Lightweight Directory Access Protocol

The Lightweight Directory Access Protocol (LDAP;  /ˈɛldæp/) is an application protocol for accessing and maintaining distributed directory information services over an Internet Protocol (IP) network.^[1] LDAP is defined in terms of ASN.1 and transmitted using BER.

Directory services may provide any organized set of records, often with a hierarchical structure, such as a corporate electronic mail directory. Similarly, a telephone directory is a list of subscribers with an address and a phone number.

LDAP is specified in a series of Internet Engineering Task Force (IETF) Standard Track Requests for comments (RFCs). The latest version is Version 3, published as RFC 4510.

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Origin and influences

Telecommunication companies' understanding of directory requirements was well developed after some 70 years of producing and managing telephone directories. These companies introduced the concept of directory services to information technology and computer networking, their input culminating in 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. Mark Wahl of Critical Angle Inc., Tim Howes, and Steve Kille started work in 1996 on a new version of LDAP, LDAPv3, under the aegis of the Internet Engineering Task Force (IETF). LDAPv3, first published in 1997, superseded LDAPv2 and added support for extensibility, integrated the Simple Authentication and Security Layer, and better aligned the protocol to the 1993 edition of X.500. Further development of the LDAPv3 specifications themselves and of numerous extensions adding features to LDAPv3 has come through the IETF.

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 relatively modest 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).

Protocol overview

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:

• 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)

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.^[3]

Directory structure

The protocol accesses LDAP directories, which follow the 1993 edition of the X.500 model:

• A directory is a tree of directory entries.
• An entry consists of a set of attributes.
• An attribute has a name (an attribute type or attribute description) and one or more values. The attributes are defined in a schema (see below).
• Each entry has a unique identifier: its Distinguished Name (DN). This consists of its Relative Distinguished Name (RDN), constructed from some attribute(s) in the entry, followed by the parent entry's DN. Think of the DN as the full file path and the RDN as its relative filename in its parent folder (e.g. if /foo/bar/myfile.txt were the DN, then myfile.txt would be the RDN).

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: [phone]
 telephoneNumber: [phone]
 mail: [e-mail]
 manager: cn=Barbara Doe,dc=example,dc=com
 objectClass: inetOrgPerson
 objectClass: organizationalPerson
 objectClass: person
 objectClass: top

"dn" is the distinguished name of the entry; it's neither an attribute nor a 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.

Operations

Expand discussion of referral responses to various operations, especially modify, for example where all modifies must be directed from replicas to a master directory.

Bind (authenticate)

The Bind operation establishes the authentication state for a connection. 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. The version is an integer and at present must be either 2 (two) or 3 (three), although the standard supports integers between 1 and 127 (inclusive) in the protocol. If the client requests a version that the server does not support, the server must set the result code in the bind response to the code for a protocol error. 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).

Search and Compare

The Search operation is used to both search for and read entries. Its parameters are:

baseObject 

The DN (Distinguished Name) of the entry at which to start the search,

scope 

What elements below the baseObject to search. This can be 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).

filter 

Criteria to use in selecting elements within scope. For example, the filter (&(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".

derefAliases 

Whether and how to follow alias entries (entries which refer to other entries),

attributes 

Which attributes to return in result entries.

sizeLimit, timeLimit 

Maximum number of entries to return, and maximum time to allow search to run. These values, however, cannot override any restrictions the server places on size limit and time limit.

typesOnly 

Return attribute types only, not attribute values.

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.

Update Data

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 meantime. Servers may implement extensions ^[4] which support this, though.

Extended operations

The Extended Operation is a generic LDAP operation which can be used to define new operations that were not part of the original protocol specification. StartTLS is one of the most significant extensions. Other examples include the Cancel and Password Modify.

StartTLS

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.

Abandon

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.

Unbind

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.^[5]

Clients can abort a session by simply closing the connection, but they should use Unbind.^[6] 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.^[7]

LDAP URLs

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.

• host is the FQDN or IP address of the LDAP server to search.
• port is the network port (default port 389) of the LDAP server.
• DN is the distinguished name to use as the search base.
• attributes is a comma-separated list of attributes to retrieve.
• scope specifies the search scope and can be "base" (the default), "one" or "sub".
• filter is a search filter. For example (objectClass=*) as defined in RFC 4515.
• extensions are extensions to the LDAP URL format.

