Test Center Rx: Swapping DHCP Servers

SAN MATEO (04/03/2000) - I'm trying to solve my DHCP [Dynamic Host Configuration Protocol] problem, and I could use your help. I have an existing PDC [primary domain controller] that was running DHCP and WINS [Windows Internet Naming Service]. But I needed to replace the machine. First I set up a new BDC [backup domain controller] and promoted it to PDC so that I could take the old PDC offline. When I set up DHCP on the new PDC, I made sure the scope and settings were the same. I changed the IP address on the new PDC to be the same as the old one, so that my clients would still be able to find the WINS.

The event viewer tells me DHCP started fine, but the problem is that my clients keep getting the message that the DHCP server is unavailable. The server keeps sending back NACKs [negative acknowledgements] to the clients. I've searched every book I can, along with Microsoft Corp.'s knowledge base.

A friend sent me this suggestion: "I believe you're getting the NACKs because the clients are requesting an address renewal, and the new DHCP server did not issue the lease, and thus will not renew. You need to identify whether the new DHCP server will lease out any fresh addresses. To try this, go to a client, run ipconfig/release, and reboot. See what happens when it comes back up. It should request a fresh lease. If this works, you'll need to do the same for each client."

Acting on his advice, I went to several clients, ran ipconfig/release, then rebooted, but it still said "DHCP server not found." Event viewer said, "DHCP issued a NACK to the client for the address it requested." Obviously, there's a problem using the same range of addresses, but I can't understand why! Can you help me figure out this problem, please?

Ed Sheaffer

Lori: The answer to your problem is not apparent in the many books I searched, or on the Microsoft Web site knowledge base. However, I can offer you some suggestions.

A NACK tells a client that it is has incorrect configuration information.

Perhaps your clients got confused because the server from which they initially received their IP address has been disabled, and the leases have not been moved to the new server. It might be necessary to move the DHCP database from the old server to the new server. This, however, requires editing the Registry.

You also might want to check whether or not the lease scope on the new machine has been activated. If a scope is deactivated, the DHCP server will send a NACK. You also want to make sure you have the latest service packs for Windows NT 4.0 at www.microsoft.com/windows/servicepacks.

There is also a slight chance that the MAC (Media Access Control) address of the server has been cached. Try assigning the server a temporary IP address, or use the network card in the previous server.

Brooks: You've definitely got a thorny problem here. My first thought is to agree with your friend, who thought that the problem came from the clients trying to renew the addresses they already had; but if you've tried the release/renew thing and it didn't help, the problem almost has to be at the server.

DHCP NACKs aren't really a problem in and of themselves; it's fairly normal for a DHCP server to have them in its event log. They are often caused by laptops whose users travel between networks and request their previous IP address when booting on a new network. The DHCP server sends back a NACK, which causes the laptop to abandon its previous IP address and restart the DHCP process with a DHCP Discover packet.

So it's easy to see why the NACKs in your situation are normal, but the clients claiming "DHCP server unavailable" is weird. This is one of those problems that a network protocol analyzer would really help troubleshoot. The way I see it, either the clients have gone berserk and aren't issuing a DHCP Discover after receiving a NACK, which is unlikely, or they are doing that but the server isn't providing an IP address in return.

I think Lori hit on the simplest, and most likely, solution: The DHCP scope may not be activated. It's easy to overlook something like that. Failing this, it's time to get out the Sniffer and really watch the conversation that's going on here.

Brooks Talley is senior business and technology architect for InfoWorld.com.

Lori Mitchell is a senior analyst in the Test Center. Send your questions for them to testcenter_rx@infoworld.com.

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