无法使用boost asio设置TCP源端口

Question

通常,作为客户端从哪个源端口发送数据并不重要,但我仍然想要进行一些测试.所以我试图将我的客户端的套接字绑定到一个特定的端口,但即使我在我的本地机器上运行客户端和服务器(使用localhost作为目标地址),服务器告诉我我的源端口是59000.我初始化我的套接字像这样:

tcp::socket socket(io_service,tcp::endpoint(tcp::v4(),2000));

有可能做我想做的事吗？我试图找出我的路由器在消息通过时是否更改了端口.这就是我正在玩的NAT遍历的东西.

Answer 1

将套接字绑定到特定端口可以完全按照问题中的说明进行:

using boost::asio::ip::tcp;
boost::asio::io_service io_service;
tcp::socket socket(io_service, tcp::endpoint(tcp::v4(), 2000));
assert(socket.local_endpoint().port() == 2000); // true

在这种情况下,socket将构造对象,打开并绑定到本地端点,地址为INADDR_ANY和port 2000.

由于建立连接的方式,本地端点可能会发生变化.当从socket.connect()或socket.async_connect()成员函数启动连接操作时,套接字将尝试连接到远程端点,必要时打开套接字.因此,在已打开的套接字上调用时,套接字的本地端点不会更改.

另一方面,当从任一函数connect()或async_connect()自由函数启动连接操作时,套接字在尝试连接到任何端点之前都会关闭.因此,套接字将绑定到未指定的端口.自由函数文档中的参数部分定义了此行为:

的socket被连接.如果socket已经打开,它将被关闭.

此外,没有干净的方法来控制此行为,因为在实现中一个接一个地调用socket.close()和socket.connect()成员函数.

以下是演示上述行为的完整示例:

#include <iostream>
#include <boost/asio.hpp>
#include <boost/bind.hpp>
#include <boost/lexical_cast.hpp>

// This example is not interested in the handlers, so provide a noop function
// that will be passed to bind to meet the handler concept requirements.
void noop() {}

// Helper function used to initialize the client socket.
void force_endpoint(boost::asio::ip::tcp::socket& client_socket)
{
  using boost::asio::ip::tcp;
  client_socket.close();
  client_socket.open(tcp::v4());
  client_socket.bind(tcp::endpoint(tcp::v4(), 2000));
  std::cout << "client socket: " << client_socket.local_endpoint()
            << std::endl;
}

int main()
{
  using boost::asio::ip::tcp;
  boost::asio::io_service io_service;

  // Create all I/O objects.
  tcp::acceptor acceptor(io_service, tcp::endpoint(tcp::v4(), 0));
  tcp::socket server_socket(io_service);
  tcp::socket client_socket(io_service);

  // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  // Initiate the connect operation directly on the socket.
  server_socket.close();
  force_endpoint(client_socket);
  acceptor.async_accept(server_socket, boost::bind(&noop));
  client_socket.async_connect(acceptor.local_endpoint(), boost::bind(&noop));

  // Print endpoints before and after running the operations.
  io_service.run();
  std::cout << "After socket.async_connect(): "
            << client_socket.local_endpoint() << std::endl;

  // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  // Initiate the connection operation with the free async_connect function.
  server_socket.close();
  force_endpoint(client_socket);
  acceptor.async_accept(server_socket, boost::bind(&noop));
  boost::asio::async_connect(
      client_socket,
      tcp::resolver(io_service).resolve(
        tcp::resolver::query("127.0.0.1", 
          boost::lexical_cast<std::string>(acceptor.local_endpoint().port()))),
      boost::bind(&noop));

  // Run the service, causing the client to connect to the acceptor.
  io_service.reset();
  io_service.run();
  std::cout << "After async_connect(): " 
            << client_socket.local_endpoint() << std::endl;
}

其中产生了以下输出:

client socket: 0.0.0.0:2000
After socket.async_connect(): 127.0.0.1:2000
client socket: 0.0.0.0:2000
After async_connect(): 127.0.0.1:53115