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Can not permanently delete BAD_ADDRESS from DHCP - Server 2012

Posted on 2016-11-04
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Last Modified: 2016-11-25
Hi,

I have about 30 BAD_ADDRESS and when I delete them from DHCP, all entries come back a few minutes later. How do I permanently delete these entries and how do they generate.  Thanks in advance.
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Question by:FredSwierczewski
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Qlemo earned 375 total points (awarded by participants)
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This are IP addresses found in use (some device responding on ping) while DHCP wants to give them out to clients. A DHCP Offer from the server always performs a ping test first, and if the IP is in use despite free in DHCP, BAD_ADDRESS is logged. I'm not certain whether the MAC address reported is useful, IIRC it is encoding a state and hence not useful for finding out which device(s) should that be.
Those devices have either not been rebooted for a long time (and not following the DHCP protocol to renew their IP after at least half of the lease time), or (more likely) use static IPs colliding with the DHCP pool.
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by:CompProbSolv
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Qlemo provided an excellent answer to which I'll add a troubleshooting method.  It's crude, but it should work.

If you're not getting any good information (MAC or IP address) of the conflicting devices, you can locate them with a bit of effort.  When it won't disturb the users (outside business hours?), note exactly how many bad addresses you have, disconnect cables from the switch and clear the bad addresses from DHCP.  Watch it for the few minutes it took for the addresses to return.  If they don't, then reconnect cables one at a time and wait a few minutes.  When you have connected a cable with an offending device, the bad addresses should return.
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by:Qlemo
Qlemo earned 375 total points (awarded by participants)
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If you've got managed switches, you can ping the IP address from a workstation, then get the MAC address with arp -a, and look up that MAC address in the switch's port table.
If the device is directly connected to that switch, a single port is the result, and you can follow up the culprit.
If the port is connected to another switch, the port table contains more than one MAC address; the connected switch needs to get checked, and so on. This of course ends at unmanaged switches.
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