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SLAPD-SQL(5) File Formats Manual SLAPD-SQL(5)
slapd-sql - SQL backend to slapd
ETCDIR/slapd.conf
The primary purpose of this slapd(8) backend is to PRESENT
information stored in some RDBMS as an LDAP subtree without any
programming (some SQL and maybe stored procedures can't be
considered programming, anyway ;).
That is, for example, when you (some ISP) have account information
you use in an RDBMS, and want to use modern solutions that expect
such information in LDAP (to authenticate users, make email
lookups etc.). Or you want to synchronize or distribute
information between different sites/applications that use RDBMSes
and/or LDAP. Or whatever else...
It is NOT designed as a general-purpose backend that uses RDBMS
instead of LMDB (as the standard MDB backend does), though it can
be used as such with several limitations. You can take a look at
http://www.openldap.org/faq/index.cgi?file=378 (OpenLDAP
FAQ-O-Matic/General LDAP FAQ/Directories vs. conventional
databases) to find out more on this point.
The idea (detailed below) is to use some meta-information to
translate LDAP queries to SQL queries, leaving relational schema
untouched, so that old applications can continue using it without
any modifications. This allows SQL and LDAP applications to
inter-operate without replication, and exchange data as needed.
The SQL backend is designed to be tunable to virtually any
relational schema without having to change source (through that
meta-information mentioned). Also, it uses ODBC to connect to
RDBMSes, and is highly configurable for SQL dialects RDBMSes may
use, so it may be used for integration and distribution of data on
different RDBMSes, OSes, hosts etc., in other words, in highly
heterogeneous environment.
This backend is experimental.
These slapd.conf options apply to the SQL backend database, which
means that they must follow a "database sql" line and come before
any subsequent "backend" or "database" lines. Other database
options not specific to this backend are described in the
slapd.conf(5) manual page.
dbname <datasource name>
The name of the ODBC datasource to use.
dbhost <hostname>
dbpasswd <password>
dbuser <username>
The three above options are generally unneeded, because
this information is taken from the datasource specified by
the dbname directive. They allow to override datasource
settings. Also, several RDBMS' drivers tend to require
explicit passing of user/password, even if those are given
in datasource (Note: dbhost is currently ignored).
These options specify SQL query templates for scoping searches.
subtree_cond <SQL expression>
Specifies a where-clause template used to form a subtree
search condition (dn="(.+,)?<dn>$"). It may differ from
one SQL dialect to another (see samples). By default, it
is constructed based on the knowledge about how to
normalize DN values (e.g. "<upper_func>(ldap_entries.dn)
LIKE CONCAT('%',?)"); see upper_func, upper_needs_cast,
concat_pattern and strcast_func in "HELPER CONFIGURATION"
for details.
children_cond <SQL expression>
Specifies a where-clause template used to form a children
search condition (dn=".+,<dn>$"). It may differ from one
SQL dialect to another (see samples). By default, it is
constructed based on the knowledge about how to normalize
DN values (e.g. "<upper_func>(ldap_entries.dn) LIKE
CONCAT('%,',?)"); see upper_func, upper_needs_cast,
concat_pattern and strcast_func in "HELPER CONFIGURATION"
for details.
use_subtree_shortcut { YES | no }
Do not use the subtree condition when the searchBase is the
database suffix, and the scope is subtree; rather collect
all entries.
These options specify SQL query templates for loading schema
mapping meta-information, adding and deleting entries to
ldap_entries, etc. All these and subtree_cond should have the
given default values. For the current value it is recommended to
look at the sources, or in the log output when slapd starts with
"-d 5" or greater. Note that the parameter number and order must
not be changed.
oc_query <SQL expression>
The query that is used to collect the objectClass mapping
data from table ldap_oc_mappings; see "METAINFORMATION
USED" for details. The default is "SELECT id, name,
keytbl, keycol, create_proc, delete_proc, expect_return
FROM ldap_oc_mappings".
at_query <SQL expression>
The query that is used to collect the attributeType mapping
data from table ldap_attr_mappings; see "METAINFORMATION
USED" for details. The default is "SELECT name, sel_expr,
from_tbls, join_where, add_proc, delete_proc, param_order,
expect_return FROM ldap_attr_mappings WHERE oc_map_id=?".
id_query <SQL expression>
The query that is used to map a DN to an entry in table
ldap_entries; see "METAINFORMATION USED" for details. The
default is "SELECT id,keyval,oc_map_id,dn FROM ldap_entries
WHERE <DN match expr>", where <DN match expr> is
constructed based on the knowledge about how to normalize
DN values (e.g. "dn=?" if no means to uppercase strings are
available; typically, "<upper_func>(dn)=?" is used); see
upper_func, upper_needs_cast, concat_pattern and
strcast_func in "HELPER CONFIGURATION" for details.
insentry_stmt <SQL expression>
The statement that is used to insert a new entry in table
ldap_entries; see "METAINFORMATION USED" for details. The
default is "INSERT INTO ldap_entries (dn, oc_map_id,
parent, keyval) VALUES (?, ?, ?, ?)".
delentry_stmt <SQL expression>
The statement that is used to delete an existing entry from
table ldap_entries; see "METAINFORMATION USED" for details.
