Multiple clients with same IP address in DHCP environment

I have a client that has a single DHCP server on a flat network.  They run a mixture of Thin Clients, Desktops, and Laptops.  On multiple occasions we have received a conflict between a Thin Client and a Laptop or Desktop.  We have not had a laptop and a desktop conflict.  The DHCP server is a Windows 2008 AD server.  I cannot explain it.
LiberatingInsightAsked:
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Mike ThomasConsultantCommented:
Do you have any rouge dhcp servers on the network with the same scope? have any of the clients been given static adresses from the dhcp scope? that's the only way this could really happen.



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Paul MacDonaldDirector, Information SystemsCommented:
[MojoTech] means rogue, not rouge.
Another possibility is that one or more of the client computers aren't DHCP but using fixed IP addresses.
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LiberatingInsightAuthor Commented:
There is only 1 DHCP server on the network.  We do have some statics but they are outside the range.  This has always happened between a thin client (DHCP) and a laptop (DHCP) or a desktop (DHCP).  We did find that on the Thin Clients they were hard coded to a set # of hrs to get new addresses where the DHCP server is set in days.  We made those 2 settings match as best as we could but the problem continues.
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DonNetwork AdministratorCommented:
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LiberatingInsightAuthor Commented:
I ran the utility and didn't get any responses back.  This one just doesn't make any sense.
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rindiCommented:
Make sure all your ports on the switches that don't connect to servers are set to Auto/Auto (speed/Duplex), and the NIC's of your PC's also use the same settings. If ports and NIC's don't have the same settings, IP conflicts can result.
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jostranderCommented:
Curious to see what other people are doing for this, so I'll join in the fun.

This happens to me a lot... when someone adds a PC to the domain & names it the same as another PC... or duplicate name errors when there is a WINS entry that matches another PC.  

Sometimes for the dupe IP addr, I'll set a fake DHCP reservation for the address and label it bad_ip.  



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moorhouselondonCommented:
>Curious to see what other people are doing for this, so I'll join in the fun.
ditto

Sounds like a problem for Wireshark to get its teeth into.  Restrict the monitoring to DHCP related requests and assignments going into and out of the DHCP server box.  

These "badly behaved" pc's, are they perchance running Vista or Win7?
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PberSolutions ArchitectCommented:
On the clients that are getting the duplicate IP warning, do a IPconfig /all and see what it reports as the DHCP server.  It should tell you if you have a rogue server around.  
Typically we've had issues with VMWARE or other virtual products installed on a clients machine causing the issue.  Your network guys should be able to track down the MAC and where it is patched out.
Also you can turn on conflict detection in DHCP.  Just load your DHCP MMC and select your DHCP server and right click and select properties.  Select the advanced tab and set conflict detection attempts to 2 or so.  This will attempt to ping the end address to see if it replies before it hands it out as a address.  If it does find it, it will mark it bad in dhcp.
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giltjrCommented:
--> We did find that on the Thin Clients they were hard coded to a set # of hrs to get new addresses where the DHCP server is set in days.  

Typically the lease time is provided by the DHCP server.  So if possible configure the thin client to use what the server provides.  If you can not do this, then set the time on the thin client to 1/2 of your server's lease time.

If your DHCP server's lease is less than the time on the thin client then the DHCP server is going to expire the lease, but the thin client will still have the address. So your server will hand out the address while the thin client still has it.
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LiberatingInsightAuthor Commented:
None of the PCs are Vista.  All are XP Pro.  I did set them to expire at 1/2 of the lease time but that still did not solve the issue.  I finally just gave up and set all the thin clients to static addresses and have not had an issue yet.  I'm guessing it has something to do with the thin clients.
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giltjrCommented:
What type of thin clients are you running?
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dan_blagutCommented:
Hello

Did you tried to put conflict detection to 1 in the server properties?

Dan
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kadadi_vIT AdminCommented:
Please check the  Router DHCP Server is disabled or not...?

Regards,
vijay
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LiberatingInsightAuthor Commented:
I have changed the conflict detection to 1 instead of the default 0.  We also have switched the thin clients to static IP addresses outside of the DHCP scope.  We have not had any issues since making both of the changes.
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PberSolutions ArchitectCommented:
If you have a highly routed network, you may want to place the conflict detection at 2.  Many times the first ping can be lost at the router if the host is not in it's ARP table.  The second ping should work though.
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