#!/usr/bin/perl
# By Michael Kennedy, michael@toronto.com
# Use or modify as you wish, but no warranty.
while (<>) {
  if (/(\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1,3})/) {
    if ($1 <= 255 && $2 <= 255 && $3 <= 255 && $4 <= 255) {
      $txtip = "$1.$2.$3.$4";
      $ip = pack 'C4',$1,$2,$3,$4;
      $hostname = (gethostbyaddr($ip,2));
      $_ =~ s/$txtip/$hostname/ if ($hostname ne "");
    }
  }
  print;
}