I wrote this a long while back just to learn the basic structs of programming with tcp/ip. Yes its very simple but this base struct has provided me with the core of alot of really cool things. (Thanks to tcp/ip Illustrated vol. 2 of course :). All this code does is construct the headers, and send a packet from a specifed source host and source port, to a specified destination host and port. Doesnt even rand the inicial sequence number...heheh. But hopefully this will be of use to someone other to myself! Just an FYI, if this looks all wierd, uhh, Im having a biotch of a time pasting this code...

#!/usr/local/bin/perl
# Dont make fun of my path

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);
}
[download]

-cleen

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