Whispers & Screams
And Other Things

Lightweight Directory Access Protocol (LDAP)

wpid-d53372ab83ca060500bfdd46e1045836ldap2Sometimes traditional network engineers who arrive at the networking industry via the world of telecommunications can often find themselves unfamiliar with certain facets of the industry. Such facets can include network security and servers. A protocol which lies at the intersection between network security  and server technology is LDAP which stands for Lightweight Directory Access Protocol.




So what is LDAP and what is it used for? Lets take a look at the protocol in some detail.


Within the OSI model, LDAP sits at layer 7 and is, as such, an application layer protocol. LDAP is also an "Open" protocol which means that its standards are public information and it is not associated with or owned by any individual commercial organisation. Its primary purpose is to act as a protocol for accessing and maintaining distributed directory information services over an IP network having been specified to act seamlessly as part of a TCP/IP modeled network.


The most common usage for LDAP is to provide a mechanism for a "single sign on" across a distributed multi facility IT estate in order to minimise the authentication across multiple services. LDAP is based on a subset of the more heavily specified and older X500 protocol which was designed to be compatible with the more abstract OSI model.


When people talk about “LDAP”, they are really talking about the complex combination of business rules, software and data that allow you to log in and get access to secure resources.


A client starts an LDAP session by connecting to an LDAP server, called a Directory System Agent (DSA), by default on TCP port and UDP port 389 and 636 for LDAPS. Global Catalog is available by default on ports 3268, and 3269 for LDAPS. The client then sends an operation request to the server, and the server sends responses in return. With some exceptions, the client does not need to wait for a response before sending the next request, and the server may send the responses in any order. All information is transmitted using Basic Encoding Rules (BER). These types of encodings are commonly called type-length-value or TLV encodings. The LDAP server hosts something called the directory-server database. As such, the LDAP protocol can be thought of loosely as a network enabled database query language.


The client may request the following operations:StartTLS — use the LDAPv3 Transport Layer Security (TLS) extension for a secure 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 a new entry
Delete an entry
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 the connection (not the inverse of Bind)



As was alluded to above, the directory-server database is indeed a database and, as a database, is structured in accordance with the rules of its own schema. The contents of the entries in an LDAP domain are governed by a directory schema, a set of definitions and constraints concerning the structure of the directory information tree (DIT).


The schema of a Directory Server defines a set of rules that govern the kinds of information that the server can hold. It has a number of elements, including:


Attribute Syntaxes—Provide information about the kind of information that can be stored in an attribute.
Matching Rules—Provide information about how to make comparisons against attribute values.
Matching Rule Uses—Indicate which attribute types may be used in conjunction with a particular matching rule.
Attribute Types—Define an object identifier (OID) and a set of names that may be used to refer to a given attribute, and associates that attribute with a syntax and set of matching rules.
Object Classes—Define named collections of attributes and classify them into sets of required and optional attributes.
Name Forms—Define rules for the set of attributes that should be included in the RDN for an entry.
Content Rules—Define additional constraints about the object classes and attributes that may be used in conjunction with an entry.
Structure Rule—Define rules that govern the kinds of subordinate entries that a given entry may have.
Attributes are the elements responsible for storing information in a directory, and the schema defines the rules for which attributes may be used in an entry, the kinds of values that those attributes may have, and how clients may interact with those values.


Clients may learn about the schema elements that the server supports by retrieving an appropriate subschema subentry.


The schema defines object classes. Each entry must have an objectClass attribute, containing named classes defined in the schema. The schema definition of the classes of an entry defines what kind of object the entry may represent - e.g. a person, organization or domain. The object class definitions also define the list of attributes that must contain values and the list of attributes which may contain values.


For example, an entry representing a person might belong to the classes "top" and "person". Membership in the "person" class would require the entry to contain the "sn" and "cn" attributes, and allow the entry also to contain "userPassword", "telephoneNumber", and other attributes. Since entries may have multiple ObjectClasses values, each entry has a complex of optional and mandatory attribute sets formed from the union of the object classes it represents. ObjectClasses can be inherited, and a single entry can have multiple ObjectClasses values that define the available and required attributes of the entry itself. A parallel to the schema of an objectClass is a class definition and an instance in Object-oriented programming, representing LDAP objectClass and LDAP entry, respectively.


Directory servers may publish the directory schema controlling an entry at a base DN given by the entry's subschemaSubentry operational attribute. (An operational attribute describes operation of the directory rather than user information and is only returned from a search when it is explicitly requested.)


Server administrators can add additional schema entries in addition to the provided schema elements. A schema for representing individual people within organizations is termed a white pages schema.


We will go on in subsequent posts to examine some of the concepts described here in more detail.
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