Chapter 20. Directory Servers

20.1. OpenLDAP

LDAP (Lightweight Directory Access Protocol) is a set of open protocols used to access centrally stored information over a network. It is based on the X.500 standard for directory sharing, but is less complex and resource-intensive. For this reason, LDAP is sometimes referred to as “X.500 Lite”.

Like X.500, LDAP organizes information in a hierarchical manner using directories. These directories can store a variety of information such as names, addresses, or phone numbers, and can even be used in a manner similar to the Network Information Service (NIS), enabling anyone to access their account from any machine on the LDAP enabled network.

LDAP is commonly used for centrally managed users and groups, user authentication, or system configuration. It can also serve as a virtual phone directory, allowing users to easily access contact information for other users. Additionally, it can refer a user to other LDAP servers throughout the world, and thus provide an ad-hoc global repository of information. However, it is most frequently used within individual organizations such as universities, government departments, and private companies.

This section covers the installation and configuration of OpenLDAP 2.4, an open source implementation of the LDAPv2 and LDAPv3 protocols.

20.1.1. Introduction to LDAP

Using a client-server architecture, LDAP provides a reliable means to create a central information directory accessible from the network. When a client attempts to modify information within this directory, the server verifies the user has permission to make the change, and then adds or updates the entry as requested. To ensure the communication is secure, the Transport Layer Security (TLS) cryptographic protocol can be used to prevent an attacker from intercepting the transmission.

Important

The OpenLDAP suite in Red Hat Enterprise Linux 6 no longer uses OpenSSL. Instead, it uses the Mozilla implementation of Network Security Services (NSS). OpenLDAP continues to work with existing certificates, keys, and other TLS configuration. For more information on how to configure it to use Mozilla certificate and key database, see How do I use TLS/SSL with Mozilla NSS.

Important

Due to the vulnerability described in Resolution for POODLE SSLv3.0 vulnerability (CVE-2014-3566) for components that do not allow SSLv3 to be disabled via configuration settings, Red Hat recommends that you do not rely on the SSLv3 protocol for security. OpenLDAP is one of the system components that do not provide configuration parameters that allow SSLv3 to be effectively disabled. To mitigate the risk, it is recommended that you use the stunnel command to provide a secure tunnel, and disable stunnel from using SSLv3. For more information on using stunnel, see the Red Hat Enterprise Linux 6 Security Guide.

The LDAP server supports several database systems, which gives administrators the flexibility to choose the best suited solution for the type of information they are planning to serve. Because of a well-defined client Application Programming Interface (API), the number of applications able to communicate with an LDAP server is numerous, and increasing in both quantity and quality.

20.1.1.1. LDAP Terminology

The following is a list of LDAP-specific terms that are used within this chapter:

entry

A single unit within an LDAP directory. Each entry is identified by its unique Distinguished Name (DN).

attribute

Information directly associated with an entry. For example, if an organization is represented as an LDAP entry, attributes associated with this organization might include an address, a fax number, etc. Similarly, people can be represented as entries with common attributes such as personal telephone number or email address.

An attribute can either have a single value, or an unordered space-separated list of values. While certain attributes are optional, others are required. Required attributes are specified using the objectClass definition, and can be found in schema files located in the /etc/openldap/slapd.d/cn=config/cn=schema/ directory.

The assertion of an attribute and its corresponding value is also referred to as a Relative Distinguished Name (RDN). Unlike distinguished names that are unique globally, a relative distinguished name is only unique per entry.

LDIF

The LDAP Data Interchange Format (LDIF) is a plain text representation of an LDAP entry. It takes the following form:

[id] dn: distinguished_name
attribute_type: attribute_value
attribute_type: attribute_value
...

The optional id is a number determined by the application that is used to edit the entry. Each entry can contain as many attribute_type and attribute_value pairs as needed, as long as they are all defined in a corresponding schema file. A blank line indicates the end of an entry.

