#!/usr/bin/perl

use Net::DNS; 

open(OUT, '>>dns.txt');

if($#ARGV < 1) {
   print "Usage: rdns.pl <dns_servers> <ip_range | cidr_block>\n";
   exit;
}

@DNSServers = split(/,/, $ARGV[0]);
if($ARGV[1] =~ /\//) {
   ($begin, $end) = iprange_cidr($ARGV[1]);
}
else {
   ($begin, $end) = iprange($ARGV[1]);
}

# instantiate the DNS resolver

$resolver = new Net::DNS::Resolver;
$resolver->nameservers(@DNSServers);

for($i = $begin; $i <= $end; $i++) {
   $ip_string = long2ip($i);
   print $ip_string, " ";
   $answer = $resolver->query($ip_string);
   if(defined($answer)) {
      $name = ($answer->answer)[0];
      ($x1, $x2, $x3, $x4, $sym_name) = split(/\t/, $name->string);
		print "$sym_name\n";
		printf(OUT "%s\n", $sys_name);
   }
   else {
      print OUT "unknown\n";
		print "unknown\n";
   }
}

sub ip2long
{
   local($ip) = @_;
   local(@octets, $octet, $long);

   @octets = split(/\./, $ip);
   $long = 0;
   foreach $octet (@octets) {
      $long <<= 8;
      $long |= $octet;
   }
   $long;
}

sub long2ip
{
   local($long) = @_;
   local($i, @octets, $retval);

   for($i = 3; $i >= 0; $i--) {
      $octets[$i] = ($long & 0xFF);
      $long >>= 8;
   }
   $retval = join('.', @octets);
}

sub iprange_cidr
{
   local($masked_ip) = @_;
   local($begin_ip, $end_ip, $begin_mask, $end_mask, $ip, $mask, $mask_bits);

   ($ip, $mask) = split(/\//, $masked_ip);
   $mask_bits = 32 - $mask;
   $end_mask = 0xFFFFFFFF;
   $begin_mask = $end_mask << $mask_bits;
   $end_mask = ~$begin_mask;
   $begin_ip = $end_ip = ip2long($ip);
   $begin_ip &= $begin_mask;
   $end_ip |= $end_mask;
   ($begin_ip, $end_ip);
}

sub iprange
{
   local($range) = @_;
   local($start, $end, $begin_ip, $end_ip);

   ($start, $end) = split(/-/, $range);
   $begin_ip = ip2long($start);
   $end_ip = ip2long($end);
   ($begin_ip, $end_ip);
}