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To find nearby routers, mobile devices send probe requests to coax the routers into sending probe responses.

Since modern devices don't use their actual MAC address anymore but instead rely on MAC address randomisation, I'd like to know what would happen if probe requests were sent from two or more devices accidentally sharing the same MAC address. From my understanding and assuming we're just transmitting the wildcard-SSID, the router would respond with a probe response as usual and the devices would still be able to start the association, using their proper MAC address. Or is the randomised MAC address still used in the later communication and would inhibit other devices with the same (false) address to connect to the same network?

Thank you so much for your responses! <3

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Any device randomizing its MAC address is responsible for the side effects, unless each MAC address has already been pre-allocated for that device (which kinda defies the randomization). Therefore, it needs to take all possible precaution to not collide with any other device. However, there's no standard protocol and the device is mostly restricted to passive discovery, which is simpler for wireless protocols and harder for wired (switched) protocols.

If MAC addresses do collide, many things can happen - from logical or physical network disconnection to intermittent problems on the transport or application layer. I don't really see a problem in probe requests alone but in anything else later on.

There are no standard procedures for randomizing MAC addresses nor for dealing with those addresses, so pretty much any assumption may be wrong at some point.

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  • Thank you for your answer! I wholeheartedly understand that there would be a problem if two devices on the same network had the same MAC. But as 1. the MAC can still be exchanged for a proper one in the association phase and 2. most devices (afaik) switch to a per-network-MAC upon connecting to a network, there really doesn't seem to be a problem with colliding MAC addresses, eh? I'd love to see some documentation on this, but wasn't able to find anything in the specification. Regarding your first paragraph; why would a device be restricted to passive discovery if a MAC collision occurred??
    – heddha
    Commented Nov 17, 2021 at 20:19
  • Yes, usually everything works out fine (and a collision isn't too likely to begin with) but after all, there's no standard or specification, so just about anything can happen. A device is restricted to passive discovery before actually using a fake address as there's no way (I can imagine) to do it actively.
    – Zac67 ♦
    Commented Nov 17, 2021 at 20:22
  • Wait - I thought the reason active discovery was used is because it's so much more energy efficient than passive discovery. Are you telling me that devices have to listen across all channels to find out whether they can use a specific MAC address (which could be considered passive discovery)? Then active discovery makes no sense anymore, particularly since APs seem to have a sending rate of 1 beacon per 100 ms! Then we could just use passive discovery instead! That's mind-boggling!
    – heddha
    Commented Nov 17, 2021 at 21:36

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