The tale of a single MAC

Steven Bellovin smb at cs.columbia.edu
Sun Jan 2 07:50:42 CST 2011


On Jan 1, 2011, at 11:33 24PM, Mark Smith wrote:

> On Sat, 01 Jan 2011 20:59:16 -0700
> Brielle Bruns <bruns at 2mbit.com> wrote:
> 
>> On 1/1/11 8:33 PM, Graham Wooden wrote:
>>> So  here is the interesting part... Both servers are HP Proliant DL380 G4s,
>>> and both of their NIC1 and NIC2 MACs addresses are exactly the same.  Not
>>> spoofd and the OS drivers are not mucking with them ... They¹re burned-in 
>>> I triple checked them in their respective BIOS screen.  I acquired these two
>>> machines at different times and both were from the grey market.  The ³What
>>> the ...² is sitting fresh in my mind ...  How can this be?
>> 
>> 
>> From the same grey market supplier?
>> 
>> I know HP has a disc they put out which updates all the firmware/bios in 
>> a specific server model, its not too far fetched that a vendor might 
>> have a modified version that also either purposely or accidentally 
>> changes the MAC address.  Off the top of my head, I'm not sure where the 
>> MAC is stored - maybe an eeprom or a portion of the bios flash.  Or, it 
>> could be botched flashing that blew away the portion of memory where 
>> that was stored and the system defaulted to a built in value.
>> 
>> Excellent example is, IIRC, the older sparc stuff, where the ethernet 
>> cards didn't have MAC addresses as part of the card, but were stored in 
>> non-volatile or battery backed memory.
> 
> This was actually the intended way to use "MAC" addresses, to used as
> host addresses rather than as individual interface addresses, according
> to the following paper -
> 
> "48-bit Absolute Internet and Ethernet Host Numbers"
> Yogan K. Dalal and Robert S. Printis, July 1981
> http://ethernethistory.typepad.com/papers/HostNumbers.pdf

Yup.
> 
> That paper also discusses why 48 bits were chosen as the size, despite
> "Ethernet systems" being limited to 1024 hosts. 
> 
> I think things evolved into MAC per NIC because when add-in NICs
> were invented there wasn't any appropriate non-volatile storage on the
> host to store the address. 
> 
On really old Sun gear, the MAC address was stored on a separate ROM chip; if the
motherboard was replaced, you'd just move the ROM chip to the new board.

I'm not sure what you mean, though, when you say "when add-in NICs were
invented" -- the Ethernet cards I used in 1982 plugged into Unibus slots
on our VAXen, so that goes back quite a ways...



		--Steve Bellovin, http://www.cs.columbia.edu/~smb









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