The Lightweight Directory Access Protocol (LDAP) (pronounced /ˈɛldæp/) is an application protocol for querying and modifying data of directory services implemented in Internet Protocol (IP) networks.[1]
A directory is a set of objects with attributes organized logically in a hierarchical manner. A simple example is the telephone directory, which consists of a list of names (of either persons or organizations) organized alphabetically, with each name having an address and phone number associated with it.
A directory information tree often reflects various political, geographic, and/or organizational boundaries, depending on the model chosen. LDAP deployments today tend to use Domain Name System (DNS) names for structuring the topmost levels of the hierarchy. Deeper inside the directory might appear entries representing people, organizational units, printers, documents, groups of people or anything else that represents a given tree entry (or multiple entries).
The latest version of LDAP is Version 3, which is specified in a series of Internet Engineering Task Force (IETF) Standard Track Requests for comments (RFCs) as detailed in RFC 4510.
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Telecommunication companies introduced the concept of directory services to information technology and computer networking, since their understanding of directory requirements was well-developed after some 70 years of producing and managing telephone directories. The culmination of this input was the comprehensive X.500 specification[2], a suite of protocols produced by the International Telecommunication Union (ITU) in the 1980s.
X.500 directory services were traditionally accessed via the X.500 Directory Access Protocol (DAP), which required the Open Systems Interconnection (OSI) protocol stack. LDAP was originally intended to be a lightweight alternative protocol for accessing X.500 directory services through the simpler (and now widespread) TCP/IP protocol stack. This model of directory access was borrowed from the DIXIE and Directory Assistance Service protocols.
Standalone LDAP directory servers soon followed, as did directory servers supporting both DAP and LDAP. The latter has become popular in enterprises, as LDAP removed any need to deploy an OSI network. Today, X.500 directory protocols including DAP can also be used directly over TCP/IP.
The protocol was originally created by Tim Howes of the University of Michigan, Steve Kille of Isode Limited, and Wengyik Yeong of Performance Systems International, circa 1993. Further development has come through the Internet Engineering Task Force.
In the early engineering stages of LDAP, it was known as Lightweight Directory Browsing Protocol, or LDBP. It was renamed with the expansion of the scope of the protocol beyond directory browsing and searching, to include directory update functions. It was given its Lightweight name because it was not as network intensive as its DAP predecessor and thus was more easily implemented over the internet due to its lightweight bandwidth usage.
LDAP has influenced subsequent Internet protocols, including later versions of X.500, XML Enabled Directory (XED), Directory Service Markup Language (DSML), Service Provisioning Markup Language (SPML), and the Service Location Protocol (SLP).
A client starts an LDAP session by connecting to an LDAP server, called a Directory System Agent (DSA), by default on TCP port 389. 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.
The client may request the following operations:
In addition the server may send "Unsolicited Notifications" that are not responses to any request, e.g. before it times out a connection.
A common alternate method of securing LDAP communication is using an SSL tunnel. This is denoted in LDAP URLs by using the URL scheme "ldaps". The default port for LDAP over SSL is 636. The use of LDAP over SSL was common in LDAP Version 2 (LDAPv2) but it was never standardized in any formal specification. This usage has been deprecated along with LDAPv2, which was officially retired in 2003.
LDAP is defined in terms of ASN.1, and protocol messages are encoded in the binary format BER. It uses textual representations for a number of ASN.1 fields/types, however.
The protocol accesses LDAP directories, which follow the 1993 edition of the X.500 model:
Be aware that a DN may change over the lifetime of the entry, for instance, when entries are moved within a tree. To reliably and unambiguously identify entries, a UUID might be provided in the set of the entry's operational attributes.
An entry can look like this when represented in LDAP Data Interchange Format (LDIF) (LDAP itself is a binary protocol):
dn: cn=John Doe,dc=example,dc=com cn: John Doe givenName: John sn: Doe telephoneNumber: +1 888 555 6789 telephoneNumber: +1 888 555 1232 mail: john@example.com manager: cn=Barbara Doe,dc=example,dc=com objectClass: inetOrgPerson objectClass: organizationalPerson objectClass: person objectClass: top
"dn" is the name of the entry; it's not an attribute nor part of the entry. "cn=John Doe" is the entry's RDN (Relative Distinguished Name), and "dc=example,dc=com" is the DN of the parent entry, where "dc" denotes 'Domain Component'. The other lines show the attributes in the entry. Attribute names are typically mnemonic strings, like "cn" for common name, "dc" for domain component, "mail" for e-mail address and "sn" for surname.