The default is "DELETE FROM ldap_entries WHERE id=?".
delobjclasses_stmt <SQL expression>
The statement that is used to delete an existing entry's ID
from table ldap_objclasses; see "METAINFORMATION USED" for
details. The default is "DELETE FROM ldap_entry_objclasses
WHERE entry_id=?".
These statements are used to modify the default behavior of the
backend according to issues of the dialect of the RDBMS. The
first options essentially refer to string and DN normalization
when building filters. LDAP normalization is more than upper- (or
lower-)casing everything; however, as a reasonable trade-off, for
case-sensitive RDBMSes the backend can be instructed to uppercase
strings and DNs by providing the upper_func directive. Some
RDBMSes, to use functions on arbitrary data types, e.g. string
constants, requires a cast, which is triggered by the
upper_needs_cast directive. If required, a string cast function
can be provided as well, by using the strcast_func directive.
Finally, a custom string concatenation pattern may be required; it
is provided by the concat_pattern directive.
upper_func <SQL function name>
Specifies the name of a function that converts a given
value to uppercase. This is used for case insensitive
matching when the RDBMS is case sensitive. It may differ
from one SQL dialect to another (e.g. UCASE, UPPER or
whatever; see samples). By default, none is used, i.e.
strings are not uppercased, so matches may be case
sensitive.
upper_needs_cast { NO | yes }
Set this directive to yes if upper_func needs an explicit
cast when applied to literal strings. A cast in the form
CAST (<arg> AS VARCHAR(<max DN length>)) is used, where
<max DN length> is builtin in back-sql; see macro
BACKSQL_MAX_DN_LEN (currently 255; note that slapd's
builtin limit, in macro SLAP_LDAPDN_MAXLEN, is set to
8192). This is experimental and may change in future
releases.
strcast_func <SQL function name>
Specifies the name of a function that converts a given
value to a string for appropriate ordering. This is used
in "SELECT DISTINCT" statements for strongly typed RDBMSes
with little implicit casting (like PostgreSQL), when a
literal string is specified. This is experimental and may
change in future releases.
concat_pattern <pattern>
This statement defines the pattern that is used to
concatenate strings. The pattern MUST contain two question
marks, '?', that will be replaced by the two strings that
must be concatenated. The default value is CONCAT(?,?); a
form that is known to be highly portable (IBM db2,
PostgreSQL) is ?||?, but an explicit cast may be required
when operating on literal strings: CAST(?||? AS
VARCHAR(<length>)). On some RDBMSes (IBM db2, MSSQL) the
form ?+? is known to work as well. Carefully check the
documentation of your RDBMS or stay with the examples for
supported ones. This is experimental and may change in
future releases.
aliasing_keyword <string>
Define the aliasing keyword. Some RDBMSes use the word
"AS" (the default), others don't use any.
aliasing_quote <string>
Define the quoting char of the aliasing keyword. Some
RDBMSes don't require any (the default), others may require
single or double quotes.
has_ldapinfo_dn_ru { NO | yes }
Explicitly inform the backend whether the dn_ru column (DN
in reverse uppercased form) is present in table
ldap_entries. Overrides automatic check (this is required,
for instance, by PostgreSQL/unixODBC). This is
experimental and may change in future releases.
fail_if_no_mapping { NO | yes }
When set to yes it forces attribute write operations to
fail if no appropriate mapping between LDAP attributes and
SQL data is available. The default behavior is to ignore
those changes that cannot be mapped. It has no impact on
objectClass mapping, i.e. if the structuralObjectClass of
an entry cannot be mapped to SQL by looking up its name in
ldap_oc_mappings, an add operation will fail regardless of
the fail_if_no_mapping switch; see section "METAINFORMATION
USED" for details. This is experimental and may change in
future releases.
allow_orphans { NO | yes }
When set to yes orphaned entries (i.e. without the parent
entry in the database) can be added. This option should be
used with care, possibly in conjunction with some special
rule on the RDBMS side that dynamically creates the missing
parent.
baseObject [ <filename> ]
Instructs the database to create and manage an in-memory
baseObject entry instead of looking for one in the RDBMS.