20.1.1.2. OpenLDAP Features

OpenLDAP suite provides a number of important features:

LDAPv3 Support — Many of the changes in the protocol since LDAP version 2 are designed to make LDAP more secure. Among other improvements, this includes the support for Simple Authentication and Security Layer (SASL), and Transport Layer Security (TLS) protocols.
LDAP Over IPC — The use of inter-process communication (IPC) enhances security by eliminating the need to communicate over a network.
IPv6 Support — OpenLDAP is compliant with Internet Protocol version 6 (IPv6), the next generation of the Internet Protocol.
LDIFv1 Support — OpenLDAP is fully compliant with LDIF version 1.
Updated C API — The current C API improves the way programmers can connect to and use LDAP directory servers.
Enhanced Standalone LDAP Server — This includes an updated access control system, thread pooling, better tools, and much more.

20.1.1.3. OpenLDAP Server Setup

The typical steps to set up an LDAP server on Red Hat Enterprise Linux are as follows:

Install the OpenLDAP suite. See Section 20.1.2, “Installing the OpenLDAP Suite” for more information on required packages.
Customize the configuration as described in Section 20.1.3, “Configuring an OpenLDAP Server”.
Start the slapd service as described in Section 20.1.4, “Running an OpenLDAP Server”.
Use the ldapadd utility to add entries to the LDAP directory.
Use the ldapsearch utility to verify that the slapd service is accessing the information correctly.

20.1.2. Installing the OpenLDAP Suite

The suite of OpenLDAP libraries and tools is provided by the following packages:

Table 20.1. List of OpenLDAP packages
Package	Description
openldap	A package containing the libraries necessary to run the OpenLDAP server and client applications.
openldap-clients	A package containing the command-line utilities for viewing and modifying directories on an LDAP server.
openldap-servers	A package containing both the services and utilities to configure and run an LDAP server. This includes the Standalone LDAP Daemon, `slapd`.
compat-openldap	A package containing the OpenLDAP compatibility libraries.

Additionally, the following packages are commonly used along with the LDAP server:

Table 20.2. List of commonly installed additional LDAP packages
Package	Description
sssd	A package containing `the System Security Services Daemon (SSSD)`, a set of daemons to manage access to remote directories and authentication mechanisms. It provides the Name Service Switch (NSS) and the Pluggable Authentication Modules (PAM) interfaces toward the system and a pluggable back-end system to connect to multiple different account sources.
mod_authz_ldap	A package containing `mod_authz_ldap`, the LDAP authorization module for the Apache HTTP Server. This module uses the short form of the distinguished name for a subject and the issuer of the client SSL certificate to determine the distinguished name of the user within an LDAP directory. It is also capable of authorizing users based on attributes of that user's LDAP directory entry, determining access to assets based on the user and group privileges of the asset, and denying access for users with expired passwords. Note that the `mod_ssl` module is required when using the `mod_authz_ldap` module.

To install these packages, use the yum command in the following form:

yum install package

For example, to perform the basic LDAP server installation, type the following at a shell prompt:

~]# yum install openldap openldap-clients openldap-servers

Note that you must have superuser privileges (that is, you must be logged in as root) to run this command. For more information on how to install new packages in Red Hat Enterprise Linux, see Section 8.2.4, “Installing Packages”.

20.1.2.1. Overview of OpenLDAP Server Utilities

To perform administrative tasks, the openldap-servers package installs the following utilities along with the slapd service:

Table 20.3. List of OpenLDAP server utilities
Command	Description
`slapacl`	Allows you to check the access to a list of attributes.
`slapadd`	Allows you to add entries from an LDIF file to an LDAP directory.
`slapauth`	Allows you to check a list of IDs for authentication and authorization permissions.
`slapcat`	Allows you to pull entries from an LDAP directory in the default format and save them in an LDIF file.
`slapdn`	Allows you to check a list of Distinguished Names (DNs) based on available schema syntax.
`slapindex`	Allows you to re-index the `slapd` directory based on the current content. Run this utility whenever you change indexing options in the configuration file.
`slappasswd`	Allows you to create an encrypted user password to be used with the `ldapmodify` utility, or in the `slapd` configuration file.
`slapschema`	Allows you to check the compliance of a database with the corresponding schema.
`slaptest`	Allows you to check the LDAP server configuration.

For a detailed description of these utilities and their usage, see the corresponding manual pages as referred to in Section 20.1.6.1, “Installed Documentation”.

