TCP/IP的三次握手

TCP是面向连接的，所谓面向连接，就是当计算机双方通信时必需先建立连接，然后数据传送，最后拆除连接三个过程

并且TCP在建立连接时又分三步走：

第一步是请求端（客户端）发送一个包含SYN即同步（Synchronize）标志的TCP报文，SYN同步报文会指明客户端使用的端口以及TCP连接的初始序号；

第二步，服务器在收到客户端的SYN报文后，将返回一个SYN+ACK的报文，表示客户端的请求被接受，同时TCP序号被加一，ACK即确认（Acknowledgement）。

第三步，客户端也返回一个确认报文ACK给服务器端，同样TCP序列号被加一，到此一个TCP连接完成。 然后才开始通信的第二步：数据处理。

这就是所说的TCP三次握手（Three-way Handshake）。

简单的说就是：（C：客户端，S：服务端）

C：SYN到S

S：如成功--返回给C（SYN+ACK）

C：如成功---返回给S（ACK）

以上是正常的建立连接方式，但如下：

假设一个C向S发送了SYN后无故消失了，那么S在发出SYN+ACK应答报文后是无法收到C的ACK报文的（第三次握手无法完成），这种情况下S一般会重试（再次发送SYN+ACK给客户端）并等待一段时间后丢弃这个未完成的连接，这段时间的长度我们称为SYN Timeout，一般来说这个时间是分钟的数量级（大约为30秒-2分钟）；一个C出现异常导致S的一个线程等待1分钟并不是什么很大的问题，但如果有一个恶意的攻击者大量模拟这种情况，S将为了维护一个非常大的半连接列表而消耗非常多的资源----数以万计的半连接，即使是简单的保存并遍历也会消耗非常多的CPU时间和内存，何况还要不断对这个列表中的IP进行SYN+ACK的重试。实际上如果S的TCP/IP栈不够强大，最后的结果往往是堆栈溢出崩溃---即使S的系统足够强大，S也将忙于处理攻击者伪造的TCP连接请求而无暇理睬客户的正常请求（毕竟C的正常请求比率非常之小），此时从正常客户的角度看来，S失去响应，这种情况我们称作：服务器端受到了SYN Flood攻击（SYN洪水攻击）。

以上的例子常被称作DoS（拒绝服务攻击）与DDoS（分布式拒绝服务攻击）

注意：其中这儿的C和S都是相对的，对于现在的计算机来讲，只要自己的计算机建立任一服务，在一定情况下都可被称为S

解决办法：在你的计算机上安装防火墙

个人防火墙一般我们可采用学习模式来让防火墙自己来进行分析

有一点得说明，防火墙是无杀毒功能的，同时也无法清除垃圾邮件，简单点讲它只起到一个过滤作用

======================

在TCP三次握手后插入伪造的TCP包

在TCP三次握手后插入伪造的TCP包
一、说明

用Socket的API Connect完成TCP建立连接的三次握手，同时子进程抓包，抓完三次握手的包后，插入第四个包即可，从对端返回的第五个包来看插入成功了，但因为插入了一个TCP包，之后的连接将发生混乱。可以将插入的那个包Data设置为HTTP Request，向WEB服务器提交请求。又如果目标系统的TCP序列号是可预计算的，那么是否可以做带伪源地址的Blind TCP three-time handshakes和插入，值得试验！

二、脚本

1、用到几个模块Net::RawIP Net::Pcap Net::PcapUtils NetPacket；
2、pretty_table()函数是我原来做的，用来在命令行下打印表格（Table）；
3、测试环境-Linux、ADSL拨号，抓包的接口是ppp0，帧的结构和Eth帧结构不同，不能使用NetPacket::Ethernet模块中的strip函数处理帧首部，根据ethereal抓包的结构，我使用unpack函数取得了帧中的IP包；

