There are many different ways to provide a directory service. Different methods allow different kinds of information to be stored in the directory, place different requirements on how that information can be referenced, queried and updated, how it is protected from unauthorized access, etc. Some directory services are local, providing service to a restricted context (e.g., the finger service on a single machine). Other services are global, providing service to a much broader context (e.g., the entire Internet). Global services are usually distributed, meaning that the data they contain is spread across many machines, all of which cooperate to provide the directory service. Typically a global service defines a uniform namespace which gives the same view of the data no matter where you are in relation to the data itself.
What kind of information can be stored in the directory? The LDAP directory service model is based on entries. An entry is a collection of attributes that has a name, called a distinguished name (DN). The DN is used to refer to the entry unambiguously. Each of the entry's attributes has a type and one or more values. The types are typically mnemonic strings, like "cn" for common name, or "mail" for email address. The values depend on what type of attribute it is. For example, a mail attribute might contain the value "email@example.com". A jpegPhoto attribute would contain a photograph in binary JPEG/JFIF format.
How is the information arranged? In LDAP, directory entries are arranged in a hierarchical tree-like structure that reflects political, geographic and/or organizational boundaries. Entries representing countries appear at the top of the tree. Below them are entries representing states or national organizations. Below them might be entries representing people, organizational units, printers, documents, or just about anything else you can think of. Figure 1 shows an example LDAP directory tree, which should help make things clear.
Figure 1: An example LDAP directory tree.
In addition, LDAP allows you to control which attributes are required and allowed in an entry through the use of a special attribute called objectclass. The values of the objectclass attribute determine the schema rules the entry must obey.
How is the information referenced? An entry is referenced by its distinguished name, which is constructed by taking the name of the entry itself (called the relative distinguished name, or RDN) and concatenating the names of its ancestor entries. For example, the entry for Barbara Jensen in the example above has an RDN of "cn=Barbara J Jensen" and a DN of "cn=Barbara J Jensen, o=U of M, c=US". The full DN format is described in RFC 1779, "A String Representation of Distinguished Names."
How is the information accessed? LDAP defines operations for interrogating and updating the directory. Operations are provided for adding and deleting an entry from the directory, changing an existing entry, and changing the name of an entry. Most of the time, though, LDAP is used to search for information in the directory. The LDAP search operation allows some portion of the directory to be searched for entries that match some criteria specified by a search filter. Information can be requested from each entry that matches the criteria.
For example, you might want to search the entire directory subtree below the University of Michigan for people with the name Barbara Jensen, retrieving the email address of each entry found. LDAP lets you do this easily. Or you might want to search the entries directly below the c=US entry for organizations with the string "Acme" in their name, and that have a fax number. LDAP lets you do this too. The next section describes in more detail what you can do with LDAP and how it might be useful to you.
How is the information protected from unauthorized access? Some directory services provide no protection, allowing anyone to see the information. LDAP provides a method for a client to authenticate, or prove its identity to a directory server, paving the way for rich access control to protect the information the server contains.
Choice of databases: Slapd comes with three different backend databases you can choose from. They are LDBM, a high-performance disk-based database; SHELL, a database interface to arbitrary UNIX commands or shell scripts; and PASSWD, a simple password file database.
Multiple database instances: Slapd can be configured to serve multiple databases at the same time. This means that a single slapd server can respond to requests for many logically different portions of the LDAP tree, using the same or different backend databases.
Generic database API: If you require even more customization, slapd lets you write your own backend database easily. Slapd consists of two distinct parts: a front end that handles protocol communication with LDAP clients; and a backend that handles database operations. Because these two pieces communicate via a well-defined C API, you can write your own customized database backend to slapd.
Access control: Slapd provides a rich and powerful access control facility, allowing you to control access to the information in your database(s). You can control access to entries based on LDAP authentication information, IP address, domain name and other criteria.
Threads: Slapd is threaded for high performance. A single multi-threaded slapd process handles all incoming requests, reducing the amount of system overhead required. Slapd will automatically select the best thread support for your platform.
Replication: Slapd can be configured to maintain replica copies of its database. This master/slave replication scheme is vital in high-volume environments where a single slapd just doesn't provide the necessary availability or reliability.
Configuration: Slapd is highly configurable through a single configuration file which allows you to change just about everything you'd ever want to change. Configuration options have reasonable defaults, making your job much easier.
Slapd also has its limitations, of course. It does not currently handle aliases, which are part of the LDAP model. The main LDBM database backend does not handle range queries or negation queries very well. These features and more will be coming in a future release.
This use of LDAP makes it easy to access the X.500 directory, but still requires a full X.500 service to make data available to the many LDAP clients being developed. As with full X.500 DAP clients, a full X.500 server is no small piece of software to run.
The stand-alone LDAP daemon, or slapd, is meant to remove much of the burden from the server side just as LDAP itself removed much of the burden from clients. If you are already running an X.500 service and you want to continue to do so, you can probably stop reading this guide, which is all about running LDAP via slapd, without running X.500. If you are not running X.500, want to stop running X.500, or have no immediate plans to run X.500, read on.
It is possible to replicate data from a slapd directory server to an X.500 DSA, which allows your organization to make your data available as part of the global X.500 directory service on a "read-only" basis. This is discussed in section 11.6.
Another way to make data in a slapd server available to the X.500 community would be by using a X.500 DAP to LDAP gateway. At this time, no such software has been written (to the best of our knowledge), but hopefully some group will see fit towrite such a gateway.
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