Re: MAC address change question


Bill Oliver wrote:
I recently travelled to Seattle for a meeting and decided to play with
changing my MAC address and seeing how that would affect my network
access.

My configuration:

HP Pavilion zd8000 laptop
Mandriva 2006 Linux

Now, if I change my MAC address, either directly using ifconfig or with
macchanger, everything *seems* to work right locally on the machine,
but none of the DHCP servers I've played with will give me an ip address.
This was true for the wireless server at my hotel room, and for the Cingular
service here at SeaTac (where I'm waiting for a plane).

What is the "real" mac address? What did you change it to? What is
the difference?

This is true even if I put the change early in the boot up sequence.

Should not make a difference.

If I change my MAC address back to the "real" hardware address, Cingular
will give me an ip address no questions asked.

They _might_ recognize the mac address as "administratively" set and
won't accept what might be a spoofed mac.

So for example, if I:

<boot up with network down>

% /etc/init.d/network start
% ifup wlan0 (I don't have my wireless come
up automatically)

I get an ip address from the dhcp server
just fine.

Then:

% /etc/init.d/network stop
% macchanger -e -a wlan0

What is your mac address at this point?

% /etc/init.d/network start
% ifup wlan0

It may be using a client-ID that the ISP is "tracking" as well as the
fact that your leases file is not in accord with the mac/client-ID now
being sent in the request. What does your leases file look like?

I get a time out and no dhcp response,

No response or no offer?

even though ifconfig wlan0 gives the
new mac number as the HWaddress.

Seems the ISP's dhcp server does not like your new mac address.

Then:

% /etc/init.d/network stop
% macchanger -m <original number>
% /etc/init.d/network start
% ifup wlan0

I get an address from the dhcp server!

It does like this mac address with a client-ID in accord with it's
records of your lease. This is speculation at this point and there are
other possible causes for the success as opposed to failure.

Everything I know to check looks fine with
respect to the change in MAC address having
been done correctly.

It's likely that the ISP's dhcp server does not like the change. This
varies _greatly_ with different ISPs or even the same ISP when they
reconfigure the dhcp server.

I don't understand why this doesn't work.
Obviously I need to learn something. Any
pointers would be appreciated!

There could be several reasons it does not work. What is your hardware
connection to the ISP's network at home? DSL? Cable modem? Dial-up?
Did you have to manually establish an account with a mac address?

As far as the hot-spot connection failing there are several, perhaps
related, reasons for that as well. Depends on how they were having you
connect. Captive portal? Account setup? Open access hot spot?
Checking your "home" account?

You might notice the same sort of anomolies working from your home
network where you are more familiar with and have greater control over
the variables involved. On the road, you have many new variables of
which you are ignorant and over which you have little or no control and
on top of that you've introduced yet another variable by changing your
mac administratively.

Bottom line is that it is hard to be certain where the "fault" lies.

Did you try using network profiles? Mandriva has profiles, doesn't it?
The best way to get a properly functioning setup that is different
depending on your location is to use profiles.

In any case, if you want to "play" with changing your mac address, I
would do it at home. It's good to know ahead of time if your "regular"
dhcp server will work with such a software change in case you really do
need to make a change to force a new lease be given to you. Handy
sometimes.

hth,
prg

.

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