207 lines
12 KiB
HTML
207 lines
12 KiB
HTML
<?xml version="1.0" encoding="utf-8"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
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"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml" lang="en-US" xml:lang="en-us">
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<head>
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<meta name="dc.language" scheme="rfc1766" content="en-us" />
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<!-- All rights reserved. Licensed Materials Property of IBM -->
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<!-- US Government Users Restricted Rights -->
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<!-- Use, duplication or disclosure restricted by -->
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<!-- GSA ADP Schedule Contract with IBM Corp. -->
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<meta name="dc.date" scheme="iso8601" content="2005-09-06" />
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<meta name="copyright" content="(C) Copyright IBM Corporation 1998, 2006" />
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<title>Directory Server (LDAP) - Directories</title>
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<link rel="stylesheet" type="text/css" href="ibmidwb.css" />
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<link rel="stylesheet" type="text/css" href="ic.css" />
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<body>
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<a id="Top_Of_Page" name="Top_Of_Page"></a><!-- Java sync-link -->
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<script language = "Javascript" src = "../rzahg/synch.js" type="text/javascript"></script>
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<a name="rzahydefdir"></a>
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<h2 id="rzahydefdir">Directories</h2>
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<p>The Directory Server allows access to a type of database that stores information
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in a hierarchical structure similar to the way that the i5/OS integrated file
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system is organized.</p>
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<p>If the name of an object is known, its characteristics can be retrieved.
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If the name of a particular individual object is not known, the directory
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can be searched for a list of objects that meet a certain requirement. Directories
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can usually be searched by specific criteria, not just by a predefined set
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of categories.</p>
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<p>A directory is a specialized database that has characteristics that set
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it apart from general purpose relational databases. A characteristic of a
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directory is that it is accessed (read or searched) much more often than it
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is updated (written). Because directories must be able to support high volumes
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of read requests, they are typically optimized for read access. Because directories
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are not intended to provide as many functions as general-purpose databases,
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they can be optimized to economically provide more applications with rapid
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access to directory data in large distributed environments.</p>
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<p>A directory can be centralized or distributed. If a directory is centralized,
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there is one directory server (or a server cluster) at one location that provides
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access to the directory. If the directory is distributed, there are multiple
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servers, usually geographically dispersed, that provide access to the directory.</p>
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<p>When a directory is distributed, the information stored in the directory
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can be partitioned or replicated. When information is partitioned, each directory
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server stores a unique and non-overlapping subset of the information. That
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is, each directory entry is stored by one and only one server. The technique
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to partition the directory is to use LDAP referrals. LDAP referrals allow
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the users to refer Lightweight Directory Access Protocol (LDAP) requests to
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either the same or different name spaces stored in a different (or same) server.
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When information is replicated, the same directory entry is stored by more
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than one server. In a distributed directory, some information can be partitioned
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and some information can be replicated.</p>
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<p>The LDAP directory server model is based on entries (which are also referred
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to as objects). Each entry consists of one or more attributes, such as a name
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or address, and a type. The types typically consist of mnemonic strings, such
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as cn for common name or mail for e-mail address.</p>
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<p>The example directory in <a href="rzahydefdir.htm#dirtree">Figure 1</a> shows an entry for Tim
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Jones that includes mail and telephoneNumber attributes. Some other possible
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attributes include fax, title, sn (for surname), and jpegPhoto.</p>
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<p>Each directory has a schema, which is a set of rules that determine the
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structure and contents of the directory. You can view the schema using the
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Web administration tool. For more information about the schema, see <a href="rzahyschema.htm#rzahyschema">Schema</a>.</p>
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<p>Each directory entry has a special attribute called objectClass. This attribute
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controls which attributes are required and allowed in an entry. In other words,
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the values of the objectClass attribute determine the schema rules the entry
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must obey.</p>
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<p>In addition to the attributes defined by the schema, entries also have
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a set of attributes that are maintained by the server. These attributes,
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known as operational attributes, include such things as when the entry was
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created and access control information. For more information about operational
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attributes, see <a href="rzahyoperational.htm#rzahyoperational">Operational attributes</a>.</p>
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<p>Traditionally, LDAP directory entries are arranged in a hierarchical structure
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that reflects political, geographic, or organizational boundaries (see <a href="rzahydefdir.htm#dirtree">Figure 1</a>). Entries that represent countries or regions appear at the
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top of the hierarchy. Entries representing states or national organizations
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occupy the second level down in the hierarchy. The entries below that can
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then represent people, organizational units, printers, documents, or other
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items.</p>
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<p>LDAP refers to entries with Distinguished Names (DNs). Distinguished names
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consist of the name of the entry itself as well as the names, in order from
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bottom to top, of the objects above it in the directory. For example, the
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complete DN for the entry in the bottom left corner of <a href="rzahydefdir.htm#dirtree">Figure 1</a> is
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cn=Tim Jones, o=IBM, c=US. Each entry has at least one attribute that is used
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to name the entry. This naming attribute is called the Relative Distinguished
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Name (RDN) of the entry. The entry above a given RDN is called its parent
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Distinguished Name. In the example above, cn=Tim Jones names the entry, so
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it is the RDN. o=IBM, c=US is the parent DN for cn=Tim Jones. For more information
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about DNs, see <a href="rzahyunderdn.htm#rzahyunderdn">Distinguished names (DNs)</a>.</p>
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<p>To give an LDAP server the capability to manage part of an LDAP directory,
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you specify the highest level parent distinguished names in the configuration
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of the server. These distinguished names are called suffixes. The server can
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access all objects in the directory that are below the specified suffix in
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the directory hierarchy. For example, if an LDAP server contained the directory
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shown in <a href="rzahydefdir.htm#dirtree">Figure 1</a>, it would need to have the suffix o=ibm, c=us
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specified in its configuration in order to be able to answer client queries
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regarding Tim Jones.</p>
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<a name="dirtree"></a>
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<div class="fignone" id="dirtree"><span class="figcap">Figure 1. LDAP directory structure</span>
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<div class="mmobj">
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<img src="rv4q100.gif" alt="An example of an LDAP directory structure" /></div></div>
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<p>You are not limited to the traditional hierarchy when structuring your
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directory. The domain component structure, for example, is gaining popularity.
