# $OpenLDAP: pkg/openldap-guide/admin/intro.sdf,v 1.45.2.11 2011/01/31 21:05:07 quanah Exp $
# Copyright 1999-2011 The OpenLDAP Foundation, All Rights Reserved.
# COPYING RESTRICTIONS APPLY, see COPYRIGHT.
H1: Introduction to OpenLDAP Directory Services

This document describes how to build, configure, and operate
{{PRD:OpenLDAP}} Software to provide directory services.  This
includes details on how to configure and run the Standalone
{{TERM:LDAP}} Daemon, {{slapd}}(8).  It is intended for new and
experienced administrators alike.  This section provides a basic
introduction to directory services and, in particular, the directory
services provided by {{slapd}}(8).  This introduction is only
intended to provide enough information so one might get started
learning about {{TERM:LDAP}}, {{TERM:X.500}}, and directory services.


H2: What is a directory service?

A directory is a specialized database specifically designed for
searching and browsing, in additional to supporting basic lookup
and update functions.

Note: A directory is defined by some as merely a database optimized
for read access.  This definition, at best, is overly simplistic.

Directories tend to contain descriptive, attribute-based information
and support sophisticated filtering capabilities.  Directories
generally do not support complicated transaction or roll-back schemes
found in database management systems designed for handling high-volume
complex updates.  Directory updates are typically simple all-or-nothing
changes, if they are allowed at all.  Directories are generally
tuned to give quick response to high-volume lookup or search
operations. They may have the ability to replicate information
widely in order to increase availability and reliability, while
reducing response time.  When directory information is replicated,
temporary inconsistencies between the replicas may be okay, as long
as inconsistencies are resolved in a timely manner.

There are many different ways to provide a directory service.
Different methods allow different kinds of information to be stored
in the directory, place different requirements on how that information
can be referenced, queried and updated, how it is protected from
unauthorized access, etc.  Some directory services are {{local}},
providing service to a restricted context (e.g., the finger service
on a single machine). Other services are global, providing service
to a much broader context (e.g., the entire Internet).  Global
services are usually {{distributed}}, meaning that the data they
contain is spread across many machines, all of which cooperate to
provide the directory service. Typically a global service defines
a uniform {{namespace}} which gives the same view of the data no
matter where you are in relation to the data itself.

A web directory, such as provided by the {{Open Directory Project}}
<{{URL:http://dmoz.org}}>, is a good example of a directory service.
These services catalog web pages and are specifically designed to
support browsing and searching.

While some consider the Internet {{TERM[expand]DNS}} (DNS) is an
example of a globally distributed directory service, DNS is not
browseable nor searchable.  It is more properly described as a
globally distributed {{lookup}} service.


H2: What is LDAP?

{{TERM:LDAP}} stands for {{TERM[expand]LDAP}}.  As the name suggests,
it is a lightweight protocol for accessing directory services,
specifically {{TERM:X.500}}-based directory services.  LDAP runs
over {{TERM:TCP}}/{{TERM:IP}} or other connection oriented transfer
services.  LDAP is an {{ORG:IETF}} Standard Track protocol and is
specified in "Lightweight Directory Access Protocol (LDAP) Technical
Specification Road Map" {{REF:RFC4510}}.

This section gives an overview of LDAP from a user's perspective.

{{What kind of information can be stored in the directory?}} The
LDAP information model is based on {{entries}}. An entry is a
collection of attributes that has a globally-unique {{TERM[expand]DN}}
(DN).  The DN is used to refer to the entry unambiguously. Each of
the entry's attributes has a {{type}} and one or more {{values}}.
The types are typically mnemonic strings, like "{{EX:cn}}" for
common name, or "{{EX:mail}}" for email address. The syntax of
values depend on the attribute type.  For example, a {{EX:cn}}
attribute might contain the value {{EX:Babs Jensen}}.  A {{EX:mail}}
attribute might contain the value "{{EX:babs@example.com}}". A
{{EX:jpegPhoto}} attribute would contain a photograph in the
{{TERM:JPEG}} (binary) format.

