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How do you find a duplicated MAC address (spoofing)?

Posted on 2007-04-06
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Last Modified: 2012-06-27
In trying to figure out why our network traffic seems to be up, I've installed NTOP on one pc and put it off of a hub to monitor the traffic.  One of several things that NTOP reports on is duplicate MAC addresses.  Several of our pc's have been red flagged as high risk because NTOP found duplicate MAC addresses which might indicate spoofing.
 
If indeed something/someone is spoofing, how to I track this down?  How do I find out if it's really a MAC address being spoofed and then find the duplicate?
 
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Question by:lloydr1l
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by:Alan Huseyin Kayahan
ID: 18864267
         *Would they be the clients which have more than 1 NIC or any other network adapter?
          Also please have a look at this link
http://www.experts-exchange.com/Security/Software_Firewalls/Q_21926836.html?sfQueryTermInfo=1+find+mac+spoof+trace
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by:lloydr1l
ID: 18864291
No, just the one.
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Expert Comment

by:Alan Huseyin Kayahan
ID: 18864788
Macpaq tool should work for you.
http://amac.paqtool.com/reprogram.htm
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by:lloydr1l
ID: 18864815
How would that help me find who is spoofing?
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Alan Huseyin Kayahan earned 250 total points
ID: 18867855
              *When you run network detection, which lists mac addresses, spoofed MACs would be easily recognized if spoofer is not a prof.
                For ex
               00-50-FC-F7-27-FD is a normal MAC address at first look. Spoofers usually spoof to thing like following
               00-12-34-AB-CD-56 and this is a suspicious MAC.
             
             *For more details please check this link
http://en.wikipedia.org/wiki/IP_Traceback
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Expert Comment

by:rickhobbs
ID: 18873915
Are you minitoring networ, server, or Internet traffic or some combination?
Where is the hub.  use a line drawing.   like

network switch---------------hub------------Internet router
                         server____/   \______monitor PC
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Author Comment

by:lloydr1l
ID: 18876174
Internet router---------Firewall--------hub-----------switch-----etc
                                                             \__monitor PC
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Expert Comment

by:rickhobbs
ID: 18876670
Are you trying to locate spoofed MAC addresses on your network or spoofed MAC addresses coming from the Internet?  If the Internet, what type of router are you using.  Hopefully Cisco.  If you are using Cisco to connect to the Internet, with the combination of CEF (Cisco Express Forwarding) and Netflow it should be fairly easy to determine what addresses are spoofed.
Check this article: http://www.cymru.com/Documents/tracking-spoofed.html
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Author Comment

by:lloydr1l
ID: 18877082
On our LAN.  When I read results using NTOP, the program reports that MAC spoofing might be going on.  If it is, I'm not sure how to locate any local spoofing, or any spoofing for that matter.
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Assisted Solution

by:rickhobbs
rickhobbs earned 250 total points
ID: 18877375
The best way would be to identify every machine by Name, Username, IP address, and MAC address.  If you can identify all the systems on your network and they have different MAC addresses, spoofing is probably NOT occurring.  Since the MAC address is only used in the initial connection to obtain an IP address, the only reason someone on your network would be spoofing would be to use someone else's IP address.  In all the yeas I have been working on networks, I have never seen that done.
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Author Comment

by:lloydr1l
ID: 19261605
Works for me, but I'll give them the points.  
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