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With this structure, entries are composed of the parts of TCP/IP domain names.
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For example, dc=ibm,dc=com might be preferable to o=ibm,c=us.</p>
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<p>Say that you want to create a directory using the domain component structure
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that will contain employee data such as names, telephone numbers, and email
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addresses. You use the suffix or naming context based on the TCP/IP domain.
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This directory might be visualized as something similar to the following:</p>
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<pre class="xmp">/
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+- ibm.com
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+- employees
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+- Tim Jones
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| 555-555-1234
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| tjones@ibm.com
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+- John Smith
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555-555-1235
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jsmith@ibm.com</pre>
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<p>When entered in the Directory Server this data might actually look similar
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to the following:</p>
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<pre class="xmp"># suffix ibm.com
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dn: dc=ibm,dc=com
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objectclass: top
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objectclass: domain
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dc: ibm
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# employees directory
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dn: cn=employees,dc=ibm,dc=com
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objectclass: top
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objectclass: container
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cn: employees
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# employee Tim Jones
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dn: cn=Tim Jones,cn=employees,dc=ibm,dc=com
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objectclass: top
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objectclass: person
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objectclass: organizationalPerson
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objectclass: inetOrgPerson
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objectclass: publisher
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objectclass: ePerson
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cn: Tim Jones
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cn: "Jones, Tim"
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sn: Jones
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givenname: Tim
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telephonenumber: 555-555-1234
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mail: tjones@ibm.com
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# employee John Smith
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dn: cn=John Smith,cn=employees,dc=ibm,dc=com
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objectclass: top
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objectclass: person
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objectclass: organizationalPerson
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objectclass: inetOrgPerson
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objectclass: publisher
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objectclass: ePerson
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cn: John Smith
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cn: "Smith, John"
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sn: Smith
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givenname: John
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telephonenumber: 555-555-1235
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mail: jsmith@ibm.com
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</pre>
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<p>You will notice that the each entry contains attribute values called objectclass.
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The objectclass values define what attributes are allowed in the entry, such
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as telephonenumber or givenname. The allowed object classes are defined in
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the schema. The schema is a set of rules that defines the type of entries
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allowed in the database.</p>
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<p><span class="bold">Directory clients and servers</span></p>
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<p>Directories are usually accessed using the client-server model of communication.
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The client and server processes might or might not be on the same machine.
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A server is capable of serving many clients. An application that wants to
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read or write information in a directory does not access the directory directly.
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Instead, it calls a function or application programming interface (API) that
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causes a message to be sent to another process. This second process accesses
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the information in the directory on behalf of the requesting application.
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The results of the read or write are then returned to the requesting application.</p>
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<p>An API defines the programming interface a particular programming language
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uses to access a service. The format and contents of the messages exchanged
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between client and server must adhere to an agreed on protocol. LDAP defines
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a message protocol used by directory clients and directory servers. There
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is also an associated LDAP API for the C language and ways to access the directory
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from a Java application using the Java Naming and Directory Interface (JNDI).</p>
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<p><span class="bold">Directory security</span></p>
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<p>A directory should support the basic capabilities needed to implement a
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security policy. The directory might not directly provide the underlying security
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capabilities, but it might be integrated with a trusted network security service
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that provides the basic security services. First, a method is needed to authenticate
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users. Authentication verifies that users are who they say they are. A user
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name and password is a basic authentication scheme. Once users are authenticated,
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it must be determined if they have the authorization or permission to perform
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the requested operation on the specific object.</p>
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<p>Authorization is often based on access control lists (ACLs). An ACL is
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a list of authorizations that might be attached to objects and attributes
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in the directory. An ACL lists what type of access each user or a group of
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users is allowed or denied. In order to make ACLs shorter and more manageable,
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users with the same access rights are often put into groups.</p>
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