This file specifies the PTR record for the local loopback interface, better known as localhost, whose network address is 127.0.0.1. The location of this file is specified in the boot file. It is vitally important to the proper operation of every name server that the 127.0.0.1 address have a PTR record pointing back to the name ``localhost.''. The name of this PTR record is always ``18.104.22.168.IN-ADDR.ARPA''. This is necessary if you want your users to be able to use hostname-authentication (hosts.equiv or ~/.rhosts) on the name ``localhost''. As implied by this PTR record, there should be a ``localhost.my.dom.ain'' A record (with address 127.0.0.1) in every domain that contains hosts. ``localhost.'' will lose its trailing dot when 22.214.171.124.in-addr.arpa is queried for; then, the DEFNAMES and/or DNSRCH resolver options will cause ``localhost'' to be evaluated as a host name in the local domain, and that means the top domains (or ideally, every domain) in your resolver's search path had better have something by that name.