帮助发送/接收 UDP 数据包 - C 套接字

Question

好的，如果你看看我之前的一些问题，我一直在努力建立一个简单的连接并使用 C 套接字运行（我对程序的整个网络方面仍然相当陌生，但每个人都必须从某个地方开始， 正确的？）。 我已经包含了到目前为止的代码，当我执行它时，我没有收到任何错误，但与此同时，我没有在另一端收到数据包。 顺便说一句，我正在用 Objective-C 编程多播套接字，“msgStatus”只是我 GUI 中的一个标签（它已正确连接，因此没有问题）。 我只是不明白我哪里错了。 有人可以帮助我或为我指明正确的方向吗？ 谢谢！

#define MAX_LEN  1024    /* maximum string size to send */
#define MIN_PORT 1024    /* minimum port allowed */
#define MAX_PORT 65535   /* maximum port allowed */
#define MYPORT 5673      /* port we will be using for our multicast socket */

    -(void)broadcastMessage {//(NSString*)msg {
        NSLog(@"broadcastMessage - Stage 1");
        NSString *msg = @"From Master";
        mc_ttl = 3; // number of node hops the message is allowed to travel across the network

        // define the port we will be using
        mc_port = MYPORT;

        /* create a socket for sending to the multicast address  */
    if ((sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0) {
        NSLog(@"ERROR: broadcastMessage - socket() failed");
        return;
    }

    mc_addr.sin_family      = AF_INET;
    mc_addr.sin_addr.s_addr = inet_addr("225.0.0.37");
    mc_addr.sin_port        = htons(mc_port);

    if (bind(sock, (struct sockaddr *) &mc_addr, sizeof(struct sockaddr_in)) < 0) {
        NSLog(@"ERROR: bind not successful");
        return;
    }

    NSLog(@"broadcastMessage - Stage 2");
    /* set the TTL (time to live/hop count) for the send */
    if ((setsockopt(sock, IPPROTO_IP, IP_MULTICAST_TTL, (void*) &mc_ttl, sizeof(mc_ttl))) < 0) {
        NSLog(@"ERROR: broadcastMessage - setsockopt() failed");
        return;
    }

    NSLog(@"broadcastMessage - Stage 3");
    /* construct a multicast address structure - erase everything in the structure first*/
    memset(&mc_addr, 0, sizeof(mc_addr));

    // prepare the message to be sent
    char send_str[MAX_LEN];

    /* clear send buffer */
    memset(send_str, 0, sizeof(send_str));

    // convert the message to a C string to send
    [msg getCString:send_str maxLength:MAX_LEN encoding:NSASCIIStringEncoding];

    //while (fgets(send_str, MAX_LEN, stdin)) {
        NSLog(@"broadcastMessage - Stage 4");
    //  send_len = strlen(send_str);

    /* send string to multicast address */
    if ((sendto(sock, send_str, sizeof(send_str), 0, (struct sockaddr *) &mc_addr, sizeof(mc_addr))) != sizeof(send_str)) {
        NSLog(@"ERROR: broadcastMessage - sendto() sent incorrect number of bytes");
        return;
    }

    NSLog(@"broadcastMessage - Stage 5");


    /* clear send buffer */
    memset(send_str, 0, sizeof(send_str));

    NSLog(@"broadcastMessage - Stage 6");
    close(sock);
}


-(void)listenForPackets {
    listeningFlag_on = 1;

    NSLog(@"listenForPackets - Stage 1");
    if ((listeningSock = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0) {
        NSLog(@"ERROR: listenForPackets - socket() failed");
        return;                         // make the method return an int instead of void and use this statement to check for errors
    }

    // set reuse port to on to allow multiple binds per host
    if ((setsockopt(listeningSock, SOL_SOCKET, SO_REUSEADDR, &listeningFlag_on, sizeof(listeningFlag_on))) < 0) {
        NSLog(@"ERROR: listenForPackets - setsockopt() failed");
        return;                         // make the method return an int instead of void and use this statement to check for errors
    }

    NSLog(@"listenForPackets - Stage 2");
    // construct a multicast address structure after erasing anything in the listeningmc_addr structure
    memset(&listeningmc_addr, 0, sizeof(listeningmc_addr));
    listeningmc_addr.sin_family      = AF_INET;
    listeningmc_addr.sin_addr.s_addr = htonl(INADDR_ANY);
    listeningmc_addr.sin_port        = htons(mc_port);

