Few doubts about ad-hoc devices

Dan Williams dcbw at redhat.com
Fri Aug 22 09:22:08 EDT 2008


On Fri, 2008-08-22 at 09:09 +0530, Raghavendra wrote:
> ----- Original Message ----- 
> From: "Dan Williams" <dcbw at redhat.com>
> To: "Raghavendra" <s.raghu at samsung.com>
> Cc: <hostap at lists.shmoo.com>
> Sent: Thursday, August 21, 2008 7:36 PM
> Subject: Re: Few doubts about ad-hoc devices
> 
> 
> > On Thu, 2008-08-21 at 16:16 +0530, Raghavendra wrote:
> >> Hi All,
> >>
> >>   I have two doubts.
> >>       *  Assume I have one wlan device enabled with ad-hoc mode and
> >>         with no-security. I am using wpa_supplicant for this purpose.
> >>         Since this is a first device to create a ad-hoc network, with
> >>         no security. It is always possible that other devices can
> >>         connect to my device easily in ad-hoc mode.  Is there a way to
> >>         know other devices connecting to this device in ad-hoc mode?
> >>         some indications/events using wpa_supplicant or some ioctls?
> >
> > In ad-hoc mode, every STA just sets a BSSID, SSID, and a channel, and
> > ignores traffic on that channel that's not addressed to the SSID + BSSID
> > that the STA is using.  Ad-Hoc means there's no central authority
> > handing out association IDs and controlling who connects and who
> > doesn't.
> >
> > It's like this: there's a really big open square in a city.  A crowd of
> > people stand in the square.  Some speak English, others speak Chinese,
> > others speak Spanish.  Anyone can walk into the square and start talking
> > in a language they can speak.  Each language is like the SSID+BSSID
> > +channel tuple, and each person is like an ad-hoc wifi node.
> >
> > To join an adhoc network, the STA just starts listening and transmitting
> > because there's nothing to gate its entry into the network.  That's the
> > definition of Ad-Hoc.  If you want security, either set an encryption
> > key or use infrastructure mode.
> >
> > The only indications you have that something else is trying to talk to
> > your STA is incoming traffic with the same SSID, BSSID, and channel as
> > your device is on.
> >
> >>       * Is it possible to hide device ssid in ad-hoc mode? so that
> >>         even if some body scans my ad-hoc device ssid should not get
> >>         displayed in scan result.
> >
> > Ad-Hoc BSSIDs are auto-generated, thus to actually join an Ad-Hoc
> > network you need to know what BSSID to connect to.  So you can hide the
> > SSID all you want by not transmitting it in the Ad-Hoc beacon (which
> > might violate standards, not sure) but then nothing else would be able
> > to join the adhoc network because there would be no way to match the
> > known SSID up with a given random adhoc BSSID.  And BSSID coalescing
> > couldn't occur because you have no idea what SSID all the other BSSIDs
> > are using, and thus you can't eventually converge on one BSSID.
> >
> > In short, no, unencrypted Ad-Hoc networks just can't do what you are
> > asking here.  At least that's my understanding of it.
> 
>  My scenario is as below:
> 
>  Assume two or more persons are near to each other in some place. They are 
> using wifi device in ad-hoc mode with no-security. In this case some unknown 
> person can easily join this network if he scans and finds that there is some 
> ad-hoc device with no-security.
> 
> In order to avoid this situation, if ssid broadcast is disabled on ad-hoc 
> wifi device. Only known persons can scan for a particular ssid and join the 
> ad-hoc network. Is this feature avaible?

No; how do you figure out the BSSID of the ad-hoc network that you need
know in order to join the network?  That BSSID is randomly generated by
the station that creates the network.  If the SSID isn't broadcast,
you'd need to probe-scan.  And since most drivers will just harvest
probe results and add those to the scan list, you've already lost
because now your network is visible.  Beacons are just _one_ place that
the SSID shows up in, so hiding the SSID from the beacons only solves
half your problem.  The SSID still needs to be sent in other situations.

Seriously, use encryption if you want privacy.  Hiding the SSID is _not_
privacy and simply causes more problems than it's worth.

Dan



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