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RE: [Xen-users] uniq mac address generator

  • To: "trilok nuwal" <tc.nuwal@xxxxxxxxx>
  • From: "Petersson, Mats" <Mats.Petersson@xxxxxxx>
  • Date: Fri, 13 Apr 2007 15:36:05 +0200
  • Cc: Xen list <Xen-users@xxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Fri, 13 Apr 2007 06:36:35 -0700
  • List-id: Xen user discussion <xen-users.lists.xensource.com>
  • Thread-index: Acd9z1/upp1qSMxVRvaR2Q85p3IoogAAA0nQ
  • Thread-topic: [Xen-users] uniq mac address generator


> -----Original Message-----
> From: trilok nuwal [mailto:tc.nuwal@xxxxxxxxx] 
> Sent: 13 April 2007 14:26
> To: Petersson, Mats
> Cc: Xen list
> Subject: Re: [Xen-users] uniq mac address generator
> So we cant insure that genearted mac is 100 % uniq in the 
> same network.
> How much the possibility of getting the same mac address. Is 
> it frequent or rare.

I just had a look at the script, and it uses "/dev/urandom", which is a
device that generates random bytes. The upper half (3 bytes) of the
mac-address is a constant (but you have two choices based on whether you
choose "static" or "random" mac-address). 

The other three bytes are taken from "urandom", which we assume is a
good random number generator, so relatively equal probability of hitting
all numbers (_NO_ random number generator is absolutely perfect, but
it's not extremely hard to make one that gives reasonably good equality
in probability). Given a 3-byte combination, you have 1 in 2^24
(16777216) to hit the same number with another machine. It's not very
likely to happen. Presumably this is only done when creating a new
domain, which isn't a very frequent occurrence [I think], which in turn
means that you're very unlikely to match with another MAC address in
your lifetime. 
> Any other thing that we can try to insure the 100% uniq addresses. 
> If we use the mac genearted by xen itself then Could we get 
> the uniqness ?

No, because it also uses a very similar technique of random numbers. 
> I dont think so it could insure uniqness. Because xen also 
> genrate the same way as easymac do .

> If we change the easymac a bit for differnet users then 
> probably we can achieve the uniqness.

If each easymac script uses different numbers (and otherwise unusued in
the mac-address range) in the three first digits, then you should have
no problem. 

The only way to entirely guarantee that YOUR mac-address is unique is to
have a world-wide store of mac-addresses that are known, and "register"
yours there to ensure that you are the owner of that address and no-one
else uses it [but that also assumes that EVERYONE has access to this
mac-address database]. But as stated before, they don't need to be
unique other than within your own network [up to the gateway]. Once it's
outside YOUR network, the mac-address is meaningless.

> Still Doubtful...
> Thanks,
> On 4/13/07, Petersson, Mats <Mats.Petersson@xxxxxxx> wrote:
>       > -----Original Message-----
>       > From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx
>       > [mailto: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> <mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx> ] On Behalf Of
>       > trilok nuwal
>       > Sent: 13 April 2007 14:13
>       > To: Xen list
>       > Subject: [Xen-users] uniq mac address generator
>       >
>       > Hi All,
>       >
>       > I am using  easymac.sh script to genearte the uniq 
> mac address.
>       > This can be downloaded from here.
>       >
>       > http://www.easyvmx.com/software/easymac.sh 
>       >
>       > But How i can insure that mac geneareted by this script will
>       > be uniq in whole world. Some body in the same network also
>       > might use the same script
>       > and can get the same  mac as i am . 
>       >
>       > can anybody tell me, how these will be uniq ?
>       It only needs to be unique to your local network - 
> anything outside that
>       will not be using MAC addresses to determine the 
> routing of the packet
>       anyways [AFAIK], so it's meaningless to aim for 
> uniqueness outside your 
>       own network (although it is of course practical to know 
> that ALL MAC
>       addresses used in the entire known world are unique).
>       Of course, if you're on a large corporate network, you 
> may still have
>       the problem that someone else uses the same easymac 
> script to generate 
>       MAC addresses...
>       --
>       Mats
>       --
>       Mats
>       >
>       > Thanks,
>       > --Trilok
>       >
>       >
>       >

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