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Subject: Re: [PATCH 1/3] nl80211: add monitor mode scan feature
To:     Johannes Berg <johannes@sipsolutions.net>
Cc:     linux-wireless@vger.kernel.org, nbd@nbd.name, lorenzo@kernel.org
References: <20200303115017.9845-1-markus.theil@tu-ilmenau.de>
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From:   Markus Theil <markus.theil@tu-ilmenau.de>
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Message-ID: <9a69f20e-8bab-33f1-b607-cfd0e6751a75@tu-ilmenau.de>
Date:   Wed, 11 Mar 2020 10:44:10 +0100
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On 11.03.20 10:36, Johannes Berg wrote:
> 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?)
Ok, I see your point. I think I'll just rework my tool in the way Steve
has already
suggested (use additional STA VIF and set it up for every active scan,
set it down afterwards).

So please feel free to drop these patches, if you haven't already :)
>
> johannes
>