Unix system administrator traditionally use NIS (Network Information Service) for name resolution and data distribution in a network. The configuration data contained in the files group, hosts, mail, netgroup, networks, passwd, printcap, protocols, rpc, and services in the /etc directory are distributed by clients all over the network. These files can be maintained without major effort because they are simple text files. The handling of larger amounts of data, however, becomes increasingly difficult due to nonexistent structuring. NIS is only designed for Unix platforms, and is not suitable as a centralized data administration tool in heterogeneous networks.

Unlike NIS, the LDAP service is not restricted to pure Unix networks. Windows servers (from 2000) support LDAP as a directory service. The application tasks mentioned above are additionally supported in non-Unix systems.

The LDAP principle can be applied to any data structure that needs to be centrally administered. A few application examples are:

This list can be extended because LDAP is extensible, unlike NIS. The clearly-defined hierarchical structure of the data eases the administration of large amounts of data, as it can be searched more easily.

To get background knowledge on how a LDAP server works and how the data is stored, it is vital to understand the way the data is organized on the server and how this structure enables LDAP to provide fast access to the data. To successfully operate an LDAP setup, you also need to be familiar with some basic LDAP terminology. This section introduces the basic layout of an LDAP directory tree and provides the basic terminology used with respect to LDAP. Skip this introductory section if you already have some LDAP background knowledge and just want to learn how to set up an LDAP environment in openSUSE. Read on at Section 4.3, “Configuring an LDAP Server with YaST” or Section 4.7, “Manually Configuring an LDAP Server”.

An LDAP directory has a tree structure. All entries (called objects) of the directory have a defined position within this hierarchy. This hierarchy is called the directory information tree (DIT). The complete path to the desired entry, which unambiguously identifies it, is called the distinguished name or DN. A single node along the path to this entry is called relative distinguished name or RDN.

The relations within an LDAP directory tree become more evident in the following example, shown in Figure 4.1, “Structure of an LDAP Directory”.

Figure 4.1. Structure of an LDAP Directory

The complete diagram is a fictional directory information tree. The entries on three levels are depicted. Each entry corresponds to one box in the image. The complete, valid distinguished name for the fictional employee Geeko Linux, in this case, is cn=Geeko Linux,ou=doc,dc=example,dc=com. It is composed by adding the RDN cn=Geeko Linux to the DN of the preceding entry ou=doc,dc=example,dc=com.

The types of objects that can be stored in the DIT are globally determined following a Schema. The type of an object is determined by the object class. The object class determines what attributes the relevant object must or can be assigned. The Schema, therefore, must contain definitions of all object classes and attributes used in the desired application scenario. There are a few common Schemas (see RFC 2252 and 2256). The LDAP RFC defines a few commonly used Schemas (see e.g., RFC4519). Additionally there are Schemas available for many other use cases (e.g., Samba, NIS replacement, etc.). It is, however, possible to create custom Schemas or to use multiple Schemas complementing each other (if this is required by the environment in which the LDAP server should operate).

Table 4.1, “Commonly Used Object Classes and Attributes” offers a small overview of the object classes from core.schema and inetorgperson.schema used in the example, including required attributes and valid attribute values.

Example 4.1, “Excerpt from schema.core” shows an excerpt from a Schema directive with explanations.

attributetype (2.5.4.11 NAME ( 'ou' 'organizationalUnitName') 
       DESC 'RFC2256: organizational unit this object belongs to' 
       SUP name ) 

...
objectclass ( 2.5.6.5 NAME 'organizationalUnit' 
       DESC 'RFC2256: an organizational unit' 
       SUP top STRUCTURAL 
       MUST ou 
MAY (userPassword $ searchGuide $ seeAlso $ businessCategory 
  $ x121Address $ registeredAddress $ destinationIndicator 
  $ preferredDeliveryMethod $ telexNumber 
  $ teletexTerminalIdentifier $ telephoneNumber 
  $ internationaliSDNNumber $ facsimileTelephoneNumber 
  $ street $ postOfficeBox $ postalCode $ postalAddress 
  $ physicalDeliveryOfficeName
  $ st $ l $ description) )
  ...

The attribute type organizationalUnitName and the corresponding object class organizationalUnit serve as an example here.

A very good introduction to the use of Schemas can be found in the OpenLDAP documentation. When installed, find it in /usr/share/doc/packages/openldap2/guide/admin/guide.html.

Use YaST to set up an LDAP server. Typical use cases for LDAP servers include the management of user account data and the configuration of mail, DNS, and DHCP servers.