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.

Schema

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:

• 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 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.

Variations

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.

Other data models

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.^[8] 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.

An example of such data model is the GLUE Schema ^[9], which is used in a distributed information system based on LDAP that enable users, applications and services to discover which services exist in a Grid infrastructure and further information about their structure and state.

Usage

An LDAP server may return referrals to other servers for requests that it cannot fulfill itself. This requires a naming structure for LDAP entries so one can find a server holding a given DN or distinguished name, a concept defined in the X.500 Directory and also used in LDAP. Another way of locating LDAP servers for an organization is a DNS server resource record (SRV).

An organization with the domain example.org may use the top level LDAP 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.

Primarily two common styles of naming are used in both X.500 [2008] and LDAPv3 which are documented in the ITU specifications and IETF RFCs. The original form takes the top level object as the country object, such as c=US, c=FR. The domain component model uses the model described above. An example of country based naming could be c=FR, o=Some Organization, ou=Some Organizational Unit L=Locality, or in the US: c=US, st=CA, o=Some Organization ou=Organizational Unit, L=Locality, and CN=Common Name.

See also

• Federated Naming Service
• Global Directory Server
• Hesiod (name service)
• Hierarchical database model
• Key server (cryptographic)
• LDAP Application Program Interface
• List of LDAP software
• Simple Authentication and Security Layer (SASL)

References

1. ^ LDAP: Framework, Practices, and Trends
2. ^ The X.500 series - ITU-T Rec. X.500 to X.521
3. ^ Tools.ietf.org
4. ^ INTERNET-DRAFT LDAP Transactions draft-zeilenga-ldap-txn-15.txt
5. ^ Tools.ietf.org
6. ^ Tools.ietf.org
7. ^ Tools.ietf.org
8. ^ Openldap.org
9. ^ [1]

• ITU-T Rec. X.680, "Abstract Syntax Notation One (ASN.1) - Specification of Basic Notation", 1994
• Basic encoding rules (BER) - ITU-T Rec. X.690, "Specification of ASN.1 encoding rules: Basic, Canonical, and Distinguished Encoding Rules", 1994
• RFC 3641 - Generic String Encoding Rules (GSER) for ASN.1 Types
• RFC 4346 - The TLS Protocol Version 1.1
• RFC 4422 - Simple Authentication and Security Layer (SASL)
• SASL mechanisms registered at IANA
• This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.

Further reading

• Arkills, B (2003). LDAP Directories Explained: An Introduction and Analysis. Addison-Wesley Professional. ISBN 020178792X. http://www.informit.com/store/product.aspx?isbn=020178792X. 
• Carter, G (2003). LDAP System Administration. O'Reilly Media. ISBN 1565924916. http://oreilly.com/catalog/9781565924918. 
• Donley, C (2002). LDAP Programming, Management, and Integration. Manning Publications. ISBN 1930110405. 
• Howes, T; Smith, M; Good, G (2003). Understanding and Deploying LDAP Directory Services. Addison-Wesley Professional. ISBN 0672323168. http://www.informit.com/store/product.aspx?isbn=0672323168. 
• Rhoton, J (1999). Programmer's Guide to Internet Mail: SMTP, POP, IMAP, and LDAP. Elsevier. ISBN 1555582125. 
• Voglmaier, R (2003). The ABCs of LDAP: How to Install, Run, and Administer LDAP Services. Auerbach Publications. ISBN 0849313465. 

External links

• Devshed.com, Understanding LDAP, A simple, light introductory tutorial for LDAP.
• Skills-1st.co.uk, LDAP schema design
• Capitalhead.com, Troubleshooting LDAP SSL connection issues between Microsoft ILM/MIIS & Novell eDirectory 8.7.3
• Prasannatech.com, LDAP schema design - A Case Study

RFCs

LDAP is specified in a series of Request for Comments documents:

