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Reference: Cisco: Internetworking Basics
The Lightweight Directory Access Protocol (LDAP) is an application
protocol used to access information in a directory over an IP network.
Most information in a directory is organized with a hierarchical
structure much like a file system.
A directory can contain any type of data but most often consists of
organizations, user accounts, address books, and other types of
resources like files and devices on a network. If you have a number
of Oracle databases in your organization, you can even become
adventurous and use LDAP directory naming to
resolve SQL*Net service names for Oracle databases
which allows storing net service names in a centralized
LDAP-compliant directory server in place of a client-side
OpenLDAP Software is a free, open source implementation of the LDAP protocol
developed by the OpenLDAP Project. Directory service is controlled by
a stand-alone LDAP daemon (
slapd) and the stand-alone LDAP
update replication daemon (
slurpd) when replication is configured.
Propagating changes using replication falls outside the scope of this document
and therefore will not include configuring the
This guide presents the steps required to install and configure OpenLDAP Software on the CentOS 5 platform. This tutorial will also work for Red Hat Enterprise Linux 5 and Oracle Linux 5.
The installation of OpenLDAP Software performed in this guide will be done using the yum package manager. Yum is available on any of the Red Hat Enterprise Linux distributions including its clones like CentOS. Building OpenLDAP Software from source will not be discussed here. If you need to compile from source, please see Building and Installing OpenLDAP Software from the OpenLDAP Software website.
Log in to the server as
root and install the basic OpenLDAP Software
package and related management tools needed for LDAP administration
from the yum repository. This will update any previously installed release
of the OpenLDAP Software packages.
The following table summarizes the OpenLDAP Software packages installed in the above step.
||Contains all configuration files, libraries, and documentation for OpenLDAP.|
||Contains files needed to host an LDAP server (
||Contains the client programs needed for accessing and modifying LDAP directories.|
||Contains access client software for using LDAP as a method of user authentication for Linux.|
After installing OpenLDAP Software, the next step is to modify the necessary configuration files to customize the LDAP server. As with any new application, it is highly recommended to understand the purpose of each configuration file and to create a backup of the original version of those configuration files before modifying them.
The following table describes the client and server configuration files used to customize OpenLDAP Software.
|Configuration File||Description||Used in this Guide?|
Main server configuration file that contains information needed by
Example DB_CONFIG file for use with
Contains all files that make up the LDAP
directory database. None of the files contained in this
directory should be manually edited. The LDAP directory
database type and location will be defined in the
Directory that contains a set of default schema specifications which
describe the different object classes that are available by default
with the OpenLDAP Software. Each set is defined in a file
Client configuration file that is used for authentication and
is required to be configured when LDAP is added as a lookup in
Used to set system-wide defaults to be applied when running
LDAP client tools like
||This configuration file controls the Name Service Switch (NSS) facility which provides information to the client to determine the sources from which to obtain name-service information in a range of categories, and in what order. Although not used in this guide, this file can be used by the client to tell the name service subsystem to use LDAP to obtain information. This is necessary when configuring LDAP authentication in the UNIX/Linux environment.|
Shell script used to start and stop the LDAP server (
In OpenLDAP, any file that begins with "
In OpenLDAP, any file that begins with "
Backup the original version of any OpenLDAP Software configuration file before making modifications.
This section provides instructions on how to configure the LDAP server by modifying
/etc/openldap/slapd.conf file. As previously noted, this file is the main
configuration file needed by the LDAP server (
slapd) and contains
server-wide directives, directory configuration information specific to your
domain and server, database type and location, and ACL definitions that define
authorization to your directory.
include directive is used to import other files.
The default configuration file includes the main schema files
which are used to tell the LDAP server the different object classes
and attributes that are available.
There is no need to modify the included default schema files for this guide.
The LDAP server can be configured to allow LDAP v2 clients
by specifying "
allow bind_v2" in the configuration file.
Without explicitly specifying this option, the LDAP server will reject any v2 clients.
If you don't know ahead of time the version of your LDAP clients, it is safe to leave it un-commented which enables v2 client access.
pidfile option specifies the absolute name of a
file what will hold the
slapd server's process ID while
argsfile option specifies the absolute name of
a file that will hold the
slapd server's command line
Both settings can be left at their default values.
