时间戳

首页 > 解决方案 > C++ UDP server send message just when get client's address config

c++ - C++ UDP server send message just when get client's address config

问题描述

My purpose is to create a UDP sender to broadcast message to a specific IP and Port, without any config requirment. I defined struct sockaddr_in si_me variable to set the server config on it and bind it:

si_me.sin_family = AF_INET;
si_me.sin_port = htons(PORT);
si_me.sin_addr.s_addr = htonl(INADDR_ANY);
if( bind(s , (struct sockaddr*)&si_me, sizeof(si_me) ) == -1)
    {
        die("bind");
    }

I expected when I send data to configured IP and Port in si_me variable, every client could receive the message but it doesn't happen and using both UDP client and nc 127.0.0.1 9090 -u command on Linux terminal, I couldn't receive any message! I found that for each client I should create a struct sockaddr_in si_other variable. The client should send me a message so I can get client's config:

if ((recv_len = recvfrom(s, buf, BUFLEN, 0, (struct sockaddr *) &si_other, (socklen_t*)&slen)) == -1)
        {
            die("recvfrom()");
        }

So now using si_other variable I can send messages successfully. I checked the si_other variable and I found the IP is the same with server IP but the Port is different for each client so because of this I couldn't send data to the client without knowing what is the destination Port!

Here is the complete source code:

int main(void)
{
    struct sockaddr_in si_me, si_other;

    int s, i, slen = sizeof(si_other) , recv_len;
    char* msg = "Hellooooo\n";
    char buf[BUFLEN] ;

    //create a UDP socket
    if ((s=socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1)
    {
        die("socket");
    }

    // zero out the structure
    memset((char *) &si_me, 0, sizeof(si_me));

    si_me.sin_family = AF_INET;
    si_me.sin_port = htons(PORT);
    si_me.sin_addr.s_addr = htonl(INADDR_ANY);

    std::cout << si_me.sin_addr.s_addr << std::endl ;
    std::cout << si_me.sin_port << std::endl ;
    std::cout << si_other.sin_addr.s_addr << std::endl ;
    std::cout << si_other.sin_port << std::endl ;
    if( bind(s , (struct sockaddr*)&si_me, sizeof(si_me) ) == -1)
        {
             die("bind");
        }

    // waiting to recieve client's message
    std::cout << "Waiting for start command ... " << std::endl ;
    if ((recv_len = recvfrom(s, buf, BUFLEN, 0, (struct sockaddr *) &si_other, (socklen_t*)&slen)) == -1)
        {
            die("recvfrom()");
        }
    std::cout << si_me.sin_addr.s_addr << std::endl ;
    std::cout << si_me.sin_port << std::endl ;
    std::cout << si_other.sin_addr.s_addr << std::endl ;
    std::cout << si_other.sin_port << std::endl ;
    long count = 0 ;
    while(0)
    {

            if (sendto(s, msg, strlen(msg), 0, (struct sockaddr*) &si_other, slen) == -1)
            {
                die("sendto()");
            }
            std::cout << "Message number " << ++ count << " has been sent" << std::endl ;

    }

    return 0;
}

So can anyone tell me how should I change the source to broadcast messages without the need of getting client message to configure its destination IP and Port?

标签： c++socketsudp

解决方案

为了发送广播消息，您必须调用setsockopt级别SOL_SOCKET和选项名称SO_BROADCAST（参见socket(7)手册页）。所以，在创建套接字之后，（在客户端），你应该有这样的代码：

int turn_on = 1;
retval = setsockopt(s, SOL_SOCKET, SO_BROADCAST, &turn_on, (socklen_t)sizeof(turn_on));

然后，发送者可以在其发送中指定一个广播地址，并且广播域内的任何站点都应该能够接收它。任何服务器都需要绑定到广播地址本身，或者INADDR_ANY为了接收这样的广播。

这通常的工作方式是：

  • 服务器创建套接字，并将其绑定到众所周知的端口INADDR_ANY
  • 客户端创建套接字，不绑定它，但添加SO_BROADCAST选项。
  • 客户端调用sendto缓冲区并sockaddr_in指定广播地址和众所周知的端口。
  • 服务器调用recvfrom和接收数据报有效载荷和返回值中的客户端地址
  • 服务器使用上面获得的客户端地址进行调用sendto（因此客户端使用的端口无关紧要——服务器将知道客户端的端口和 IP 地址）。
  • 客户端recvfrom在同一个套接字上调用以接收来自服务器的回复消息

通常有两个可用的广播地址：一个特定于子网（由 显示ifconfig）——例如 192.168.1.255——和全局广播地址——255.255.255.255。

监听INADDR_ANY应该允许您接收发送到广播地址（或属于服务器机器的单播地址）的数据报。

推荐阅读