Subject: Re: NL80211_SCAN_FLAG_RANDOM_ADDR ?
From:   Ben Greear <greearb@candelatech.com>
To:     Denis Kenzior <denkenz@gmail.com>, linux-wireless@vger.kernel.org
References: <e09a9bc7-9774-334b-53e8-13e1d781bee3@gmail.com>
 <913e7464-ea6e-bb22-141a-ab76019bed3f@candelatech.com>
Organization: Candela Technologies
Message-ID: <fcd9fcc2-86dd-8cd7-1822-efb7df498cf8@candelatech.com>
Date:   Thu, 11 Apr 2019 16:20:58 -0700
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On 4/11/19 4:19 PM, Ben Greear wrote:
> On 4/11/19 3:30 PM, Denis Kenzior wrote:
>> Hi,
>>
>> I've been poking around at how this flag is used and I noticed this check in net/wireless/nl80211.c:
>>
>> nl80211_check_scan_flags()
>>
>>          if (*flags & NL80211_SCAN_FLAG_RANDOM_ADDR) {
>>                  int err;
>>
>>                  if (!(wiphy->features & randomness_flag) ||
>>                      (wdev && wdev->current_bss))
>>                          return -EOPNOTSUPP;
>>
>>
>> The above disallows the use of RANDOM_ADDR for scans while connected. The nl80211.h uapi header seems to concur:
>>
>>   "@NL80211_FEATURE_SCAN_RANDOM_MAC_ADDR: This device/driver supports using a random MAC address during scan (if the device is unassociated);"
>>
>> However, if I create a P2P Device (in addition to the default STA device), the kernel happily lets me scan on the wdev while the STA interface is connected.
>>
>> sudo iw phy0 interface add p2p type __p2pdev
>> sudo iw wdev 0x2 p2p start
>> sudo iw wdev 0x2 scan randomize
>>
>> So the immediate question I have is, should the RANDOM_ADDR flag indeed be limited to unassociated STA interfaces?  It would seem the hardware is capable 
>> randomizing even when connected? Please educate me :)
> 
> You can be sure that each driver/hardware has its own bugs and limitations related to this.
> 
> Ath10k wave 1 and wave 2 that I am aware of would ignore and/or not ACK probe responses
> sent back to an MAC address that is not that of the station itself.  And changing the mac of a station
> would require complete re-association AFAIK.  That is likely just one of the many issues.

I should add:  If you really want to scan in this manner, you could just create a new station vdev with
random addr and have it do the scanning, then delete it when done?  The original station will continue on
its way unmolested.

Thanks,
Ben


-- 
Ben Greear <greearb@candelatech.com>
Candela Technologies Inc  http://www.candelatech.com