modify mac address with multiple interfaces on a device

modify mac address with multiple interfaces on a device

[ferran]

Hi there,

I have noticed that having more than one interface pointing to the same
physical device, modifying the address of any of these interfaces gets
limited to the last 3 bits of the address, thus giving only 8 possible values.

Moreover, the mac address is set after the previous interface's one. Let's
put it in a example (output copied by hand)

---

$ iw dev
phy#0
     Interface wlan0
            ifindex 7
            type managed
$ sudo ip link set wlan0 down
$ sudo ip link set wlan0 address 00:11:22:33:44:55
[ mac is changed ]
$ sudo iw phy phy0 interface add mywlan type managed
$ iw dev
phy#0
     Interface mywlan
            ifindex 9
            type managed
     Interface wlan0
            ifindex 7
            type managed
$ ip link
[ ... ]
7: wlan0: [...]
    link/ether 00:11:22:33:44:55 [...]
7: mywlan: [...]
    link/ether 00:11:22:33:44:56 [...]

$ sudo ip link set wlan0 down
$ sudo ip link set wlan0 address aa:bb:cc:dd:ee:ff
RTNETLINK answers: Invalid argument

$ sudo ip link set wlan0 address 00:11:22:33:44:5f
RTNETLINK answers: Invalid argument

$ sudo ip link set wlan0 address 00:11:22:33:44:57
[ mac is changed ]

$ sudo iw mywlan del
$ sudo ip link set wlan0 address aa:bb:cc:dd:ee:ff

$ ip link
[ ... ]
7: wlan0: [...]
    link/ether aa:bb:cc:dd:ee:ff [...]

---

However, in another machine, using an iwlwifi card, I can change mac
addresses disregarding other interfaces on same physical device. Another
fact, when adding a new interface to a phy, the assigned mac address is
derived from the factory immutable one, instead of from the other interface's.

---

Where can I confirm this behaviour, is it docummented? Am I right saying
that this depends on the actual driver? And how much difference is between
one and another?

Thank you very much

Re: modify mac address with multiple interfaces on a device

[Ben Greear]

On 05/27/2015 11:44 AM, ferran wrote:
> Hi there,
> 
> I have noticed that having more than one interface pointing to the same
> physical device, modifying the address of any of these interfaces gets
> limited to the last 3 bits of the address, thus giving only 8 possible values.
> 
> Moreover, the mac address is set after the previous interface's one. Let's
> put it in a example (output copied by hand)

This does depend on the driver.  ath9k, for instance, has no limitations.

There are some subtitles relating to the ath9k BSSID mask that mean you often only
want to change only bytes 1 and 2 (leaving 0, 3, 4, 5 alone), but it is not
a hard requirement.

Thanks,
Ben

> 
> ---
> 
> $ iw dev
> phy#0
>      Interface wlan0
>             ifindex 7
>             type managed
> $ sudo ip link set wlan0 down
> $ sudo ip link set wlan0 address 00:11:22:33:44:55
> [ mac is changed ]
> $ sudo iw phy phy0 interface add mywlan type managed
> $ iw dev
> phy#0
>      Interface mywlan
>             ifindex 9
>             type managed
>      Interface wlan0
>             ifindex 7
>             type managed
> $ ip link
> [ ... ]
> 7: wlan0: [...]
>     link/ether 00:11:22:33:44:55 [...]
> 7: mywlan: [...]
>     link/ether 00:11:22:33:44:56 [...]
> 
> $ sudo ip link set wlan0 down
> $ sudo ip link set wlan0 address aa:bb:cc:dd:ee:ff
> RTNETLINK answers: Invalid argument
> 
> $ sudo ip link set wlan0 address 00:11:22:33:44:5f
> RTNETLINK answers: Invalid argument
> 
> $ sudo ip link set wlan0 address 00:11:22:33:44:57
> [ mac is changed ]
> 
> $ sudo iw mywlan del
> $ sudo ip link set wlan0 address aa:bb:cc:dd:ee:ff
> 
> $ ip link
> [ ... ]
> 7: wlan0: [...]
>     link/ether aa:bb:cc:dd:ee:ff [...]
> 
> ---
> 
> However, in another machine, using an iwlwifi card, I can change mac
> addresses disregarding other interfaces on same physical device. Another
> fact, when adding a new interface to a phy, the assigned mac address is
> derived from the factory immutable one, instead of from the other interface's.
> 
> ---
> 
> Where can I confirm this behaviour, is it docummented? Am I right saying
> that this depends on the actual driver? And how much difference is between
> one and another?
> 
> Thank you very much
> 
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-- 
Ben Greear <greearb@candelatech.com>
Candela Technologies Inc  http://www.candelatech.com