Trying to install a new child domain in an existing forest I received this error from DCPROMO:
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Active Directory Installation Wizard
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The wizard cannot gain access to the list of domains in the forest.
Active Directory Installation Wizard
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The wizard cannot gain access to the list of domains in the forest.
This condition may be caused by a DNS lookup problem. For information about troubleshooting common DNS lookup problems, please see the following Microsoft Web site: http://go.microsoft.com/fwlink/?LinkId=5171
The error is:
The RPC server is unavailable.
A trace of the network traffic during the dcpromo process revealed a connection attempt from the local computer to one of the DCs in the root domain using the computername and username of the local computer. This of course fails since the local computer is to become the first domain controller in the new child domain and thus is in a workgroup. I could not see exactly what the local server was trying to connect to, so I started authenticating to the most common ones: IPC$, C$ and ADMIN$. Turns out it was ADMIN$ and after I ran this command, I was able to continue the dcpromo process:
net use \<name of DC in root domain>admin$ /user:<root domain><administrator in root domain>
Just as dcpromo was starting (after the summary screen) I received another error:
Managing the network session with \<name of DC in root domain> failed
“Multiple connections to a server or shared resource by the same user, using more than one user name, are not allowed. Disconnect all previous connections to the server or shared resource and try again.”
Dcpromo failed after this.
I restarted dcpromo and ran it all the way up to the summary screen. Before I hit Finish I ran this command to delete all connections to remote servers:
net use * /delete
Dcpromo could now successfully complete.
The Dcpromo log was also valuable in troubleshooting this, it can be found in %systemroot%debug.