LDAP
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 and as such one can use it for any kind of data, not just directory services. Often, institutions adopt an LDAP server to authenticate users for various internet applications (e.g. an LMS), to manage access to central systems and to manage the email and phone directory. So it's a kind of all-in-one solution to deal with users and their attributes (e-mail, address, user identification, office numbers, hobbies,....)
See also: digital identity
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), one 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, ...)
Setting up a good LDAP server is a very difficult task and probably needs about two weeks of reading and testing (or more !). Setting up a simple one, e.g. to manage a Phone,Mail,address and URL directory for students can be done much faster. Most Unix-based systems do include for example a copy of the OpenLDAP server. This article is not complete, but can give you an idea of what LDAP is about. See readings and links for more ...
Architecture overview
In LDAP world, a directory is defined as follows:
- By its structure
- It is a tree of objects called entries (like a file system or the Windows registry). LDAP is kind of hierarchical database, called a data information tree (DIT).
- By having entries
- Each entry is a collection of attributes.
- Entries are instances of ObjectClasses (at least one), i.e. definitions of required or optional attributes So attributes are members of object classes.
- 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 having typed attributes
- Each attribute has a name (also called type) and can have one or several values.
- Values must be of a certain data type (e.g. a case-insensitive string, a phone number).
Here is a picture from IBM's LDAP Redbook defining entries and attributes:
- Standardization
- Both ObjectClasses and Attributes must be defined in a schema, else an LDAP server will not accept entries.
- LDAP Schemas are files with object classes and attribute definitions. Schemas can be built on top of other schemas.
- Usually, Object classes must contain certain attributes and can contain others
- For most kinds of directories, there exist a series of international standards (schemas) that define both object classes and attribute names and value types. On top of these you can your own schema files.
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:. Various LDAP entries are simply defined by a blank line (?) and the start of a new dn:
Examples from TECFA's very small LDAP server:
An organization:
dn: o=tecfa.unige.ch objectclass: top objectclass: organization o: tecfa.unige.ch
An 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 title: Assistant ......
Note: Since this server is just for internal use, there was no need to make the base dn more complicated than the one of our main webserver's domain name.
LDIF can be used to define entries, but also to modify entries. Here is an example:
dn: uid=jermann,o=tecfa.unige.ch changetype: modify add: studentCategory studentCategory: studentCategory staf - add: studentPromotion studentPromotion: A -
Not exactly text editor friendly...
Directory information - entries and schemas
The Directory Information Tree (DIT) is the hierarchy of objects that make up the local directory structure. More than one DIT may be supported by an LDAP server.
Typical object classes
Standard Object Classes that you may find in a typical LDAP server. The difference between an attribute and an object is that objects have several attributes, attributes only can have values. Objects can (and usually do) inherit from other objects.
objectclass: top
- Persons
objectclass: person objectclass: organizationalPerson objectclass: organizationalRole objectclass: inetOrgPerson
- Units
objectclass: country objectclass: locality objectclass: organization objectclass: organizationalUnit objectclass: domain
Then, you may find any number of other things like organizational roles, rooms, computers, documents, accounts, whatever ....
- Related to some application
objectclass: nsCalAdmin objectclass: groupOfUniqueNames (A list of user names (dn) plus owner, etc) objectclass: nginfo (Newsgroup)
Definition of object classes
These definitions must be loaded as configuration files into the LDAP server. It's like the equivalent of SQL database, table and field definitions. So it's not content, but a schema. Most organizations simply adopt schemas that are defined as standards. This way you can be sure to be able to exchange data or to interface with special clients (like email programs).
The Schema definition language is ASN.1, Abstract Syntax Notation-1 (X.691).
Example of an Object class definition hierarchy:
- Each object has a unique number and a human readable name
- DESC: is a description
- SUP: defines the super-class the object inherits from
- MUST: Defines the list of mandatory attributes (separated by $)
- MAY: Defines the list of optional attributes.
