nl80211, wext, dhclient

Jouni Malinen j at
Fri Sep 4 17:44:36 EDT 2015

On Fri, Sep 04, 2015 at 10:33:40PM +0200, Simon Kitching wrote:
> I added ap_scan=2 due to various advice on the internet, and due to
> comments in the wpa_supplicant sourcecode.

If those were talking about the nl80211 driver interface, they were not
very good advice.. ap_scan=2 does not really have any good use with that
driver interface.

> From wpa_supplicant/config_ssid.h:
>      * scan_ssid can be used to scan for APs using hidden SSIDs.
>      * Note: Many drivers do not support this. ap_mode=2 can be used with
>      * such drivers to use hidden SSIDs.

That "many drivers" does not include nl80211-based interface..
scan_ssid=1 is the way to do this with nl80211.

> From wpa_supplicant/config.h:
>      * 2: like 0, but associate with APs using security policy and SSID
>      * (but not BSSID); this can be used, e.g., with ndiswrapper and NDIS
>      * drivers to enable operation with hidden SSIDs and optimized roaming;
>      * in this mode, the network blocks in the configuration are tried
>      * one by one until the driver reports successful association; each

Neither ndiswrapper not NDIS have anything to do with nl80211..

> So the only outstanding issue is a minor one: that wext works with
> "ap_scan=2" but nl80211 does not (and fails without error-message).

Outstanding issue? This difference in the driver interfaces is by
design.. Or well, ignoring that lack of a clear debug log message.. In
theory, I'd like to prevent ap_scan=2 completely with nl80211, but
unfortunately, there has been some products using that configuration and
if those still exist, automatically ignoring that parameter is somewhat
inconvenient since it could potentially cause issues with whatever the
use case was that originally made the design use ap_scan=2.

I guess wpa_supplicant could complain in debug log if ap_scan=2 is used
with nl80211 if that would make it any easier to figure out that that
combination is unlikely to be a good idea.

Jouni Malinen                                            PGP id EFC895FA

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