Although not included by default in the configuration file, logging
can be setup using the
loglevel directive. This directive
specifies the level at which debugging statements and operation statistics
should be logged. Log levels may be specified as integers or by keyword; however,
most people stick to using
loglevel integers. Multiple log levels may be used
and the levels are additive. For example, when setting multiple log levels,
add up the integers for the levels you want to use and use that integer when
loglevel directive. I typically specify logging
for trace function calls, connection management, ACL processing,
and stats log connections/operations/results so
the log level is determined by adding the integers 1, 8, 128, and 256 together
loglevel of 393. The standard (default) log level is 256. The
possible log level values are described in the following table.
|-1||Any||enable all debugging|
|1||(0x1 trace)||trace function calls|
|2||(0x2 packets)||debug packet handling|
|4||(0x4 args)||heavy trace debugging (function args)|
|8||(0x8 conns)||connection management|
|16||(0x10 BER)||print out packets sent and received|
|32||(0x20 filter)||search filter processing|
|64||(0x40 config)||configuration file processing|
|128||(0x80 ACL)||access control list processing|
|256||(0x100 stats)||stats log connections/operations/results|
|512||(0x200 stats2)||stats log entries sent|
|1024||(0x400 shell)||print communication with shell backends|
|2048||(0x800 parse)||print entry parsing debugging|
|4096||(0x1000 cache)||caching (unused)|
|8192||(0x2000 index)||data indexing (unused)|
|16384||(0x4000 sync)||LDAPSync replication|
|32768||(0x8000 none)||only messages that get logged whatever log level is set|
Another setting related to logging that is not included by default is the
By default, log entries for LDAP are sent to syslog. While this has its benefits, I find
it more convenient to direct log entries to a file using the
Below are the logging options I used for the LDAP server configuration described
in this guide.
When specifying the
logfile directive, you need to
make certain that the log file exists prior to starting the LDAP server.
Another global directive that should be set and is not included by default
password-hash. This option configures one or more hashes to be
used in generation of user passwords stored in the
during processing of LDAP Password Modify Extended Operations (RFC 3062).
This allows the directory server to handle hashing instead of the client.
The password hash to use for new passwords must be one of
password-hash directive is not specified, the default is
Including this option in the configuration file conforms to best practices and will be specified in this guide using the SSHA hashing algorithm.
moduleload option to specify the name of a dynamically
loadable module to load. The filename may be an absolute path name or a
simple filename. Non-absolute names are searched for in the directories
specified by the
modulepath option. This option and the
option are only usable if
slapd was compiled with
A default OpenLDAP Software installation includes a number of module load statements; all of which are commented out. Loading modules falls outside the scope of this guide and will not be covered.
By default, LDAP network connections are not encrypted. All network transmission is performed in plain text. Enabling Transport Layer Security (TLS) provides added security through its framework by requiring a valid X.509 certificate on the server. Client certificates are optional.
Although highly recommended with a production directory server, building
slapd with support for TLS is beyond the scope of this
guide and will not be covered.
The next section allows you to specify any global ACLs. The default configuration file includes several ACL statements that are commented out.
Adding the following ACLs are common when the directory is being used for authentication.
This permission grants anonymous users to log in, and all authenticated users to read any entries in the directory. For example, users may lookup other users' profiles.
This permission lets users update their own
passwords (i.e., using
Determining an LDAP domain for an enterprise takes careful consideration.
This is where questions regarding how you intend to use the directory and how
the data will be organized to enhance efficiency needs to be answered.
The domain used in this guide to serve LDAP will simply be the name of my
website idevelopment.info. The LDAP domain gets specified using the
suffix directive. Also discussed in this section is how to
configure the administrator of the directory (
suffix option is your base Distinguished Name (DN) and
names the domain for which the LDAP server provides information. You
can set this to anything you like, but remember that it will be included
in the DN for each and every record in your directory.
Edit the line that specifies
and change it accordingly. In this guide, I am using the domain suffix
"dc=idevelopment,dc=info". Note that it is a convention not to
have any white space characters in this string.
Below suffix is the
rootdn entry is the full
Distinguished Name (DN) for the user who is unrestricted by access controls
or administrative limit parameters set for operations on the LDAP directory.
rootdn user can be thought of as the root user for the LDAP
directory. The default is
Manager, but you can change that to
root or any other value. Note that this entry is not the same
as the root user account on your Linux/UNIX operating system. If you choose to add
the root user account in LDAP, it will be a different user than the
For the domain used in this example, I specified
rootpw option is the password for the
that you specified in the previous step. Although you can include
the password in clear text, it is not recommended for obvious
security reasons. To encrypt the password, OpenLDAP Software comes with a
slappasswd which accepts clear text passwords and converts
them. In addition,
slappasswd can also be passed the hash method you want to use.
The most basic hash method is
crypt, but it is not very
secure. There are two other methods available, and each has two variations.
The first is the standard
MD5 encryption. You can also use
MD5 with a salt, which is designated as
SMD5. The most secure
SHA-1 encryption. If you want to include a salt,
which is recommended, it is designated as
SSHA is the
default hash method, so you shouldn't need to pass in this option.
