[kismac] Re: SSID field in scanning reports?

  • From: Bob Cunningham <bob@xxxxxxxxxx>
  • To: kismac@xxxxxxxxxxxxx
  • Date: Sat, 06 Dec 2003 09:29:09 -1000

802.11 networks come in two main flavors.  "Managed"
or "infrastructure" networks (that Kismac identifies
as <managed>) involve one or more dedicated "access points".
In an "ad hoc" network (Kismac: <ad hoc>), individual computers
communicate directly, without any access point involved.

In a managed network, an access point sends "beacon frame" broadcasts, typically every 100ms (i.e., 10 times/second) containing data that identify the properties of that particular 802.11 network (e.g., whether WEP is required, etc.). In an "ad hoc" networks with no access points, one of the individual stations (usually the first one that has ad hoc mode turned on) takes on the chore of sending the beacon frames instead.

By default, the SSID is included in those beacon frame broadcasts.
If it is, then Kismac can easily and quickly determine the SSID.

Many models of access points have an option to "hide" the SSID,
which simply means that the AP won't include the SSID in the
beacon frames it sends.  However, the SSID will still show
up in some other types of 802.11 frames that are sent less
often (e.g., "association request",  "reassociation request",
"probe response", and probably a few other types of frames).
In that case (particularly on an essentially idle network),
it can take quite a while for Kismac to detect the SSID when
listening passively.

Kismac may show <hidden ssid>, <no ssid> or blank when it
hasn't yet detected an SSID.  I'm not precisely sure when
each of those is shown (and mileage has varied between different
versions of Kismac).

A "probe request frame" is sent by computers trying to find
access points.  Asking for access points with a specific SSID
(or via "broadcast SSID" to try to get a response from any
access point within range).  Access points send "probe response
frames" back.  (However, hidden access points will generally
not respond to probes to "broadcast SSID".)  The Kismac
<probe> shows probes.

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