DNS forwarding is inherent in the NIS name service. The proper format for the hosts line in a NIS-primary switch file to enable DNS forwarding is:
hosts: nis [NOTFOUND=return] files
If an NIS client is using the DNS forwarding capability of a NIS-compatible NIS+ server, its nsswitch.conf file should not have hosts: nis dns files as the syntax for the hosts file. This is because DNS forwarding automatically forwards host requests to DNS and this syntax would cause the NIS+ server to forward unsuccessful requests to the DNS servers twice, which would reduce performance. To take best advantage of DNS forwarding, use the default syntax for the nsswitch.nis file.