Relay backend sets up a relay virtual database that allows
to access other databases in the same instance of slapd
through different naming contexts and remapping attribute 
values.

The DN rewrite, filter rewrite and attributeType/objectClass
mapping is done by means of the rewrite-remap overlay.

The database containing the real naming context can be
explicitly selected by means of the "relay" directive,
which must contain the naming context of the target 
database.  This also causes the rewrite-remap overlay 
to be automatically instantiated.  If the optional keyword 
"massage" is present, the rewrite-remap overlay is 
automatically configured to map the virtual to the real 
naming context and vice-versa; in this case, the "suffix" 
directive must precede the "relay" directive.

Otherwise, the rewrite-remap overlay must be explicitly
instantiated, by using the "overlay" directive, as 
illustrated below.  This allows much more freedom in target 
database selection and DN rewriting.

If the "relay" directive is not present, the backend is
not bound to a single target database; on the contrary,
the target database is selected on a per-operation basis.

This allows, for instance, to relay one database for 
authentication and anothe for search/modify, or allows
to use one target for persons and another for groups
and so on.

# automatically massage from virtual to real naming context
database	relay
suffix		"dc=virtual,dc=naming,dc=context"
relay		"dc=real,dc=naming,dc=context" massage

# explicitly massage (same as above)
database	relay
suffix		"dc=virtual,dc=naming,dc=context"
overlay		rewrite-remap
suffixmassage	"dc=virtual,dc=naming,dc=context" \
			"dc=real,dc=naming,dc=context"

# old fashioned suffixalias, applied also to DN-valued attributes
# from virtual to real naming context, but not the reverse...
database	relay
suffix		"dc=virtual,dc=naming,dc=context"
overlay		rewrite-remap
rewriteEngine	on
rewriteContext	default
rewriteRule	"(.*)dc=virtual,dc=naming,dc=context$" \
			"$1dc=real,dc=naming,dc=context"