c - 原始套接字 : sendto() and recvfrom() not working

Question

我正在尝试使用 RAW 套接字编写客户端/服务器应用程序。

存在多个问题:

1. 当客户端使用sendto()方法向服务器发送消息时，sendto()返回错误invalid argument em> 方法。 为什么会出现此错误消息？。相应的代码标记在错误1部分下。 sendto() 的代码在这篇文章中进行了注释。

2. 由于我已经注释了发送消息部分，因此客户端应该等待消息； recvfrom() 是一个阻塞系统调用。相反，recvfrom() 总是返回一条消息E。 此消息从哪里到达？。相应的代码标记为ERROR 2。

3. 如果我将socket()中的协议(protocol)(第三个)参数更改为0或IPPROTO_RAW我收到协议(protocol)不支持的错误。 为什么会出现这些错误？

操作系统是Ubuntu。

    #include <stdio.h>
    #include <stdlib.h>
    #include <unistd.h>

    #include <sys/socket.h> // For the socket () etc. functions.
    #include <netinet/in.h> // For IPv4 data struct..
    #include <string.h>     // For memset.
    #include <arpa/inet.h>  // For inet_pton ().

    #define BUF_SIZE 30

    void main ()

    {
        int rst; // Return status of functions.
    /**************** Create a socket. *******************************/
    int sfd; // Socket file descriptor.
    sfd = socket (AF_INET, SOCK_RAW, IPPROTO_UDP); /* 
            * AF_INET --> IPv4, SOCK_RAW for Raw socket,  
            * 0 --> for any protocol. */
    if (sfd == -1) 
    {
        perror ("Client: socket error");
        exit (1);
    }


    /*********** Server's address ***********************************/
    struct sockaddr_in srv_addr;
    socklen_t addrlen = sizeof (struct sockaddr_in); 

    // Initializing the server's address to zero.
    memset (&srv_addr, 0, addrlen); 

    srv_addr.sin_family = AF_INET;    // Address is in IPv4 format.
    // srv_addr.sin_port   = htons (0);  // Port number of the server. 


    rst = inet_pton (AF_INET, "127.0.0.1", &srv_addr.sin_addr); /* Note
            * that third field should point to an in_addr (in6_addr). */
    if (rst <= 0)
    {
        perror ("Client Presentation to network address conversion.\n");
        exit (1);
    }        



    /****************** ERROR 1 ************************************
    ******************* Sending message to the server. *************/
    const int flags = 0;
    const char *msg = "Hello";
    /* rst = sendto (sfd, msg, strlen(msg)+1, flags, 
                    (struct sockaddr *) &srv_addr, 
                    sizeof (struct sockaddr_in));
    if (rst < 0)
    {
        perror ("Client: Sendto function call failed");
        exit (1);
    }
    else 
        printf ("Client: Sent data size = %d\n", rst);

    */


    /******************* ERROR 2 ***********************************
     ******************* Receiving message from server. ************/
    // Initializing the server's address to zero.
    memset (&srv_addr, 0, addrlen); 

    char buf[BUF_SIZE] = {'\0'};

    rst = recvfrom (sfd, buf, BUF_SIZE, flags, 
                    (struct sockaddr *) &srv_addr, 
                    &addrlen);
    if (rst < 0)
    {
        perror ("Client: couldn't receive");
        exit (1);
    }
    printf ("Message from server = |%s|\n", buf);

    /* Address of the server. */
    const char *buf2 = inet_ntop (AF_INET, 
                       (struct sockaddr *) &srv_addr, buf, BUF_SIZE);
    if (buf2 == NULL)
    {
        perror ("Client: Conversion of sender's address to presentation failed");
        exit (1);
    }

    printf ("Servers address,  = %s\n", buf2);
    close (sfd);

}

Answer 1

SOCK_RAW 不适用于 UDP。 SOCK_DGRAM 是正确的。有关教程，请参阅:

a tutorial from Rutgers