{{How is the information arranged?}} In LDAP, directory entries
are arranged in a hierarchical tree-like structure.  Traditionally,
this structure reflected the geographic and/or organizational
boundaries.  Entries representing countries appear at the top of
the tree. Below them are entries representing states and national
organizations. Below them might be entries representing organizational
units, people, printers, documents, or just about anything else
you can think of.  Figure 1.1 shows an example LDAP directory tree
using traditional naming.

!import "intro_tree.png"; align="center"; \
	title="LDAP directory tree (traditional naming)"
FT[align="Center"] Figure 1.1: LDAP directory tree (traditional naming)

The tree may also be arranged based upon Internet domain names.
This naming approach is becoming increasing popular as it allows
for directory services to be located using the {{DNS}}.
Figure 1.2 shows an example LDAP directory tree using domain-based
naming.

!import "intro_dctree.png"; align="center"; \
	title="LDAP directory tree (Internet naming)"
FT[align="Center"] Figure 1.2: LDAP directory tree (Internet naming)

In addition, LDAP allows you to control which attributes are required
and allowed in an entry through the use of a special attribute
called {{EX:objectClass}}.  The values of the {{EX:objectClass}}
attribute determine the {{schema}} rules the entry must obey.

{{How is the information referenced?}} An entry is referenced by
its distinguished name, which is constructed by taking the name of
the entry itself (called the {{TERM[expand]RDN}} or RDN) and
concatenating the names of its ancestor entries. For example, the
entry for Barbara Jensen in the Internet naming example above has
an RDN of {{EX:uid=babs}} and a DN of
{{EX:uid=babs,ou=People,dc=example,dc=com}}. The full DN format is
described in {{REF:RFC4514}}, "LDAP: String Representation of
Distinguished Names."

{{How is the information accessed?}} LDAP defines operations for
interrogating and updating the directory.  Operations are provided
for adding and deleting an entry from the directory, changing an
existing entry, and changing the name of an entry. Most of the
time, though, LDAP is used to search for information in the directory.
The LDAP search operation allows some portion of the directory to
be searched for entries that match some criteria specified by a
search filter. Information can be requested from each entry that
matches the criteria.

For example, you might want to search the entire directory subtree
at and below {{EX:dc=example,dc=com}} for people with the name
{{EX:Barbara Jensen}}, retrieving the email address of each entry
found. LDAP lets you do this easily.  Or you might want to search
the entries directly below the {{EX:st=California,c=US}} entry for
organizations with the string {{EX:Acme}} in their name, and that
have a fax number. LDAP lets you do this too. The next section
describes in more detail what you can do with LDAP and how it might
be useful to you.

{{How is the information protected from unauthorized access?}} Some
directory services provide no protection, allowing anyone to see
the information. LDAP provides a mechanism for a client to authenticate,
or prove its identity to a directory server, paving the way for
rich access control to protect the information the server contains.
LDAP also supports data security (integrity and confidentiality)
services.


H2: When should I use LDAP?

This is a very good question. In general, you should use a Directory
server when you require data to be centrally managed, stored and accessible via
standards based methods. 

Some common examples found throughout the industry are, but not limited to:

* Machine Authentication
* User Authentication
* User/System Groups
* Address book
* Organization Representation
* Asset Tracking
* Telephony Information Store
* User resource management
* E-mail address lookups
* Application Configuration store
* PBX Configuration store
* etc.....

There are various {{SECT:Distributed Schema Files}} that are standards based, but
you can always create your own {{SECT:Schema Specification}}.

There are always new ways to use a Directory and apply LDAP principles to address
certain problems, therefore there is no simple answer to this question.

If in doubt, join the general LDAP forum for non-commercial discussions and 
information relating to LDAP at: 
{{URL:http://www.umich.edu/~dirsvcs/ldap/mailinglist.html}} and ask

H2: When should I not use LDAP?