Important

Although only root can run slapadd, the slapd service runs as the ldap user. Because of this, the directory server is unable to modify any files created by slapadd. To correct this issue, after running the slapd utility, type the following at a shell prompt:

~]# chown -R ldap:ldap /var/lib/ldap

Warning

To preserve the data integrity, stop the slapd service before using slapadd, slapcat, or slapindex. You can do so by typing the following at a shell prompt:

~]# service slapd stop
Stopping slapd:                                            [  OK  ]

For more information on how to start, stop, restart, and check the current status of the slapd service, see Section 20.1.4, “Running an OpenLDAP Server”.

20.1.2.2. Overview of OpenLDAP Client Utilities

The openldap-clients package installs the following utilities which can be used to add, modify, and delete entries in an LDAP directory:

Table 20.4. List of OpenLDAP client utilities
Command	Description
`ldapadd`	Allows you to add entries to an LDAP directory, either from a file, or from standard input. It is a symbolic link to `ldapmodify -a`.
`ldapcompare`	Allows you to compare given attribute with an LDAP directory entry.
`ldapdelete`	Allows you to delete entries from an LDAP directory.
`ldapexop`	Allows you to perform extended LDAP operations.
`ldapmodify`	Allows you to modify entries in an LDAP directory, either from a file, or from standard input.
`ldapmodrdn`	Allows you to modify the RDN value of an LDAP directory entry.
`ldappasswd`	Allows you to set or change the password for an LDAP user.
`ldapsearch`	Allows you to search LDAP directory entries.
`ldapurl`	Allows you to compose or decompose LDAP URLs.
`ldapwhoami`	Allows you to perform a `whoami` operation on an LDAP server.

With the exception of ldapsearch, each of these utilities is more easily used by referencing a file containing the changes to be made rather than typing a command for each entry to be changed within an LDAP directory. The format of such a file is outlined in the man page for each utility.

20.1.2.3. Overview of Common LDAP Client Applications

Although there are various graphical LDAP clients capable of creating and modifying directories on the server, none of them is included in Red Hat Enterprise Linux. Popular applications that can access directories in a read-only mode include Mozilla Thunderbird, Evolution, or Ekiga.

20.1.3. Configuring an OpenLDAP Server

By default, OpenLDAP stores its configuration in the /etc/openldap/ directory. Table 20.5, “List of OpenLDAP configuration files and directories” highlights the most important files and directories within this directory.

Table 20.5. List of OpenLDAP configuration files and directories
Path	Description
`/etc/openldap/ldap.conf`	The configuration file for client applications that use the OpenLDAP libraries. This includes `ldapadd`, `ldapsearch`, Evolution, etc.
`/etc/openldap/slapd.d/`	The directory containing the `slapd` configuration.

In Red Hat Enterprise Linux 6, the slapd service uses a configuration database located in the /etc/openldap/slapd.d/ directory and only reads the old /etc/openldap/slapd.conf configuration file if this directory does not exist. If you have an existing slapd.conf file from a previous installation, you can either wait for the openldap-servers package to convert it to the new format the next time you update this package, or type the following at a shell prompt as root to convert it immediately:

~]# slaptest -f /etc/openldap/slapd.conf -F /etc/openldap/slapd.d/

The slapd configuration consists of LDIF entries organized in a hierarchical directory structure, and the recommended way to edit these entries is to use the server utilities described in Section 20.1.2.1, “Overview of OpenLDAP Server Utilities”.

Important

An error in an LDIF file can render the slapd service unable to start. Because of this, it is strongly advised that you avoid editing the LDIF files within the /etc/openldap/slapd.d/ directory directly.

20.1.3.1. Changing the Global Configuration

Global configuration options for the LDAP server are stored in the /etc/openldap/slapd.d/cn=config.ldif file. The following directives are commonly used:

olcAllows

The olcAllows directive allows you to specify which features to enable. It takes the following form:

olcAllows: feature

It accepts a space-separated list of features as described in Table 20.6, “Available olcAllows options”. The default option is bind_v2.

Table 20.6. Available olcAllows options
Option	Description
`bind_v2`	Enables the acceptance of LDAP version 2 bind requests.
`bind_anon_cred`	Enables an anonymous bind when the Distinguished Name (DN) is empty.
`bind_anon_dn`	Enables an anonymous bind when the Distinguished Name (DN) is not empty.
`update_anon`	Enables processing of anonymous update operations.
`proxy_authz_anon`	Enables processing of anonymous proxy authorization control.

