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Duplicate Mac Address

Posted on 2011-03-11
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Last Modified: 2012-05-11
Hello,

We have had our network running with about 30 computers bought 5 years ago.  Anyway, everything has been fine until today.  We have two computers grabbing the same IP address from DHCP because they both have the same Mac address.  The Mac address on both computers also is odd.  It starts with 40 and the other 28 omputers bought at the same time start with 00.  We have had to replace some hard drives and even mother boards over the years, but nothing in the past several months.  I am really concerned that we have a hacker or virus running around, so any help/answers would be greatly appreciated.  Thanks.
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Question by:rcblevins
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by:Ernie Beek
ID: 35110082
There are possibilities to manually put in a mac address in the network drivers. Perhaps somebody has been experimanting with that?
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by:MikeKane
ID: 35110095
Hardware should not have the same MAC addresses on the NIC.    IT should be really easy to find the dupes since your switch would complain about flapping between 2 ports.  

I would guess that there may be a virtual machine that was cloned running on 2 workstations.    But that should be very evident.  
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by:LHT_ST
ID: 35110102
mac spoofing can be done if the user is intelligent enough.
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by:Ernie Beek
ID: 35110115
If you have the first six characters of the mac address you could try to figure out what it is: http://www.coffer.com/mac_find/
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by:nsx106052
ID: 35110154
I would try to see if you can pin point the location of both computers then check the actual MAC address.  Once done verify the actual MAC addresses.  If one doesn't match then I would scan for hacking utilities.  Malwarebytes or CCcleaner should be able to find them.  There are several utilities that can spoof MAC addresses.  

I would also check to make sure none of these machines are virtual.  If so you might need to create another NIC and disable the current one.
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by:rfc1180
ID: 35113200
Depeding on the equipment you have, the images you are running, mitigating MAC spoof is a fairly simple process and can be addressed with DHCP Snooping: http://en.wikipedia.org/wiki/DHCP_snooping

As already mentioned, get down to the root cause to investigate to determine if the issue was related to experimentation, accidental, or malacious. If you switches are managed switches, you should be able to view the MAC/CAM table of the switches and locate the MAC to a specific switchport on the switch.

What type of switch equipment do you have connected to the network?
If the switches are not managed, then your only ooption is to investigate by visiting each computer and checking the properties of the card or TCP/IP network settings (ipconfig /all, etc).

Billy
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rcblevins earned 0 total points
ID: 35127639
Thanks to all for your help and suggestions.  Finally found the issue and resolution at the link below.  We have Dell SX270 computers and the mac address was the exact same that's specified in the link below.  Very wierd problem.   Thanks again.

http://forums.techguy.org/networking/272551-mac-address-changed-spontaneously.html
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by:Qlemo
ID: 35360941
This question has been classified as abandoned and is being closed as part of the Cleanup Program. See my comment at the end of the question for more details.
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