[nobody7290]

Thanks,
So I am not alone, and not a complete dumbass.

In this case this was someone with a macbook who had internetsharing running. A private PC.
Looks like I really have to get a Layer-2 switch to solve this problem. The only other option is to make windows 2008 send an email if the service stops. Then at least the problem is noticed.

I wonder what microsoft thought about when they implemented the dhcp services. The way this works has not changed from maybe windows 2000.
From the technical aspect always a little catastrophe will happen when someone connects a second DHCP server, but, only for those PCs, which are connected to the network afterwards, or, those who need to renew the lease, and, only while the server is connected. Any other will continue to function.

But, the Microsoft behaviour to completely give up silently in favour of a foreign device/server makes things worse. Because of this, problems will arise hours later (even, when the rouge dchp server has left the network).
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