三、源代码

#!/usr/bin/perl
#By i_am_jojo@msn.com, 2005/04
use strict;
use warnings;

use Net::RawIP;
use Net::PcapUtils;
use NetPacket::Ethernet;
use NetPacket::IP;
use NetPacket::TCP;

use Socket;
use Getopt::Std;
use POSIX qw(strftime);

my %opts;
getopts('ht:p:u:n:', /%opts);

print_help() and exit if(defined($opts{'h'}));
print_help() and exit if(not defined($opts{'t'}) or not defined($opts{'p'}));

die "/tInvalid Target Ipaddress!/n"
if(defined($opts{'t'}) and $opts{'t'} !~ m/^/d+./d+./d+./d+$/);

die "/tInvalid Service Port!/n"
if(defined($opts{'p'}) and $opts{'p'} !~ m/^/d+$/);

my $request;
if(defined($opts{'u'})) {
$request = "GET $opts{'u'} HTTP/1.1/r/n";
$request.= "Accept: text/html; text/plain/r/n";
$request.= "/r/n";
} else {
$request = "GET / HTTP/1.1/r/n";
$request.= "Accept: text/html; text/plain/r/n";
$request.= "/r/n";
}

my $child = fork();

if($child == 0) {
#child process
my ($next_packet, %next_header);
my ($frame_hdr, $ip_packet);
my ($ip_obj, $tcp_obj);
my $counter = 0;

my $pkt_descriptor = Net::PcapUtils::open(
FILTER => 'ip',
PROMISC => 0,
DEV => 'ppp0',
#DEV => 'eth0'
);

die "Net::PcapUtils::open returned: $pkt_descriptor/n" if (!ref($pkt_descriptor));
print strftime '%Y/%m/%d %H:%M:%S, ', localtime and print "begin sniffing .../n";

while(($next_packet, %next_header) = Net::PcapUtils::next($pkt_descriptor)) {
($frame_hdr, $ip_packet) = unpack 'H32a*', $next_packet;
$ip_obj = NetPacket::IP->decode($ip_packet);
#$ip_obj = NetPacket::IP->decode(NetPacket::Ethernet::eth_strip($next_packet));

next if ($ip_obj->{'proto'} != 6);
next if (($ip_obj->{'src_ip'} ne $opts{'t'})
and ($ip_obj->{'dest_ip'} ne $opts{'t'}));

$tcp_obj = NetPacket::TCP->decode($ip_obj->{'data'});
next if (($tcp_obj->{'src_port'} ne $opts{'p'})
and ($tcp_obj->{'dest_port'} ne $opts{'p'}));

$counter++;

print "==ID.$counter==", '=' x 60, "/n";
print get_ip_hdr($ip_obj);
print get_tcp_hdr($tcp_obj);
if($tcp_obj->{'data'}) {
my $data;
$data = unpack 'a*', $tcp_obj->{'data'};
$data =~ s/[/r][/n]//g;
print pretty_table('TCP data', [$data]);
}

if($counter == 3) {
my $a = new Net::RawIP;
$a->set({
'ip' => {
'id' => $ip_obj->{'id'} + 1,
'saddr' => $ip_obj->{'src_ip'},
'daddr' => $ip_obj->{'dest_ip'}
},
'tcp' => {
'source' => $tcp_obj->{'src_port'},
'dest' => $tcp_obj->{'dest_port'},
'seq' => $tcp_obj->{'seqnum'},
'ack_seq' => $tcp_obj->{'acknum'},
'window' => $tcp_obj->{'winsize'},
'data' => $request,
'psh' => 1,
'ack' => 1
}
});
$a->send;
}
last if($counter == 5);
}
exit;
} else {
sleep(1);
my $trans_serv = getprotobyname('tcp');
my $dest_sockaddr = sockaddr_in($opts{'p'}, inet_aton($opts{'t'}));

socket(TCP_SOCK, PF_INET, SOCK_STREAM, $trans_serv);
connect(TCP_SOCK, $dest_sockaddr);
sleep(1);
#close TCP_SOCK;
}

exit;

sub print_help {
print <<HELP

%./iamFool.pl [-h] <-t,-p,-u,-n>
-h print help
-t target ipaddr
-p service port
-u requested url

by:i_am_jojo/@msn.com

HELP
}

sub get_ip_hdr {
my $ip_obj = shift;
my @ip_hdr;