Example 20.1. Using the olcAllows directive

olcAllows: bind_v2 update_anon

olcConnMaxPending

The olcConnMaxPending directive allows you to specify the maximum number of pending requests for an anonymous session. It takes the following form:

olcConnMaxPending: number

The default option is 100.

Example 20.2. Using the olcConnMaxPending directive

olcConnMaxPending: 100

olcConnMaxPendingAuth

The olcConnMaxPendingAuth directive allows you to specify the maximum number of pending requests for an authenticated session. It takes the following form:

olcConnMaxPendingAuth: number

The default option is 1000.

Example 20.3. Using the olcConnMaxPendingAuth directive

olcConnMaxPendingAuth: 1000

olcDisallows

The olcDisallows directive allows you to specify which features to disable. It takes the following form:

olcDisallows: feature

It accepts a space-separated list of features as described in Table 20.7, “Available olcDisallows options”. No features are disabled by default.

Table 20.7. Available olcDisallows options
Option	Description
`bind_anon`	Disables the acceptance of anonymous bind requests.
`bind_simple`	Disables the simple bind authentication mechanism.
`tls_2_anon`	Disables the enforcing of an anonymous session when the STARTTLS command is received.
`tls_authc`	Disallows the STARTTLS command when authenticated.

Example 20.4. Using the olcDisallows directive

olcDisallows: bind_anon

olcIdleTimeout

The olcIdleTimeout directive allows you to specify how many seconds to wait before closing an idle connection. It takes the following form:

olcIdleTimeout: number

This option is disabled by default (that is, set to 0).

Example 20.5. Using the olcIdleTimeout directive

olcIdleTimeout: 180

olcLogFile

The olcLogFile directive allows you to specify a file in which to write log messages. It takes the following form:

olcLogFile: file_name

The log messages are written to standard error by default.

Example 20.6. Using the olcLogFile directive

olcLogFile: /var/log/slapd.log

olcReferral

The olcReferral option allows you to specify a URL of a server to process the request in case the server is not able to handle it. It takes the following form:

olcReferral: URL

This option is disabled by default.

Example 20.7. Using the olcReferral directive

olcReferral: ldap://root.openldap.org

olcWriteTimeout

The olcWriteTimeout option allows you to specify how many seconds to wait before closing a connection with an outstanding write request. It takes the following form:

olcWriteTimeout

This option is disabled by default (that is, set to 0).

Example 20.8. Using the olcWriteTimeout directive

olcWriteTimeout: 180

20.1.3.2. Changing the Database-Specific Configuration

By default, the OpenLDAP server uses Berkeley DB (BDB) as a database back end. The configuration for this database is stored in the /etc/openldap/slapd.d/cn=config/olcDatabase={2}bdb.ldif file. The following directives are commonly used in a database-specific configuration:

olcReadOnly

The olcReadOnly directive allows you to use the database in a read-only mode. It takes the following form:

olcReadOnly: boolean

It accepts either TRUE (enable the read-only mode), or FALSE (enable modifications of the database). The default option is FALSE.

Example 20.9. Using the olcReadOnly directive

olcReadOnly: TRUE

olcRootDN

The olcRootDN directive allows you to specify the user that is unrestricted by access controls or administrative limit parameters set for operations on the LDAP directory. It takes the following form:

olcRootDN: distinguished_name

It accepts a Distinguished Name (DN). The default option is cn=Manager,dc=my-domain,dc=com.

Example 20.10. Using the olcRootDN directive

olcRootDN: cn=root,dc=example,dc=com

olcRootPW

The olcRootPW directive allows you to set a password for the user that is specified using the olcRootDN directive. It takes the following form:

olcRootPW: password

It accepts either a plain text string, or a hash. To generate a hash, type the following at a shell prompt:

~]$ slappaswd
New password: 
Re-enter new password: 
{SSHA}WczWsyPEnMchFf1GRTweq2q7XJcvmSxD

Example 20.11. Using the olcRootPW directive

olcRootPW: {SSHA}WczWsyPEnMchFf1GRTweq2q7XJcvmSxD

olcSuffix

The olcSuffix directive allows you to specify the domain for which to provide information. It takes the following form:

olcSuffix: domain_name

It accepts a fully qualified domain name (FQDN). The default option is dc=my-domain,dc=com.

