How to handle multiple SSIDs with the same BSS

Dan Williams dcbw at redhat.com
Fri Dec 25 18:12:30 EST 2009


On Fri, 2009-12-25 at 15:29 -0800, Marcel Holtmann wrote:
> Hi Dan,
> 
> > > > > >>> BSS 00:1a:a1:53:50:a0 (on wlan0)
> > > > > >>> 	SSID: Private
> > > > > >>> BSS 00:1a:a1:53:50:a0 (on wlan0)
> > > > > >>> 	SSID: Public
> > > > > >>> When using the wpa_supplicant D-Bus interface, every BSS can only be
> > > > > >>> listed once and so we either see "Private" or "Public", but never
> > > > > >>> actually both of them. How should this be handled.
> > > > > >> I wasn't aware that there is this kind of limitation in the D-Bus
> > > > > >> interface. It is clearly broken if it cannot include both of those
> > > > > >> networks.
> > > > > > 
> > > > > > from what I have read so far while going through the code, it is not
> > > > > > possible. The BSSID is used as unique path in D-Bus terms and because of
> > > > > > that there is no way both networks could be listed at the same time.
> > > > > > 
> > > > > Precisely. Path of DBus object representing BSS is based on BSSID. Since
> > > > > the path must be unique we can't have both networks listed.
> > > > > I think that we can change a path format quite easy in the new DBus API,
> > > > > but I'm afraid that changes in the old API may may brake it.
> > > > > Actually, I was sure that BSSID is an unique identifier, but obviously
> > > > > we need to think out something else. Is the pair of BSSID and SSID
> > > > > unique? I'm not sure.
> > > > 
> > > > I missed the beginning of this, but the BSSID *should* be unique unless
> > > > someone is spoofing their mac.  If it is two separate APs each
> > > > providing the same SSID, the BSSID should be the mac and that should be
> > > > unique.  If it is virtual APs providing the same SSID, the BSSID should
> > > > be unique.  I apologize for missing the beginning of this, but
> > > > something seems strange with the AP setup.
> > > 
> > > the BSSID is unique, but an access point can broadcast two different
> > > SSIDs via the same BSSID.
> > > 
> > > This is not about the same SSID. We are talking about different SSIDs
> > > from the same access point.
> > 
> > This used to be used for Cisco APs where the main broadcasted SSID was
> > the "public" SSID (Guest SSID) and there was a second non-broadcast SSID
> > using the same BSSID.  Of course, the public SSID shows up in the scan
> > results unless you probe-scan for the hidden one.  Yay.
> 
> I have all variations of these Cisco APs around. When both are hidden
> then you are also going to have fun. So far it is just pure luck which
> one shows up first. Especially with Intel wireless cards where you
> always get the full set of scan results after a probe scan.

Yup; internally I believe mac80211 (if it doesn't it should) stores
things as a tuple of (ssid,bssid).  We should be carrying that unique ID
through the whole stack, from driver -> supplicant -> connection
manager.

Dan




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