push @ip_hdr, [qw(ver tos flags id src_ip proto)];
push @{$ip_hdr[1]}, $ip_obj->{$_} foreach (qw(ver tos flags id src_ip proto));
push @ip_hdr, [qw(hlen len foffset ttl dest_ip cksum)];
push @{$ip_hdr[3]}, $ip_obj->{$_} foreach (qw(hlen len foffset ttl dest_ip cksum));

return pretty_table('IP Header', @ip_hdr);
}

sub get_tcp_hdr {
my $tcp_obj = shift;
my @tcp_hdr;

push @tcp_hdr, [qw(src_port seqnum hlen flags)];
push @{$tcp_hdr[1]}, $tcp_obj->{$_} foreach (qw(src_port seqnum hlen flags));
push @tcp_hdr, [qw(dest_port acknum reserved winsize)];
push @{$tcp_hdr[3]}, $tcp_obj->{$_} foreach (qw(dest_port acknum reserved winsize));

return pretty_table('TCP Header', @tcp_hdr);
}

sub pretty_table {
# prettyTable($aString, @aList); @aList = ( [...], [...] );
# by i_am_jojo@msn.com
my ($title, @data) = @_;
my @temp;
my @max_length;
my $row_length;
my $indent = 4;
my $the_table;

foreach my $col (0..$#{$data[0]}) { push @{$temp[$col]}, $_->[$col] foreach (@data); }
$max_length[$_] = length( (sort{length($b) <=> length($a)} @{$data[$_]} )[0]) + 2 foreach (0..$#data);
$row_length+= $max_length[$_] foreach (0..$#{$temp[0]});
$row_length+= $#data;

$the_table = ' ' x $indent.'+'.'-' x $row_length."+/n";
$the_table.= ' ' x $indent.'| '.$title.' ' x ($row_length - length($title) - 1)."|/n";
foreach my $row (0..$#temp) {
$the_table.= ' ' x $indent;
$the_table.= '+'.'-' x $max_length[$_] foreach (0.. $#{$temp[0]});
$the_table.= "+/n";
$the_table.= ' ' x $indent;
$the_table.= '| '.@{$temp[$row]}[$_].' ' x ($max_length[$_] - length(@{$temp[$row]}[$_]) - 1) foreach (0.. $#{$temp[0]});
$the_table.= "|/n";
}
$the_table.= ' ' x $indent;
$the_table.= '+'.'-' x $max_length[$_] foreach (0.. $#{$temp[0]});
$the_table.= "+/n";

