| Deployment GuideChapter 17. Directory ServersLDAP (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. 17.1.1. Introduction to LDAPUsing a client/server architecture, LDAP provides 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 Secure Sockets Layer ( SSL) or Transport Layer Security ( TLS) cryptographic protocols can be used to prevent an attacker from intercepting the transmission. 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, refer to How do I use TLS/SSL with Mozilla NSS. 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. 17.1.1.1. LDAP TerminologyThe 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.
17.1.1.2. OpenLDAP FeaturesOpenLDAP 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), Transport Layer Security ( TLS), and Secure Sockets Layer ( SSL) 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.
17.1.1.3. OpenLDAP Server SetupThe typical steps to set up an LDAP server on Red Hat Enterprise Linux are as follows: 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. 17.1.2. Installing the OpenLDAP SuiteThe suite of OpenLDAP libraries and tools is provided by the following packages: Table 17.1. List of OpenLDAP packages Package | Description |
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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 . | openldap-servers-sql | A package containing the SQL support module. | compat-openldap | A package containing the OpenLDAP compatibility libraries. |
Additionally, the following packages are commonly used along with the LDAP server: Table 17.2. List of commonly installed additional LDAP packages Package | Description |
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nss-pam-ldapd | A package containing nslcd , a local LDAP name service that allows a user to perform local LDAP queries. | 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, refer to Section 6.2.4, "Installing Packages". 17.1.2.1. Overview of OpenLDAP Server UtilitiesTo perform administrative tasks, the openldap-servers package installs the following utilities along with the slapd service: Table 17.3. List of OpenLDAP server utilities Command | Description |
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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. |
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 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 ] 17.1.2.2. Overview of OpenLDAP Client UtilitiesThe openldap-clients package installs the following utilities which can be used to add, modify, and delete entries in an LDAP directory: Table 17.4. List of OpenLDAP client utilities Command | Description |
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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. 17.1.2.3. Overview of Common LDAP Client ApplicationsAlthough 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. 17.1.3. Configuring an OpenLDAP ServerBy default, the OpenLDAP configuration is stored in the /etc/openldap/ directory. The following table highlights the most important directories and files within this directory: Table 17.5. List of OpenLDAP configuration files and directories Path | Description |
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/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. |
Note that OpenLDAP no longer reads its configuration from the /etc/openldap/slapd.conf file. Instead, it uses a configuration database located in the /etc/openldap/slapd.d/ directory. If you have an existing slapd.conf file from a previous installation, you can convert it to the new format by running the following command: ~]# slaptest -f /etc/openldap/slapd.conf -F /etc/openldap/slapd.d/ 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/ directly. 17.1.3.1. Changing the Global ConfigurationGlobal 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 . . . . . .
Table 17.6. Available olcAllows options Option | Description |
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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 17.1. Using the olcAllows directive olcAllows: bind_v2 update_anon
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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 17.2. Using the olcConnMaxPending directive
-
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 17.3. 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 . . . . . .
Table 17.7. Available olcDisallows options Option | Description |
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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 17.4. Using the olcDisallows directive
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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 17.5. Using the olcIdleTimeout directive
-
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 17.6. 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 17.7. 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 17.8. Using the olcWriteTimeout directive
17.1.3.2. Changing the Database-Specific ConfigurationBy 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={1}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 17.9. Using the olcReadOnly directive
-
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,dn=my-domain,dc=com . Example 17.10. Using the olcRootDN directive olcRootDN: cn=root,dn=example,dn=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 17.11. 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 17.12. Using the olcSuffix directive olcSuffix: dc=example,dc=com
17.1.3.3. Extending SchemaSince OpenLDAP 2.3, the /etc/openldap/slapd.d/ 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, refer to http://www.openldap.org/doc/admin/schema.html. 17.1.4. Running an OpenLDAP ServerThis 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, refer to Chapter 10, Services and Daemons. 17.1.4.1. Starting the ServiceTo 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 17.1.4.2. Stopping the ServiceTo 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 17.1.4.3. Restarting the ServiceTo 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. 17.1.4.4. Checking the Service StatusTo check whether the service is running, type the following at a shell prompt: ~]# service slapd status slapd (pid 3672) is running... 17.1.5. Configuring a System to Authenticate Using OpenLDAP~]# yum install openldap openldap-clients nss-pam-ldapd 17.1.5.1. Migrating Old Authentication Information to LDAP FormatThe 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 Table 17.8. Commonly used LDAP migration scripts Existing Name Service | Is LDAP Running? | Script to Use |
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/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, refer to the README and the migration-tools.txt files in the /usr/share/doc/migrationtools-version / directory. 17.1.6. Additional ResourcesThe 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. 17.1.6.1. Installed DocumentationThe 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
- 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.
17.1.6.2. Useful Websites- OpenLDAP by Example by John Terpstra and Benjamin Coles; Prentice Hall.
A collection of practical exercises in the OpenLDAP deployment. - Implementing LDAP by Mark Wilcox; Wrox Press, Inc.
A book covering LDAP from both the system administrator's and software developer's perspective. - Understanding and Deploying LDAP Directory Services by Tim Howes et al.; Macmillan Technical Publishing.
A book covering LDAP design principles, as well as its deployment in a production environment.
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