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From:   Matthias May <matthias.may@westermo.com>
Cc:     linux-wireless@vger.kernel.org
Subject: Re: ath9k: unable to connect to hidden SSID on DFS channel
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From: Matthias May <matthias.may@westermo.com>
To: =?UTF-8?B?TcOhdGjDqSBLb3Bww6FueQ==?= <mathekoppany@gmail.com>
Cc: linux-wireless@vger.kernel.org
Message-ID: <1c9e0038-d40d-40a0-ac70-5bfcc8d8b95d@westermo.com>
Subject: Re: ath9k: unable to connect to hidden SSID on DFS channel
References: <CALOm-MsEcY7=MwfoEzhCQqSUM6qANy=oP5xYmoPqHAaADcscwA@mail.gmail.com>
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On 10/11/21 12:46 PM, M=C3=A1th=C3=A9 Kopp=C3=A1ny wrote:
> Hello,
>=20
> I have a project where I want to establish Wi-Fi connection to a
> hidden SSID, on DFS channel.
>=20
> I am running wpa_supplicant 2.9 on a device with:
> "Linux 4.9.51-yocto-standard armv7l" and
> "Network controller: Qualcomm Atheros AR958x 802.11abgn Wireless
> Network Adapter (rev 01)"
>=20
> Sounds an old project, I know, but want to bring this alive, if
> possible. Unfortunately, connection to a hidden SSID on DFS channel
> seems not working.
>=20
> However, it works on a different, newer device, which has
> "Linux raspberrypi 5.10.17-v7l+ armv7l" and
> "PCI bridge: Broadcom Limited Device 2711 (rev 10)".
>=20
> The wpa_supplicant on the two devices is mostly idem: same v2.9, same
> runtime configuration, almost all config variables (wpa_cli dump)
> match (not seen relevant difference). There might be compile option
> difference, but don't think they are relevant.
> The only thing I see is with driver flags, where the newer device
> supports flags like DFS_OFFLOAD, BSS_SELECTION, or
> 4WWAY_HANDSHAKE_PSK, but the older device does not offer these.
>=20
> So I suspect the issue is due to some of the above missing features in
> my ath9k driver (have default driver provided by kernel).
>=20
> Questions:
> - is it possible to enable the above driver flags/features for ath9k?
> If so, how to enable and are there patches for these?
> - any hint where to look in ath9k driver for passive scanning
> implementation? Is there another module/location where passive
> scanning is implemented?
> - any suggestions for a solution to my issue?
>=20
> Thanks,
> Koppany
>=20

Hi

Clients not being able to find hidden APs on DFS frequencies is not a bug=
, but by design.

Facts:
* When enabling hidden SSID on an AP, the SSID is not sent in the beacons=
=2E
* Clients (non-master devices) are not allowed to transmit on a DFS frequ=
ency unless an AP (master device) tells them it
is ok.

This means that a client can find new APs only by scanning passively.
Since the beacons do not contain the SSID, the client will never find the=
 AP it is looking for.

Some ways around:
* Your client is a master and does radar detection (with all its downside=
s), thus it is allowed to probe actively
* You hack your beacons and add a custom SSID which allows the client to =
get the SSID passively (hack both sides)
* Your device behaves illegal and sends probe requests on DFS frequencies=

* ???

BR
Matthias


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