If the (optional) <filename> argument is given, the entry
is read from that file in LDIF(5) format; otherwise, an
entry with objectClass extensibleObject is created based on
the contents of the RDN of the baseObject. This is
particularly useful when ldap_entries information is stored
in a view rather than in a table, and union is not
supported for views, so that the view can only specify one
rule to compute the entry structure for one objectClass.
This topic is discussed further in section "METAINFORMATION
USED". This is experimental and may change in future
releases.
create_needs_select { NO | yes }
Instructs the database whether or not entry creation in
table ldap_entries needs a subsequent select to collect the
automatically assigned ID, instead of being returned by a
stored procedure.
fetch_attrs <attrlist>
fetch_all_attrs { NO | yes }
The first statement allows one to provide a list of
attributes that must always be fetched in addition to those
requested by any specific operation, because they are
required for the proper usage of the backend. For
instance, all attributes used in ACLs should be listed
here. The second statement is a shortcut to require all
attributes to be always loaded. Note that the dynamically
generated attributes, e.g. hasSubordinates, entryDN and
other implementation dependent attributes are NOT generated
at this point, for consistency with the rest of slapd.
This may change in the future.
check_schema { YES | no }
Instructs the database to check schema adherence of entries
after modifications, and structural objectClass chain when
entries are built. By default it is set to yes.
sqllayer <name> [...]
Loads the layer <name> onto a stack of helpers that are
used to map DNs from LDAP to SQL representation and vice-
versa. Subsequent args are passed to the layer
configuration routine. This is highly experimental and
should be used with extreme care. The API of the layers is
not frozen yet, so it is unpublished.
autocommit { NO | yes }
Activates autocommit; by default, it is off.
Almost everything mentioned later is illustrated in examples
located in the servers/slapd/back-sql/rdbms_depend/ directory in
the OpenLDAP source tree, and contains scripts for generating
sample database for Oracle, MS SQL Server, mySQL and more
(including PostgreSQL and IBM db2).
The first thing that one must arrange is what set of LDAP object
classes can present your RDBMS information.
The easiest way is to create an objectClass for each entity you
had in ER-diagram when designing your relational schema. Any
relational schema, no matter how normalized it is, was designed
after some model of your application's domain (for instance,
accounts, services etc. in ISP), and is used in terms of its
entities, not just tables of normalized schema. It means that for
every attribute of every such instance there is an effective SQL
query that loads its values.
Also you might want your object classes to conform to some of the
standard schemas like inetOrgPerson etc.
Nevertheless, when you think it out, we must define a way to
translate LDAP operation requests to (a series of) SQL queries.
Let us deal with the SEARCH operation.
Example: Let's suppose that we store information about persons
working in our organization in two tables:
PERSONS PHONES
---------- -------------
id integer id integer
first_name varchar pers_id integer references persons(id)
last_name varchar phone
middle_name varchar
...
(PHONES contains telephone numbers associated with persons). A
person can have several numbers, then PHONES contains several
records with corresponding pers_id, or no numbers (and no records
in PHONES with such pers_id). An LDAP objectclass to present such
information could look like this:
person
-------
MUST cn
MAY telephoneNumber $ firstName $ lastName
...
To fetch all values for cn attribute given person ID, we construct
the query:
SELECT CONCAT(persons.first_name,' ',persons.last_name)
AS cn FROM persons WHERE persons.id=?
for telephoneNumber we can use:
SELECT phones.phone AS telephoneNumber FROM persons,phones
WHERE persons.id=phones.pers_id AND persons.id=?
If we wanted to service LDAP requests with filters like
(telephoneNumber=123*), we would construct something like:
SELECT ... FROM persons,phones
WHERE persons.id=phones.pers_id
AND persons.id=?
AND phones.phone like '%1%2%3%'
(note how the telephoneNumber match is expanded in multiple
wildcards to account for interspersed ininfluential chars like
spaces, dashes and so; this occurs by design because
telephoneNumber is defined after a specially recognized syntax).
So, if we had information about what tables contain values for
each attribute, how to join these tables and arrange these values,
we could try to automatically generate such statements, and
translate search filters to SQL WHERE clauses.
