Overlays in OpenLDAP

Or when is an overlay an overlay?

Overlays and ModuleLoad

Overlays can be either dynamic or static. If the overlays are dynamic they need a moduleload directive (or olcModuleLoad if using cn=config). If the overlays are static they are built-in to slapd, they do not need a moduleload directive and indeed they will fail if you do add one because the overlay does not exist.

OK. So how do know whether overlays are static or dynamic?

You need to know how slapd was built - specifically the configure directives used. To build any overlays there needs to be a --enable-modules configure directive - this is normally defaulted on most builds far as we can tell. Then each overlay will have a separate configure directive which may be:

# the example uses accesslog for illustration, may be syncprov etc.
--enable-accesslog
# builds statically - no moduleload required
--enable-access=mod
# builds dynamically - moduleload required

To find how your slap was built:

# for FreeBSD assuming build from ports:
cd /usr/ports/net/openldap2X-server/work/openldap-X.X.X
vi config.log
# the whole configure clause is stored in this file at the top

# if you used'make install clean' this file has been deleted
# to re-create use 'make configure'
# then 'make clean' to remove

# RPM based distributions
# find the .spec file used to build slapd
# edit the file and look for the configure command line 
# followed by its associated directives

Perhaps the easiest way is simply to look for the overlays - if they exist you need a moduleload directive if not you don't. The overlays are normally in [bsd] /usr/local/libexec/openldap and [fc] /usr/libexec/openldap or /usr/sbin/openldap.

Finally you will find on most platforms that overlays are built with .la (library) or .la and .so (shared object library) file suffixes. Which to use - try .la and if it fails use .so.