LDAP

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Definition

The Lightweight Directory Access Protocol (LDAP) is a client-server protocol for querying and modifying a directory service. It represents a kind of hierarchical database.

LDAP has become the de facto access method for directory information, much the same as the Domain Name System (DNS) is used for IP address look-up. Often LDAP is also used to authenticate users, i.e. instead of authenticating users with password files or custom databases (in the case of portails), on may ask an LDAP server to match a username with a password.

LDAP is a vendor-independent, open, network protocol standard and thus is as platform-independent as you can get. LDAP is supported by a lot of vendors (Netscape, Sun, Microsoft, Novell, IBM, ...)

Architecture overview

In LDAP world, a directory is defined as follows:

By its structure

• It is a tree of entries (like a file system or the Windows registry)

By having entries

• Each entry is a collection of attributes
• Each entry has a unique identifier: its Distinguished Name (DN). It is constructed as a list of some attributes. DN's must be unambiguous, e.g. an organisation can choose as DN for its users the email address or a department name + Unix login.
• Entries can be typed with ObjectClasses, i.e. a schema that allows to define which attributes are required and which are optional.

Entries having typed attributes

• Each attribute has a name, called type and can have several values.
• Each value must be of a certain type (e.g. a case-insensitive string, a phone number).

Here is a picture from IBM's LDAP Redbook defining entries and attributes:

LDAP Entries and attributes (IBM Redbook)

Standardization

• Both ObjectClasses and Attributes must be defined in a schema, else and LDAP server will not accept entries.
• Object classes must contain certain attributes and can contain others
• For most kinds of directories, there exist a series of international standards that define both object classes and attribute names and value types.

Directory information - entries

Object Classes

Standard Object Classes are taken from X.500, they include

Alias
Country
Locality
Organization
Organizational Unit
Person

Distinguished Name

Each entry must have a Distinguished Name (DN). It's composed of the entry's relative distinguised name and all of the ancestors of the entry up to the root of the DIT (Directory Information Tree).

Example:

dn: uid=roiron,o=tecfa.unige.ch

Some common Attribute types

From X500 (I believe), e.g. see Summary of the X.500(96) User Schema for use with LDAPv3 (rfc2256). Each attribute value must respect some defined syntax.

  cn               CommonName (in principle: givenname   SN)
  co               Country (or sometimes c?)
  dc               DomainComponent
  description      Describes the Entry
  dn               DistinguishedName (Owner)
  drink            favorite drink of a Person
  employeeType
  fax              facsimileTelephoneNumber
  givenname        First Name
  homePhone
  homePostalAddress (each line must be separated with a $)
  keywords         keywords for the entry.
  l                Locality Name
  labeledURI       URL that is relevant in some way to the entry
  mail             Email
  mailAlternateAddress
  manager          dn of the entry's manager
  member           dn for each member of the group
  memberURL        URL associated with each member of a group
  mobile           entry's mobile or cellular phone number
  o                Organization Name
  organizationalStatus person's role in an organization
  ou               Organizational Unit Name
  personalTitle    like Mr.
  postalAddress    (each line must be separated with a $)
  roomNumber       room number of an object
  sa               Street Address
  secretary        entry's secretary or administrative assistant
  seeAlso          related information
  sn               SurName
  st               State or Province Name
  street           entry's house number and street name
  telephonenumber
  title            Job Title
  userClass        Specifies a category of computer user
  userpassword

The LDIF Format

LDIF is the text format that can be used to export/import information from/into a directory server. The LDAP directory server itself uses some kind of binary format.

As you can see, each entry has at least a dn: and an objectclass:

Examples:

A organization:

dn: o=tecfa.unige.ch
objectclass: top
objectclass: organization
o: tecfa.unige.ch

Organizational Unit:

dn: ou=tecfa,o=tecfa.unige.ch
objectclass: top
objectclass: organizationalUnit
ou: tecfa
description: TECFA

A person:

dn: uid=roiron,o=tecfa.unige.ch
userpassword: ....
objectclass: top
objectclass: person
objectclass: organizationalPerson
objectclass: inetOrgPerson
objectclass: nsCalUser
givenname: Cyril
sn: Roiron
cn: Cyril Roiron
uid: roiron
mail: roiron@fapse.unige.ch
title: Assistant
telephonenumber: 9696

LDAP Search

Search Filters

Note that search can be performed on any subtree of the directory tree. See for instance the LDAP URL examples below.

Syntax:

attribute OPERATOR value

Operators:

 =    equal
 >=   bigger than (including alphabetic)
 <=
 =*   all entries that have this attribute
 ~=   aprroximate match

 &    and, entries match ALL criteria
 |    or, one of entries must match
 !    not

Example:

(| (sn=roiron) (&ou=tecfa) (sn=muller))
.. returns all roiron   all muller that are members of tecfa

http://tecfa.unige.ch/tecfa-people/ldap.html

Documentation

Indexes for Documentation

• RFCs can be found in several places, e.g. at http://www.umich.edu/~dirsvcs/ldap/doc/, at Critical Angle, at X.500 and LDAP: Raw Bibliography of Relevant RFCs, ..

