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

From: jb-gnumlists
Subject: Re: [Qemu-discuss] any restriction on the guest mac addr when using tap networking?
Date: Mon, 29 Apr 2013 02:26:14 +0200
User-agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:17.0) Gecko/20130328 Thunderbird/17.0.5

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

Jakob Bohm, CIO, partner, WiseMo A/S. http://www.wisemo.com
Transformervej 29, 2730 Herlev, Denmark. direct: +45 31 13 16 10 <call:+4531131610>
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