To store such information, we add three more tables to our schema
and fill it with data (see samples):
ldap_oc_mappings (some columns are not listed for clarity)
---------------
id=1
name="person"
keytbl="persons"
keycol="id"
This table defines a mapping between objectclass (its name held in
the "name" column), and a table that holds the primary key for
corresponding entities. For instance, in our example, the person
entity, which we are trying to present as "person" objectclass,
resides in two tables (persons and phones), and is identified by
the persons.id column (that we will call the primary key for this
entity). Keytbl and keycol thus contain "persons" (name of the
table), and "id" (name of the column).
ldap_attr_mappings (some columns are not listed for clarity)
-----------
id=1
oc_map_id=1
name="cn"
sel_expr="CONCAT(persons.first_name,' ',persons.last_name)"
from_tbls="persons"
join_where=NULL
************
id=<n>
oc_map_id=1
name="telephoneNumber"
sel_expr="phones.phone"
from_tbls="persons,phones"
join_where="phones.pers_id=persons.id"
This table defines mappings between LDAP attributes and SQL
queries that load their values. Note that, unlike LDAP schema,
these are not attribute types - the attribute "cn" for "person"
objectclass can have its values in different tables than "cn" for
some other objectclass, so attribute mappings depend on
objectclass mappings (unlike attribute types in LDAP schema, which
are indifferent to objectclasses). Thus, we have oc_map_id column
with link to oc_mappings table.
Now we cut the SQL query that loads values for a given attribute
into 3 parts. First goes into sel_expr column - this is the
expression we had between SELECT and FROM keywords, which defines
WHAT to load. Next is table list - text between FROM and WHERE
keywords. It may contain aliases for convenience (see examples).
The last is part of the where clause, which (if it exists at all)
expresses the condition for joining the table containing values
with the table containing the primary key (foreign key equality
and such). If values are in the same table as the primary key,
then this column is left NULL (as for cn attribute above).
Having this information in parts, we are able to not only
construct queries that load attribute values by id of entry (for
this we could store SQL query as a whole), but to construct
queries that load id's of objects that correspond to a given
search filter (or at least part of it). See below for examples.
ldap_entries
------------
id=1
dn=<dn you choose>
oc_map_id=...
parent=<parent record id>
keyval=<value of primary key>
This table defines mappings between DNs of entries in your LDAP
tree, and values of primary keys for corresponding relational
data. It has recursive structure (parent column references id
column of the same table), which allows you to add any tree
structure(s) to your flat relational data. Having id of
objectclass mapping, we can determine table and column for primary
key, and keyval stores value of it, thus defining the exact tuple
corresponding to the LDAP entry with this DN.
Note that such design (see exact SQL table creation query) implies
one important constraint - the key must be an integer. But all
that I know about well-designed schemas makes me think that it's
not very narrow ;) If anyone needs support for different types for
keys - he may want to write a patch, and submit it to OpenLDAP
ITS, then I'll include it.
Also, several users complained that they don't really need very
structured trees, and they don't want to update one more table
every time they add or delete an instance in the relational
schema. Those people can use a view instead of a real table for
ldap_entries, something like this (by Robin Elfrink):
CREATE VIEW ldap_entries (id, dn, oc_map_id, parent, keyval)
AS
SELECT 0, UPPER('o=MyCompany,c=NL'),
3, 0, 'baseObject' FROM unixusers WHERE userid='root'
UNION
SELECT (1000000000+userid),
UPPER(CONCAT(CONCAT('cn=',gecos),',o=MyCompany,c=NL')),
1, 0, userid FROM unixusers
UNION
SELECT (2000000000+groupnummer),
UPPER(CONCAT(CONCAT('cn=',groupname),',o=MyCompany,c=NL')),
2, 0, groupnummer FROM groups;
If your RDBMS does not support unions in views, only one
objectClass can be mapped in ldap_entries, and the baseObject
cannot be created; in this case, see the baseObject directive for
a possible workaround.
Having meta-information loaded, the SQL backend uses these tables
to determine a set of primary keys of candidates (depending on
search scope and filter). It tries to do it for each objectclass
registered in ldap_objclasses.
Example: for our query with filter (telephoneNumber=123*) we would
get the following query generated (which loads candidate IDs)
SELECT ldap_entries.id,persons.id, 'person' AS objectClass,
ldap_entries.dn AS dn
FROM ldap_entries,persons,phones
WHERE persons.id=ldap_entries.keyval
AND ldap_entries.objclass=?
AND ldap_entries.parent=?
AND phones.pers_id=persons.id
AND (phones.phone LIKE '%1%2%3%')
(for ONELEVEL search) or "... AND dn=?" (for BASE search) or "...
