Introduction to Directory Services

Red Hat Directory Server (Directory Server) provides a centralized directory service for your intranet, network, and extranet information. Directory Server integrates with existing systems and acts as a centralized repository for the consolidation of employee, customer, supplier, and partner information. You can extend Directory Server to manage user profiles and preferences, as well as extranet user authentication.

This chapter describes the basic ideas you need to understand before designing your directory. It includes the following sections:

What Is a Directory Service?

The term directory service means the collection of software, hardware, and processes that store information about your enterprise, subscribers, or both and make that information available to users. A directory service consists of at least one instance of Directory Server and one or more directory client programs. Client programs can access names, phone numbers, addresses, and other data stored in the directory.

One common directory service is a Domain Name System (DNS) server. A DNS server maps a computer hostname to an IP address. Thus, all of the computing resources (hosts) become clients of the DNS server. The mapping of hostnames allows users of your computing resources to easily locate computers on your network by remembering hostnames rather than numerical IP addresses.

However, the DNS server stores only two types of information: names and IP addresses. A true directory service stores virtually unlimited types of information.

Directory Server stores all of your information in a single, network-accessible repository. The following are a few examples of the kinds of information you might store in a directory:

Directory Server serves the needs of a wide variety of applications. It also provides a standard protocol and application programming interfaces (APIs) to access the information it contains.

The following sections describe global directory services and the Lightweight Data Access Protocol (LDAP).

About Global Directory Services

Directory Server provides global directory services, meaning it provides information to a wide variety of applications. Until recently, many applications came bundled with their own proprietary databases. While a proprietary database can be convenient if you use only one application, multiple databases become an administrative burden if the databases manage the same information.

For example, suppose your network supports three different proprietary email systems, each system with its own proprietary directory service. If users change their passwords in one directory, the changes are not automatically replicated in the others. Managing multiple instances of the same information results in increased hardware and personnel costs, a problem referred to as the n + 1 directory problem.

A global directory service solves the n+1 directory problem by providing a single, centralized repository of directory information that any application can access. However, giving a wide variety of applications access to the directory requires a network-based means of communicating between the applications and the directory. Directory Server uses LDAP (Lightweight Directory Access Protocol) to give applications access to its global directory service.

About LDAP

LDAP provides a common language that client applications and servers use to communicate with one another. LDAP is a "lightweight" version of the Directory Access Protocol (DAP) used by the ISO X.500 standard. DAP gives any application access to the directory via an extensible and robust information framework but at an expensive administrative cost. DAP uses a communications layer that is not the Internet standard TCP/IP protocol and has complicated directory-naming conventions.

LDAP preserves the best features of DAP while reducing administrative costs. LDAP uses an open directory access protocol running over TCP/IP and uses simplified encoding methods. It retains the X.500 standard data model and can support millions of entries for a modest investment in hardware and network infrastructure.

Introduction to Directory Server

Directory Server includes the directory itself, the server-side software that implements the LDAP protocol, and a graphical user interface that allows end-users to search and change entries in the directory. You can purchase other LDAP client programs or write your own using the LDAP client SDK included with the Directory Server product.

Without adding other LDAP client programs, Directory Server can provide the foundation for your intranet or extranet. Every Directory Server and compatible server applications use the directory as a central repository for shared server information, such as employee, customer, supplier, and partner data.

You can use Directory Server to manage extranet user-authentication, create access control, set up user preferences, and centralize user management. In hosted environments, partners, customers, and suppliers can manage their own portions of the directory, reducing administrative costs.

When you install Directory Server, the following components are installed on your machine:

For more information about the command-line tools, refer to Red Hat Directory Server Configuration, Command, and File Reference.

This guide talks about the core Directory Server and the plug-ins it uses for doing its work. The next sections describe Directory Server in more detail. The topics discussed are:

Overview of Directory Server Architecture

Overview of the Server Front-End

The server front-end of Directory Server manages communications with directory client programs. Directory Server functions as a daemon. Multiple client programs can speak to the server in LDAP. They can communicate using LDAP over TCP/IP. The connection can also be protected with SSL/TLS, depending on whether the client negotiates the use of Transport Layer Security (TLS) for the connection.

