Formerly known as Yellow Pages (yp), Network Information Service (NIS) provides a simple network look-up service that consists of databases and processes. An NIS domain consists of a client and one or more servers. Clients use the NIS protocol to look up information stored in NIS databases, which are replicated among servers. A single master server is used to update databases; subordinate (slave) servers provide read-only services. Databases are synchronized by copying them from master servers to subordinate servers periodically or upon change.
A database served by NIS is called an NIS map. The NIS lookup calls require a map (database) name and an NIS domain name. An NIS domain consists of a collection of such maps. In addition to map databases, the master NIS server maintains the timestamp that indicates when each database was last modified.
Each database is created from a source file. For example, passwd.byname and passwd.byuid databases are created from the source file named passwd. When source files change, administrators either execute a shell script called ypmake or a makefile to update the databases. In addition, the makefiles used to create the databases push the databases to subordinate NIS servers using yppush. The transfer uses ypxfr, a program that runs on the subordinate servers.
Clients can use many functions and remote procedure calls to connect to the network lookup service, including yp_match, yp_first, yp_next, yp_all, yp_order, yp_master, yperr_string, and ypprot_err. In addition there are usually many tools for administering NIS, such as ypwhich, yppoll, ypset, ypcat, and domainname.