A server holds a subtree starting from a specific entry, e.g. "dc=example,dc=com" and its children. Servers may also hold references to other servers, so an attempt to access "ou=department,dc=example,dc=com" could return a referral or continuation reference to a server which holds that part of the directory tree. The client can then contact the other server. Some servers also support chaining, which means the server contacts the other server and returns the results to the client.
LDAP rarely defines any ordering: The server may return the values of an attribute, the attributes in an entry, and the entries found by a search operation in any order. This follows from the formal definitions - an entry is defined as a set of attributes, and an attribute is a set of values, and sets need not be ordered.
The client gives each request a positive Message ID, and the server response has the same Message ID. The response includes a numeric result code which indicates success, some error condition or some other special cases. Before the response, the server may send other messages with other result data - for example each entry found by the Search operation is returned in such a message.
The StartTLS operation establishes Transport Layer Security (the descendant of SSL) on the connection. It can provide data confidentiality (to protect data from being observed by third parties) and/or data integrity protection (which protects the data from tampering). During TLS negotiation the server sends its X.509 certificate to prove its identity. The client may also send a certificate to prove its identity. After doing so, the client may then use SASL/EXTERNAL. By using the SASL/EXTERNAL, the client requests the server derive its identity from credentials provided at a lower level (such as TLS). Though technically the server may use any identity information established at any lower level, typically the server will use the identity information established by TLS.
Servers also often support the non-standard "LDAPS" ("Secure LDAP", commonly known as "LDAP over SSL") protocol on a separate port, by default 636. LDAPS differs from LDAP in two ways: 1) upon connect, the client and server establish TLS before any LDAP messages are transferred (without a StartTLS operation) and 2) the LDAPS connection must be closed upon TLS closure.
LDAPS was used with LDAPv2, because the StartTLS operation had not yet been defined. The use of LDAPS is deprecated, and modern software should only use StartTLS .
The Bind operation authenticates the client to the server. Simple Bind can send the user's DN and password in plaintext, so the connection should be protected using Transport Layer Security (TLS). The server typically checks the password against the userPassword attribute in the named entry. Anonymous Bind (with empty DN and password) resets the connection to anonymous state. SASL (Simple Authentication and Security Layer) Bind provides authentication services through a wide range of mechanisms, e.g. Kerberos or the client certificate sent with TLS.
Bind also sets the LDAP protocol version. Normally clients should use LDAPv3, which is the default in the protocol but not always in LDAP libraries.
Bind had to be the first operation in a session in LDAPv2, but is not required in LDAPv3 (the current LDAP version).
The Search operation is used to both search for and read entries. Its parameters are:
BaseObject (search just the named entry, typically used to read one entry), singleLevel (entries immediately below the base DN), or wholeSubtree (the entire subtree starting at the base DN).(&(objectClass=person)(|(givenName=John)(mail=john*))) will select "persons" (elements of objectClass person) who either have the given name "John" or an e-mail address that begins with the string "john".The server returns the matching entries and potentially continuation references. These may be returned in any order. The final result will include the result code.
The Compare operation takes a DN, an attribute name and an attribute value, and checks if the named entry contains that attribute with that value.
Add, Delete, and Modify DN - all require the DN of the entry that is to be changed.
Modify takes a list of attributes to modify and the modifications to each: Delete the attribute or some values, add new values, or replace the current values with the new ones.
Add operations also can have additional attributes and values for those attributes.
Modify DN (move/rename entry) takes the new RDN (Relative Distinguished Name), optionally the new parent's DN, and a flag which says whether to delete the value(s) in the entry which match the old RDN. The server may support renaming of entire directory subtrees.
An update operation is atomic: Other operations will see either the new entry or the old one. On the other hand, LDAP does not define transactions of multiple operations: If you read an entry and then modify it, another client may have updated the entry in the mean time. Servers may implement extensions [3] which support this, however.
