NIS stands for Network Information Service. It is an RPC service, called ypserv, which is used in conjunction with portmap and other related services to distribute maps of usernames, passwords, and other sensitive information to any computer claiming to be within its domain.
An NIS server is comprised of several applications. They include the following:
/usr/sbin/rpc.yppasswdd — Also called the yppasswdd service, this daemon allows users to change their NIS passwords.
/usr/sbin/rpc.ypxfrd — Also called the ypxfrd service, this daemon is responsible for NIS map transfers over the network.
/usr/sbin/yppush — This application propagates changed NIS databases to multiple NIS servers.
/usr/sbin/ypserv — This is the NIS server daemon.
NIS is rather insecure by todays standards. It has no host authentication mechanisms and passes all of its information over the network unencrypted, including password hashes. As a result, extreme care must be taken to set up a network that uses NIS. Further complicating the situation, the default configuration of NIS is inherently insecure.
It is recommended that anyone planning to implement an NIS server first secure the portmap service as outlined in Section 5.2, “Securing Portmap”, then address the following issues, such as network planning.