<?xml version="1.0" encoding="UTF-8"?>

<chapter xml:id='chap-understanding-ldap'

xmlns='http://docbook.org/ns/docbook' version='5.0' xml:lang='en'

xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance'

xsi:schemaLocation='http://docbook.org/ns/docbook

http://docbook.org/xml/5.0/xsd/docbook.xsd'>

<title>Understanding LDAP</title>

<indexterm>

<primary>LDAP</primary>

<secondary>About</secondary>

</indexterm>

<para>A directory resembles a dictionary or a phone book. If you know a

word, you can look it up its entry in the dictionary to learn its definition

or its pronunciation. If you know a name, you can look it up its entry in the

phone book to find the telephone number and street address associated with the

name. If you are bored, curious, or have lots of time, you can also read

through the dictionary, phone book, or directory, entry after entry.</para>

<para>Where a directory differs from a paper dictionary or phone book is

in how entries are indexed. Dictionaries typically have one index: words

in alphabetical order. Phone books, too: names in alphabetical order.

Directories entries on the other hand are often indexed for multiple

attributes, names, user identifiers, email addresses, telephone numbers.

This means you can look up a directory entry by the name of the user the

entry belongs to, but also by her user identifier, her email address, or

her telephone number, for example.</para>

<section xml:id="ldap-directory-history">

<title>How Directories &amp; LDAP Evolved</title>

<para>Phone companies have been managing directories for many decades. The

Internet itself has relied on distributed directory services like DNS since

the mid 1980s.</para>

<para>It was not until the late 1980s, however, that experts from what is now

the International Telecommunications Union brought forth the X.500 set of

international standards, including Directory Access Protocol. The X.500

standards specify Open Systems Interconnect (OSI) protocols and

data definitions for general-purpose directory services. The X.500 standards

were designed to meet the needs of systems built according to the X.400

standards, covering electronic mail services.</para>

<para>Lightweight Directory Access Protocol has been around since the early

1990s. LDAP was originally developed as an alternative protocol that would

allow directory access over Internet protocols rather than OSI protocols,

and be lightweight enough for desktop implementations. By the mid 1990s, LDAP

directory servers became generally available and widely used.</para>

<para>Until the late 1990s, LDAP directory servers were designed primarily

with quick lookups and high availability for lookups in mind. LDAP directory

servers replicate data, so when an update is made, that update gets pushed

out to other peer directory servers. Thus if one directory server goes down,

lookups can continue on other servers. Furthermore, if a directory service

needs to support more lookups, the administrator can simply add another

directory server to replicate with its peers.</para>

<para>As organizations rolled out larger and larger directories serving more

and more applications, they discovered that they needed high availability

not only for lookups, but also for updates. Around the year 2000 directories

began to support multi-master replication, that is replication with multiple

read-write servers. Soon thereafter the organizations with the very largest

directories started to need higher update performance as well as

availability.</para>

<para>The OpenDJ code base began in the mid 2000s, when engineers solving the

update performance issue decided the cost of adapting the existing C-based

directory technology for high performance updates would be higher than the

cost of building a next generation, high performance directory using Java

technology.</para>

</section>

<section xml:id="directory-data">

<title>Data In LDAP Directories</title>

<indexterm>

<primary>LDAP</primary>

<secondary>Data</secondary>

</indexterm>

<indexterm>

<primary>Attributes</primary>

</indexterm>

<para>LDAP directory data is organized into entries, similar to the entries

for words in the dictionary, or for subscriber names in the phone book.

A sample entry follows.</para>

<programlisting language="ldif">

dn: uid=bjensen,ou=People,dc=example,dc=com

uid: bjensen

cn: Babs Jensen

cn: Barbara Jensen

facsimileTelephoneNumber: +1 408 555 1992

gidNumber: 1000

givenName: Barbara

homeDirectory: /home/bjensen

l: Cupertino

mail: bjensen@example.com

objectClass: inetOrgPerson

objectClass: organizationalPerson

objectClass: person

objectClass: posixAccount

objectClass: top

ou: People

ou: Product Development

roomNumber: 0209

sn: Jensen

telephoneNumber: +1 408 555 1862

uidNumber: 1076

</programlisting>

<para>Barbara Jensen's entry has a number of attributes, such as

<literal>uid: bjensen</literal>,

<literal>telephoneNumber: +1 408 555 1862</literal>, and

<literal>objectClass: posixAccount</literal><footnote><para>The

<literal>objectClass</literal> attribute type indicates which types of

attributes are allowed and optional for the entry. As the entries object

classes can be updated online, and even the definitions of object classes

and attributes are expressed as entries that can be updated online, directory

data is extensible on the fly.</para></footnote>. When you look up her entry

in the directory, you specify one or more attributes and values to match.