objectclass ( 2.5.6.6 NAME 'person' DESC 'RFC2256: a person' SUP top STRUCTURAL MUST ( sn $ cn ) MAY ( userPassword $ telephoneNumber $ seeAlso $ description ) ) objectclass ( 2.5.6.7 NAME 'organizationalPerson' DESC 'RFC2256: an organizational person' SUP person STRUCTURAL MAY ( title $ x121Address $ registeredAddress $ destinationIndicator $ preferredDeliveryMethod $ telexNumber $ teletexTerminalIdentifier $ telephoneNumber $ internationaliSDNNumber $ facsimileTelephoneNumber $ street $ postOfficeBox $ postalCode $ postalAddress $ physicalDeliveryOfficeName $ ou $ st $ l ) ) objectclass ( 2.16.840.1.113730.3.2.2 NAME 'inetOrgPerson' DESC 'RFC2798: Internet Organizational Person' SUP organizationalPerson STRUCTURAL MAY ( audio $ businessCategory $ carLicense $ departmentNumber $ displayName $ employeeNumber $ employeeType $ givenName $ homePhone $ homePostalAddress $ initials $ jpegPhoto $ labeledURI $ mail $ manager $ mobile $ o $ pager $ photo $ roomNumber $ secretary $ uid $ userCertificate $ x500uniqueIdentifier $ preferredLanguage $ userSMIMECertificate $ userPKCS12 ) )
Here is the formal definition of object class (RFC 2252):
ObjectClassDescription = "(" whsp numericoid whsp ; ObjectClass identifier [ "NAME" qdescrs ] [ "DESC" qdstring ] [ "OBSOLETE" whsp ] [ "SUP" oids ] ; Superior ObjectClasses [ ( "ABSTRACT" / "STRUCTURAL" / "AUXILIARY" ) whsp ] ; default structural [ "MUST" oids ] ; AttributeTypes [ "MAY" oids ] ; AttributeTypes whsp ")"
Distinguished Names
Each entry must have a Distinguished Name (DN). It's composed of the entry's relative distinguished 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 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
Example of my own person (some attributes taken away):
dn: uid=schneide,o=tecfa.unige.ch userPassword:: e1NIQxxxxxxxxxxxxxxxxxDRSSHoxczg9 objectclass: top objectclass: person objectclass: organizationalPerson objectclass: inetOrgPerson objectclass: tdsTe{{under construction}} cfaPerson givenname: Daniel sn: Schneider cn: Daniel Schneider uid: schneide mail: Daniel.Schneider@tecfa.unige.ch telephonenumber: +41 (22) 379 93 77 title: MER description: Maitre d'enseignement et de recherche ...... homepostaladdress: Forget it l: Geneve tdsTecfaHomePage: http://tecfa.unige.ch/tecfa-people/schneider.html [Home Page at Tecfa] personaltitle: Dr. postalcode: CH-1227 street: 54 route des Acacias
Definition of attributes
Like object classes, each attribute also must be defined. When you define your own object classes and attributes, you should use standard datatypes.
Examples:
This is the definition of the street attibute:
- It has a unique number
- Two alternative names (usually a short and long one)
- DESC: description referring to a standard
- EQUALITY: E.g. case unsensitive
- SUBSTR: same
- SYNTAX: A number or a name that refers to a data format.
- SUP: An attribute definition it may inherit from
- SINGLE-VALUE: If present, you can't enter more than once this attribute.
attributetype ( 2.5.4.9 NAME ( 'street' 'streetAddress' ) DESC 'RFC2256: street address of this object' EQUALITY caseIgnoreMatch SUBSTR caseIgnoreSubstringsMatch SYNTAX 1.3.6.1.4.1.1466.115.121.1.15{128} ) attributetype ( 2.5.4.10 NAME ( 'o' 'organizationName' ) DESC 'RFC2256: organization this object belongs to' SUP name ) attributetype ( 2.5.4.41 NAME 'name' EQUALITY caseIgnoreMatch SUBSTR caseIgnoreSubstringsMatch SYNTAX 1.3.6.1.4.1.1466.115.121.1.15{32768} ) attributetype ( 2.16.840.1.113730.3.1.3 NAME 'employeeNumber' DESC 'RFC2798: numerically identifies an employee within an organization' EQUALITY caseIgnoreMatch SUBSTR caseIgnoreSubstringsMatch SYNTAX 1.3.6.1.4.1.1466.115.121.1.15 SINGLE-VALUE )
Here is the formal definition of attribute from RFC 2252
AttributeTypeDescription = "(" whsp numericoid whsp ; AttributeType identifier [ "NAME" qdescrs ] ; name used in AttributeType [ "DESC" qdstring ] ; description [ "OBSOLETE" whsp ] [ "SUP" woid ] ; derived from this other ; AttributeType [ "EQUALITY" woid ; Matching Rule name [ "ORDERING" woid ; Matching Rule name [ "SUBSTR" woid ] ; Matching Rule name [ "SYNTAX" whsp noidlen whsp ] ; Syntax OID [ "SINGLE-VALUE" whsp ] ; default multi-valued [ "COLLECTIVE" whsp ] ; default not collective [ "NO-USER-MODIFICATION" whsp ]; default user modifiable [ "USAGE" whsp AttributeUsage ]; default userApplications whsp ")"
Directory Standards
In the OPENLDAP server (2004) I found these:
- corba.schema: Corba Object (RFC 2714)
- core.schema: OpenLDAP "core":
- cosine.schema: COSINE Pilot, This is the biggest file and contains RFC 1274: X.500 Cosine and Internet schema. It is based on other RFC's: RFC 987, RFC 822, RFC 1279, ....
- inetorgperson.schema: InetOrgPerson
- java.schema: Java Object (RFC 2713)
- misc.schema: Miscellaneous Schema (experimental)
- nis.schema: Network Information Service
- openldap.schema: OpenLDAP Project (FYI)
Overview:
- RFC 4519 - Lightweight Directory Access Protocol (LDAP): Schema for User Applications, 2006 (supercedes RFC 2256).
LDAP Search
LDAP servers must implement query functionalities.
Search Filters
(this section needs to be expanded)
Search can be performed on any subtree of the directory tree. See for instance the LDAP URL examples below.
Filter Syntax: RFC 1558
Simplified syntax:
attribute OPERATOR value
Operators you can use (there may be others).
= 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 and all muller that are members of tecfa
LDAP URLs
See: RFC 1959
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://someserver.org/o=tecfa.unige.ch??sub? ... most everything in our server
- ldap://someserver.org/o=tecfa.unige.ch??sub?(sn=*) .. all things that have sn (Surnames)
- ldap://someserver.org/o=tecfa.unige.ch??one?(sn=*) .. one of all things that have sn (Surnames)
- ldap://someserver.org/o=tecfa.unige.ch??one?(objectClass=person)... Persons only
- ldap://someserver.org/o=tecfa.unige.ch??one?(&(objectClass=person)(sn=s*))
- ldap://someserver.org/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://someserver.org/o=tecfa.unige.ch?mail?one?(objectClass=person) Prints entries (uid) mail
- ldap://someserver.org/o=tecfa.unige.ch?mail?one?(&(objectClass=person)(sn=s*))
- ldap://someserver.org/o=tecfa.unige.ch?mail?one?(&(objectClass=person)(sn=s*))] Print Email for all persons who's surname starts with "s"
Restrictions
- ldap://someserver.org/o=tecfa.unige.ch?cn,labeledUri,mail?sub?(&(studentCategory=staf)(studentpromotion=D)) * ldap://someserver.org/o=tecfa.unige.ch?cn,labeledUri,mail?sub?(&(studentCategory=staf)(studentpromotion=D))]
Documentation
Indexes for Documentation
- RFCs can be found in several places, e.g. at http://www.umich.edu/~dirsvcs/ldap/doc/ or
- Jeff Hodge's LDAP Roadmap & FAQ
Specifications
Some RFC's (there are many more, see the LDAP Wikipedia article.
- 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 (RFC 2256)
Other Stuff
- The UniCode doc
Programmer's Tutorials
- The JNDI Tutorial Building directory-enabled Java applications by by Rosanna Lee (at Sun)
- With PHP (missing)
Documentation about schemas
e.g. X500
Articles and tutorials
(not complete !!)
- LDAP for Rocket Scientists. Best resource according to Daniel K. Schneider. Also includes a glossary
- IBM's LDAP Redbook (HTML/PDF Format). Also good.
- 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
- LDAP (Wikipedia)
- LDAP Data Interchange Format (Wikipedia)
- LDAP: Framework, Practices, and Trends
- Understanding LDAP (DevShed introductory article)
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
- 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
- Apache Directory (In 2007, the favorite one for Daniel K. Schneider). Java-based, cross-platform
- JXplorer (Java-based, not tested).
- LDAP Administration Tool (lat). Linux. Tested with Ubuntu 7.x. Works, but DKS preferes Apache Directory.
- LDAP Admin Windows. Freeware. (not tested)
- MaXware Directory Explorer 4.0 IE/Outlook plugin.(not tested)
- GQ - GTK-based LDAP client. You will have to compile it...
Others:
- Netscape Communicator used to be LDAP aware. The LDAP URLs did work. Can't use LDAP URL's with Mozilla /Firefox anymore (nov. 2007).
- 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. Works but the URL is dead and you will now have to buy it...
Servers
- OpenLDAP. Distributed as package for most Linuxes. On Solaris available in the /opt/sfw directory.
- Sun, Novell and Microsoft (Active Directory) ship their own LDAP servers with their OSs
- To play on windows, try one of these (not tested)
LDAP Development Libraries
- PHP has an LDAP library included
- You may find php classes in various places. The PHP library is not exactly end-programmer friendly.