1 端口映射

举个例子来说明一下端口映射的作用。

有A、B、C三台计算机,A、B互通,B、C互通,但是A、C不通,这个时候在C上开了一个Web服务,如何让A访问C的Web服务?

最简单有效的办法就是在B上开一个端口映射服务,然后让A访问B的某个端口,B将这个端口上的所有流量全部转发到C的Web服务端口上,同时将C上Web服务返回的流量也全部转发给A。这样对A来说,以B为跳板,实现了间接访问C上Web服务的目的。

2 实现流程

端口映射的原理并不复杂,本文以TCP为例介绍一下实现过程,简单画了个时序图(如下),这里就不再用文字赘述了。

TCP Mapping Runtime

需要注意的是,由于端口映射只是单纯的流量转发,对应用层数据不进行处理,所以对于多通道协议是无法支持的(如FTP协议)。

3 代码示例

按照上面的流程,Python实现如下(建议从后向前看):

# -*- coding: utf-8 -*-
# tcp mapping created by hutaow(hutaow.com) at 2014-08-31

import socket
import threading

# 端口映射配置信息
CFG_REMOTE_IP = '192.168.0.10'
CFG_REMOTE_PORT = 22
CFG_LOCAL_IP = '0.0.0.0'
CFG_LOCAL_PORT = 10022

# 接收数据缓存大小
PKT_BUFF_SIZE = 2048

# 调试日志封装
def send_log(content):
    print content
    return

# 单向流数据传递
def tcp_mapping_worker(conn_receiver, conn_sender):
    while True:
        try:
            data = conn_receiver.recv(PKT_BUFF_SIZE)
        except Exception:
            send_log('Event: Connection closed.')
            break

        if not data:
            send_log('Info: No more data is received.')
            break

        try:
            conn_sender.sendall(data)
        except Exception:
            send_log('Error: Failed sending data.')
            break

        # send_log('Info: Mapping data > %s ' % repr(data))
        send_log('Info: Mapping > %s -> %s > %d bytes.' % (conn_receiver.getpeername(), conn_sender.getpeername(), len(data)))

    conn_receiver.close()
    conn_sender.close()

    return

# 端口映射请求处理
def tcp_mapping_request(local_conn, remote_ip, remote_port):
    remote_conn = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

    try:
        remote_conn.connect((remote_ip, remote_port))
    except Exception:
        local_conn.close()
        send_log('Error: Unable to connect to the remote server.')
        return

    threading.Thread(target=tcp_mapping_worker, args=(local_conn, remote_conn)).start()
    threading.Thread(target=tcp_mapping_worker, args=(remote_conn, local_conn)).start()

    return

# 端口映射函数
def tcp_mapping(remote_ip, remote_port, local_ip, local_port):
    local_server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    local_server.bind((local_ip, local_port))
    local_server.listen(5)

    send_log('Event: Starting mapping service on ' + local_ip + ':' + str(local_port) + ' ...')

    while True:
        try:
            (local_conn, local_addr) = local_server.accept()
        except KeyboardInterrupt, Exception:
            local_server.close()
            send_log('Event: Stop mapping service.')
            break

        threading.Thread(target=tcp_mapping_request, args=(local_conn, remote_ip, remote_port)).start()

        send_log('Event: Receive mapping request from %s:%d.' % local_addr)

    return

# 主函数
if __name__ == '__main__':
    tcp_mapping(CFG_REMOTE_IP, CFG_REMOTE_PORT, CFG_LOCAL_IP, CFG_LOCAL_PORT)

源文件下载:链接

4 运行

运行效果如下,192.168.0.20通过连接映射服务器的10022端口,成功访问192.168.0.10的SSH服务(22端口):

TCP Mapping Runtime


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