History
Telecommunication companies' understanding of directory requirements were well developed after some 70 years of producing and managing telephone directories. These companies introduced the concept of directory services toProtocol overview
A client starts an LDAP session by connecting to an LDAP server, called a Directory System Agent (DSA), by default on TCP and UDPDirectory structure
The protocol provides an interface with directories that follow the 1993 edition of the X.500 model: * 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).
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), a plain text format (as opposed a 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: [email protected] 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 is 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 that 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
Add
The ADD operation inserts a new entry into the directory-server database. If the distinguished name in the add request already exists in the directory, then the server will not add a duplicate entry but will set the result code in the add result to decimal 68, "entryAlreadyExists". * LDAP-compliant servers will never dereference the distinguished name transmitted in the add request when attempting to locate the entry, that is, distinguished names are never de-aliased. * LDAP-compliant servers will ensure that the distinguished name and all attributes conform to naming standards. * The entry to be added must not exist, and the immediate superior must exist.dn: uid=user,ou=people,dc=example,dc=com changetype: add objectClass:top objectClass:person uid: user sn: last-name cn: common-name userPassword: passwordIn the above example,
uid=user,ou=people,dc=example,dc=com
must not exist, and ou=people,dc=example,dc=com
must exist.
Bind (authenticate)
When an LDAP session is created, that is, when an LDAP client connects to the server, the authentication state of the session is set to anonymous. The BIND operation establishes the authentication state for a session. Simple BIND and SASL PLAIN can send the user's DN and password inuserPassword
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 Delete
To delete an entry, an LDAP client transmits a properly formed delete request to the server. * A delete request must contain the distinguished name of the entry to be deleted * Request controls may also be attached to the delete request * Servers do not dereference aliases when processing a delete request * Only leaf entries (entries with no subordinates) may be deleted by a delete request. Some servers support an operational attributehasSubordinates
whose value indicates whether an entry has any subordinate entries, and some servers support an operational attribute numSubordinates
indicating the number of entries subordinate to the entry containing the numSubordinates
attribute.
* Some servers support the subtree delete request control permitting deletion of the DN and all objects subordinate to the DN, subject to access controls. Delete requests are subject to access controls, that is, whether a connection with a given authentication state will be permitted to delete a given entry is governed by server-specific access control mechanisms.
Search and compare
The Search operation is used to both search for and read entries. Its parameters are: ; baseObject : The name of the base object entry (or possibly the root) relative to which the search is to be performed. ; scope : What elements below the baseObject to search. This can beBaseObject
(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
) where the matching rules for givenName
and mail
determine whether the values for those attributes match the filter assertion. Note that a common misconception is that LDAP data is case-sensitive, whereas in fact matching rules and ordering rules determine matching, comparisons, and relative value relationships. If the example filters were required to match the case of the attribute value, an ''extensible match filter'' must be used, for example, (&(objectClass=person)(, (givenName:caseExactMatch:=John)(mail:caseExactSubstringsMatch:=john*)))
; derefAliases : Whether and how to follow alias entries (entries that 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.
Modify
The MODIFY operation is used by LDAP clients to request that the LDAP server make changes to existing entries. Attempts to modify entries that do not exist will fail. MODIFY requests are subject to access controls as implemented by the server. The MODIFY operation requires that the distinguished name (DN) of the entry be specified, and a sequence of changes. Each change in the sequence must be one of: * add (add a new value, which must not already exist in the attribute) * delete (delete an existing value) * replace (replace an existing value with a new value) LDIF example of adding a value to an attribute:dn: dc=example,dc=com changetype: modify add: cn cn: the-new-cn-value-to-be-added -To replace the value of an existing attribute, use the
replace
keyword. If the attribute is multi-valued, the client must specify the value of the attribute to update.
To delete an attribute from an entry, use the keyword delete
and the changetype designator modify
. If the attribute is multi-valued, the client must specify the value of the attribute to delete.
There is also a Modify-Increment extension which allows an incrementable attribute value to be incremented by a specified amount. The following example using LDIF increments employeeNumber
by 5
:
dn: uid=user.0,ou=people,dc=example,dc=com changetype: modify increment: employeeNumber employeeNumber: 5 -When LDAP servers are in a replicated topology, LDAP clients should consider using the post-read control to verify updates instead of a search after an update. The post-read control is designed so that applications need not issue a search request after an update – it is bad form to retrieve an entry for the sole purpose of checking that an update worked because of the replication
Modify DN
Modify DN (move/rename entry) takes the new RDN (Relative Distinguished Name), optionally the new parent's DN, and a flag that indicates whether to delete the value(s) in the entry that 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 that support this, though.Extended operations
The Extended Operation is a generic LDAP operation that can define new operations that were not part of the original protocol specification. StartTLS is one of the most significant extensions. Other examples include 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. Some "LDAPS" client libraries only encrypt communication; they do not check the host name against the name in the supplied certificate.Abandon
The Abandon operation requests that the server abort an operation named by a message ID. The server need not honor the request. Neither Abandon nor a successfully abandoned operation send a response. A similar Cancel extended operation 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. Clients can abort a session by simply closing the connection, but they should use Unbind. 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.URI scheme
An LDAP uniform resource identifier (URI) scheme exists, which clients support in varying degrees, and servers return in referrals and continuation references (see RFC 4516): ldap://host:port/DN?attributes?scope?filter?extensions Most of the components described below are optional. * ''host'' is the FQDN or IP address of the LDAP server to search. * ''port'' is the(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
URI 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 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 anVariations
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 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 and/or authorization (what actions a given already-authenticated user can do on what service). For example in Active Directory Kerberos is used in the authentication step, while LDAP is used in the authorization step. An example of such data model is the GLUE Schema,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 distinguished name (DN), 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 record (SRV). An organization with the domain example.org may use the top level LDAP DNdc=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 008and LDAPv3. These 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 l=Locality, ou=Some Organizational Unit, o=Some Organization, c=FR
, or in the US: cn=Common Name, l=Locality, ou=Some Organizational Unit, o=Some Organization, st=CA, c=US
.
See also
*References
Sources
* ITU-T Rec. X.680, "Abstract Syntax Notation One (ASN.1) - Specification of Basic Notation", 1994 *Further reading
* * * * * *External links
*List of public LDAP Servers (2013): {{DEFAULTSORT:Ldap Application layer protocols Directory services Internet protocols Internet Standards Open Group standards