The directory server then returns entries with attribute values that match

what you specified.</para>

<para>The attributes you search for are indexed in the directory, so the

directory server can retrieve them more quickly.<footnote><para>Attribute

values do not have to be strings. Some attribute values are pure binary like

certificates and photos.</para></footnote></para>

<para>The entry also has a unique identifier, shown at the top of the entry,

<literal>dn: uid=bjensen,ou=People,dc=example,dc=com</literal>. DN stands

for distinguished name. No two entries in the directory have the same

distinguished name. Yet, DNs are typically composed of case insensitive

attributes.<footnote><para>Sometimes your distinguished names include

characters that you must escape. The following example shows an entry that

includes escaped characters in the DN.</para>

<screen>

$ <userinput>ldapsearch --port 1389 --baseDN dc=example,dc=com "(uid=escape)"</userinput>

<computeroutput>dn: cn=\" # \+ \, \; \&lt; = \&gt; \\ DN Escape Characters,dc=example,dc=com

objectClass: person

objectClass: inetOrgPerson

objectClass: organizationalPerson

objectClass: top

givenName: " # + , ; &lt; = &gt; \

uid: escape

cn: " # + , ; &lt; = &gt; \ DN Escape Characters

sn: DN Escape Characters

mail: escape@example.com</computeroutput>

</screen></footnote></para>

<para>LDAP entries are arranged hierarchically in the directory. The

hierarchical organization resembles a file system on a PC or a web server,

often imagined as an upside-down tree structure, looking similar to a

pyramid. <footnote><para>Hence pyramid icons are associated with directory

servers.</para></footnote> The distinguished name consists of components

separated by commas,

<literal>uid=bjensen,ou=People,dc=example,dc=com</literal>. The names are

little-endian. The components reflect the hierarchy of directory entries.</para>

<mediaobject xml:id="figure-data-organization">

<alt>Directory data hierarchy as seen in OpenDJ Control Panel.</alt>

<imageobject>

<imagedata fileref="images/data-organization.png" format="PNG" />

</imageobject>

<textobject>

<para>You can see the hierarchy of directory data in the left pane of

the Manage Entries browser.</para>

</textobject>

</mediaobject>

<para>Barbara Jensen's entry is located under an entry with DN

<literal>ou=People,dc=example,dc=com</literal>, an organization unit and

parent entry for the people at Example.com. The

<literal>ou=People</literal> entry is located under the entry with DN

<literal>dc=example,dc=com</literal>, the base entry for Example.com.

DC stands for domain component. The directory has other base entries, such

as <literal>cn=config</literal>, under which the configuration is accessible

through LDAP. A directory can serve multiple organizations, too. You might

find <literal>dc=example,dc=com</literal>,

<literal>dc=mycompany,dc=com</literal>, and

<literal>o=myOrganization</literal> in the same LDAP directory.

Therefore when you look up entries, you specify the base DN to look under

in the same way you need to know whether to look in the New York, Paris,

or Tokyo phone book to find a telephone number.<footnote>

<para>The root entry for the directory, technically the entry with DN

<literal>""</literal> (the empty string), is called the root DSE, and

contains information about what the server supports, including the other

base DNs it serves.</para></footnote></para>

<para>

A directory server stores two kinds of attributes in a directory entry:

<firstterm>user attributes</firstterm>

and <firstterm>operational attributes</firstterm>.

User attributes hold the information for users of the directory.

All of the attributes shown in the entry at the outset of this section

are user attributes.

Operational attributes hold information used by the directory itself.

Examples of operational attributes include

<literal>entryUUID</literal>, <literal>modifyTimestamp</literal>,

and <literal>subschemaSubentry</literal>.

When an LDAP search operation finds an entry in the directory,

the directory server returns all the visible user attributes

unless the search request restricts the list of attributes

by specifying those attributes explicitly.

The directory server does not however return any operational attributes

unless the search request specifically asks for them.

Generally speaking, applications should change only user attributes,

and leave updates of operational attributes to the server,

relying on public directory server interfaces to change server behavior.

An exception is access control instruction (<literal>aci</literal>) attributes,

which are operational attributes used to control access to directory data.

