#!/usr/bin/perl

  use Socket;

  $src_host = $ARGV[0]; # The source IP/Hostname
  $src_port = $ARGV[1]; # The Source Port
  $dst_host = $ARGV[2]; # The Destination IP/Hostname
  $dst_port = $ARGV[3]; # The Destination Port.

   if(!defined $src_host or !defined $src_port or !defined $dst_host or !defined $dst_port) {
     print "Usage: $0 <source host> <source port> <dest host> <dest port>\n";
     exit;
   } 
   else {
    main();
   }
   
  sub main {
   my $src_host = (gethostbyname($src_host))[4];
   my $dst_host = (gethostbyname($dst_host))[4];

   socket(RAW, AF_INET, SOCK_RAW, 255) || die $!;
   setsockopt(RAW, 0, 1, 1);
   
   my ($packet) = makeheaders($src_host, $src_port, $dst_host, $dst_port);
   my ($destination) = pack('Sna4x8', AF_INET, $dst_port, $dst_host);
   send(RAW,$packet,0,$destination);
  }

  sub makeheaders {
   local($src_host,$src_port,$dst_host,$dst_port) = @_;
   my $zero_cksum = 0;
   # Lets construct the TCP half
   my $tcp_proto          = 6;
   my ($tcp_len)          = 20;
   my $syn                = 13456;
   my $ack                = 0;
   my $tcp_headerlen      = "5";
   my $tcp_reserved       = 0;
   my $tcp_head_reserved  = $tcp_headerlen .
                            $tcp_reserved;
   my $tcp_urg            = 0; # Flag bits
   my $tcp_ack            = 0; # eh no
   my $tcp_psh            = 0; # eh no
   my $tcp_rst            = 0; # eh no
   my $tcp_syn            = 1; # yeah lets make a connexion! :)
   my $tcp_fin            = 0;
   my $null               = 0;
   my $tcp_win            = 124;
   my $tcp_urg_ptr        = 0;
   my $tcp_all            = $null . $null .
                            $tcp_urg . $tcp_ack .
                            $tcp_psh . $tcp_rst .
                            $tcp_syn . $tcp_fin ;

   # In order to calculate the TCP checksum we have
   # to create a fake tcp header, hence why we did
   # all this stuff :) Stevens called it psuedo headers :)

   my ($tcp_pseudo) = pack('a4a4CCnnnNNH2B8nvn',
    $tcp_len,$src_port,$dst_port,$syn,$ack,
    $tcp_head_reserved,$tcp_all,$tcp_win,$null,$tcp_urg_ptr);

   my ($tcp_checksum) = &checksum($tcp_pseudo);

   # Now lets construct the IP packet
   my $ip_ver             = 4;
   my $ip_len             = 5;
   my $ip_ver_len         = $ip_ver . $ip_len;
   my $ip_tos             = 00;
   my ($ip_tot_len)       = $tcp_len + 20;
   my $ip_frag_id         = 19245;
   my $ip_frag_flag       = "010";
   my $ip_frag_oset       = "0000000000000";
   my $ip_fl_fr           = $ip_frag_flag . $ip_frag_oset;
   my $ip_ttl             = 30;

   # Lets pack this baby and ship it on out!
   my ($pkt) = pack('H2H2nnB16C2na4a4nnNNH2B8nvn',
    $ip_ver_len,$ip_tos,$ip_tot_len,$ip_frag_id,
    $ip_fl_fr,$ip_ttl,$tcp_proto,$zero_cksum,$src_host,
    $dst_host,$src_port,$dst_port,$syn,$ack,$tcp_head_reserved,
    $tcp_all,$tcp_win,$tcp_checksum,$tcp_urg_ptr);

   return $pkt;
  }

  sub checksum {
   # This of course is a blatent rip from _the_ GOD,
   # W. Richard Stevens.
    
   my ($msg) = @_;
   my ($len_msg,$num_short,$short,$chk);
   $len_msg = length($msg);
   $num_short = $len_msg / 2;
   $chk = 0;
   foreach $short (unpack("S$num_short", $msg)) {
    $chk += $short;
   }
   $chk += unpack("C", substr($msg, $len_msg - 1, 1)) if $len_msg % 2;
   $chk = ($chk >> 16) + ($chk & 0xffff);
   return(~(($chk >> 16) + $chk) & 0xffff);
  }