Example 20.12. Using the olcSuffix directive

olcSuffix: dc=example,dc=com

20.1.3.3. Extending Schema

Since OpenLDAP 2.3, the /etc/openldap/slapd.d/cn=config/cn=schema/ directory also contains LDAP definitions that were previously located in /etc/openldap/schema/. It is possible to extend the schema used by OpenLDAP to support additional attribute types and object classes using the default schema files as a guide. However, this task is beyond the scope of this chapter. For more information on this topic, see http://www.openldap.org/doc/admin/schema.html.

20.1.4. Running an OpenLDAP Server

This section describes how to start, stop, restart, and check the current status of the Standalone LDAP Daemon. For more information on how to manage system services in general, see Chapter 12, Services and Daemons.

20.1.4.1. Starting the Service

To run the slapd service, type the following at a shell prompt:

~]# service slapd start
Starting slapd:                                            [  OK  ]

If you want the service to start automatically at the boot time, use the following command:

~]# chkconfig slapd on

Note that you can also use the Service Configuration utility as described in Section 12.2.1.1, “Enabling and Disabling a Service”.

20.1.4.2. Stopping the Service

To stop the running slapd service, type the following at a shell prompt:

~]# service slapd stop
Stopping slapd:                                            [  OK  ]

To prevent the service from starting automatically at the boot time, type:

~]# chkconfig slapd off

Alternatively, you can use the Service Configuration utility as described in Section 12.2.1.1, “Enabling and Disabling a Service”.

20.1.4.3. Restarting the Service

To restart the running slapd service, type the following at a shell prompt:

~]# service slapd restart
Stopping slapd:                                            [  OK  ]
Starting slapd:                                            [  OK  ]

This stops the service, and then starts it again. Use this command to reload the configuration.

20.1.4.4. Checking the Service Status

To check whether the service is running, type the following at a shell prompt:

~]# service slapd status
slapd (pid  3672) is running...

20.1.5. Configuring a System to Authenticate Using OpenLDAP

In order to configure a system to authenticate using OpenLDAP, make sure that the appropriate packages are installed on both LDAP server and client machines. For information on how to set up the server, follow the instructions in Section 20.1.2, “Installing the OpenLDAP Suite” and Section 20.1.3, “Configuring an OpenLDAP Server”. On a client, type the following at a shell prompt:

~]# yum install openldap openldap-clients sssd

Chapter 13, Configuring Authentication provides detailed instructions on how to configure applications to use LDAP for authentication.

20.1.5.1. Migrating Old Authentication Information to LDAP Format

The migrationtools package provides a set of shell and Perl scripts to help you migrate authentication information into an LDAP format. To install this package, type the following at a shell prompt:

~]# yum install migrationtools

This will install the scripts to the /usr/share/migrationtools/ directory. Once installed, edit the /usr/share/migrationtools/migrate_common.ph file and change the following lines to reflect the correct domain, for example:

# Default DNS domain
$DEFAULT_MAIL_DOMAIN = "example.com";

# Default base
$DEFAULT_BASE = "dc=example,dc=com";

Alternatively, you can specify the environment variables directly on the command line. For example, to run the migrate_all_online.sh script with the default base set to dc=example,dc=com, type:

~]# export DEFAULT_BASE="dc=example,dc=com" \
/usr/share/migrationtools/migrate_all_online.sh

To decide which script to run in order to migrate the user database, see Table 20.8, “Commonly used LDAP migration scripts”.

Table 20.8. Commonly used LDAP migration scripts
Existing Name Service	Is LDAP Running?	Script to Use
`/etc` flat files	yes	`migrate_all_online.sh`
`/etc` flat files	no	`migrate_all_offline.sh`
NetInfo	yes	`migrate_all_netinfo_online.sh`
NetInfo	no	`migrate_all_netinfo_offline.sh`
NIS (YP)	yes	`migrate_all_nis_online.sh`
NIS (YP)	no	`migrate_all_nis_offline.sh`

For more information on how to use these scripts, see the README and the migration-tools.txt files in the /usr/share/doc/migrationtools-version/ directory.

20.1.6. Additional Resources

The following resources offer additional information on the Lightweight Directory Access Protocol. Before configuring LDAP on your system, it is highly recommended that you review these resources, especially the OpenLDAP Software Administrator's Guide.

