Return to the Terminal application and type "vi /etc/hostconfig" to edit the IP forwarding. Scroll down the file and look for a line that says "IPFORWARDING=-NO-". Invoke the replace mode in the vi editor with "cw" and use the arrow keys to scroll to the letter "N" of the word "N". Replace "N" with "YES" and the line should read "IPFORWARDING=-YES-". Type "ZZ" to save and exit vi. Restart the Mac and you can configure the client machines to join this WLAN.

Onwards to work

In today's business, the ability to use tools of any kind to increase value and productivity is key, and it is no longer fashionable nor practical to be practising MIS professionals who simply specify what to buy. Maybe it's time to look in dusty corners and salvage usable hardware to keep costs down.