Figure 4.2. YaST LDAP Server Configuration

Figure 4.3. YaST LDAP Server—New Database

To set up an LDAP server for user account data, make sure the yast2-ldap-server and openldap2 packages and packages they depend on are installed. Then proceed as follows:

Figure 4.4. YaST LDAP Server Configuration

For changes or additional configuration start the LDAP server module again and in the left pane expand Global Settings to make subentries visible—see Figure 4.4, “YaST LDAP Server Configuration”:

Add Schema files to be included in the server's configuration by selecting Schema Files in the left part of the dialog. The default selection of schema files applies to the server providing a source of YaST user account data.

YaST allows to add traditional Schema files (usually with a name ending in .schema) or LDIF files containing Schema definitions in OpenLDAP's LDIF Schema format.

Figure 4.5. YaST LDAP Server Database Configuration

To configure the databases managed by your LDAP server, proceed as follows:

If you have not opted for password policies, your server is ready to run at this point. If you chose to enable password policies, proceed with the configuration of the password policy in detail. If you chose a password policy object that does not yet exist, YaST creates one:

To edit a previously created database, select its base DN in the tree to the left. In the right part of the window, YaST displays a dialog similar to the one used for the creation of a new database (with the main difference that the base DN entry is grayed out and cannot be changed).

After leaving the LDAP server configuration by selecting Finish, you are ready to go with a basic working configuration for your LDAP server. To fine-tune this setup, make use of OpenLDAP's dynamic configuration backend.

The OpenLDAP's dynamic configuration backend stores the configuration in an LDAP database. That database consists of a set of .ldif files in /etc/openldap/slapd.d. There is no need to access these files directly. To access the settings you can either use the YaST LDAP server module (the yast2-ldap-server package) or an LDAP client such as ldapmodify or ldapsearch. For more information on the dymanic configuration of OpenLDAP, see the OpenLDAP Administration Guide.

YaST includes a module to set up LDAP-based user management. If you did not enable this feature during the installation, start the module by selecting Network Services+LDAP Client. YaST automatically enables any PAM and NSS-related changes as required by LDAP and installs the necessary files. Simply connect your client to the server and let YaST manage users over LDAP. This basic setup is described in Section 4.4.1, “Configuring Basic Settings”.

Use the YaST LDAP client to further configure the YaST group and user configuration modules. This includes manipulating the default settings for new users and groups and the number and nature of the attributes assigned to a user or group. LDAP user management allows you to assign far more and different attributes to users and groups than traditional user or group management solutions. This is described in Section 4.4.2, “Configuring the YaST Group and User Administration Modules”.

Configuring Basic Settings

The basic LDAP client configuration dialog (Figure 4.6, “YaST: LDAP Client Configuration”) opens during installation if you choose LDAP user management or when you select Network Services+LDAP Client in the YaST Control Center in the installed system.

Figure 4.6. YaST: LDAP Client Configuration

To authenticate users of your machine against an OpenLDAP server and to enable user management via OpenLDAP, proceed as follows:

1. Click Use LDAP to enable the use of LDAP. Select Use LDAP but Disable Logins if instead you want to use LDAP for authentication, but do not want other users to log in to this client.

2. Enter the IP address of the LDAP server to use.

3. Enter the LDAP Base DN to select the search base on the LDAP server. To retrieve the base DN automatically, click Fetch DN. YaST then checks for any LDAP database on the server address specified above. Choose the appropriate base DN from the search results given by YaST.

4. If TLS or SSL-protected communication with the server is required, select LDAP TLS/SSL. Click Download CA Certificate to download a certificate in PEM format from a URL.

5. Select Start Automounter to mount remote directories on your client, such as a remotely managed /home.

6. Select Create Home Directory on Login to have a user's home automatically created on the first user login.

7. Click Ok to apply your settings.

To modify data on the server as administrator, click Advanced Configuration. The following dialog is split into two tabs. See Figure 4.7, “YaST: Advanced Configuration”.

Figure 4.7. YaST: Advanced Configuration

1. In the Client Settings tab, adjust the following settings according to your needs:

   1. If the search base for users, passwords, and groups differs from the global search base specified in the LDAP base DN, enter these different naming contexts in User Map, Password Map, and Group Map.

   2. Specify the password change protocol. The standard method to use whenever a password is changed is crypt, meaning that password hashes generated by crypt are used. For details on this and other options, refer to the pam_ldap man page.

   3. Specify the LDAP group to use with Group Member Attribute. The default value for this is member.

   4. If a secure connection requires certificate checking, specify where your CA Certificate File in PEM format is located. Or specify a directory with certificates.

