Hesiod (name service)

[1] It uses DNS functionality to provide access to databases of information that change infrequently.

Frequently an LDAP server is used to distribute the same kind of information that Hesiod does.

However, because Hesiod can leverage existing DNS servers, deploying it to a network is fairly easy.

The Domain Name System has a special class of service for Hesiod's purpose.

Then What happens here is that the foo and the passwd are combined with the lhs and rhs values in the /etc/hesiod.conf file to create a fully qualified name of foo.passwd.ns.example.net.