Re: ath11k: Question about active scanning of DFS channels

[Jeff Johnson <jeff.johnson@oss.qualcomm.com>]

On 9/25/2026 7:00 AM, Alexander Wilhelm wrote:
> Based on my research, DFS channels cannot be scanned passively, so I used a
> sniffer to verify that the "directed" probe requests with given SSID were
> actually being transmitted.

In general that isn't correct.

For starters, everything related to DFS is gated by regulatory rules. So the
operation of the client device first depends upon whether it is configured for
operation in a given country, or if it is configured for World Mode.

If configured for World Mode, then initially DFS channels can only be scanned
passively. A client device in World Mode must not transmit on a DFS channel
unless under the control of a master device. To do otherwise violates
regulatory rules in many countries. So the normal process is to perform
passive scan on DFS channels and process Country elements in Beacon frames in
order to determine the country in which the client is operating (aka perform
an 802.11d scan). Then the client can switch from World behavior to
country-specific behavior.

But even when operating in country-specific mode, whether by the initial
configuration or by processing Country elements, a client cannot transmit on a
DFS channel unless it is under control of a master device. If a client
receives a beacon on a DFS channel, then that channel is considered to be
under control of the master device that sent the beacon, and only at that
point can a client device actively probe/authenticate/associate.

Further complicating the issue is that some regulatory bodies have rules that
govern under what conditions Country elements can be used to determine the
current operating country. In particular, the FCC has rules which require a
device to maintain all FCC restrictions unless multiple criteria are met (I
don't remember the exact rules but seem to remember that at a minimum there
cannot be any master devices sending beacons with US Country elements and
there must be multiple master devices sending beacons with the same non-US
Country element before the device can be consider to be located in that non-US
country.

/jeff