It's an obnoxious regression since provisioning a domain is easy, well documented, and impressively fool-proof, but joining an existing domain is less so. Especially since samba-tool purports to allow such a feat, but it doesn't actually work.
# server services = rpc, .....
# dcerpc endpoint servers = epmapper, .....
Joined 'M1' to dns domain 'samdom.example.com'
No DNS domain configured for m1. Unable to perform DNS Update.
DNS update failed: NT_STATUS_INVALID_PARAMETER
net rpc rights list accounts -U'SAMDOM\administrator'
[users]
path = /var/smb/users
read only = No
browseable = Yes
vfs objects = zfsacl recycle
recycle:keeptree = On
recycle:versions = On
recycle:maxsize = 10485760
recycle:repository = .recycle
recycle:exclude = *.tmp, *.bak
recycle
VFS does NOT work on DC (tested on net/samba41 and net/samba42).