Re: [Bridge] Question about using brctl and changing MAC addresses

[Philip Prindeville <philipp_subx@redfish-solutions.com>]

Stephen Hemminger wrote:
> On Mon, 10 Mar 2008 11:05:53 -0700
> Philip Prindeville <philipp_subx@redfish-solutions.com> wrote:
>
>   
>> I'm using Linux 2.6.20 (yes, I'll be upgrading to 2.6.24 as soon as it's
>> stable).
>>
>> I have 4 VIA rhine ethernet controllers on my Soekris net5501, plus an
>> ADSL PCI card (a Sangoma S-518) that runs in AAL5-SNAP mode, so it looks
>> like an Ethernet controller.
>>
>> I'm trying to bridge eth0 and w1ad (the ADSL interface) into "br0".  And
>> I'm trying to force the traffic on br0 to have the MAC address that my
>> ISP insists I use (i.e. that of the crappy little Westell 6100 modem
>> they sent me).
>>
>> The issue is the following.  If I do:
>>
>> ifconfig w1ad down hw ether x:x:x:x:x:x up
>>
>> followed by either:
>>
>> brctl addbr br0
>> brctl addif br0 eth0
>> brctl addif br0 w1ad
>>
>> or:
>>
>> brctl addbr br0
>> brctl addif br0 w1ad
>> brctl addif br0 eth0
>>
>> the bridge ends up taking the MAC address of eth0 either way, which I
>> don't get (how does it decide which to use?).
>>
>> I can clone the same MAC address to both eth0 and w1ad, but I don't know
>> if that would cause me any problems or not (I haven't read the STP spec
>> in about 15 years).
>>
>>     
>
> Having the same mac address on both just is going to be problematic (impossible
> to do STP), so don't if you don't have to.
> But you can set address of bridge and eth0 to any address you want. So set these
> to the ISP assigned address.  You probably will have to do it after bridge
> is created.
>
> brctl addbr br0
> brctl addif br0 w1ad br0
> ifconfig eth0 hw ether x:x:x:x:x:x up
> ifconfig w1ad up
> ifconfig br0 hw ether x:x:x:x:x:x up
>
> ifconfig br0 A.B.C.D netmask 255.255.255.0 
>   

That seems to not work.  I get:

# ifconfig br0 hw ether "00:18:3A:55:15:35"
ifconfig: SIOCSIFHWADDR: Cannot assign requested address
#

Any ideas why?  I'm doing this before an IP address has even been 
assigned...

-Philip