Peter D. Hedderley
Technology Specialist & IT Consultant

"Software is like a lucid dream, limited only by our imagination and ingenuity
- we are the architects, sculptors, rule-makers and rule-breakers."
Peter D. Hedderley.

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Chasing the MAC

We had an interesting experience at work this week. Something which shouldn't have happened - did - and it had been causing me no end of troubles over the past months in a VMWare virtual machine. The only problem was - we didn't know what was causing it.

So, what was going on?

The symptoms were as follows:

* Periodic disconnection from MS SQL Servers (connection reset).
* Periodic blue screens with errors from the NDIS drivers.
* Strange PING reports when using ping -t to continuously ping a server.

What had actually happened and how did we solve it?

Well, it turns out the the strange PING behaviour was pure fluke and had nothing to do with the real problem - however it did point us (luckily) in the right direction. As we thought the problem was related to the physical hardware, we moved the virtual machine to a different host - however the problem did not go away.

During our tests, we had stopped the virtual machine. Accidentally, we found out that when we tried pinging the IP address of the virtual machine (which was off and should not have been reachable!) that the machine could be reached! So how is that possible? A virtual machine that is switched off - replies!?

Some time ago, we cloned the virtual machine for a specific purpose. However, the MAC address of the clone was the same as that of my original virtual machine. Normally, this would not have bothered us as it was never our plan to have both virtual machines in the same physical network. It was deliberate that we kept the MAC addresses the same - so that machine/license identities were not damaged (we had no intention of using the machines at the same time, so there was no risk of license infringement).

When someone did put the machine back on the same network (which they shouldn't have done - but still - that's not the issue here) - a can of worms was opened and my original virtual machine ended up in chaos.

Needless to say, now that the unwanted clone is now gone, my virtual machine is behaving as it should be and all is right again with the world.

What can we learn from this? Virtual machines introduce situations which would never normally occur in the “physical world” and when cloning virtual machines, be careful!

Published on 08.03.2013 -

articles/chasing_the_mac.txt · Last modified: 2013/03/08 11:18 by Peter Hedderley