Nsswitch.conf

The Name Service Switch (NSS) is a facility in Unix-like operating systems that provides a variety of sources for common configuration databases and name resolution mechanisms. These sources include local operating system files (such as /etc/passwd, /etc/group, and /etc/hosts), the Domain Name System (DNS), the Network Information Service (NIS), and LDAP.

A system administrator usually configures the operating system's name services using the file /etc/nsswitch.conf. This lists databases (such as passwd, shadow and group) and one or more sources for obtaining that information. Examples for sources are files for local files, ldap for the , nis for the Network Information Service, nisplus for NIS+, and 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. A typical file might look like:

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

Earlier Unix-like systems, with the notable exception of Ultrix, 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 first developed NSS for their Solaris operating system, but subsequently programmers ported it to many other operating systems including FreeBSD, NetBSD, Linux, HP-UX, IRIX and AIX.

...
Wikipedia