Re: [Qemu-discuss] any restriction on the guest mac addr when using tap networking?

[jb-gnumlists]

On 28-04-2013 07:49, Xeslaro wrote:
>    when using tap networking, everything works perfectly when i don't specify a 
> specific mac addr for the guest, but when i choose a mac addr for the 
> guest(just 6 random bytes), strange things happen. sometimes it works, 
> sometimes it don't.
>    when i dump the traffic on the tap device, the host obviously could receive 
> the packets sent by the guest because it answers all the arp requests, but the 
> guest seems couldn't receive anything at all as it still send arp request and 
> the packets it received is zero from the status of the connection(it's running 
> windows xp).
>    so i guess there's some limitation about the mac addr i could assign to the 
> guest? i googled but didn't find anything related.
>    p.s. the host is running linux 3.9.0-rc6+ and qemu-1.4.1
I have not seen your particular situation before, but there are 3 rules that
I know about for MAC addresses:

- In the first byte, the least significant bit should be 0 for addresses 
that
 can be assigned to an adapter.
- In the first byte, the second-to-least significant bit (the 2's bit) 
should
 be 1 for made up addresses.
- If the first byte second-to-least significant bit is not set, then the 
first
 3 bytes should be a vendor prefix assigned to someone who will let you
 misuse their prefix for virtual machines, there are very few of those.

So the second hex digit should be 2, 6, A or E and odd values are extra bad.

Good: 12:34:56:78:9a:bc
Bad: 14:34:56:78:9a:bc
Very bad: 13:34:56:78:9a:bc

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Jakob Bohm, CIO, partner, WiseMo A/S. http://www.wisemo.com
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