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Windows 2012 R2 DHCP server BAD_ADDRESS, confict detection OFF, how does it detect conflicts?

Question

Monday, August 5, 2019 10:01 AM

We have a setup with two DHCP servers running in active-standby failover mode, both are Windows 2012 R2 and have all patches applied including KB2919355. For some reason on some of the scopes DHCP is servicing we experience BAD_ADDRESS ip's being put in those scopes, ultimately depleting the scopes.

We have conflict detection turned OFF.

Now I understand that if conflict detection is turned ON these addresses could emerge if there are, for example, workstations were given a static ip in the scope. But conflict detection is turned OFF. So how does the DHCP service detect there is a conflict?

We have not been able to find any useful information on this, does anyone know how DHCP marks ip's as conflicting in this situation?

The ARP table of the DHCP servers do not contain any information on the ip's being a BAD_ADDRESS so that is not used. The DHCP servers should not PING the address as conflict detection is turned off so that is not used. 

Funny thing is that the addresses marked as BAD_ADDRESS have a MAC address containing 8 digits and if we recalculate this it translates back to the ip address that is marked as BAD_ADDRESS so this does not help in troubleshooting or finding offending devices.

All replies (3)

Tuesday, August 6, 2019 6:43 AM ✅Answered

Hi,

Thanks for your question.

Firstly, In addition to DHCP server, the DHCP client will send Address Resolution Protocol request (ARP) to do conflict detection during the process of DHCP client requesting IP address.

DHCP BAD_ADDRESS may generate from the following causes.

  1. In general, when an IP conflict is detected, DHCP BAD_ADDRESS temporary are created by the DHCP server. Please make sure that those addresses are not being used.                                                                             
  2. You have one multihomed DHCP client and one DHCP server. Both network adapters on the DHCP client and the network adapter on the DHCP server are connected to the same network segment.                                   
  3. There may be 2 DHCP servers are present on the nertwork and DHCP Duplicate IP detection marks the IP address as BAD_ADDRESS, When the response is “Host unreachable”.                                                          
  4. The router has ARP Cached enabled.                                                                                                              
  5. The router has the  BOOTP relay agent enabled.                                                                                          
  6. Whether the scope IP addresses have been assigned manually.

We could try the following resolutions:

  1. Make sure only one DHCP server provide service in the network.                                                                        
  2. Make sure the DHCP server is not running on a multihomed computer.                                                             
  3. Check the router settings.                                                                                                                            
  4. Check the following link settings, Maybe it will resolve the issue.                                                                  http://technet.microsoft.com/en-us/library/dd183587(v=ws.10).aspx                                                           
  5. You have a DHCP failover, you could check the following link to fix:                   https://blogs.technet.microsoft.com/teamdhcp/2014/02/24/dhcp-failover-fixes-in-kb-2919393-for-windows-server-2012-and-kb-2919355-for-windows-server-2012-r2/                                       
  6. Could you check the event viewer for more error message so that we could find more clue.                            
  7. Meanwhile, you can refer to the following link:

       https://social.technet.microsoft.com/Forums/windowsserver/en-US/b2967383-a515-4f55-9a4c-519d82dd7e17/bad-address-in-dhcp?forum=winserveripamdhcpdns

       https://social.technet.microsoft.com/Forums/windowsserver/en-US/0ef9b4b1-f14d-4e55-8760-150ddd22420b/dhcp-server-bad-address-issue?forum=winserverNIS

       https://social.technet.microsoft.com/Forums/windowsserver/en-US/d59ae9e8-7efd-4d48-9639-1dae3deac5ad/dhcp-server-generating-bad-address?forum=winserveripamdhcpdns

Hope this helps.

Best regards,

Hollis

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Thursday, August 8, 2019 8:48 AM

Good call on the client doing an arp request as well, we setup wireshark to check what the clients is actually doing when getting an ip address and duplicate addresses seem to be generated by offline clients in tha subnet that probably have WOL enabled and we think these clients keep their addresses retained on the NIC past the lease time of the DHCP server which in turn seem to cause the issues we are seeing.

Thursday, August 8, 2019 9:09 AM

Hi,

I am glad to hear that the information helps you. If you have any another question, please feel free to post in the forum.

Best regards,

Hollis

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