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I have a Hyper-V Host with a couple of VMs a guest.

One of the guest is the DC. There is only on physical server, so I have only one DC as a Vm (I know, it is not a good solution to have only one DC, but we are limited to on physical server).

All works fine, until I have to reeboot the Hyper-V host. After a reboot of the host, all VMs are running, but the DHCP-Server in the DC-VM is not giving ip addresses to clients. I have to manually reboot the DC-VM.

I have already looked into the event-log, but cannot find some hints.

Does anybody know why me DC is not working properly

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Have you tried using Wireshark to diagnose the problem? Despite the recommendation that you not run DHCP on DCs, I do; and I have several on Hyper-V VMs, they all work fine so far. – Chris S Oct 4 '12 at 13:41

This is why it's particularly bad to have your only Domain Controller as a virtual machine too, by the way.

What happens when you have to reboot the host, and the only Domain Controller goes down? Oh, right, the host and the other domain-joined VMs can't authenticate against the domain when they reboot, because the Domain Controller isn't available yet. They'll error out after being unable to contact the domain, which is the likely cause of the errors you're seeing.

Get yourself a second Domain Controller, and make it a physical machine this time.

And, as mentioned in the comments, you shouldn't run DHCP off a Domain Controller. It causes problems, like you're seeing.

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I know, that it would be better to have a second DC. But the problem is not, that the other machines are not working after reboot, the problem is, that the DC is bnot working after reboot of the host. – Chris Oct 4 '12 at 10:59
@Chris Not to doubt your word, but the symptoms you describe could just as well (if not better) be explained by the clients erroring out upon not being able to contact "the domain." – HopelessN00b Oct 4 '12 at 11:02
I agree with you, that it could cause errors when clients are starting and the DC is not reachable. But what I do not understand is, that when I restart the host, wait until the DC is running again and then restart the clients, that then they will not get a new IP by DHCP. I have to restart the the DC, then I can get a new IP with ipconfig/renew. So in my opinion there is something broken in the DC after Host restart. – Chris Oct 4 '12 at 11:07
@Chris Oh, I missed that. You shouldn't run DHCP off a Domain Controller. It causes problems, like you're seeing. – HopelessN00b Oct 4 '12 at 11:17
Ah, ok, I will try it with a separate DHCP-Server. Thanks. – Chris Oct 4 '12 at 11:19

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