   5. If the LDAP server still uses LDAPv2, enable the use of this protocol version by selecting LDAP Version 2.

2. In Administration Settings, adjust the following settings:

   1. Set the base for storing your user management data via Configuration Base DN.

   2. Enter the appropriate value for Administrator DN. This DN must be identical with the rootdn value specified in /etc/openldap/slapd.conf to enable this particular user to manipulate data stored on the LDAP server. Enter the full DN (such as cn=Administrator,dc=example,dc=com) or activate Append Base DN to have the base DN added automatically when you enter cn=Administrator.

   3. Check Create Default Configuration Objects to create the basic configuration objects on the server to enable user management via LDAP.

   4. If your client machine needs to act as a file server for home directories across your network, check Home Directories on This Machine.

   5. Use the Password Policy section to select, add, delete, or modify the password policy settings to use. The configuration of password policies with YaST is part of the LDAP server setup.

   6. Click OK to leave the Advanced Configuration, then Finish to apply your settings.

Use Configure User Management Settings to edit entries on the LDAP server. Access to the configuration modules on the server is then granted according to the ACLs and ACIs stored on the server. Follow the procedures outlined in Section 4.4.2, “Configuring the YaST Group and User Administration Modules”.

Configuring the YaST Group and User Administration Modules

Use the YaST LDAP client to adapt the YaST modules for user and group administration and to extend them as needed. Define templates with default values for the individual attributes to simplify the data registration. The presets created here are stored as LDAP objects in the LDAP directory. The registration of user data is still done with the regular YaST modules for user and group management. The registered data is stored as LDAP objects on the server.

Figure 4.8. YaST: Module Configuration

The dialog for module configuration (Figure 4.8, “YaST: Module Configuration”) allows the creation of new modules, selection and modification of existing configuration modules, and design and modification of templates for such modules.

To create a new configuration module, proceed as follows:

1. In the LDAP Client Configuration click Advanced Configuration, then open the Administration Settings tab. Click Configure User Management Settings and enter the LDAP server credentials.

2. Click New and select the type of module to create. For a user configuration module, select suseUserConfiguration and for a group configuration choose suseGroupConfiguration.

3. Choose a name for the new template (e.g., userConfig). The content view then features a table listing all attributes allowed in this module with their assigned values. Apart from all set attributes, the list also contains all other attributes allowed by the current Schema, but currently not used.

4. Accept the preset values or adjust the defaults to use in group and user configurations by selecting the relevant attribute, pressing Edit, and entering the new value. Rename a module by changing the cn attribute of the module. Clicking Delete deletes the currently selected module.

5. After you click OK, the new module is added to the selection menu.

The YaST modules for group and user administration embed templates with standard values. To edit a template associated with a configuration module, start the object template configuration (Figure 4.9, “YaST: Configuration of an Object Template”):

1. In the Module Configuration dialog, click Configure Template.

2. Determine the values of the general attributes assigned to this template according to your needs or leave them empty. Empty attributes are deleted on the LDAP server.

3. Modify, delete, or add new default values for new objects (user or group configuration objects in the LDAP tree).

Figure 4.9. YaST: Configuration of an Object Template

Connect the template to its module by setting the susedefaulttemplate attribute value of the module to the DN of the adapted template.

The default values for an attribute can be created from other attributes by using a variable instead of an absolute value. For example, when creating a new user, cn=%sn %givenName is created automatically from the attribute values for sn and givenName.

Once all modules and templates are configured correctly and ready to run, new groups and users can be registered in the usual way with YaST.

The actual registration of user and group data differs only slightly from the procedure when not using LDAP. The following instructions relate to the administration of users. The procedure for administering groups is analogous.

Figure 4.10. YaST: Additional LDAP Settings

The initial input form of user administration offers LDAP Options. This allows you to apply LDAP search filters to the set of available users (or go to the module for the configuration of LDAP users and groups by selecting LDAP User and Group Configuration).

To conveniently browse the LDAP directory tree and all its entries, use the YaST LDAP Browser:

YaST uses OpenLDAP's dynamic configuration database (back-config) to store the LDAP server's configuration. For details about the dynamic configuration backend please see the slapd-config(5) man page or the OpenLDAP Software 2.4 Administrator's Guide located at /usr/share/doc/packages/openldap2/guide/admin/guide.html on this system if the openldap2 package is installed.

