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Re: how to detect MAC address

 
 
Rene
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      06-28-2003
Sudsy <(E-Mail Removed)> wrote:
> I'm amazed by how many times this topic arises. I can only surmise
> that people are trying to use the MAC address for some licensing
> scheme. I almost hate to tell them that there are ethernet cards
> out there which allow you to reconfigure the MAC address. As long


Well that's a driver / software thingie. Almost any card can do it.
Actually, I've never seen one that couldn't do it.

It is just that the rumor that the MAC addresses are unique and
unchangeable without an eprom burner still floats around.

But that is simply wrong.

The MAC address *is* set statically in a NIC, so much is true. It is
further true that in most cases the MAC address can only be changed by
exchanging the eeprom. However, the MAC address is only read out of the
card once at boot time (or rather, the time when the NICs driver starts and
initializes the card) and then the MAC address is stored in memory in the
OS network driver.

The driver puts together data packets, not the NIC, the NIC has to bring
the data on the wire, but the OS "fills in" the data into the frame. So its
a pure driver thing. All you need is a driver which allows you to override
the default value that is provided by the NICs eeprom.

And most drivers do, since without that feature, you couldn't do failover
and other high-availability functions.

> as you don't have more than one machine on the LAN segment with
> the same MAC then you don't have a problem with routing.


You can always override the value and create a collision by hand. There are
some very specific and rare cases where it makes sense to do so. Generally
its a very bad thing. But it can be done. You shouldn't though, unless you
know exactly what you do.

The bottom line for everyone else is: Trusting on a static never changing
MAC address is no more secure than trusting on a static, never changing IP
address. (there are people around who tell you to configure a firewall to
filter on MAC address instead of IP addresses, since MAC addresses are
static. This is very untrue)

CU

Rene

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