1.3. LDAP backends, objects and attributes
The LDAP server daemon is called Slapd. Slapd
supports a variety of different database backends which you can use.
They include the primary choice BDB, a high-performance transactional database backend; LDBM, a lightweight DBM based backend; SHELL, a backend interface to arbitrary shell scripts and PASSWD, a simple backend interface to the passwd(5) file.
BDB utilizes Sleepycat Berkeley DB 4. LDBM utilizes
either Berkeley DB or GDBM.
BDB transactional backend is suited for multi-user read/write database access,
with any mix of read and write operations. BDB is used in applications that require:
Transactions, including making multiple changes to the
database atomically and rolling back uncommitted changes when necessary.
Ability to recover from systems crashes and hardware failures without
losing any committed transactions.
In this document I assume that you choose the BDB database.
To import and export directory information between LDAP-based directory servers,
or to describe a set of changes which are to be applied to a directory, the
file format known as LDIF, for LDAP Data Interchange Format, is typically used.
A LDIF file stores information in object-oriented hierarchies of entries. The
LDAP software package you're going to get comes with an utility to convert LDIF
files to the BDB format
A common LDIF file looks like this:
dn: o=TUDelft, c=NL
dn: cn=Luiz Malere, o=TUDelft, c=NL
cn: Luiz Malere
As you can see each entry is uniquely identified by a distinguished name, or
DN. The DN consists of the name of the entry plus a path of names tracing the
entry back to the top of the directory hierarchy (just like a tree).
In LDAP, an object class defines the collection of attributes that can be used
to define an entry. The LDAP standard provides these basic types of object classes:
Groups in the directory, including unordered lists of individual objects
or groups of objects.
Locations, such as the country name and description.
Organizations in the directory.
People in the directory.
An entry can belong to more than one object class. For example, the entry for a
person is defined by the person object class, but may also be
defined by attributes in the inetOrgPerson, groupOfNames, and organization objectclasses.
The server's object class structure (it's schema) determines the total list of required and
allowed attributes for a particular entry.
Directory data is represented as attribute-value pairs. Any specific piece of
information is associated with a descriptive attribute.
For instance, the commonName, or cn, attribute is used to store a person's name
. A person named Jonas Salk can be represented in the directory as
Each person entered in the directory is defined by the collection of attributes
in the person object class. Other attributes used to define this
entry could include:
Required attributes include the attributes that must be present in entries
using the object class. All entries require the objectClass attribute, which
lists the object classes to which an entry belongs.
Allowed attributes include the attributes that may be present in entries using
the object class. For example, in the person object class, the cn and sn
attributes are required. The description, telephoneNumber, seeAlso, and
userpassword attributes are allowed but are not required.
Each attribute has a corresponding syntax definition. The syntax definition
describes the type of information provided by the attribute, for instance:
ces case exact string (case must match during comparisons).
cis case ignore string (case is ignored during comparisons).
tel telephone number string (like cis but blanks and dashes `- ' are
ignored during comparisons).
dn distinguished name.
Note: Usually objectclass and attribute definitions reside on schema files, on the
subdirectory schema under the OpenLDAP installation home.