.TH SLAPD-RELAY 5 "RELEASEDATE" "OpenLDAP LDVERSION"
.SH NAME
slapd-relay \- relay backend to slapd
.SH SYNOPSIS
ETCDIR/slapd.conf
.SH DESCRIPTION
The primary purpose of this
.BR slapd (8)
backend is to map a naming context defined in a database 
running in the same 
.BR slapd (8)
instance into a virtual naming context, with attributeType
and objectClass manipulation, if required.
It requires the
.B rwm
.BR overlay .
.LP
This backend and the above mentioned overlay are experimental.
.SH CONFIGURATION
The following
.B slapd.conf
directives apply to the relay backend database.
That is, they must follow a "database relay" line and come before any
subsequent "backend" or "database" lines.
Other database options are described in the
.BR slapd.conf (5)
manual page; only the
.B suffix
directive is required by the 
.I relay
backend.
.TP
.B relay <real naming context> [massage]
The naming context of the database that is presented 
under a virtual naming context.
The presence of this directive implies that one specific database,
i.e. the one serving the
.BR "real naming context" ,
will be presented under a virtual naming context.
This directive automatically instantiates the 
.IR "rwm overlay" .
If the optional
.B massage
keyword is present, the suffix massaging is automatically
configured as well; otherwise, specific massaging instructions
are required by means of the
.I rewrite
directives described in
.BR slapo-rwm (5).

.SH ACCESS RULES
One important issue is that access rules are based on the identity
that issued the operation.
After massaging from the virtual to the real naming context, the
frontend sees the operation as performed by the identity in the
real naming context.
Moreover, since
.B back-relay
bypasses the real database frontend operations by short-circuiting
operations thru the internal backend API, the original database
access rules do not apply but in selected cases, i.e. when the
backend itself applies access control.
As a consequence, the instances of the relay database must provide
own access rules that are consistent with those of the original
database, possibly adding further specific restrictions.
So, access rules in the
.B relay
database must refer to identities in the real naming context.
Examples are reported in the EXAMPLES section.

.SH SCENARIOS
.LP
If no
.B relay
directive is given, the 
.I relay
database does not refer to any specific database, but the most
appropriate one is looked-up after rewriting the request DN
for the operation that is being handled.
.LP
This allows to write carefully crafted rewrite rules that
cause some of the requests to be directed to one database, and
some to another; e.g., authentication can be mapped to one 
database, and searches to another, or different target databases
can be selected based on the DN of the request, and so.
.LP
Another possibility is to map the same operation to different 
databases based on details of the virtual naming context,
e.g. groups on one database and persons on another.
.LP
.SH Caveats
The
.B rwm overlay
is experimental.
.LP
.SH EXAMPLES
To implement a plain virtual naming context mapping
that refers to a single database, use
.LP
.nf
  database        relay
  suffix          "dc=virtual,dc=naming,dc=context"
  relay           "dc=real,dc=naming,dc=context" massage
.fi
.LP
To implement a plain virtual naming context mapping
that looks up the real naming context for each operation, use
.LP
.nf
  database        relay
  suffix          "dc=virtual,dc=naming,dc=context"
  overlay         rwm
  suffixmassage   "dc=real,dc=naming,dc=context"
.fi
.LP
This is useful, for instance, to relay different databases that
share the terminal portion of the naming context (the one that
is rewritten).
.LP
To implement the old-fashioned suffixalias, e.g. mapping
the virtual to the real naming context, but not the results
back from the real to the virtual naming context, use
.LP
.nf
  database        relay
  suffix          "dc=virtual,dc=naming,dc=context"
  relay           "dc=real,dc=naming,dc=context"
  rewriteEngine   on
  rewriteContext  default
  rewriteRule     "dc=virtual,dc=naming,dc=context"
          "dc=real,dc=naming,dc=context" ":@"
  rewriteContext  searchFilter
  rewriteContext  searchEntryDN
  rewriteContext  searchAttrDN
  rewriteContext  matchedDN
.fi
.LP
Note that the virtual database is bound to a single real database,
so the 
.B rwm overlay
is automatically instantiated, but the rewrite rules 
are written explicitly to map all the virtual to real 
naming context data flow, but none of the real to virtual.
.LP
Access rules:
.LP
.nf
  database        bdb
  suffix          "dc=example,dc=com"
  # skip...
  access to dn.subtree="dc=example,dc=com"
          by dn.exact="cn=Supervisor,dc=example,dc=com" write
          by * read

  database        relay
  suffix          "o=Example,c=US"
  relay           "dc=example,dc=com" massage
  # skip ...
  access to dn.subtree="o=Example,c=US"
          by dn.exact="cn=Supervisor,dc=example,dc=com" write
          by dn.exact="cn=Relay Supervisor,dc=example,dc=com" write
          by * read
.fi
.LP
Note that, in both databases, the identities (the 
.B <who> 
clause) are in the
.BR "real naming context" ,
i.e.
.BR "`dc=example,dc=com'" ,
while the targets (the 
.B <what> 
clause) are in the
.B real
and in the
.BR "virtual naming context" ,
respectively.
.SH ACCESS CONTROL
The
.B relay
backend does not honor any of the access control semantics described in
.BR slapd.access (5);
all access control is delegated to the relayed database(s).
Only
.B read (=r)
access to the
.B entry
pseudo-attribute and to the other attribute values of the entries
returned by the
.B search
operation is honored, which is performed by the frontend.
.SH FILES
.TP
ETCDIR/slapd.conf
default slapd configuration file
.SH SEE ALSO
.BR slapd.conf (5),
.BR slapo-rwm (5),
.BR slapd (8).