When you start finding yourself bending the directory to do what you require,
maybe a redesign is needed. Or if you only require one application to use and 
manipulate your data (for discussion of LDAP vs RDBMS, please read the 
{{SECT:LDAP vs RDBMS}} section).

It will become obvious when LDAP is the right tool for the job.


H2: How does LDAP work?

LDAP utilizes a {{client-server model}}. One or more LDAP servers
contain the data making up the directory information tree ({{TERM:DIT}}).
The client connects to servers and asks it a question.  The server
responds with an answer and/or with a pointer to where the client
can get additional information (typically, another LDAP server).
No matter which LDAP server a client connects to, it sees the same
view of the directory; a name presented to one LDAP server references
the same entry it would at another LDAP server.  This is an important
feature of a global directory service.


H2: What about X.500?

Technically, {{TERM:LDAP}} is a directory access protocol to an
{{TERM:X.500}} directory service, the {{TERM:OSI}} directory service.
Initially, LDAP clients accessed gateways to the X.500 directory service.
This gateway ran LDAP between the client and gateway and X.500's
{{TERM[expand]DAP}} ({{TERM:DAP}}) between the gateway and the
X.500 server.  DAP is a heavyweight protocol that operates over a
full OSI protocol stack and requires a significant amount of
computing resources.  LDAP is designed to operate over
{{TERM:TCP}}/{{TERM:IP}} and provides most of the functionality of
DAP at a much lower cost.

While LDAP is still used to access X.500 directory service via
gateways, LDAP is now more commonly directly implemented in X.500
servers. 

The Standalone LDAP Daemon, or {{slapd}}(8), can be viewed as a
{{lightweight}} X.500 directory server.  That is, it does not
implement the X.500's DAP nor does it support the complete X.500
models.

If you are already running a X.500 DAP service and you want to
continue to do so, you can probably stop reading this guide.  This
guide is all about running LDAP via {{slapd}}(8), without running
X.500 DAP.  If you are not running X.500 DAP, want to stop running
X.500 DAP, or have no immediate plans to run X.500 DAP, read on.

It is possible to replicate data from an LDAP directory server to
a X.500 DAP {{TERM:DSA}}.  This requires an LDAP/DAP gateway.
OpenLDAP Software does not include such a gateway.


H2: What is the difference between LDAPv2 and LDAPv3?

LDAPv3 was developed in the late 1990's to replace LDAPv2.
LDAPv3 adds the following features to LDAP:

 * Strong authentication and data security services via {{TERM:SASL}}
 * Certificate authentication and data security services via {{TERM:TLS}} (SSL)
 * Internationalization through the use of Unicode
 * Referrals and Continuations
 * Schema Discovery
 * Extensibility (controls, extended operations, and more)

LDAPv2 is historic ({{REF:RFC3494}}).  As most {{so-called}} LDAPv2
implementations (including {{slapd}}(8)) do not conform to the
LDAPv2 technical specification, interoperability amongst
implementations claiming LDAPv2 support is limited.  As LDAPv2
differs significantly from LDAPv3, deploying both LDAPv2 and LDAPv3
simultaneously is quite problematic.  LDAPv2 should be avoided.
LDAPv2 is disabled by default.


H2: LDAP vs RDBMS

This question is raised many times, in different forms. The most common, 
however, is: {{Why doesn't OpenLDAP drop Berkeley DB and use a relational 
database management system (RDBMS) instead?}} In general, expecting that the 
sophisticated algorithms implemented by commercial-grade RDBMS would make 
{{OpenLDAP}} be faster or somehow better and, at the same time, permitting 
sharing of data with other applications.

The short answer is that use of an embedded database and custom indexing system 
allows OpenLDAP to provide greater performance and scalability without loss of 
reliability. OpenLDAP uses Berkeley DB concurrent / transactional 
database software. This is the same software used by leading commercial 
directory software.

Now for the long answer. We are all confronted all the time with the choice 
RDBMSes vs. directories. It is a hard choice and no simple answer exists.