When communication takes place with TLS, the communication is usually encrypted. In the future, when DNS security is present, TLS used in conjunction with secured DNS will provide confirmation to client applications that they are binding to the correct server. If clients have been issued certificates, TLS can be used by Directory Server to confirm that the client has the right to access the server. TLS and its predecessor SSL are used to perform other security activities such as message integrity checks, digital signatures, and mutual authentication between servers.

Multiple clients can bind to the server at the same time over the same network because the Directory Server is a multi-threaded application. As directory services grow to include larger numbers of entries or larger numbers of clients spread out geographically, they also include multiple Directory Servers placed in strategic places around the network.

Server Plug-ins Overview

Directory Server relies on plug-ins. A plug-in is a way to add functionality to the core server. For example, a database is a plug-in.

A plug-in can be disabled. When disabled, the plug-in's configuration information remains in the directory, but its function will not be used by the server. Depending upon what you want your directory to do, you can choose to enable any of the plug-ins provided with Directory Server.

Red Hat Professional Services can write custom plug-ins for your Directory Server deployment. Contact Red Hat Professional Services for more information.

Overview of the Basic Directory Tree

The directory tree, also known as a directory information tree or DIT, mirrors the tree model used by most filesystems, with the tree's root, or first entry, appearing at the top of the hierarchy. At installation, Directory Server creates a default directory tree.

At installation, the directory contains up to four subtrees under your root suffix:


Note	When you install another instance of Directory Server, you can specify that it does not contain the o=NetscapeRoot information, that it uses the configuration directory (or the o=NetscapeRoot subtree) present on another server. See the Red Hat Directory Server Installation Guide for more information about deciding upon the location of your configuration directory.

You can build on the default directory tree to add any data relevant to your directory installation. An example of a directory tree for example.com Corporation follows:

Directory Server Data Storage

Your directory data is stored in an LDBM database. The LDBM database is implemented as a plug-in that is automatically installed with the directory and is enabled by default.

The database is the basic unit of storage, performance, replication, and indexing. You can do operations like importing, exporting, backing up, restoring, and indexing on the database.

By default, Directory Server uses a single database to contain the directory tree. This database can manage millions of entries. The default database supports advanced methods of backing up and restoring your data, so that your data is not at risk.


Note	For database files that are larger than 2Gbytes, the machine must be configured to support large files. You can do this by choosing largefile on Solaris and vxfs filesystem with largefiles option on HP UX. Linux handles large files without having to configure the filesystem.

You can choose to use multiple databases to support your Directory Server. You can distribute your data across the databases, allowing the server to hold more data than can be stored in a single database.

About Directory Entries

LDIF is a standard text-based format for describing directory entries. An entry is a group of lines in the LDIF file that contains information about an object, such as a person in your organization or a printer on your network. Information about the entry is represented in the LDIF file by a set of attributes and their values. Each entry has an object class attribute that specifies the kind of object the entry describes and defines the set of additional attributes it contains. Each attribute describes a particular trait of an entry.

For example, an entry might be of an object class organizationalPerson, indicating that the entry represents a person within a particular organization. This object class allows the givenname and telephoneNumber attributes. The values assigned to these attributes give the name and phone number of the person represented by the entry.

Directory Server also uses read-only attributes that are calculated by the server. These attributes are called operational attributes. There are also some operational attributes that can be set by the administrator, for access control and other server functions.

Entries are stored in a hierarchical structure in the directory tree. In LDAP, you can query an entry and request all entries below it in the directory tree. This subtree is called the base distinguished name, or base DN. For example, if you make an LDAP search request specifying a base DN of ou=people, dc=example,dc=com, then the search operation examines only the ou=people subtree in the dc=example,dc=com directory tree.

