Fwd: OMAPI Host Object and Duplicate MACs in different subnets

[Caciano Machado]

Hi,
I'm managing to enable duplicate MACs in different subnets for ISC
DHCP with OMAPI host entries. After some workarounds I've got some
progress. Here is a patch (for dhcp 3.1.1) with the modifications I've
done. With this, I can add more than one IP for each MAC. I just
removed some tests that were avoiding the duplication and I don't know
what are the side effects :-). Although, the host entries are
activated only after a DHCP restart. How can I enable the "online"
activation of these entries? Any suggestions?

Regards,
Caciano

---------- Forwarded message ----------
From: Simon Hobson <dhcp1 at thehobsons.co.uk>
Date: Wed, Jul 9, 2008 at 3:48 AM
Subject: Re: OMAPI Host Object and Duplicate MACs in different subnets
To: dhcp-users at isc.org


At 17:38 +0200 8/7/08, Stéphane PURNELLE wrote:
>
> It's possible to have multiple same MAC address ?

Officially no, in practice yes.

Every MAC address is supposed to be unique - manufacturers are
supposed to obtain one (or more depending on quantity) 3 byte prefix
from the registry, and are then supposed to track which combination of
the last 3 bytes they have used.

In practice there are a number of problems :

'cheap' manufacturers who don't (or didn't) grasp the importance of
this and so just made up some numbers

'not quite so cheap' manufacturers who bothered to get a prefix, but
then didn't track what they've used

'cock ups' that can happen to even the biggest of manufacturers. I
heard that for a while, Dell were duplicating one value in every 257
machines shipped (ie of the 256 values for the last byte, one got used
twice).

'helpful' users or admins that reprogram a card to match what they just took out



In small networks it doesn't matter as the chances of getting a
duplicate are slim. But as the network gets bigger, the chances of
having a problem go up. Some (like the Dell problem) only show up if
you buy a lot of machines at the same time - like a local university
with a large flat network and did an update by buying thousands of
machines in one order (I know one of the admins and they were 'a bit
puzzled' before they figured out the problem !)