use Net::DNS;

my %name_cache = ();

sub go_names {
    my $host      = shift;
    my $already   = shift;

    return @{ $name_cache{$host} } if defined $name_cache{$host};

    my $resolver  = new Net::DNS::Resolver;
    my $query     = $resolver->search($host) or return (undef, undef);
    my @answer    = $query->answer or return (undef, undef);
    my $rr        = shift @answer;
    my $is_ip     = $rr->type eq "A";
    my $is_ptr    = $rr->type eq "PTR";

    return (undef, undef) if not ($is_ip or $is_ptr);

    my @ret = (($is_ip) ? ($rr->address, $host) : ($host, $rr->ptrdnam
+e));

    return &go_names($ret[0], 1) if $ret[1] !~ /[.]/ and not $already;
+  # canonize iff needed and not already recursing.

    $name_cache{$host} = [ @ret ];

    return @ret;
}