WARNING - OLD ARCHIVES

This is an archived copy of the Xen.org mailing list, which we have preserved to ensure that existing links to archives are not broken. The live archive, which contains the latest emails, can be found at http://lists.xen.org/
   
 
 
Xen 
 
Home Products Support Community News
 
   
 

xen-users

RE: [Xen-users] uniq mac address generator

To: "trilok nuwal" <tc.nuwal@xxxxxxxxx>
Subject: RE: [Xen-users] uniq mac address generator
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
Envelope-to: www-data@xxxxxxxxxxxxxxxxxx
In-reply-to: <f58fc26d0704130626n42ea7f5fn229d0e6fd0ce565d@xxxxxxxxxxxxxx>
List-help: <mailto:xen-users-request@lists.xensource.com?subject=help>
List-id: Xen user discussion <xen-users.lists.xensource.com>
List-post: <mailto:xen-users@lists.xensource.com>
List-subscribe: <http://lists.xensource.com/cgi-bin/mailman/listinfo/xen-users>, <mailto:xen-users-request@lists.xensource.com?subject=subscribe>
List-unsubscribe: <http://lists.xensource.com/cgi-bin/mailman/listinfo/xen-users>, <mailto:xen-users-request@lists.xensource.com?subject=unsubscribe>
Sender: xen-users-bounces@xxxxxxxxxxxxxxxxxxx
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 .

Exactly. 
> 
> 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.

--
Mats
> 
> 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
>       >
>       >
>       >
>       
>       
>       
> 
> 
> 



_______________________________________________
Xen-users mailing list
Xen-users@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-users