	Upgrading an Old OpenLDAP Installation
YaST does not use /etc/openldap/slapd.conf to store the OpenLDAP configuration anymore. In case of a system upgrade, a copy of the original /etc/openldap/slapd.conf file will get created as /etc/openldap/slapd.conf.YaSTsave.

To conveniently access the configuration backend, you use SASL external authentication. For example, the following ldapsearch command executed as root can be used to show the complete slapd configuration:

ldapsearch -Y external -H ldapi:/// -b cn=config

OpenLDAP offers a series of tools for the administration of data in the LDAP directory. The four most important tools for adding to, deleting from, searching through and modifying the data stock are explained in this section.

Inserting Data into an LDAP Directory

Once your LDAP server is correctly configured (it features appropriate entries for suffix, directory, rootdn, rootpw and index), proceed to entering records. OpenLDAP offers the ldapadd command for this task. If possible, add the objects to the database in bundles (for practical reasons). LDAP is able to process the LDIF format (LDAP data interchange format) for this. An LDIF file is a simple text file that can contain an arbitrary number of attribute and value pairs. The LDIF file for creating a rough framework for the example in Figure 4.1, “Structure of an LDAP Directory” would look like the one in Example 4.2, “An LDIF File”.

	Encoding of LDIF Files
LDAP works with UTF-8 (Unicode). Umlauts must be encoded correctly. Otherwise, avoid umlauts and other special characters or use iconv to convert the input to UTF-8.

Example 4.2. An LDIF File

# The Organization
dn: dc=example,dc=com
objectClass: dcObject
objectClass: organization
o: Example dc: example

# The organizational unit development (devel)
dn: ou=devel,dc=example,dc=com
objectClass: organizationalUnit
ou: devel

# The organizational unit documentation (doc)
dn: ou=doc,dc=example,dc=com
objectClass: organizationalUnit
ou: doc

# The organizational unit internal IT (it)
dn: ou=it,dc=example,dc=com
objectClass: organizationalUnit
ou: it

Save the file with the .ldif suffix then pass it to the server with the following command:

ldapadd -x -D dn_of_the_administrator -W -f file.ldif

-x switches off the authentication with SASL in this case. -D declares the user that calls the operation. The valid DN of the administrator is entered here just like it has been configured in slapd.conf. In the current example, this is cn=Administrator,dc=example,dc=com. -W circumvents entering the password on the command line (in clear text) and activates a separate password prompt. The -f option passes the filename. See the details of running ldapadd in Example 4.3, “ldapadd with example.ldif”.

Example 4.3. ldapadd with example.ldif

ldapadd -x -D cn=Administrator,dc=example,dc=com -W -f example.ldif 

Enter LDAP password: 
adding new entry "dc=example,dc=com" 
adding new entry "ou=devel,dc=example,dc=com" 
adding new entry "ou=doc,dc=example,dc=com" 
adding new entry "ou=it,dc=example,dc=com"

The user data of individuals can be prepared in separate LDIF files. Example 4.4, “LDIF Data for Tux” adds Tux to the new LDAP directory.

Example 4.4. LDIF Data for Tux

# coworker Tux
dn: cn=Tux Linux,ou=devel,dc=example,dc=com
objectClass: inetOrgPerson
cn: Tux Linux
givenName: Tux
sn: Linux
mail: tux@example.com
uid: tux
telephoneNumber: +49 1234 567-8

An LDIF file can contain an arbitrary number of objects. It is possible at once to pass directory branches (entirely or in part) to the server as shown in the example of individual objects. If it is necessary to modify some data relatively often, a fine subdivision of single objects is recommended.

# coworker Tux
dn: cn=Tux Linux,ou=devel,dc=example,dc=com 
changetype: modify
replace: telephoneNumber 
telephoneNumber: +49 1234 567-10

ldapmodify -x -D cn=Administrator,dc=example,dc=com -W -f tux.ldif

ldapsearch -x -b dc=example,dc=com "(objectClass=*)"

ldapdelete -x -D cn=Administrator,dc=example,dc=com -W cn=Tux \
Linux,ou=devel,dc=example,dc=com

More complex subjects (like SASL configuration or establishment of a replicating LDAP server that distributes the workload among multiple slaves) were omitted from this chapter. Find detailed information about both subjects in the OpenLDAP 2.4 Administrator's Guide—see at OpenLDAP 2.4 Administrator's Guide.

The Web site of the OpenLDAP project offers exhaustive documentation for beginner and advanced LDAP users:

Printed literature about LDAP:

The ultimate reference material for the subject of LDAP are the corresponding RFCs (request for comments), 2251 to 2256.