The Name Service Switch is a facility in Unix-like

Unix-like

A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....

 operating system

Operating system

An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...

s that provides a variety of sources for common configuration databases and name resolution mechanisms. Such source databases include local operating system files, for example, /etc/passwd, /etc/group, and /etc/hosts, the Domain Name System

Domain name system

The Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities...

 (DNS), the Network Information Service

Network Information Service

The Network Information Service, or NIS is a client–server directory service protocol for distributing system configuration data such as user and host names between computers on a computer network...

 (NIS), LDAP, and others.

A system administrator

System administrator

A system administrator, IT systems administrator, systems administrator, or sysadmin is a person employed to maintain and operate a computer system and/or network...

 usually configures the operating system's name services using the file /etc/nsswitch.conf. This lists databases (such as passwd

Passwd (database)

passwd is a name service database used to store user account information on Unix-like operating systems.The sources for the passwd database are configured, like other name service databases, in nsswitch.conf.-Fetching a specific user entry:For a specific user called 'joe':getent passwd joeFor a...

, shadow and group

Group (database)

group is a name service database used to store group information on Unix-like operating systems.The sources for the group database are configured, like other name service databases, in nsswitch.conf....

) and one or more sources for obtaining that information. Examples for sources are files for local files, ldap for the Lightweight Directory Access Protocol

Lightweight Directory Access Protocol

The Lightweight Directory Access Protocol is an application protocol for accessing and maintaining distributed directory information services over an Internet Protocol network...

, nis for the Network Information Service

Network Information Service

The Network Information Service, or NIS is a client–server directory service protocol for distributing system configuration data such as user and host names between computers on a computer network...

, nisplus for NIS+

Nisplus

NIS+ is a directory service developed by Sun Microsystems to replace its older 'NIS' . It is designed to eliminate the need for duplication across many computers of configuration data such as user accounts, host names and addresses, printer information and NFS disk mounts on individual systems,...

, wins for Windows Internet Name Service).

The nsswitch.conf file has line entries for each service consisting of a database name in the first field, terminated by a colon, and a list of possible source databases mechanisms in the second field.
An example file is the following:

passwd:     files ldap

shadow:     files

group:      files ldap



hosts:      dns nis files



ethers:     files nis

netmasks:   files nis

networks:   files nis

protocols:  files nis

rpc:        files nis

services:   files nis



automount:  files

aliases:    files

The order of the services listed determines in which order NSS will attempt to use those services to resolve queries on the specified database.

Implementation

The service switch facility is implemented within the C library, so that calls to functions such as getent

Getent

getent is a unix command that helps a user get entries in a number of important text files called databases. This includes the passwd and group databases which store user information – hence getent is a common way to look up user details on Unix...

 resolve to the appropriate NSS module. This assures that existing applications that use the NSS routines do not require any changes to operate with NSS.

History

Earlier Unix-like systems, with the notable exception of Ultrix

Ultrix

Ultrix was the brand name of Digital Equipment Corporation's native Unix systems. While ultrix is the Latin word for avenger, the name was chosen solely for its sound.-History:...

, either accessed only the configuration files or had hard-coded rules for files or databases to access; Ultrix offered nearly identical functionality to the Name Service Switch, using the configuration file /etc/svc.conf.

Sun Microsystems

Sun Microsystems

Sun Microsystems, Inc. was a company that sold :computers, computer components, :computer software, and :information technology services. Sun was founded on February 24, 1982...

 first developed NSS for their Solaris operating system, but subsequently programmers ported it to many other operating systems including FreeBSD

FreeBSD

FreeBSD is a free Unix-like operating system descended from AT&T UNIX via BSD UNIX. Although for legal reasons FreeBSD cannot be called “UNIX”, as the direct descendant of BSD UNIX , FreeBSD’s internals and system APIs are UNIX-compliant...

, NetBSD

NetBSD

NetBSD is a freely available open source version of the Berkeley Software Distribution Unix operating system. It was the second open source BSD descendant to be formally released, after 386BSD, and continues to be actively developed. The NetBSD project is primarily focused on high quality design,...

, GNU/Linux, HP-UX

HP-UX

HP-UX is Hewlett-Packard's proprietary implementation of the Unix operating system, based on UNIX System V and first released in 1984...

, IRIX

IRIX

IRIX is a computer operating system developed by Silicon Graphics, Inc. to run natively on their 32- and 64-bit MIPS architecture workstations and servers. It was based on UNIX System V with BSD extensions. IRIX was the first operating system to include the XFS file system.The last major version...

 and AIX

AIX operating system

AIX AIX AIX (Advanced Interactive eXecutive, pronounced "a i ex" is a series of proprietary Unix operating systems developed and sold by IBM for several of its computer platforms...

.

See also

• Passwd (database)
  Passwd (database)
  passwd is a name service database used to store user account information on Unix-like operating systems.The sources for the passwd database are configured, like other name service databases, in nsswitch.conf.-Fetching a specific user entry:For a specific user called 'joe':getent passwd joeFor a...
  
  
• Shadow (database)
• Group (database)
  Group (database)
  group is a name service database used to store group information on Unix-like operating systems.The sources for the group database are configured, like other name service databases, in nsswitch.conf....
  
  
• Pluggable Authentication Modules
  Pluggable Authentication Modules
  Pluggable authentication modules are a mechanism to integrate multiple low-level authentication schemes into a high-level application programming interface . It allows programs that rely on authentication to be written independent of the underlying authentication scheme...
  
  
• BSD Authentication
  BSD Authentication
  BSD Authentication, otherwise known as BSD Auth, is an authentication framework and software API employed by some Unix-like operating systems, specifically OpenBSD and BSD/OS, and accompanying system and application software such as OpenSSH and Apache...
  
  
• Name server
  Name server
  In computing, a name server is a program or computer server that implements a name-service protocol. It maps a human-recognizable identifier to a system-internal, often numeric, identification or addressing component....
  
  

External links

• Name Service Switch implementation in the GNU C Library
• NSS module supporting LDAP: nss_ldap
• Another NSS module supporting LDAP: nss-ldapd
• NSS module supporting AFS: nss_afs