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In computer networking, the Lightweight Directory Access Protocol, or LDAP ("ell-dap"), is a networking protocol for querying and modifying directory services running over TCP/IP. A directory is a set of information with similar attributes organised in a logical and hierarchical manner. The most common example is the telephone directory, which consists of a series of names (either of a person or organisation) organised alphabetically, with an address and phone number attached. Origin and influences Unsurprisingly, telecommunication companies introduced the concept of directories to information technology (computer networking). No doubt their understanding of directory requirements were heavily influenced by 70 odd years of producing and managing telephone directories. The culmination of this input was the X.500 specification, produced by the International Telecommunication Union (ITU) at a time when information technology (computers) were an organised and well-considered venture (aka pre-public-access Internet). By most accounts the X.500 protocol was an extremely well engineered and thoroughly thought out specification. However, it suffered from being so thorough that even in the late '90s there were very few implementations of it and those that existed required significant computing power. Unfortunately the computing resources required to implement and deploy X.500 at the time were prohibitively expensive, and so LDAP, the "Lightweight" Directory Access Protocol was developed. LDAP reduced its "weight" by only implementing a subset of the X.500 protocol and was originally designed as a gateway to X.500 servers rather than a complete standalone service. An LDAP directory 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 which represents a given tree entry (or multiple entries). Its current version is LDAPv3. LDAPv3 is specified in a series of IETF Standard Track RFCs as detailed in RFC 4510. LDAP was originally intended to be a lightweight alternative protocol for accessing X.500 directory services. X.500 directory services were traditionally accessed via the X.500 Directory Access Protocol, or DAP, which required the cumbersome Open Systems Interconnection (OSI) protocol stack. With LDAP, a client could access these directory services without all of the overhead. This model of directory access was borrowed from DIXIE and the Directory Assistance Service. Standalone LDAP directory servers soon followed, as did directory servers supporting both DAP and LDAP. The former has become popular in enterprises as they 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 and Wengyik Yeong of Performance Systems International, circa 1993. Further development has been done via the Internet Engineering Task Force (IETF). Note that in the early engineering stages of LDAP, it was known as Lightweight Directory Browsing Protocol or LDBP. It was renamed as the scope of the protocol was expanded to not only include directory browsing functions (i.e., search) but also directory update functions (i.e., modify). LDAP has influenced subsequent Internet protocols, including later versions of X.500, XML Enabled Directory (XED), Directory Services 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, by default on TCP port 389. The client then sends operation requests to the server, and the server sends responses in turn. With some exceptions the client need not wait for a response before sending the next request, and the server may send the responses in any order. The basic operations are, in order: 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 standard port for LDAP over SSL is 636. 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. Directory structure The protocol accesses LDAP directories, which follow 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 a full filename and the RDN as a relative filename in a folder. 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 may be provided in the set of the entry's operational attributes. An entry can look like this when represented in LDIF format (LDAP itself is a binary protocol): dn: cn=John Doe,dc=example,dc=com cn: John Doe givenName: John sn: Doe telephoneNumber: +1 555 6789 telephoneNumber: +1 555 1234 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, and "dc=example,dc=com" is the DN of the parent entry. The other lines show the attributes in the entry. Attribute names are typically mnemonic strings, like "cn" for common name, "dc" for domain component, and "mail" for e-mail address. 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=Some 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 in an attribute, the attributes in an entry, and the entries found by a search operation in any order. Operations 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 indicating 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. Stub for 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 authenticates the client to the server. Simple Bind sends the user's DN and password - in cleartext, 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. StartTLS The StartTLS operation establishes Transport Layer Security (the descendant of SSL) on the connection. That can provide data confidentiality (cannot be read by third parties) and/or data integrity protection (protect 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 to have this identity used in determining the identity used in making LDAP authorization decisions. Servers also often support the non-standard "LDAPS" ("Secure LDAP", commonly known as "LDAP over SSL") protocol on a separate port, by default 636. The LDAPS differs from LDAP in two ways: 1) upon connect, the client and server establish TLS before any LDAP messages are transferred (without a Start TLS operation) and 2) the LDAPS connection must be closed upon TLS closure. Search and Compare The Search operation is used to both search for and read entries. Its parameters are: The server returns the matching entries and maybe continuation references (in any order), followed by the final result with 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 operations 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 which support this, however. Extended operations The Extended Operation is a generic LDAP operation which can be used to define new operations. Examples include the Cancel, Password Modify and Start TLS operations. Abandon The Abandon operation requests that the server aborts 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: It is not the opposite of the Bind operation. Clients can abort a session by simply closing the connection, but they should use Unbind. Otherwise the server cannot tell the difference between a failed network connection (or a truncation attack) and a discourteous client. 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" where most components after "ldap://" can be omitted. attributes is a comma-separated list of attributes to retrieve. scope can be "base" (the default), "one" or "sub". filter e.g, (objectClass= As in other URLs, special characters must be escaped with %hex format. There is a similar non-standard "ldaps:" URL scheme for LDAP over SSL. 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. "ldap:///dc=example,dc=com??sub?(givenName=John)" searches for him in the default server. Schema The contents of the entries in a subtree is governed by a schema. The schema defines the attribute types that directory entries can contain. An attribute definition includes a syntax, and most non-binary values in LDAPv3 use UTF-8 string syntax. For example, a "mail" attribute might contain the value "user@example.com". A "jpegPhoto" attribute would contain photograph(s) in binary JPEG/JFIF format. A "member" attribute contains the DNs of other directory entries. Attribute definitions also specify whether the attribute is single-valued or multi-valued, how to search/compare the attribute (e.g. case-sensitive vs. case-insensitive and whether substring matching is supported), etc. The schema defines object classes. Each entry must have an objectClass attribute, containing named classes defined in the schema. Typically, the objectClass attribute is multivalued, and contains the class "top" as well as some number of other classes. The schema definition of the class of an entry defines what kind of object the entry may represent - e.g. a person, organization or domain. Practically this means that membership in a particular class gives the entry the option of containing one set of attributes (optional attributes), and the obligation of containing another set (mandatory or required attributes). 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 belong to multiple classes, each entry has a complex of optional and mandatory attribute sets formed from the union of the object classes it represents. The schema also includes various other information controlling directory entries. Most schema elements have a name and a globally unique Object identifier (OID). Many directory servers publish the directory schema itself as a collection of LDAP entries rooted at the base DN cn=schema. Server administrators can define their own schemas in addition to the standard ones. 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. 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. Terminology The LDAP terminology one can encounter is quite a mess. 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. The "uid" attribute should hold user names rather than numeric user IDs. Supporting vendors LDAP has gained wide support from vendors such as: Also the Apache HTTP Server used as a proxy (by the module mod_proxy) supports LDAP. LDAP implementations See also Articles LDAP fora RFCs LDAP is currently 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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