In computing, directory service or name service maps the names of network resources to their respective network addresses. It is a shared information infrastructure for locating, managing, administering and organizing everyday items and network resources, which can include volumes, folders, files, printers, users, groups, devices, telephone numbers and other objects. A directory service is a critical component of a network operating system. A directory server is a server which provides such a service. Each resource on the network is considered an object by the directory server. Information about a particular resource is stored as a collection of attributes associated with that resource or object.
Contents
- Comparison with relational databases
- Replication and distribution
- Implementations
- LDAP implementations
- Unix OSs
- References
A directory service defines a namespace for the network. The namespace is used to assign a "name" (unique identifier) to each of the objects. Directories typically have a set of rules determining how network resources are named and identified, which usually includes a requirement that the identifiers be unique and unambiguous. When using a directory service, a user does not have to remember the physical address of a network resource; providing a name locates the resource. Some directory services include access control provisions, limiting the availability of directory information to authorized users.
Comparison with relational databases
Several things distinguish a directory service from a relational database. Data can be redundant if it aids performance.
Directory schemas are object classes, attributes, name bindings and knowledge (namespaces) where an object class has:
Attributes are sometimes multi-valued, allowing multiple naming attributes at one level (such as machine type and serial number concatenation, or multiple phone numbers for "work phone"). Attributes and object classes are standardized throughout the industry, and formally registered with the IANA for their object ID. Therefore, directory applications try to reuse standard classes and attributes to maximize the benefit of existing directory-server software.
Object instances are slotted into namespaces; each object class inherits from its parent object class (and ultimately from the root of the hierarchy), adding attributes to the must-may list. Directory services are often central to the security design of an IT system and have a correspondingly-fine granularity of access control.
Replication and distribution
Replication and distribution have distinct meanings in the design and management of a directory service. Replication is used to indicate that the same directory namespace (the same objects) are copied to another directory server for redundancy and throughput reasons; the replicated namespace is governed by the same authority. Distribution is used to indicate that multiple directory servers in different namespaces are interconnected to form a distributed directory service; each namespace can be governed by a different authority.
Implementations
Directory services were part of an Open Systems Interconnection (OSI) initiative for common network standards and multi-vendor interoperability. During the 1980s, the ITU and ISO created a set of standards for directory services, initially to support the requirements of inter-carrier electronic messaging and network-name lookup. The Lightweight Directory Access Protocol (LDAP) is based on the X.500 directory-information services, using the TCP/IP stack and an X.500 Directory Access Protocol (DAP) string-encoding scheme on the Internet.
Systems developed before the X.500 include:
LDAP implementations
LDAP/X.500-based implementations include:
Open-source tools to create directory services include OpenLDAP, the Kerberos protocol and Samba software, which can function as a Windows domain controller with Kerberos and LDAP back ends. Administration is by GOsa or Samba SWAT.
Unix OSs
Name services on Unix systems are typically configured through nsswitch.conf. Information from name services can be retrieved with getent.