Re: virtual interface mac adress

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On Dec 4, 2005, at 2:41 PM, H. Peter Anvin wrote:

Followup to: <Pine.LNX. [email protected]>
By author:    Rik van Riel <[email protected]>
In newsgroup: linux.dev.kernel

On Sun, 4 Dec 2005, anil dahiya wrote:

I want to assign mac addres to virtual adpater and mac
address should be like that if it should not create
problem in arp resoultion(i.e. mac address should be
as real card which able to comunicate  on lan )

You may be able to get away with using a MAC address
inside the OUI range that XenSource registered.

Any MAC with bit 0 clear and bit 1 set in the first octet is "local
use"; the best thing to do (unless you have your own OUI) is just to
pick a random address inside this range.  You should only run into
collision problems when you get close to 2^23 hosts on a network.

Theoretically that is true, however there are usages that have been approved that violate that principal. One was for TI Token Ring chips. They were completely unable to use "global" MAC addresses - the local bit always had to be set. Since TI could/would not fix their chips, using the local address became allowed for a universally unique address.

This method was later used by Apple on Ethernet for their DOS card. The Macintosh environment would get the global address and the DOS card would get the local one through the shared ethernet port. You might think that you can ignore the token ring case, but you'd be wrong - there are ethernet/token ring bridges deployed. The Apple case is also best not ignored. I don't know how many others may be doing similar things.

So, I would not advise anyone to simply believe that they can use the entire local MAC address space safely. You are also very likely to have trouble if there is any DECnet usage in the area. Anyone else notice that DECnet kernel patch recently? Someone must still be using it...

This is an instance where Linus' comment a few weeks ago regarding specs vs. reality comes into play. This is kind of an obscure area so not a whole lot of people know about some of these things. Don't believe everything you read in magazines regarding MAC addresses either. I've seen some very bad advice there from time to time in this particular area.

I would recommend using the same MAC address with the local bit set (as Apple did) for a single additional address. If you need more addresses and need them to be visible on the LAN, I don't know of a reliable, generic solution off the top of my head.

--
Mark Rustad, [email protected]

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