AND dn LIKE '%?'" (for SUBTREE)
Then, for each candidate, we load the requested attributes using
per-attribute queries like
SELECT phones.phone AS telephoneNumber
FROM persons,phones
WHERE persons.id=? AND phones.pers_id=persons.id
Then, we use test_filter() from the frontend API to test the entry
for a full LDAP search filter match (since we cannot effectively
make sense of SYNTAX of corresponding LDAP schema attribute, we
translate the filter into the most relaxed SQL condition to filter
candidates), and send it to the user.
ADD, DELETE, MODIFY and MODRDN operations are also performed on
per-attribute meta-information (add_proc etc.). In those fields
one can specify an SQL statement or stored procedure call which
can add, or delete given values of a given attribute, using the
given entry keyval (see examples -- mostly PostgreSQL, ORACLE and
MSSQL - since as of this writing there are no stored procs in
MySQL).
We just add more columns to ldap_oc_mappings and
ldap_attr_mappings, holding statements to execute (like
create_proc, add_proc, del_proc etc.), and flags governing the
order of parameters passed to those statements. Please see
samples to find out what are the parameters passed, and other
information on this matter - they are self-explanatory for those
familiar with the concepts expressed above.
First of all, let's recall that among other major differences to
the complete LDAP data model, the above illustrated concept does
not directly support such features as multiple objectclasses per
entry, and referrals. Fortunately, they are easy to adopt in this
scheme. The SQL backend requires that one more table is added to
the schema: ldap_entry_objectclasses(entry_id,oc_name).
That table contains any number of objectclass names that
corresponding entries will possess, in addition to that mentioned
in mapping. The SQL backend automatically adds attribute mapping
for the "objectclass" attribute to each objectclass mapping that
loads values from this table. So, you may, for instance, have a
mapping for inetOrgPerson, and use it for queries for "person"
objectclass...
Referrals used to be implemented in a loose manner by adding an
extra table that allowed any entry to host a "ref" attribute,
along with a "referral" extra objectClass in table
ldap_entry_objclasses. In the current implementation, referrals
are treated like any other user-defined schema, since "referral"
is a structural objectclass. The suggested practice is to define
a "referral" entry in ldap_oc_mappings, holding a naming
attribute, e.g. "ou" or "cn", a "ref" attribute, containing the
url; in case multiple referrals per entry are needed, a separate
table for urls can be created, where urls are mapped to the
respective entries. The use of the naming attribute usually
requires to add an "extensibleObject" value to
ldap_entry_objclasses.
As previously stated, this backend should not be considered a
replacement of other data storage backends, but rather a gateway
to existing RDBMS storages that need to be published in LDAP form.
The hasSubordinates operational attribute is honored by back-sql
in search results and in compare operations; it is partially
honored also in filtering. Owing to design limitations, a (brain-
dead?) filter of the form (!(hasSubordinates=TRUE)) will give no
results instead of returning all the leaf entries, because it
actually expands into ... AND NOT (1=1). If you need to find all
the leaf entries, please use (hasSubordinates=FALSE) instead.
A directoryString value of the form "__First___Last_" (where
underscores mean spaces, ASCII 0x20 char) corresponds to its
prettified counterpart "First_Last"; this is not currently honored
by back-sql if non-prettified data is written via RDBMS; when non-
prettified data is written through back-sql, the prettified values
are actually used instead.
When the ldap_entry_objclasses table is empty, filters on the
objectClass attribute erroneously result in no candidates. A
workaround consists in adding at least one row to that table, no
matter if valid or not.
The proxy cache overlay allows caching of LDAP search requests
(queries) in a local database. See slapo-pcache(5) for details.
There are example SQL modules in the slapd/back-sql/rdbms_depend/
directory in the OpenLDAP source tree.
The sql backend honors access control semantics as indicated in
slapd.access(5) (including the disclose access privilege when
enabled at compile time).
ETCDIR/slapd.conf
default slapd configuration file
slapd.conf(5), slapd(8).
This page is part of the OpenLDAP (an open source implementation
of the Lightweight Directory Access Protocol) project.
Information about the project can be found at
⟨http://www.openldap.org/⟩. If you have a bug report for this
manual page, see ⟨http://www.openldap.org/its/⟩. This page was
obtained from the project's upstream Git repository
⟨https://git.openldap.org/openldap/openldap.git⟩ on 2025-08-11.
(At that time, the date of the most recent commit that was found
in the repository was 2025-08-05.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
OpenLDAP LDVERSION RELEASEDATE SLAPD-SQL(5)
Pages that refer to this page: slapd.access(5), slapd.backends(5), slapo-pcache(5)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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