• RFC 4510 - LDAP: Technical Specification Road Map (Obsoletes: RFC 2251, RFC 2252, RFC 2253, RFC 2254, RFC 2255, RFC 2256, RFC 2829, RFC 2830, RFC 3377, RFC 3771)
• RFC 4511 - LDAP: The Protocol (Obsoletes RFC 2251, RFC 2830 & RFC 3771)
• RFC 4512 - LDAP: Directory Information Models (Obsoletes RFC 2251, RFC 2252, RFC 2256 & RFC 3674)
• RFC 4513 - LDAP: Authentication Methods and Security Mechanisms (Obsoletes RFC 2251, RFC 2829 & RFC 2830)
• RFC 4514 - LDAP: String Representation of Distinguished Names (Obsoletes RFC 2253)
• RFC 4515 - LDAP: String Representation of Search Filters (Obsoletes RFC 2254)
• RFC 4516 - LDAP: Uniform Resource Locator (Obsoletes RFC 2255)
• RFC 4517 - LDAP: Syntaxes and Matching Rules (Obsoletes RFC 2252 & RFC 2256, Updates RFC 3698)
• RFC 4518 - LDAP: Internationalized String Preparation
• RFC 4519 - LDAP: Schema for User Applications (Obsoletes RFC 2256, Updates RFC 2247, RFC 2798 & RFC 2377)

The following RFCs detail LDAP-specific Best Current Practices:

• RFC 4520 (also BCP 64) - Internet Assigned Numbers Authority (IANA) Considerations for the Lightweight Directory Access Protocol (LDAP) (replaced RFC 3383)
• RFC 4521 (also BCP 118) - Considerations for Lightweight Directory Access Protocol (LDAP) Extensions

The following is a partial list of RFCs specifying LDAPv3 extensions:

• RFC 2247 - Use of DNS domains in distinguished names (Updated by RFC 4519 & RFC 4524)
• RFC 2307 - Using LDAP as a Network Information Service
• RFC 2589 - LDAPv3: Dynamic Directory Services Extensions
• RFC 2649 - LDAPv3 Operational Signatures
• RFC 2696 - LDAP Simple Paged Result Control
• RFC 2798 - inetOrgPerson LDAP Object Class (Updated by RFC 3698, RFC 4519 & RFC 4524)
• RFC 2830 - LDAPv3: Extension for Transport Layer Security
• RFC 2849 - The LDAP Data Interchange Format (LDIF)
• RFC 2891 - Server Side Sorting of Search Results
• RFC 3045 - Storing Vendor Information in the LDAP root DSE
• RFC 3062 - LDAP Password Modify Extended Operation
• RFC 3296 - Named Subordinate References in LDAP Directories
• RFC 3671 - Collective Attributes in LDAP
• RFC 3672 - Subentries in LDAP
• RFC 3673 - LDAPv3: All Operational Attributes
• RFC 3687 - LDAP Component Matching Rules
• RFC 3698 - LDAP: Additional Matching Rules
• RFC 3829 - LDAP Authorization Identity Controls
• RFC 3866 - Language Tags and Ranges in LDAP
• RFC 3909 - LDAP Cancel Operation
• RFC 3928 - LDAP Client Update Protocol
• RFC 4370 - LDAP Proxied Authorization Control
• RFC 4373 - LBURP
• RFC 4403 - LDAP Schema for UDDI
• RFC 4522 - LDAP: Binary Encoding Option
• RFC 4523 - LDAP: X.509 Certificate Schema
• RFC 4524 - LDAP: COSINE Schema (replaces RFC 1274)
• RFC 4525 - LDAP: Modify-Increment Extension
• RFC 4526 - LDAP: Absolute True and False Filters
• RFC 4527 - LDAP: Read Entry Controls
• RFC 4528 - LDAP: Assertion Control
• RFC 4529 - LDAP: Requesting Attributes by Object Class
• RFC 4530 - LDAP: entryUUID
• RFC 4531 - LDAP Turn Operation
• RFC 4532 - LDAP Who am I? Operation
• RFC 4533 - LDAP Content Sync Operation
• RFC 4876 - Configuration Profile Schema for LDAP-Based Agents
• RFC 5020 - LDAP entryDN Operational Attribute

LDAPv2 was specified in the following RFCs:

• RFC 1777 - Lightweight Directory Access Protocol (replaced RFC 1487)
• RFC 1778 - The String Representation of Standard Attribute Syntaxes (replaced RFC 1488)
• RFC 1779 - A String Representation of Distinguished Names (replaced RFC 1485)

LDAPv2 was moved to historic status by the following RFC:

• RFC 3494 - Lightweight Directory Access Protocol version 2 (LDAPv2) to Historic Status