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Hack 20 Find All Available Wireless Networks

Locate all wireless networks in range without
installing any additional software.

So, you've got a
laptop. You've got a wireless card. The card might
even be built into your laptop. You know there are wireless networks
in your area. How do you find them? You might even have an external
antenna connected to your wireless card, hoping to establish a longer
distance connection. How do you find that network a half-mile away?

If you are connected to a wireless network already, you could
download a tool like NetStumbler [Hack #21], but this
requires a network connection and you don't have one
yet.

All of the major operating systems have integrated software that
allows you to discover wireless networks and obtain some status
information about the currently connected network.

Windows XP

If a wireless access point is in
range of your wireless card, Windows XP by default will attempt to
automatically connect to the access point. It will inform you using a
pop up above the task bar, which says, "One or more
wireless connections are available."

Clicking on the network icon opens a window titled
"Wireless Network Connection," as
shown in Figure 3-1.

This window lists any wireless networks that are in range of your
wireless card. In this example, there are three within range. The
window also shows you that the selected wireless network requires the
use of a WEP key [Hack #86] in order to join the
network.

In order to join this network, you would need to type in the WEP key
and then confirm the key by retyping. Once done, you would click on
Connect. The window will close, and the network
icon in the task bar should say "Wireless Network
Connection (network name)". The icon also displays
the wireless network speed and signal strength.

As shown in Figure 3-1, if you have difficulty connecting to any of
the listed networks, you can click on the
Advanced button, which opens a Wireless Networks
window (Figure 3-2).

This window again shows the available
wireless networks. It also shows a list of
"preferred networks" that can be
added by the user. This is important to know if your wireless access
point does not broadcast the SSID, saving you from repeatedly having
to type in the name of the otherwise-invisible network and, indeed,
needing to remember its name in the first place. Many access points
have the ability to disable SSID broadcast as a security feature
(so-called "closed" networks). This
means that you need to know the SSID so you can add a preferred
network (assuming, of course, that you aren't using
a passive monitor like Kismet [Hack #31] or KisMac
[Hack #24]).

At the top of this window is the checkbox: Use Windows to
configure my wireless network settings. If this box is
checked, Windows will automatically attempt to connect to any
wireless networks listed in your preferred networks. If no preferred
networks are available, it will provide you with a list of available
wireless networks as shown above.

To get status on the wireless
network to which you are currently connected, right-click on the
network icon in the task bar and select Status.
A typical status screen is shown in Figure 3-3.

While this gives you some basic connection information, it
doesn't show you actual signal strength in dB, which
would be very useful for testing wireless connections. You also do
not get any information on signal-to-noise ratio. Clicking on the
Support tab gives you IP addressing information
for this wireless card.

Mac OS X

For Apple notebooks with a built-in
AirPort card, all wireless
configuration is handled through the System Preferences
(System Preferences
Network), as shown in Figure 3-4.

You will likely have two available network cards. Click the
Show pull-down menu for a choice of adapters,
including Built-in
Ethernet and
AirPort. Select AirPort. To
get to the wireless network settings, select the
AirPort tab.

I'll come back to some of this later. Right now, you
should be mostly concerned with the Show AirPort status in
menu bar setting, which should be checked. Once you check
this box and close the configuration window, you'll
see a new icon in the menu bar (Figure 3-5). The
first thing you'll want to do is click the menu bar
icon and select the option to turn on the AirPort card.

Once the AirPort card is on, you'll be able to see a
list of available networks; you can select any of these. If a

password
(WEP key) is required for the selected network,
you'll be prompted for it.

To
connect to a network that is not listed, click on
Other.... You will be presented with the
Closed Network box, as shown in Figure 3-6.

Here you
can enter the network name (SSID) of the wireless network you want to
join and the password (WEP key), if one is required. This is how you
can join networks that do not broadcast their SSID.

Once you've either selected an available network or
entered information for another network not listed,
you'll see which network is currently connected by
using the AirPort menu bar (Figure 3-7).

The AirPort software offers a signal strength meter, though it is
rather limited in its granularity. Click the AirPort icon in the menu
bar and select Internet Connect;
you'll see a window similar to that shown in Figure 3-8.

Combined with the lack of a connector for external antennas, this
severely limits the AirPort wireless card as a useful tool for
testing wireless network connections. For more advanced diagnostics,
you might want to take a look [Hack #22].

Linux

Using wireless
networking cards in Linux can require a good deal of work, depending
on your particular Linux distribution. We're not
going to cover that here. This assumes you have
PCMCIA support for
your wireless card, the Wireless Extensions in your kernel,
and the Wireless Tools package installed. Both
the Wireless Tools and kernel patches for the Wireless Extension can
be found at http://www.hpl.hp.com/personal/Jean_Tourrilhes/Linux/Toolsl.
The v.14 Wireless Extensions are included in the 2.4.20 kernel, and
v.16 are included in the 2.4.21 kernel.

The Wireless Tools package does come with many distributions. It
provides four command-line tools:

iwconfig

Allows you to manipulate the basic wireless parameters

iwlist

Allows you to list addresses, frequencies, bit-rates, and more

iwspy

Allows you to get per node link quality

iwpriv

Allows you to manipulate the Wireless Extensions specific to a driver

iwlist is the tool you need at the command line
to show you available wireless networks. To enable scanning, use
this:

# iwlist wlan0 scanning

This gives you detailed information about all detected networks and
is supported in the newer versions of the Wireless Extensions/Tools.
You'll see output similar to the following:

wlan0   Scan completed :
Cell 01 - Address: 00:02:6F:01:76:31
ESSID:"NoCat "
Mode:Master
Frequency:2.462GHz
Quality:0/92 Signal level:-50 dBm Noise level:-100 dBm
Encryption key:off
Bit Rate:1Mb/s
Bit Rate:2Mb/s
Bit Rate:5.5Mb/s
Bit Rate:11Mb/s

If there are multiple access points visible from your machine, you
receive detailed information on each one. Once
you've found the access point you need to connect
to, you can use iwconfig to tell your card about
it.

Anyone who works with wireless networks in Linux will likely be
looking for a more powerful link state monitoring tool. Be
sure to take a [Hack #33] if you need more
functionality than the simple command-line tools provide.

Roger Weeks