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在本科做毕设的时候就接触到TCP代理这东西，当时需要使用代理来对发送和收到的数据做修改，同时使用代理也让我对HTTP协议有了更深的了解。
下面讲一下用python做一个tcp代理的基本原理

TCP Proxy用到的一个主要的东西就是socket。proxy通过socket分别对localhost和remotehost做连接，然后可以对通过proxy的流量和数据进行分析。

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__author__ = 'seven'
import sys
import socket
import threading
def hexdump(src, length=16):
    result = []
    digits = 4 if isinstance(src, unicode) else 2
    for i in xrange(0, len(src), length):
        s = src[i:i + length]
        hexa = b' '.join(["%0*X" % (digits, ord(x)) for x in s])
        text = b''.join([x if 0x20 <= ord(x) < 0x7F else b'.' for x in s])
        result.append(b"%04X  %-*s  %s" % (i, length * (digits + 1), hexa, text))
    print b'\n'.join(result)
def receive_from(connection):
    buffer = ""
    # We set a 2 second time out depending on your target this may need to be adjusted
    connection.settimeout(2)
    try:
        while True:
            data = connection.recv(4096)
            if not data:
                break
            buffer += data
    except:
        pass
    return buffer
def request_handler(buffer):
    # perform packet mofifications
    return buffer
def response_handler(buffer):
    # perform pakect modifications
    return buffer
def proxy_handler(client_socket, remote_host, remote_port, receive_first):
    remote_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    remote_socket.connect((remote_host, remote_port))
    if receive_first:
        remote_buffer = receive_from(remote_socket)
        hexdump(remote_buffer)
        remote_buffer = response_handler(remote_buffer)
        if len(remote_buffer):
            print "[<==] Sending %d bytes to localhost." % len(remote_buffer)
            client_socket.send(remote_buffer)
    while True:
        local_buffer = receive_from(client_socket)
        if len(local_buffer):
            print "[==>] Received %d bytes from localhost." % len(local_buffer)
            hexdump(local_buffer)
            local_buffer = request_handler(local_buffer)
            remote_socket.send(local_buffer)
            print "[==>] Sent to remote."
        remote_buffer = receive_from(remote_socket)
        if len(remote_buffer):
            print "[<==] Received %d bytes from remote." % len(remote_buffer)
            hexdump(remote_buffer)
            remote_buffer = response_handler(remote_buffer)
            client_socket.send(remote_buffer)
            print "[<==] Sent to localhost."
        if not len(local_buffer) or not len(remote_buffer):
            client_socket.close()
            remote_socket.close()
            print "[*] No more data. Closing connections."
            break
def server_loop(local_host, local_port, remote_host, remote_port, receive_first):
    server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    try:
        server.bind((local_host, local_port))
    except:
        print "[!!] Failed to listen on %s:%d" % (local_host, local_port)
        print "[!!] Check for other listening sockets or correct permissions."
        sys.exit(0)
    print "[*] Listening on %s:%d" % (local_host, local_port)
    server.listen(5)
    while True:
        client_socket, addr = server.accept()
        print "[==>] Received incoming connection from %s:%d" % (addr[0], addr[1])
        proxy_thread = threading.Thread(target=proxy_handler,
                                        args=(client_socket, remote_host, remote_port, receive_first))
        proxy_thread.start()
def main():
    if len(sys.argv[1:]) != 5:
        print "Usage: ./tcp proxy.py [localhost] [localport] [remotehost] [remoteport] [receive_first]"
        print "Example: ./tcp proxy.py 127.0.0.1 9000 10.12.132.1 9000 True"
        sys.exit(0)
    local_host = sys.argv[1]
    local_port = int(sys.argv[2])
    remote_host = sys.argv[3]
    remote_port = int(sys.argv[4])
    receive_first = sys.argv[5]
    if "True" in receive_first:
        receive_first = True
    else:
        receive_first = False
    server_loop(local_host, local_port, remote_host, remote_port, receive_first)
main()

在request_handler和response_handler函数中就可以对收到的数据进行修改。

高大上的hexdump函数来自http://code.activestate.com/recipes/142812-hex-dumper/，表示不怎么看得懂..如果有看得懂的朋友欢迎和我交流

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