However, all entries are not automatically returned in response to an LDAP search. This is because Directory Server supports a new kind of entry, entries of the object class ldapsubentry. An ldapsubentry entry represents an administrative object; for example, entries used to define a role or a class of service are of the ldapsubentry type. Entries of type ldapsubentry are not returned in response to normal search requests. To receive these entries, clients need to search specifically for entries of the ldapsubentry object class.

Distributing Directory Data

When you store various parts of your tree in separate databases, your directory can process client requests in parallel, improving performance. You can also store your databases on different machines, to further improve performance.

To connect your distributed data, you can create a special entry in a subtree of your directory. All LDAP operations attempted below this entry are sent to a remote machine where the entry is actually stored. This method is called chaining.

Chaining is implemented in the server as a plug-in. The plug-in is enabled by default. Using this plug-in, you create database links, special entries that point to data stored remotely. When a client application requests data from a database link, the database link retrieves the data from the remote database and returns it to the client.

Directory Design Overview

The previous sections described directory services in general and the Directory Server in particular. Now, it is time to consider the design of your own directory service.

Planning your directory service before actual deployment is the most important task to ensure the success of your directory. During your directory design, you will gather data about your directory requirements, such as environment and data sources, your users, and the applications that will use your directory. With this data, you can design a directory service that meets your needs.

However, keep in mind that the flexibility of Directory Server allows you to rework your design to meet unexpected or changing requirements, even after you deploy Directory Server.

Design Process Outline

Your directory is designed to support one or more directory-enabled applications. These applications have requirements of the data you store in your directory, such as format. Your directory schema determines the characteristics of the data stored in your directory. The standard schema shipped with Directory Server is introduced in chapter 3, "How to Design the Schema, along with describing how to customize the schema and providing tips for maintaining consistent schema.

Once you decide what data your directory contains, you need to organize and reference that data. This is the purpose of the directory tree. In chapter 4, "Designing the Directory Tree, the directory tree is introduced, and you are guided through the design of your data hierarchy. Sample directory tree designs are also provided.

Topology design involves determining how you divide your directory tree among multiple physical Directory Servers and how these servers communicate with one another. The general principles behind topology design, using multiple databases, the mechanisms available for linking your distributed data together, and how the directory itself keeps track of distributed data are all described in chapter 5, "Designing the Directory Topology.

With replication, multiple Directory Servers maintain the same directory data to increase performance and provide fault tolerance, as described in chapter 6, "Designing the Replication Process. This chapter describes how replication works, what kinds of data you can replicate, common replication scenarios, and tips for building a highly available directory service.

Finally, you need to plan how to protect the data in the directory and design the other aspects of your service to meet the security requirements of your users and applications, chapter 7, "Designing a Secure Directory. This chapter covers common security threats, an overview of security methods, steps in analyzing your security needs, and tips for designing access controls and protecting the integrity of your directory data.

Deploying Your Directory

After you have designed your directory service, you start the deployment phase. The deployment phase consists of the following steps:

Piloting Your Directory

The first step of the deployment phase is installing a server instance as a pilot and testing whether your service can handle your user load. If the service is not adequate as it is, adjust your design and pilot it again. Adjust your pilot design until you have a robust service you can confidently introduce to your enterprise.

For a comprehensive overview of creating and implementing a directory pilot, refer to Understanding and Deploying LDAP Directory Services (T. Howes, M. Smith, G. Good, Macmillan Technical Publishing, 1999).

Putting Your Directory into Production

Once you have piloted and tuned the service, you need to develop and execute a plan for taking the directory service from a pilot to production. Create a production plan that includes the following:

For information on installing your directory service, refer to Red Hat Directory Server Installation Guide. For information on administering and maintaining your directory, refer to Red Hat Directory Server Administrator's Guide.

Other General Directory Resources

For more information about directories, LDAP, and LDIF, take a look at the following:

Chapter 1