20.1.6.1. Installed Documentation

The following documentation is installed with the openldap-servers package:

/usr/share/doc/openldap-servers-version/guide.html: A copy of the OpenLDAP Software Administrator's Guide.
/usr/share/doc/openldap-servers-version/README.schema: A README file containing the description of installed schema files.

Additionally, there is also a number of manual pages that are installed with the openldap, openldap-servers, and openldap-clients packages:

Client Applications

man ldapadd — Describes how to add entries to an LDAP directory.
man ldapdelete — Describes how to delete entries within an LDAP directory.
man ldapmodify — Describes how to modify entries within an LDAP directory.
man ldapsearch — Describes how to search for entries within an LDAP directory.
man ldappasswd — Describes how to set or change the password of an LDAP user.
man ldapcompare — Describes how to use the ldapcompare tool.
man ldapwhoami — Describes how to use the ldapwhoami tool.
man ldapmodrdn — Describes how to modify the RDNs of entries.

Server Applications

man slapd — Describes command-line options for the LDAP server.

Administrative Applications

man slapadd — Describes command-line options used to add entries to a slapd database.
man slapcat — Describes command-line options used to generate an LDIF file from a slapd database.
man slapindex — Describes command-line options used to regenerate an index based upon the contents of a slapd database.
man slappasswd — Describes command-line options used to generate user passwords for LDAP directories.

Configuration Files

man ldap.conf — Describes the format and options available within the configuration file for LDAP clients.
man slapd-config — Describes the format and options available within the configuration directory.

20.1.6.2. Useful Websites

http://www.openldap.org/doc/admin24/: The current version of the OpenLDAP Software Administrator's Guide.

Chapter 20. Directory Servers

20.1. OpenLDAP

20.1.1. Introduction to LDAP

20.1.1.1. LDAP Terminology

20.1.1.2. OpenLDAP Features

20.1.1.3. OpenLDAP Server Setup

20.1.2. Installing the OpenLDAP Suite

20.1.2.1. Overview of OpenLDAP Server Utilities

20.1.2.2. Overview of OpenLDAP Client Utilities

20.1.2.3. Overview of Common LDAP Client Applications

20.1.3. Configuring an OpenLDAP Server

20.1.3.1. Changing the Global Configuration

20.1.3.2. Changing the Database-Specific Configuration

20.1.3.3. Extending Schema

20.1.4. Running an OpenLDAP Server

20.1.4.1. Starting the Service

20.1.4.2. Stopping the Service

20.1.4.3. Restarting the Service

20.1.4.4. Checking the Service Status

20.1.5. Configuring a System to Authenticate Using OpenLDAP

20.1.5.1. Migrating Old Authentication Information to LDAP Format

20.1.6. Additional Resources

20.1.6.1. Installed Documentation

20.1.6.2. Useful Websites

Learn

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Chapter 20. Directory Servers

20.1. OpenLDAP

20.1.1. Introduction to LDAP

20.1.1.1. LDAP Terminology

20.1.1.2. OpenLDAP Features

20.1.1.3. OpenLDAP Server Setup

20.1.2. Installing the OpenLDAP Suite

20.1.2.1. Overview of OpenLDAP Server Utilities

20.1.2.2. Overview of OpenLDAP Client Utilities

20.1.2.3. Overview of Common LDAP Client Applications

20.1.3. Configuring an OpenLDAP Server

20.1.3.1. Changing the Global Configuration

20.1.3.2. Changing the Database-Specific Configuration

20.1.3.3. Extending Schema

20.1.4. Running an OpenLDAP Server

20.1.4.1. Starting the Service

20.1.4.2. Stopping the Service

20.1.4.3. Restarting the Service

20.1.4.4. Checking the Service Status

20.1.5. Configuring a System to Authenticate Using OpenLDAP

20.1.5.1. Migrating Old Authentication Information to LDAP Format

20.1.6. Additional Resources

20.1.6.1. Installed Documentation

20.1.6.2. Useful Websites

20.1.6.3. Related Books

Learn

Try, buy, & sell

Communities

About Red Hat Documentation

Making open source more inclusive

About Red Hat

Red Hat legal and privacy links

Red Hat legal and privacy links