#!/usr/bin/env perl
#
# ipnum2name - translate IP addresses in the input to DNS names
#
# $Id$

# when a lookup fails, returns the IP address itself
# caches all results

use warnings;
use strict;

use Socket qw( getaddrinfo getnameinfo );

my %ipnum2name;

$| = 1;    # force output line flushing

my $ip4_rx = qr/([0-9]{1,3}(?:\.[0-9]{1,3}){3})/i;
my $ip6_rx = qr/([0-9a-f]*:[0-9a-f:]+)/i;
# not entirely accurate for IPv6, see https://stackoverflow.com/questions/53497/regular-expression-that-matches-valid-ipv6-addresses

sub ipnum2name
# thank you, https://www.perlmonks.org/?node_id=938198
{
  my ($num) = @_;

  my ( $err, @addrs ) = getaddrinfo( $num, 0 );

  if ($err)
  {
    $num;
  }
  elsif ( my ( $err, $hostname ) = getnameinfo( $addrs[0]->{addr} ) )
  {
    $err ? $num : $hostname;
  }
}

sub lookup
{
  my ($num) = @_;

  if (!defined $ipnum2name{$num})
  {
    $ipnum2name{$_} = ipnum2name($_)
  }

  $ipnum2name{$_}
}

while (<>)
{
  chomp;
  my @line = split( /$ip4_rx/, $_ );
  $_ = join('', map { /$ip4_rx/ ? lookup($_) : $_ } @line);
  @line = split( /$ip6_rx/, $_ );
  print join('', map { /$ip6_rx/ ? lookup($_) : $_ } @line), "\n";
}