There is one and only one PTR record for a given IP

Not on this planet.

use strict;
use Socket;

my $addr = shift || '172.17.0.1';
my ($name,$aliases) = gethostbyaddr(inet_aton($addr), AF_INET);

print "address $addr has the following names:\n";
print "\t$_\n" for ($name, split / /, $aliases);

[download]

When run against my DNS server, produces:

address 172.17.0.1 has the following names:
        host1.example.com
        host2.example.com
        host3.example.com
        host4.example.com
        host5.example.com

[download]

In other words, a PTR lookup can return a result set of more than one record. Many buggy applications make the invalid assumption that no more than one result will be returned. But of course, no Perlmonk ever would :)

gethostbyaddr is a bit naughty by referring to one as the name, and the others, aliases. According the the RFC (if I remember correctly, it's been a while), they are all equally "at the same level". It's up to the application to sift through them and find the one that suits its needs.

• another intruder with the mooring in the heart of the Perl

In reply to Re^2: Retrieving all hostnames for an IP address... by grinder
in thread Retrieving all hostnames for an IP address... by RMaxwell

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