`man xinetd.conf' says nearly `only_from' (paragraph d):

d) a  host name.  When a connection is made to xinetd, a reverse
    lookup is performed, and the canonical name returned is  com-
    pared  to  the  specified 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.

When running an own named (nameserver is 127.0.0.1) use of e.g

| only_from = host.local.zone

results in a parsing error during startup. 

This happens because xinetd starts at position 50, but named laterly at pos
55. So DNS lookups done by xinetd will fail.