It is tempting to think that having a RDBMS backend to the directory solves all 
problems. However, it is a pig. This is because the data models are very 
different. Representing directory data with a relational database is going to 
require splitting data into multiple tables.

Think for a moment about the person objectclass. Its definition requires 
attribute types objectclass, sn and cn and allows attribute types userPassword, 
telephoneNumber, seeAlso and description. All of these attributes are multivalued, 
so a normalization requires putting each attribute type in a separate table.

Now you have to decide on appropriate keys for those tables. The primary key 
might be a combination of the DN, but this becomes rather inefficient on most 
database implementations.

The big problem now is that accessing data from one entry requires seeking on 
different disk areas. On some applications this may be OK but in many 
applications performance suffers.

The only attribute types that can be put in the main table entry are those that 
are mandatory and single-value. You may add also the optional single-valued 
attributes and set them to NULL or something if not present.

But wait, the entry can have multiple objectclasses and they are organized in 
an inheritance hierarchy. An entry of objectclass organizationalPerson now has 
the attributes from person plus a few others and some formerly optional attribute 
types are now mandatory.

What to do? Should we have different tables for the different objectclasses? 
This way the person would have an entry on the person table, another on 
organizationalPerson, etc. Or should we get rid of person and put everything on 
the second table?

But what do we do with a filter like (cn=*) where cn is an attribute type that 
appears in many, many objectclasses. Should we search all possible tables for 
matching entries? Not very attractive.

Once this point is reached, three approaches come to mind. One is to do full 
normalization so that each attribute type, no matter what, has its own separate 
table. The simplistic approach where the DN is part of the primary key is 
extremely wasteful, and calls for an approach where the entry has a unique 
numeric id that is used instead for the keys and a main table that maps DNs to 
ids. The approach, anyway, is very inefficient when several attribute types from 
one or more entries are requested. Such a database, though cumbersomely, 
can be managed from SQL applications.

The second approach is to put the whole entry as a blob in a table shared by all 
entries regardless of the objectclass and have additional tables that act as 
indices for the first table. Index tables are not database indices, but are 
fully managed by the LDAP server-side implementation. However, the database 
becomes unusable from SQL. And, thus, a fully fledged database system provides 
little or no advantage. The full generality of the database is unneeded. 
Much better to use something light and fast, like Berkeley DB. 

A completely different way to see this is to give up any hopes of implementing 
the directory data model. In this case, LDAP is used as an access protocol to 
data that provides only superficially the directory data model. For instance, 
it may be read only or, where updates are allowed, restrictions are applied, 
such as making single-value attribute types that would allow for multiple values. 
Or the impossibility to add new objectclasses to an existing entry or remove 
one of those present. The restrictions span the range from allowed restrictions 
(that might be elsewhere the result of access control) to outright violations of 
the data model. It can be, however, a method to provide LDAP access to preexisting 
data that is used by other applications. But in the understanding that we don't
really have a "directory".

Existing commercial LDAP server implementations that use a relational database 
are either from the first kind or the third. I don't know of any implementation 
that uses a relational database to do inefficiently what BDB does efficiently.
For those who are interested in "third way" (exposing EXISTING data from RDBMS 
as LDAP tree, having some limitations compared to classic LDAP model, but making 
it possible to interoperate between LDAP and SQL applications):

OpenLDAP includes back-sql - the backend that makes it possible. It uses ODBC + 
additional metainformation about translating LDAP queries to SQL queries in your 
RDBMS schema, providing different levels of access - from read-only to full 
access depending on RDBMS you use, and your schema.

For more information on concept and limitations, see {{slapd-sql}}(5) man page, 
or the {{SECT: Backends}} section. There are also several examples for several 
RDBMSes in {{F:back-sql/rdbms_depend/*}} subdirectories. 


H2: What is slapd and what can it do?

