/usr/share/samba/setup/provision_basedn.ldif is in samba4 4.0.0~alpha18.dfsg1-4ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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## Domain Naming Context
################################
dn: ${DOMAINDN}
objectClass: top
objectClass: domaindns
instanceType: 5
objectSid: ${DOMAINSID}
nTSecurityDescriptor:: ${DESCRIPTOR}
${DOMAINGUID}
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