/usr/sbin/slappasswd to generate an encrypted
password for the
Manager (or root) LDAP user. When prompted, type and then re-type the password.
The program displays the resulting encrypted password version
for you to copy into the configuration file.
Copy the generated hash and then add it to the
The information in an LDAP directory tree is stored in a directory database. There are two Berkley DB types that are available by default. BDB is the original Berkley DB, and HDB is the newer variation. Both are good for smaller networks, but HDB is a bit faster when you have a large number of users. If you will be using LDAP on a larger scale, consider using HDB for greater efficiency.
database directive to select the LDAP directory database type
directory directive to specify the location of the backend database.
The example presented in this guide uses the bdb database located at /var/lib/ldap.
In addition to BDB and HDB, the OpenLDAP Software data storage type can also be
configured using Oracle, MySQL, PostgreSQL, and several other backend databases; however,
those will not be discussed in this guide.
Using OpenLDAP Software with a BDB backend requires a
configuration file for optimum performance. An example
DB_CONFIG file exists
/etc/openldap/DB_CONFIG.example. To create an LDAP database
configuration file for BDB, simply copy the example configuration file to the
LDAP directory database location as follows:
The final step is to verify the file system directory for the LDAP directory database
is owned by the correct user and group. As with all
externally-accessible daemons running on your system, LDAP should never
be run as the root user for security reasons. Because of this, OpenLDAP Software
automatically creates a user and group on the local system; both named
The owner and group for the database directory should be
The rest of the
slapd.conf file can be left at defaults.
The only other part of the configuration file to point out is the
section at the bottom. Here you can specify indices for the directory database just
like you would create indices in any relational database. This provides better
lookup efficiency that can speed up a client's ability to access the most commonly
used attributes in the directory. It's common to index entries like
loginShell to provide faster access to those attributes. You don't need
nor would you want to index fields that are rarely accessed since it requires
more memory to do so.
For clarity, I am displaying the entire contents of the
/etc/openldap/slapd.conf file used in this guide with non-default (modified)
entries highlighted in blue.
After saving the LDAP configuration file, test the
configuration using slaptest.
This will tell you if you have issues with your config file. Initially it
may say that it cannot find the file
id2entry.bdb along with
other possible errors. This specific warning is OK. The
file will be created the first time the LDAP server is run. The results
should indicate that a test without checking for those files would have
This section contains instructions for starting and stopping the LDAP server.
The LDAP server is now ready to be started for the first time.
Modify the LDAP server to start when the machine boots.
Running the LDAP server the first time may give you the following lines:
This message should disappear the next time
slapd is restarted.
The LDAP server can be stopped using the following:
/etc/openldap/ldap.conf configuration file is used to
set system-wide defaults to be applied when running LDAP clients.
It is common to set these system-wide defaults on both client
and server hosts configured to use LDAP.
Modify the LDAP client configuration file (
and change the URI and BASE options for your LDAP server.
At this point the LDAP server is running and the backend database was created; however, the directory doesn't contain any data. The directory starts out completely empty, without even a root structure present. Initializing the directory with a root record and other supporting directory sub-structures (i.e., sub-directories) is required before you can add any user data.
Initialize the new LDAP directory using the following document:
Jeffrey Hunter is an Oracle Certified Professional, Java Development Certified Professional, Author, and an Oracle ACE. Jeff currently works as a Senior Database Administrator for The DBA Zone, Inc. located in Pittsburgh, Pennsylvania. His work includes advanced performance tuning, Java and PL/SQL programming, developing high availability solutions, capacity planning, database security, and physical / logical database design in a UNIX / Linux server environment. Jeff's other interests include mathematical encryption theory, tutoring advanced mathematics, programming language processors (compilers and interpreters) in Java and C, LDAP, writing web-based database administration tools, and of course Linux. He has been a Sr. Database Administrator and Software Engineer for over 20 years and maintains his own website site at: http://www.iDevelopment.info. Jeff graduated from Stanislaus State University in Turlock, California, with a Bachelor's degree in Computer Science and Mathematics.
Copyright (c) 1998-2018 Jeffrey M. Hunter. All rights reserved.
All articles, scripts and material located at the Internet address of http://www.idevelopment.info is the copyright of Jeffrey M. Hunter and is protected under copyright laws of the United States. This document may not be hosted on any other site without my express, prior, written permission. Application to host any of the material elsewhere can be made by contacting me at firstname.lastname@example.org.
I have made every effort and taken great care in making sure that the material included on my web site is technically accurate, but I disclaim any and all responsibility for any loss, damage or destruction of data or any other property which may arise from relying on it. I will in no case be liable for any monetary damages arising from such loss, damage or destruction.
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