</para>

</section>

<section xml:id="ldap-client-server-communication">

<title>LDAP Client &amp; Server Communication</title>

<indexterm>

<primary>LDAP</primary>

<secondary>Connected protocol</secondary>

</indexterm>

<para>You may be used to web service client server communication, where

each time the web client has something to request of the web server, a

connection is set up and then torn down. LDAP has a different model. In

LDAP the client application connects to the server and authenticates, then

requests any number of operations, perhaps processing results in between

requests, and finally disconnects when done.</para>

<indexterm>

<primary>Authentications</primary>

</indexterm>

<indexterm>

<primary>Searches</primary>

</indexterm>

<indexterm>

<primary>Comparisons</primary>

</indexterm>

<indexterm>

<primary>Modifications</primary>

</indexterm>

<indexterm>

<primary>Adds</primary>

</indexterm>

<indexterm>

<primary>Deletes</primary>

</indexterm>

<indexterm>

<primary>Renames</primary>

</indexterm>

<itemizedlist xml:id="standard-ldap-operations">

<para>The standard operations are as follows.</para>

<listitem>

<para>Bind (authenticate). The first operation in an LDAP session usually

involves the client binding to the LDAP server, with the server

authenticating the client.<footnote><para>If the client does not bind

explicitly, the server treats the client as an anonymous client. The client

can also bind again on the same connection.</para></footnote> Authentication

identifies the client's identity in LDAP terms, the identity which is later

used by the server to authorize (or not) access to directory data that the

client wants to lookup or change.</para>

</listitem>

<listitem>

<para>Search (lookup). After binding, the client can request that the server

return entries based on an LDAP filter, which is an expression that the

server uses to find entries that match the request, and a base DN under

which to search. For example, to lookup all entries for people with email

address <literal>bjensen@example.com</literal> in data for Example.com,

you would specify a base DN such as

<literal>ou=People,dc=example,dc=com</literal> and the filter

<literal>(mail=bjensen@example.com)</literal>.</para>

</listitem>

<listitem>

<para>Compare. After binding, the client can request that the server

compare an attribute value the client specifies with the value stored

on an entry in the directory.</para>

<para>This operation is not used as commonly as others.</para>

</listitem>

<listitem>

<para>Modify. After binding, the client can request that the server

change one or more attribute values stored on an entry. Often administrators

do not allow clients to change directory data, so request that your

administrator set appropriate access rights for your client application if

you want to update data.</para>

</listitem>

<listitem>

<para>Add. After binding, the client can request to add one or more

new LDAP entries to the server. </para>

</listitem>

<listitem>

<para>Delete. After binding, the client can request that the server

delete one or more entries. To delete and entry with other entries

underneath, first delete the children, then the parent.</para>

</listitem>

<listitem>

<para>Modify DN. After binding, the client can request that the server

change the distinguished name of the entry. In other words, this renames

the entry or moves it to another location. For example, if Barbara

changes her unique identifier from <literal>bjensen</literal> to something

else, her DN would have to change. For another example, if you decide

to consolidate <literal>ou=Customers</literal> and

<literal>ou=Employees</literal> under <literal>ou=People</literal>

instead, all the entries underneath must change distinguished names.

<footnote><para>Renaming entire branches of entries can be a major

operation for the directory, so avoid moving entire branches if you

can.</para></footnote></para>

</listitem>

<listitem>

<para>Unbind. When done making requests, the client can request an

unbind operation to end the LDAP session.</para>

</listitem>

<listitem>

<para>Abandon. When a request seems to be taking too long to complete,

or when a search request returns many more matches than desired, the client

can send an abandon request to the server to drop the operation in

progress.</para>

</listitem>

</itemizedlist>

</section>

<section xml:id="standard-ldapv3-extensions">

<title>Standard LDAPv3 &amp; Extensions</title>

<para>LDAP has standardized two mechanisms for extending what directory

servers can do beyond the basic operations listed above. One mechanism

involves using LDAP controls. The other mechanism involves using LDAP extended

operations.</para>

<indexterm>

<primary>Controls</primary>

<secondary>About</secondary>

</indexterm>

<para>LDAP controls are information added to an LDAP message to further

specify how an LDAP operation should be processed. For example, the

Server Side Sort Request Control modifies a search to request that the

directory server return entries to the client in sorted order. The Subtree

Delete Request Control modifies a delete to request that the server

also remove child entries of the entry targeted for deletion.</para>

<para>The directory server can also send response controls in some cases to

indicate that the response contains special information. Examples include

responses for entry change notification, password policy, and paged results.</para>

<indexterm>

<primary>Extended operations</primary>

<secondary>About</secondary>

</indexterm>

<para>LDAP extended operations are additional LDAP operations not included

in the original standard list. For example, the Cancel Extended Operation

works like an abandon operation, but finishes with a response from the

server after the cancel is complete. The StartTLS Extended Operation allows

a client to connect to a server on an unsecure port, but then start

Transport Layer Security negotiations to protect communications.</para>

<para>Both LDAP controls and extended operations are demonstrated later in

this guide. OpenDJ directory server supports many LDAP controls and a few

LDAP extended operations, controls and extended operations matching those

demonstrated in this guide.</para>

</section>

</chapter>