The Extended Operation is a generic LDAP operation which can be used to define new operations. Examples include the Cancel and Password Modify.
The Abandon operation requests that the server abort an operation named by a message ID. The server need not honor the request. Unfortunately, neither Abandon nor a successfully abandoned operation send a response. A similar Cancel extended operation has therefore been defined which does send responses, but not all implementations support this.
The Unbind operation abandons any outstanding operations and closes the connection. It has no response. The name is of historical origin, and is not the opposite of the Bind operation.[4]
Clients can abort a session by simply closing the connection, but they should use Unbind.[5] Unbind allows the server to gracefully close the connection and free resources that it would otherwise keep for some time until discovering the client had abandoned the connection. It also instructs the server to cancel operations that can be canceled, and to not send responses for operations that cannot be canceled.[6]
An LDAP URL format exists which clients support in varying degree, and which servers return in referrals and continuation references (see RFC 4516):
ldap://host:port/DN?attributes?scope?filter?extensions
Most of the components, which are described below, are optional.
(objectClass=*) as defined in RFC 4515.For example, "ldap://ldap.example.com/cn=John%20Doe,dc=example,dc=com" refers to all user attributes in John Doe's entry in ldap.example.com, while "ldap:///dc=example,dc=com??sub?(givenName=John)" searches for the entry in the default server (note the triple slash, omitting the host, and the double question mark, omitting the attributes). As in other URLs, special characters must be percent-encoded.
There is a similar non-standard ldaps: URL scheme for LDAP over SSL. This should not be confused with LDAP with TLS, which is achieved using the StartTLS operation using the standard ldap: scheme.
The contents of the entries in a subtree are governed by a schema known as a 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:
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 which 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.
A lot of the server operation is left to the implementor or administrator to decide. Accordingly, servers may be set up to support a wide variety of scenarios.
For example, data storage in the server is not specified - the server may use flat files, databases, or just be a gateway to some other server. Access control is not standardized, though there has been work on it and there are commonly used models. Users' passwords may be stored in their entries or elsewhere. The server may refuse to perform operations when it wishes, and impose various limits.
Most parts of LDAP are extensible. Examples: One can define new operations. Controls may modify requests and responses, e.g. to request sorted search results. New search scopes and Bind methods can be defined. Attributes can have options that may modify their semantics.
As LDAP has gained momentum, vendors have provided it as an access protocol to other services. The implementation then recasts the data to mimic the LDAP/X.500 model, but how closely this model is followed varies. For example, there is software to access SQL databases through LDAP, even though LDAP does not readily lend itself to this.[7] X.500 servers may support LDAP as well.
Similarly, data which were previously held in other types of data stores are sometimes moved to LDAP directories. For example, Unix user and group information can be stored in LDAP and accessed via PAM and NSS modules. LDAP is often used by other services for authentication.
Since an LDAP server can return referrals to other servers for requests the server itself will not/can not serve, a naming structure for LDAP entries is needed so one can find a server holding a given DN. Since such a structure already exists in the Domain name system (DNS), servers' top level names often mimic DNS names, as they do in X.500.
If an organization has domain name example.org, its top level LDAP entry will typically have the DN dc=example,dc=org (where dc means domain component). If the LDAP server is also named ldap.example.org, the organization's top level LDAP URL becomes ldap://ldap.example.org/dc=example,dc=org.
Below the top level, the entry names will typically reflect the organization's internal structure or needs rather than DNS names.
The LDAP terminology one can encounter is rather cumbersome. Some of this is due to misunderstandings, other examples are due to its historical origins, others arise when used with non-X.500 services that use different terminology. For example, "LDAP" is sometimes used to refer to the protocol, other times to the protocol and the data. An "LDAP directory" may be the data or also the access point. An "attribute" may be the attribute type, or the contents of an attribute in a directory, or an attribute description (an attribute type with options). An "anonymous" and an "unauthenticated" Bind are different Bind methods that both produce anonymous authentication state, so both terms are being used for both variants.
LDAP is specified in a series of Request for Comments documents:
The following RFCs detail LDAP-specific Best Current Practices:
The following is a partial list of RFCs specifying LDAPv3 extensions:
LDAPv2 was specified in the following RFCs:
LDAPv2 was moved to historic status by the following RFC:
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