{{slapd}}(8) is an LDAP directory server that runs on many different
platforms. You can use it to provide a directory service of your
very own.  Your directory can contain pretty much anything you want
to put in it. You can connect it to the global LDAP directory
service, or run a service all by yourself. Some of slapd's more
interesting features and capabilities include:

{{B:LDAPv3}}: {{slapd}} implements version 3 of {{TERM[expand]LDAP}}.
{{slapd}} supports LDAP over both {{TERM:IPv4}} and {{TERM:IPv6}}
and Unix {{TERM:IPC}}.

{{B:{{TERM[expand]SASL}}}}: {{slapd}} supports strong authentication
and data security (integrity and confidentiality) services through
the use of SASL.  {{slapd}}'s SASL implementation utilizes {{PRD:Cyrus
SASL}} software which supports a number of mechanisms including
{{TERM:DIGEST-MD5}}, {{TERM:EXTERNAL}}, and {{TERM:GSSAPI}}.

{{B:{{TERM[expand]TLS}}}}: {{slapd}} supports certificate-based
authentication and data security (integrity and confidentiality)
services through the use of TLS (or SSL).  {{slapd}}'s TLS
implementation can utilize {{PRD:OpenSSL}}, {{PRD:GnuTLS}},
or {{PRD:MozNSS}} software.

{{B:Topology control}}: {{slapd}} can be configured to restrict
access at the socket layer based upon network topology information.
This feature utilizes {{TCP wrappers}}.

{{B:Access control}}: {{slapd}} provides a rich and powerful access
control facility, allowing you to control access to the information
in your database(s). You can control access to entries based on
LDAP authorization information, {{TERM:IP}} address, domain name
and other criteria.  {{slapd}} supports both {{static}} and {{dynamic}}
access control information.

{{B:Internationalization}}: {{slapd}} supports Unicode and language
tags.

{{B:Choice of database backends}}: {{slapd}} comes with a variety
of different database backends you can choose from. They include
{{TERM:BDB}}, a high-performance transactional database backend;
{{TERM:HDB}}, a hierarchical high-performance transactional
backend; {{SHELL}}, a backend interface to arbitrary shell scripts;
and PASSWD, a simple backend interface to the {{passwd}}(5) file.
The BDB and HDB backends utilize {{ORG:Oracle}} {{PRD:Berkeley
DB}}.

{{B:Multiple database instances}}: {{slapd}} can be configured to
serve multiple databases at the same time. This means that a single
{{slapd}} server can respond to requests for many logically different
portions of the LDAP tree, using the same or different database
backends.

{{B:Generic modules API}}:  If you require even more customization,
{{slapd}} lets you write your own modules easily. {{slapd}} consists
of two distinct parts: a front end that handles protocol communication
with LDAP clients; and modules which handle specific tasks such as
database operations.  Because these two pieces communicate via a
well-defined {{TERM:C}} {{TERM:API}}, you can write your own
customized modules which extend {{slapd}} in numerous ways.  Also,
a number of {{programmable database}} modules are provided.  These
allow you to expose external data sources to {{slapd}} using popular
programming languages ({{PRD:Perl}}, {{shell}}, and {{TERM:SQL}}).

{{B:Threads}}: {{slapd}} is threaded for high performance.  A single
multi-threaded {{slapd}} process handles all incoming requests using
a pool of threads.  This reduces the amount of system overhead
required while providing high performance.

{{B:Replication}}: {{slapd}} can be configured to maintain shadow
copies of directory information.  This {{single-master/multiple-slave}}
replication scheme is vital in high-volume environments where a
single {{slapd}} installation just doesn't provide the necessary availability
or reliability.  For extremely demanding environments where a
single point of failure is not acceptable, {{multi-master}} replication
is also available.  {{slapd}} includes support for {{LDAP Sync}}-based
replication.

{{B:Proxy Cache}}: {{slapd}} can be configured as a caching
LDAP proxy service.

{{B:Configuration}}: {{slapd}} is highly configurable through a
single configuration file which allows you to change just about
everything you'd ever want to change.  Configuration options have
reasonable defaults, making your job much easier. Configuration can
also be performed dynamically using LDAP itself, which greatly
improves manageability.

