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。
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)
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):