openldap/doc/guide/intro.sdf

# Copyright 1999, The OpenLDAP Foundation, All Rights Reserved.
# COPYING RESTRICTIONS APPLY, see COPYRIGHT.
H1: Introduction to slapd and slurpd

This document describes how to build, configure, and run the stand-alone
LDAP daemon ({{I:slapd}}) and the stand-alone LDAP update replication
daemon ({{I:slurpd}}). It is intended for newcomers and experienced
administrators alike. This section provides a basic introduction to directory
service, and the directory service provided by {{I:slapd}} in particular.



H2: What is a directory service?

A directory is like a database, but tends to contain more descriptive,
attribute-based information. The information in a directory is generally read
much more often than it is written. As a consequence, directories don't
usually implement the complicated transaction or roll-back schemes regular
databases use for doing high-volume complex updates. Directory updates
are typically simple all-or-nothing changes, if they are allowed at all.
Directories are 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 OK, as long as they get in sync eventually.

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 {{I: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 {{I: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 {{I:namespace}}
which gives the same view of the data no matter where you are in relation to
the data itself.



H2: What is LDAP?

{{I:Slapd}}'s model for directory service is based on a global directory model
called LDAP, which stands for the Lightweight Directory Access Protocol.
LDAP is a directory service protocol that runs over TCP/IP. The nitty-gritty
details of LDAP are defined in RFC 1777 "The Lightweight Directory Access
Protocol." This section gives an overview of LDAP from a user's perspective.

{{I:What kind of information can be stored in the directory?}} The LDAP directory
service model is based on {{I:entries}}. An entry is a collection of 
attributes that has a name, called a {{I:distinguished name}} (DN). 
The DN is used to refer to the entry unambiguously. Each of the 
entry's attributes has a {{I:type}} and one or
more {{I:values}}. 
The types are typically mnemonic strings, like "{{EX:cn}}" for common
name, or "{{EX:mail}}" for email address. The values depend on what type of
attribute it is. For example, a {{EX:mail}} attribute might contain the value
"{{EX:babs@openldap.org}}". A {{EX:jpegPhoto}} attribute would contain 
a photograph in binary JPEG/JFIF format.

{{I:How is the information arranged?}} In LDAP, directory entries are arranged in
a hierarchical tree-like structure that reflects political, geographic and/or
organizational boundaries. Entries representing countries appear at the top
of the tree. Below them are entries representing states or national
organizations. Below them might be entries representing people,
organizational units, printers, documents, or just about anything else you can
think of. Figure 1 shows an example LDAP directory tree, which should help
make things clear.


!import "figure_1.gif"; align="center"; title="An example LDAP directory tree"
FT: Figure 1: An example LDAP directory tree.


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

{{I: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 relative distinguished name, or RDN) and concatenating the
names of its ancestor entries. For example, the entry for Barbara Jensen in
the example above has an RDN of "{{EX:cn=Barbara J Jensen}}" and a DN of
"{{EX:cn=Barbara J Jensen, o=OpenLDAP Project, c=US}}". The full DN format is
described in RFC 1779, "A String Representation of Distinguished Names."

{{I: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 below the
OpenLDAP Project for people with the name 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 c=US entry for
organizations with the string "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.

{{I:How is the information protected from unauthorized access?}} Some directory
services provide no protection, allowing anyone to see the information. LDAP
provides a method 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.



H2: How does LDAP work?

LDAP directory service is based on a {{I:client-server}} model. One or more
LDAP servers contain the data making up the LDAP directory tree. An LDAP
client connects to an LDAP server and asks it a question. The server
responds with the answer, or with a pointer to where the client can get more
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, like LDAP.




H2: What is slapd and what can it do?

{{I:Slapd}} is an LDAP directory server that runs on many different UNIX
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:Choice of databases}}: {{I:Slapd}} comes with three different backend 
databases you can choose from. They are LDBM, a high-performance disk-based
database; SHELL, a database interface to arbitrary UNIX commands or shell
scripts; and PASSWD, a simple password file database.

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

{{B:Generic database API}}: If you require even more customization, {{I:slapd}} 
lets you write your own backend database easily. {{I:Slapd}} 
consists of two distinct parts: a front end that handles protocol 
communication with LDAP clients; and a backend that handles database 
operations. Because these two pieces communicate via a well-defined 
C API, you can write your own customized database backend to {{I:slapd}}.

{{B:Access control}}: {{I: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 authentication information, IP address, domain name and other criteria.

{{B:Threads}}: {{I:Slapd}} is threaded for high performance. A 
single multi-threaded {{I:slapd}} process handles all incoming 
requests, reducing the amount of system overhead required. {{I:Slapd}}
will automatically select the best thread support for your platform.

{{B:Replication}}: {{I:Slapd}} can be configured to maintain replica 
copies of its database. This master/slave replication scheme is 
vital in high-volume environments where a single {{I:slapd}} just 
doesn't provide the necessary availability or reliability.

{{B:Configuration}}: {{I: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.

{{I:Slapd}} also has its limitations, of course. It does not 
currently handle aliases, which are part of the LDAP model. The 
main LDBM database backend does not handle range queries or negation 
queries very well. These features and more will be coming in a future release.



H2: What about X.500?

LDAP was originally developed as a front end to X.500, the OSI directory
service. X.500 defines the Directory Access Protocol (DAP) for clients to
use when contacting directory servers. DAP is a heavyweight protocol that
runs over a full OSI stack and requires a significant amount of computing
resources to run. LDAP runs directly over TCP and provides most of the
functionality of DAP at a much lower cost.

This use of LDAP makes it easy to access the X.500 directory, but still
requires a full X.500 service to make data available to the many LDAP clients
being developed. As with full X.500 DAP clients, a full X.500 server is no
small piece of software to run.

The stand-alone LDAP daemon, or {{I:slapd}}, is meant to remove much of the
burden from the server side just as LDAP itself removed much of the burden
from clients. If you are already running an X.500 service and you want to
continue to do so, you can probably stop reading this guide, which is all
about running LDAP via {{I:slapd}}, without running X.500. If you are not running
X.500, want to stop running X.500, or have no immediate plans to run X.500,
read on.

It is possible to replicate data from a {{I:slapd}} directory 
server to an X.500 DSA, which allows your organization to make your 
data available as part of the global X.500 directory service on a 
"read-only" basis. This is discussed in section 11.6.

Another way to make data in a {{I:slapd}} server available to the X.500
community would be by using a X.500 DAP to LDAP gateway. At this time, no
such software has been written (to the best of our knowledge), but hopefully
some group will see fit towrite such a gateway.



H2: What is slurpd and what can it do?

{{I:Slurpd}} is a UNIX daemon that helps {{I:slapd}} provide 
replicated service. It is responsible for distributing changes made 
to the master {{I:slapd}} database out to the various {{I:slapd}} 
replicas. It frees {{I:slapd}} from having to worry that some
replicas might be down or unreachable when a change comes through;
{{I:slurpd}} handles retrying failed requests automatically. 
{{I:Slapd}} and {{I:slurpd}} communicate through a simple text 
file that is used to log changes.

PB: