Lightweight Directory Access Protocol (LDAP)

Before going into the LDAP, we have to look into what are Directory Services and X500 Model.

Directory Service

• Directory Service can be characterized as a hierarchical tree-based naming system that offers numerous distributed system advantages:
• Unique names can be determined by concatenating hierarchical naming components starting at a root node (e.g., "com")
• Object-oriented schema and data model supports very fast search operations (uses Key-based querying).
• Due named sub-tree partitions, the features like Distribution, Replication and Delegated or Autonomous Administration are supported.
• Supports Authentication and Fine-Grained Access Control mechanisms.
• Few examples of directories are DNS, NOS, Novell's NDS, Microsoft's Active Directory.
• Directories are designed and optimized for more READ/ SEARCH operations, than WRITE/ UPDATE operations.
• In real world example, a telephone directory is a directory system that contains a list of subscribers with an address and a phone number in a dictionary format, than in a tree-based fashion.

Telephone Directory

X500 Model

• X.500 is a series of computer networking naming standards and models for Directory Service.
• The primary concept of X.500 is that there is a single Directory Information Tree (DIT), a hierarchical organization of Entries and these can be called using:

• Distinguished Name (DN) is a unique Distinguished Name. Much like a "path" to a filename in a file system.

• For eg: cn=alice, ou=People, dc=example.com

• Relative Distinguished Name (RDN) is a component of the distinguished name. Much like a "filename" in a file system.

• For eg: cn=alice, ou=People is a RDN relative to the root RDN  dc=example.com

Directory Information Tree (DIT)

• DNs are also called as naming contexts, because it describes the namespace where the entry lives.
• X500 Model suggest the way how directory service can be designed in a DIT to have a database to fetch information for a given entry. 
• Directory Access Protocol (DAP) is one of the protocol defined in X.500 that says how to access the entries in the DIT on TCP/IP stack.

LDAP

• The Lightweight Directory Access Protocol (LDAP) is an open, vendor-neutral, industry standard Application Layer protocol interface for accessing and maintaining distributed directory information services over an Internet Protocol (IP) network.
• The latest specification is Version 3, published as RFC 4511.
• LDAP is based on:
• A simpler subset of standards within X.500 model, hence sometimes LDAP is called as X.500-lite.
• DAP to access X500 model directory, hence it is called as Lightweight DAP.

Naming Model

• An Attribute defines as a piece of information that the directory entries contain. 
• Attribute is similar to DB column name.
• The most widely used naming attributes are defined in the following table:

Attr.	Abbreviations	Explanations	Examples
dc	domainComponent	An element of a DNS domain name	dc=acme,dc=com
uid	userid	A person's account name	uid=jdoe
cn	commonName	Full name of a person, group, device, etc.	cn=John Doe
l	localityName	Name of a geographic region	l=Bay Area
st	stateOrProvinceName	State or Province	st=CA
o	organizationName	Organization name	o=Acme
ou	organizationalUnitName	Organization unit name	ou=Sales
c	countryName	Two letter country code	c=US

Schema Model

• An LDAP schema is a set of rules that define what can be stored as entries in a LDAP directory. 
• We have similar concept in RDBMS, where DB schema contains information about database structure, tables, columns, data types and constraints.
• The schema model consists of two types of elements: 
• Object Classes: 
• Defines as a placeholder for attributes. 
• Similar to DB tables.
• Object classes come in one of three kinds: 

Abstract	Structural	Auxiliary
Defines an attribute or set of attributes that all object classes in an object class structure inherit. Every object class structure must have an abstract object class as the top-level object class.	Defines an object entry type. Every entry must contain at least one structural object class. A structural object class inherits either from top or from another structural object class.	An auxiliary object class adds attributes to another object class. Useful to define a set of attributes used by multiple object classes.
There are only two abstract classes: The top class is present in every entry, and it requires the objectClass attribute be present. The alias requires the aliasedObjectName attribute be present.	For example:  Object classes such as: organization, person,  organizationalPerson  or device.	For example: Object class strongAuthenticationUser  allows the attribute userCertificate;binary  be present, but this class could be used in an entry with object class person, device or some other structural class.

Object Classes in a Class Diagram

• Attribute Types: 
• Defines the data types of attribute values. 
• Similar to DB column's data types.
• Attribute Type Definition specifies:

Syntax	Matching Rules
Defines the data format in which an attribute value is stored.	A matching rule encapsulates a set of logic that may be used to perform some kind of matching operation against two LDAP values
For example: Directory String, Integer, and JPEG are examples of standard LDAP syntaxes.	For example: Comparison, Sort and Order

Protocol Overview

• A client starts an LDAP session by connecting to an LDAP server, called a Directory System Agent (DSA), by default on TCP and UDP port 389, or on port 636 for LDAPS. 
• The client may request the following LDAP operations:

Operations	Explanations
StartTLS	Use the LDAPv3-TLS connection
Bind	Authenticate and specify LDAP protocol version
Search	Search for and/or retrieve directory entries
Compare	Test if a named entry contains a given attribute value
Add	Add a new entry
Delete	Delete an entry
Modify	Modify an entry
Modify Distinguished Name (DN)	Move or rename an entry
Abandon	Abort a previous request
Extended Operation	Generic operation used to define other operations
Unbind	Close and exit the connection

LDIF

• LDAP Data Interchange Format (LDIF) is a standard text-based representation for LDAP data.
• LDIF conveys directory content as a set of records, one record for each entry.
• It also represents update requests, such as Add, Modify, Delete, and Rename, as a set of records, one record for each update request.
• This is an example of a simple directory entry with several attributes, represented as a record in LDIF:

dn: cn=John Doe,dc=example,dc=com

cn: John Doe

givenName: John

sn: Doe

telephoneNumber: +1 800 800 8888

telephoneNumber: +1 800 800 9999

mail: jdoe@example.com

manager: cn=Eliza Beth,dc=example,dc=com

objectClass: inetOrgPerson

objectClass: organizationalPerson

objectClass: person

objectClass: top

Other Useful Notes

• In LDAP 3.0, there is a special root called rootDSE (where DSE stands for “DSA-specific entry”) is defined as the root of the DIT server. 
• The purpose of the rootDSE is to provide data about the directory server itself.
• i.e., rootDSE is not part of any namespace, but it contains namingContexts, which provides a list of all DNs in the the DIT.
• Directory servers publish internal schema as an entry in the directory. 
• It can be retrieved by LDAP clients performing a baseObject search on the a special entry that is defined by the directory server to publish schema information (e.g., cn=schema), with the attributes attributeTypes and objectClasses specified as part of the search criteria.
• Each LDAP directory has some default schema, which developers can customize, or "extend," by adding elements to it.