Re: Scanning for infrastructure SSIDs during in active AP mode connection

[Dan Williams <dcbw redhat com>]

On Mon, 2018-10-01 at 18:54 +0200, Tobias Grasse | glancr wrote:
> Hello everyone,
>
> is it possible to scan for nearby WiFi SSIDs (i.e. get “nmcli d wifi
> list” to output available networks) while network-manager has an
> active WiFi connection in AP mode?

It *highly* depends on the driver, and some drivers can do this without
dropping clients.  Many cannot because when when scanning the device is
off the AP channel for a given amount of time, and at any time
(including while the AP is off-channel) a client could try to talk to
the AP.

The Linux kernel has the NL80211_FEATURE_AP_SCAN flag that drivers set
to indicate they can support AP-mode scanning, but only the ath10k and
tiwlan drivers set this flag.

> My usecase: An IoT device creates a setup WiFi (nmcli connection with
> mode “ap”). During setup, the user enters the credentials for their
> home WiFi so that the device can close the AP and connect to the
> infrastructure network. 

For most drivers, this would likely cause the client to disconnect from
the setup AP since it may take multiple seconds for the setup AP to
connect to a WiFi network (including authentication and DHCP).

> I couldn’t find conclusive info in the manpages, a similar question
> on SO has no answers yet and when I try it out ( duh :-D ), “nmcli d
> wifi list” lists only the AP WiFi while the AP connection is active.
> Once I deactivate the AP connection, the command returns all nearby
> networks as expected. Just want to make sure that I’m not missing
> something. 

You're not.

> Unfortunately, I cannot assume that the device has two WiFi
> interfaces, and the one it has only supports simultaneous modes on
> the same channel. 

Yeah, that's the only way around it with many devices, is to have the
setup AP be on the same channel as the other AP you want to connect to,
and then the device doesn't have to jump to another channel.  But of
course you don't know that during setup so that doesn't work...

Dan