When using a straight LDAP backend in Samba4 (passdb backend = ldapsam:ldap://<server>) running on RHEL6 the ldapsam.so isn't found and I get the following error in the smbd.log:
No builtin nor plugin backend for ldapsam found
The workaround is a simple
cp -v /usr/lib64/samba/pdb/ldap.so /usr/lib64/samba/pdb/ldapsam.so
I've had to do this on a couple of servers before and believe this should be fixed upstream in core samba.
Here is the exact package version:
rpm -qi samba4
Name : samba4 Relocations: (not relocatable)
Version : 4.0.0 Vendor: Red Hat, Inc.
Release : 63.el6_5.rc4 Build Date: Fri 01 Aug 2014 21:00:25 BST
Install Date: Tue 02 Dec 2014 16:51:29 GMT Build Host: x86-029.build.eng.bos.redhat.com
Group : System Environment/Daemons Source RPM: samba4-4.0.0-63.el6_5.rc4.src.rpm
Size : 1125820 License: GPLv3+ and LGPLv3+
Signature : RSA/8, Tue 05 Aug 2014 06:27:40 BST, Key ID 199e2f91fd431d51
Packager : Red Hat, Inc. <http://bugzilla.redhat.com/bugzilla>
please report this bug to Red Hat if there is still such a problem with their packages.