    // bind multicast address to socket
    if ((bind(listeningSock, (struct sockaddr *) &listeningmc_addr, sizeof(listeningmc_addr))) < 0) {
        NSLog(@"ERROR: listenForPackets - bind() failed");
        return;                         // make the method return an int instead of void and use this statement to check for errors
    }

    //******************************************************************************************************************************
    //******************************************************************************************************************************
    NSString *ipAddress = [[NSString alloc] initWithString:self.getIPAddress];
    const char *tmp = [ipAddress UTF8String];
    listeningMc_addr_str = tmp;

    printf("%s\n", listeningMc_addr_str);

    listeningMc_req.imr_multiaddr.s_addr = inet_addr("225.0.0.37");
    listeningMc_req.imr_interface.s_addr = htonl(INADDR_ANY);


    // send an ADD MEMBERSHIP message via setsockopt
    if ((setsockopt(listeningSock, IPPROTO_IP, IP_ADD_MEMBERSHIP, (void*) &listeningMc_req, sizeof(listeningMc_req))) < 0) {
        NSLog(@"ERROR: listenForPackets - setsockopt() failed");
        int err = errno;
        NSLog(@"errno - %i", err);
        NSLog(@"Error = %s", strerror(err));
        perror("ERROR");
        return;                         // make the method return an int instead of void and use this statement to check for errors
    }

    NSLog(@"listenForPackets - Stage 3");
    for (;;) {          // loop forever

        // clear the receive buffers & structs
        memset(listeningRecv_str, 0, sizeof(listeningRecv_str));
        listeningFrom_len = sizeof(listeningFrom_addr);
        memset(&listeningFrom_addr, 0, listeningFrom_len);

        // block waiting to receive a packet
        if ((listeningRecv_len = recvfrom(listeningSock, listeningRecv_str, MAX_LEN, 0, (struct sockaddr*)&listeningFrom_addr, &listeningFrom_len)) < 0) {
            NSLog(@"ERROR: listenForPackets - recvfrom() failed");
            return;                     // make the method return an int instead of void and use this statement to check for errors
        }
        NSLog(@"listenForPackets - Stage 4");

        NSString *tmpy = [[NSString alloc] initWithCString:listeningRecv_str encoding:NSASCIIStringEncoding];
            msgStatus.text = tmpy;
            NSLog(@"ERROR");
        }
        // received string
        printf("Received %d bytes from %s: ", listeningRecv_len, inet_ntoa(listeningFrom_addr.sin_addr));
        printf("%s", listeningRecv_str);
    }

    // send a DROP MEMBERSHIP message via setsockopt
    if ((setsockopt(listeningSock, IPPROTO_IP, IP_DROP_MEMBERSHIP, (void*) &listeningMc_req, sizeof(listeningMc_req))) < 0) {
        NSLog(@"ERROR: listenForPackets - setsockopt() failed");
        //return 1;                         // make the method return an int instead of void and use this statement to check for errors
    }

    close(listeningSock);
    NSLog(@"listenForPackets - Stage 5 - Complete");    
}

这是我用来提取 IP 地址的代码。

-(NSString *)getIPAddress {
    NSString *address = @"error";
    struct ifaddrs *interfaces; // = NULL;
    struct ifaddrs *temp_addr; // = NULL;
    int success = 0;

    // retrieve the current interfaces - returns 0 on success
    success = getifaddrs(&interfaces);
    if (success == 0)  
    {
        // Loop through linked list of interfaces
        temp_addr = interfaces;
        while(temp_addr != NULL)  
        {
            if(temp_addr->ifa_addr->sa_family == AF_INET)
            {
                // Check if interface is en0 which is the wifi connection on the iPhone  
                if([[NSString stringWithUTF8String:temp_addr->ifa_name] isEqualToString:@"en0"])  
                {
                    // Get NSString from C String
                    address = [NSString stringWithUTF8String:inet_ntoa(((struct sockaddr_in *)temp_addr->ifa_addr)->sin_addr)];
                }
            }
            temp_addr = temp_addr->ifa_next;
        }
    }

    // Free memory
    freeifaddrs(interfaces); 
    return address; 
}

Answer 1

在侦听器中，我认为您需要设置

listeningMc_req.imr_interface.s_addr = htonl(INADDR_ANY);

...，因为这也是您绑定套接字的接口。 根据您是否在单个主机上运行所有内容，您可能需要考虑环回接口并绑定到 INADDR_ANY 即可做到这一点。

Answer 2

您和目的地之间有路由器吗？ 如果是这样，则需要完成一些工作来告诉路由器您想要订阅源并告诉路由器您将发送源。

我将首先对连接进行 tcpdump，以确保数据包首先离开您的计算机。