Duplicate IP Addresses After Upgrading Clients to SP2

 Last reviewed: March 5, 1998
Article ID: Q173753
The information in this article applies to:
  • Microsoft Windows NT Workstation version 4.0
  • Microsoft Windows NT Server version 4.0

SYMPTOMS

After you upgrade your Windows NT dynamic host control protocol (DHCP) clients to Service Pack 2 or later, and you are using non-Windows NT DHCP servers, you may begin receiving error messages indicating duplicate IP addresses on your network, displayed by DHCP clients.

CAUSE

Windows NT Service Pack 2 includes an update to the DHCP client that allows the client to send a new packet type to the DHCP server. This new packet, DHCP DECLINE, lacks the Client ID in the DHCP Options field. Some non- Windows NT DHCP servers may not process the DECLINE packet, which can create duplicate IP addresses on the network.

RESOLUTION

To resolve this problem, obtain the following fix or wait from the next Windows NT service pack.

STATUS

Microsoft has confirmed this to be a problem in Windows NT version 4.0. A supported fix is now available, but has not been fully regression tested and should be applied only to systems experiencing this specific problem. Unless you are severely impacted by this specific problem, Microsoft recommends that you wait for the next Service Pack that contains this fix. Contact Microsoft Technical Support for more information.

MORE INFORMATION

Windows NT Service Pack 2 includes an update to the DHCP client that improves the handling of duplicate IP addresses by workstations. Upon receiving a DHCP lease, it uses address resolution protocol (ARP) to determine the assigned IP address. If a response is received (indicating that the address is already in use by another device on the network), it sends a DHCP decline to the server. A Windows NT DHCP Server then marks the address as "bad" and does not attempt to assign that IP address to any other device. The client resends a DHCP DISCOVER packet and starts the lease process again.

RFC 2131 contains the following statement about the required use of client ID in Section 2, page 9: 

   If a client uses a 'client identifier' in one message, it
   MUST use that same identifier in all subsequent messages, to
   ensure that all servers correctly identify the client.
For additional information, please see the following article in the Microsoft Knowledge Base: 

   ARTICLE-ID: Q161430
   TITLE     : DHCP: Detecting and Flagging Duplicate IP Addresses
This problem will not arise if you accept the media access control address as the default client ID (CID) and use a Windows NT DHCP Server, because the Windows NT DHCP server will look elsewhere in the packet for the media access control address of the client. If you choose to implement custom CIDs or use a third-party DHCP server, the IP address will not be marked as a Bad_Address. There will be duplicate IP addresses on the network if the DHCP server does not have duplicate address detection enabled.

For additional information, please see the following article in the Microsoft Knowledge Base: 

   ARTICLE-ID: Q172408
   TITLE: DHCP: Customer DHCP Client Identifiers for Windows NT

Additional query words: sp2 client sp-2
Keywords : kbbug4.00 kbfix4.00.sp3 ntnetserv NTSrvWkst nttcp kb3rdparty
Version : WinNT:4.0
Platform : winnt
Issue type : kbbug
Solution Type : kbfix

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Last reviewed: March 5, 1998
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