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Hack 22 Network Detection on Mac OS X

Find out everything you ever wanted to know
about the networks available in your area.

If you are simply looking for any
available network, you can usually get by with the built-in AirPort
client. But if you are building your own network, or troubleshooting
someone else's, you need much more detail than the
standard clients provide. In particular, knowing which networks are
in range and which channels they are using can be invaluable when
determining where to put your own equipment. Here are two very
easy-to-use survey tools for OS X that give you a far better idea of
what's really going on.

MacStumbler

Sharing nothing but a name with the
very popular NetStumbler [Hack #21],
MacStumbler (http://www.macstumbler.com/) is probably the
most popular network scanner for OS X. It is simple to use, and
provides the details that you are most likely interested in:

available networks, the channels they
use, and their received signal strength. It also displays received
noise, whether WEP is enabled, and a
bunch of other useful details. See Figure 3-14.

Like many OS X apps, MacStumbler is capable of

text-to-speech,
so it will even speak the ESSIDs of networks that it finds as they
appear. Although it is still in beta, I have found MacStumbler to be
a very reliable tool. It currently supports network scanning using
only the built-in AirPort card.

iStumbler

Another popular network discovery tool is
iStumbler (http://homepage.mac.com/alfwatt/istumbler/).
This tool is even simpler than MacStumbler, in that there is really
nothing to configure. Just fire it up and it will find all available
networks for you, complete with a real-time

signal and noise meter.

As you can see in Figure 3-15, there are plans for
GPS support in the next release, but as of v0.6b and at the time of
this writing, the Coordinates field is meaningless. Like MacStumbler,
iStumbler supports scanning only when using the built-in AirPort
card.

These tools will both find all available networks quickly, and will
keep historical logs if you need to monitor wireless networks over
time. If you need to find all available networks in range, either of
these tools are ideal.

MacStumbler and iStumbler work by actively sending out
probe requests to all available
access points. The access
points respond to the probes (as they would for any legitimate
wireless client), and this information is then collected, sorted, and
displayed by the scanners. Unfortunately, neither of these tools will
find "closed" networks, since they
don't respond to probe requests. This is an
unfortunate side effect for people who choose to hide their networks.
Since it isn't easy to tell what channel they are
using, it is very likely that someone nearby will choose to use the
same (or an adjacent) channel for their own network. This causes
undesirable
interference for everybody.
To detect "closed" networks, you
need a passive scanner, such as KisMAC [Hack #24] or Kismet
[Hack #31].