Nagios Support Forum

only_from determines the remote hosts to which the particular
service is available. Its value is a list of IP
addresses which can be specified in any combination of
the following ways:

a) a numeric address in the form of %d.%d.%d.%d. If
the rightmost components are 0, they are treated
as wildcards (for example, 128.138.12.0 matches
all hosts on the 128.138.12 subnet). 0.0.0.0
matches all Internet addresses. IPv6 hosts may
be specified in the form of abcd:ef01:6789.
The rightmost rule for IPv4 addresses does not
apply to IPv6 addresses.

b) a factorized address in the form of
%d.%d.%d.{%d,%d,...}. There is no need for all 4
components (i.e. %d.%d.{%d,%d,...%d} is also ok).
However, the factorized part must be at the end
of the address. This form does not work for IPv6
hosts.

c) a network name (from /etc/networks). This form
does not work for IPv6 hosts.

d) a host name. When a connection is made to
xinetd, a reverse lookup is performed, and the
canonical name returned is compared to the speci‐
fied host name. You may also use domain names in
the form of .domain.com. If the reverse lookup
of the client’s IP is within .domain.com, a match
occurs.

e) an ip address/netmask range in the form of
1.2.3.4/32. IPv6 address/netmask ranges in the
form of 1234::/46 are also valid.

Specifying this attribute without a value makes the
service available to nobody.