There are two options to ensure that the dNSHostName attribute is populated:
Join using a user with administrator rights.
Run the following command as root on the server to set the attribute after the computer object has been created by the join:
# /opt/quest/bin/vastool -u host/ setattrs host/ dNSHostName <server FQDN>
Replacing <server FQDN> with the correct Fully Qualified Domain Name entry for the server.