Specifications

Some RFC's (there are many more !)

• Lightweight Directory Access Protocol (RFC-1777)
• LDAP URL Format (RFC-1959)
• String Representation of LDAP Search Filters (RFC-1558)
• Summary of the X.500(96) User Schema for use with LDAPv3 (rfc2256)

Other Stuff

• The UniCode doc

FAQs

• LDAP FAQ at Innosoft
• Jeff Hodge's LDAP Roadmap & FAQ

Programmer's Tutorials

• The JNDI Tutorial Building directory-enabled Java applications by by Rosanna Lee (at Sun)

Articles

• Introduction to Directories and the Lightweight Directory Access Protocol (Jeff Hodges@Stanford). Good set of introductory slides
• Why do I need a Directory when I could use a Relational Database? Powerpoint slides from a talk given at Stanford
• An Internet Approach To Directories (Netscape specific, but has general value)
• IBM's LDAP Redbook (PDF Format). EXCELLENT !
• LDAP: The next-generation directory? SunWorld Article. Good overview, includes pointers to on-line specs

Links

• LDAP Central. Good and large Index (has most major links)
• Netscape's Directory Developer Centeral. Good ressource (with a lot of Netscape centered information of course, but more ...)
• LDAP Quellen
• Webopedia's LDAP Page
• Innosoft's LDAP World (no longer fully maintained ?)
• OpenLDAP
• University of Michigan's Lightweight Directory Access Protocol
• LDAP at Yahoo
• Mark Wilcox's List o' Links on LDAP minimalistic presentation, but good stuff
• LDAP Resources A short rated list from B. Foote"Directory" Entries at Dmoz

Software

Clients

• Netscape Communicator used to be LDAP aware. The LDAP URLs work and its mail client can access LDAP servers.
• Most mail clients can access directory services (but not edit LDAP directories or make custom queries for other information than names, firstnames, emails and such).
• LDAP Browser/Editor Java-based GUI
• LDAP Web Exploter. (PHP) Under development, dead ?
• gq - The Gentleman's LDAP client Recent X Client (needs gtk installed). Works fine (but I did not figure out how to edit so far empty attributes)
• Under development: Plums (Java/Swing)
• Python cgi client (ldap-client-cgi.py)

LDAP Development Libraries

• PHP has an LDAP library included

LDAP URLs

See: LDAP URL Format (RFC-1959)

Filter Syntax (much simplified, see also RFC-1558):

ldap://SERVER/BASE_DN/?ATTRIBUTES?ITEMS?FILTER

SERVER     = ldap server URL
BASE_DN    = The Base DN
ATTRIBUTES = What attributes to return for found entries
ITEMS      = How many (of the same) attributes to return
FILTER     = Entries must have these attribute value pairs

Some LDAP queries printing WHOLE entries

• ldap://tecfa2.unige.ch/o=tecfa.unige.ch??sub? ... most everything in our server
• ldap://tecfa2.unige.ch/o=tecfa.unige.ch??sub?(sn=*) .. all things that have sn (Surnames)
• ldap://tecfa2.unige.ch/o=tecfa.unige.ch??one?(sn=*) .. one of all things that have sn (Surnames)
• ldap://tecfa2.unige.ch/o=tecfa.unige.ch??one?(objectClass=person)... Persons only
• [ldap://tecfa2.unige.ch/o=tecfa.unige.ch??one?(&(objectClass=person)(sn=s*)) ldap://tecfa2.unige.ch/o=tecfa.unige.ch??one?(&(objectClass=person)(sn=s*))] (Almost) full entries for persons who's surname starts with "s"

Some queries printing MUCH less:

• ldap://tecfa2.unige.ch/o=tecfa.unige.ch?mail?one?(objectClass=person) Prints entries (uid) mail
• [ldap://tecfa2.unige.ch/o=tecfa.unige.ch?mail?one?(&(objectClass=person)(sn=s*)) ldap://tecfa2.unige.ch/o=tecfa.unige.ch?mail?one?(&(objectClass=person)(sn=s*))] Print Email for all persons who's surname starts with "s"

Restrict search to organizational units (mhh something I don't like here)

• [ldap://tecfa2.unige.ch/o=tecfa.unige.ch?cn,labeledUri,mail?sub?(ou=staf) ldap://tecfa2.unige.ch/o=tecfa.unige.ch?cn,mail,labeledUri?sub?(ou=staf)]. Shows Common Name Emails labelled URLs of all the members of the "staf" Organizational Unit.
• [ldap://tecfa2.unige.ch/o=tecfa.unige.ch?cn,labeledUri,mail?sub?(&(studentCategory=staf)(studentpromotion=D)) ldap://tecfa2.unige.ch/o=tecfa.unige.ch?cn,labeledUri,mail?sub?(&(studentCategory=staf)(studentpromotion=D))] These are custom entries attached to the tecfaPerson Class

PHP

• [/guides/php/examples/ldap/ php-ldap example dir at Tecfa] See also the PHP Manual
• LDAP Web Exploter. Under development ?

D.K.S.