If the /var/yp/securenets file is blank or does not exist (as is the case after a default installation), NIS listens to all networks. One of the first things to do is to put netmask/network pairs in the file so that ypserv only responds to requests from the appropriate network.

Below is a sample entry from a /var/yp/securenets file:

255.255.255.0     192.168.0.0

This technique does not provide protection from an IP spoofing attack, but it does at least place limits on what networks the NIS server services.