return $the_table;
}

四、结果举例

==Result eXample==

2005/05/02 21:51:23, begin sniffing ...
==ID.1==============================================================
+---------------------------------------------------+
| IP Header |
+--------+---------------+---------+----------------+
| ver | 4 | hlen | 5 |
+--------+---------------+---------+----------------+
| tos | 0 | len | 60 |
+--------+---------------+---------+----------------+
| flags | 2 | foffset | 0 |
+--------+---------------+---------+----------------+
| id | 20682 | ttl | 64 |
+--------+---------------+---------+----------------+
| src_ip | 218.11.149.14 | dest_ip | 64.233.189.104 |
+--------+---------------+---------+----------------+
| proto | 6 | cksum | 31878 |
+--------+---------------+---------+----------------+
+------------------------------------------+
| TCP Header |
+----------+------------+-----------+------+
| src_port | 32851 | dest_port | 80 |
+----------+------------+-----------+------+
| seqnum | 1104143983 | acknum | 0 |
+----------+------------+-----------+------+
| hlen | 10 | reserved | 0 |
+----------+------------+-----------+------+
| flags | 2 | winsize | 5808 |
+----------+------------+-----------+------+
==ID.2==============================================================
+---------------------------------------------------+
| IP Header |
+--------+----------------+---------+---------------+
| ver | 4 | hlen | 5 |
+--------+----------------+---------+---------------+
| tos | 0 | len | 44 |
+--------+----------------+---------+---------------+
| flags | 0 | foffset | 0 |
+--------+----------------+---------+---------------+
| id | 63029 | ttl | 241 |
+--------+----------------+---------+---------------+
| src_ip | 64.233.189.104 | dest_ip | 218.11.149.14 |
+--------+----------------+---------+---------------+
| proto | 6 | cksum | 26154 |
+--------+----------------+---------+---------------+
+------------------------------------------------+
| TCP Header |
+----------+------------+-----------+------------+
| src_port | 80 | dest_port | 32851 |
+----------+------------+-----------+------------+
| seqnum | 3660731207 | acknum | 1104143984 |
+----------+------------+-----------+------------+
| hlen | 6 | reserved | 0 |
+----------+------------+-----------+------------+
| flags | 18 | winsize | 4356 |
+----------+------------+-----------+------------+
==ID.3==============================================================
+---------------------------------------------------+
| IP Header |
+--------+---------------+---------+----------------+
| ver | 4 | hlen | 5 |
+--------+---------------+---------+----------------+
| tos | 0 | len | 40 |
+--------+---------------+---------+----------------+
| flags | 2 | foffset | 0 |
+--------+---------------+---------+----------------+
| id | 20684 | ttl | 64 |
+--------+---------------+---------+----------------+
| src_ip | 218.11.149.14 | dest_ip | 64.233.189.104 |
+--------+---------------+---------+----------------+
| proto | 6 | cksum | 31896 |
+--------+---------------+---------+----------------+
+------------------------------------------------+
| TCP Header |
+----------+------------+-----------+------------+
| src_port | 32851 | dest_port | 80 |
+----------+------------+-----------+------------+
| seqnum | 1104143984 | acknum | 3660731208 |
+----------+------------+-----------+------------+
| hlen | 5 | reserved | 0 |
+----------+------------+-----------+------------+
| flags | 16 | winsize | 5808 |
+----------+------------+-----------+------------+
==ID.4==============================================================
+---------------------------------------------------+
| IP Header |
+--------+---------------+---------+----------------+
| ver | 4 | hlen | 5 |
+--------+---------------+---------+----------------+
| tos | 16 | len | 89 |
+--------+---------------+---------+----------------+
| flags | 2 | foffset | 0 |
+--------+---------------+---------+----------------+
| id | 20685 | ttl | 64 |
+--------+---------------+---------+----------------+
| src_ip | 218.11.149.14 | dest_ip | 64.233.189.104 |
+--------+---------------+---------+----------------+
| proto | 6 | cksum | 31830 |
+--------+---------------+---------+----------------+
+------------------------------------------------+
| TCP Header |
+----------+------------+-----------+------------+
| src_port | 32851 | dest_port | 80 |
+----------+------------+-----------+------------+
| seqnum | 1104143984 | acknum | 3660731208 |
+----------+------------+-----------+------------+
| hlen | 5 | reserved | 0 |
+----------+------------+-----------+------------+
| flags | 24 | winsize | 5808 |
+----------+------------+-----------+------------+
+--------------------------------------------+
| TCP data |
+--------------------------------------------+
| GET / HTTP/1.1Accept: text/html; text/plai |
+--------------------------------------------+
==ID.5==============================================================
+---------------------------------------------------+
| IP Header |
+--------+----------------+---------+---------------+
| ver | 4 | hlen | 5 |
+--------+----------------+---------+---------------+
| tos | 0 | len | 40 |
+--------+----------------+---------+---------------+
| flags | 0 | foffset | 0 |
+--------+----------------+---------+---------------+
| id | 47931 | ttl | 241 |
+--------+----------------+---------+---------------+
| src_ip | 64.233.189.104 | dest_ip | 218.11.149.14 |
+--------+----------------+---------+---------------+
| proto | 6 | cksum | 41256 |
+--------+----------------+---------+---------------+
+------------------------------------------------+
| TCP Header |
+----------+------------+-----------+------------+
| src_port | 80 | dest_port | 32851 |
+----------+------------+-----------+------------+
| seqnum | 3660731208 | acknum | 1104144033 |
+----------+------------+-----------+------------+
| hlen | 5 | reserved | 0 |
+----------+------------+-----------+------------+
| flags | 16 | winsize | 4356 |
+----------+------------+-----------+------------+
===End===