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Message-ID: <75fcee24b6eacd59473731087f854dfc451cab40.camel@sipsolutions.net>
Subject: Re: [PATCH 1/3] nl80211: add monitor mode scan feature
From:   Johannes Berg <johannes@sipsolutions.net>
To:     Markus Theil <markus.theil@tu-ilmenau.de>
Cc:     linux-wireless@vger.kernel.org, nbd@nbd.name, lorenzo@kernel.org
Date:   Wed, 11 Mar 2020 10:36:15 +0100
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References: <20200303115017.9845-1-markus.theil@tu-ilmenau.de>
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On Wed, 2020-03-04 at 10:30 +0100, Markus Theil wrote:
> 
> I used this patchset for some time to build a combined monitoring and
> channel survey tool.
> A mt76 based USB dongle is used, because mt76 supports the channel
> survey cmd which yields the channel's busy time.
> At the same time, the tool listens for beacons over the monitor
> interface to log available networks and their RSSI to the user.
> When the user changes into heatmap mode, he can issue active scan
> requests at different points on the floor plan. All of this this
> can be done with a single interface in monitor mode. I change the
> frequency of the monitoring interface periodically to passively listen
> at other channels.
> 
> nl80211 is able to receive scan results in my setup. "iw dev $MON_IF
> scan" also works as expected.
> 
> A combination of an unassociated STA VIF and a monitor VIF (scan on STA
> VIF, receive Wi-Fi frames on monitor VIF) does not work, because
> I cannot freely set the channel on an unassociated STA VIF. Trying to
> set the operating frequency on the monitor VIF also fails, when the STA
> VIF is up.
> Bringing the STA VIF up would be needed for scanning on it.
> 
> Therefore this patches solved my solely monitoring-oriented use-case.

Yeah, I kinda see where you're coming from, but I still don't think this
makes a lot of sense. I'm actually surprised that "iw dev mon scan"
works (in that it prints results) - would have expected the results to
just go nowhere?

And as you say, you're actually sending probe requests here, while you
cannot even set the MAC address properly on a radiotap monitor
interface.

I don't really think we should mix all these concepts, the more things
we try to allow in monitor mode, the more confusing that becomes (is it
active now? or passive?)

johannes