Normaly, ypserv will read the file /var/yp/securenets. The file will
look like:

# allow connections from local host -- necessary
255.255.255.255 127.0.0.1     
# allow connections from any host
# on the 131.234.223.0 network
255.255.255.0   131.234.223.0 
# allow connections from any host
# between 131.234.214.0 to 131.234.215.255
255.255.254.0   131.234.214.0 


An  expression  of  the  form  `m.m.m.m n.n.n.n' is interpreted as a 
`netmask network' pair. A host address is matched if `network' is equal 
to the  bitwise AND of the address and the `netmask'. If you doesen't 
have such a file or the file is empty, and you don't enable the tcp 
wrappers, all hosts in the world could connect to your sever. This
file uses the same notation as the SunOS/Solaris ypserv. FreeBSD ypserv 
use a different notation, be carefull with copying this file.

If you enable the tcp wrappers security lookups in the Makefile, then the
server `ypserv' will use your /etc/hosts.allow and /etc/hosts.deny files
(which most people already have) and not the /var/yp/securenets. You must 
have the library libwrap.a and log_tcp.h from the tcp_wrappers source 
distribution. I used version 7.4 of the wrappers.

You can enable the tcp wrappers with the configure option: 
configure --enable-tcp-wrapper[=BASE-DIR],
where BASE-DIR is an optional parameter and includes the path to the
library and the includefiles.

Read the tcp_wrappers manpages on how to enable/disable hosts from querying
your server.

One note of warning. Always use IP numbers instead of hostnames, because
otherwise gethostbyname() will then call the server `ypserv', and the server
is_host_valid() function will call gethostbyname() which will then call the
server `ypserv' ad infinitum.... The tcp_wrappers let you easily specify
a domain in number notation for all hostnames contained in the last octet.
So for example in a class C network 192.16.192.0, to allow all local hosts
to query your server, but deny everyone else you can specify the following
lines:

In /etc/hosts.allow

	ypserv: 192.16.192.

and the /etc/hosts.deny file

	ypserv: ALL