来自特定 IP 地址的多播数据

Question

例如，我有同一台服务器 172.17.8.90 172.19.8.100 的两个不同网段 IP。 现在，我希望我的程序仅从 172.19.8.100 这个 ip 发送多播。 目前我的程序正在从 172.17.8.90 发送多播，因此不允许发送多播，我想要来自特定 IP 的多播数据

#include <stdio.h>      /* for fprintf() */
#include <sys/socket.h> /* for socket(), connect(), send(), and recv() */
#include <arpa/inet.h>  /* for sockaddr_in and inet_addr() */
#include <stdlib.h>     /* for atoi() and exit() */
#include <string.h>     /* for memset() */
#include <unistd.h>     /* for sleep() */

void DieWithError(char *errorMessage)  /* External error handling function */
{
     fprintf(stderr,"Dieing baby\n");
}

int main(int argc, char *argv[])
{
  int sock;                         /* Socket */
  struct sockaddr_in multicastAddr; /* Multicast address */
  char *multicastIP;                /* IP Multicast address */
  unsigned short multicastPort;     /* Server port */
  char *sendString;                 /* String to multicast */
  unsigned char multicastTTL;       /* TTL of multicast packets */
  unsigned int sendStringLen;       /* Length of string to multicast */
  if ((argc < 4) || (argc > 5))         /* Test for correct number of parameters */
  {
     fprintf(stderr,"Usage:  %s <Multicast Address> <Port> <Send String> [<TTL>]\n",
     argv[0]);
     exit(1);
  }
  multicastIP = argv[1];            /* First arg:  multicast IP address */ 
  multicastPort = atoi(argv[2]);    /* Second arg:  multicast port */
  sendString = argv[3];             /* Third arg:  String to multicast */
  if (argc == 5)                     /* Is TTL specified on command-line? */
    multicastTTL = atoi(argv[4]);  /* Command-line specified TTL */
  else
    multicastTTL = 1;              /* Default TTL = 1 */
  /* Create socket for sending/receiving datagrams */
  if ((sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0)
     DieWithError("socket() failed");
  /* Set TTL of multicast packet */
  if (setsockopt(sock, IPPROTO_IP, IP_MULTICAST_TTL, (void *) &multicastTTL, 
      sizeof(multicastTTL)) < 0)
      DieWithError("setsockopt() failed");
 /* Construct local address structure */
  memset(&multicastAddr, 0, sizeof(multicastAddr));   /* Zero out structure */
  multicastAddr.sin_family = AF_INET;                 /* Internet address family */
  multicastAddr.sin_addr.s_addr = inet_addr(multicastIP);/* Multicast IP address */
  multicastAddr.sin_port = htons(multicastPort);         /* Multicast port */
  sendStringLen = strlen(sendString);  /* Find length of sendString */
  for (;;) /* Run forever */
  {
    /* Multicast sendString in datagram to clients every 3 seconds */
    if (sendto(sock, sendString, sendStringLen, 0, (struct sockaddr *) 
          &multicastAddr, sizeof(multicastAddr)) != sendStringLen)
       DieWithError("sendto() sent a different number of bytes than expected");
       sleep(3);
  }
}

Answer 1

当发送多播数据包时，操作系统将默认选择一个特定的接口进行发送。 要更改传出接口，您需要设置IP_MULTICAST_IF选项：

struct in_addr multi_out;
multi_out.s_addr = inet_addr("172.19.8.100");
if (setsockopt(sock,IPPROTO_IP,IP_MULTICAST_IF, (char *)&multi_out, sizeof(multi_out)) == -1) {
    perror("error setting IP_MULTICAST_IF option");
    exit(1);
}

还要确保使用上述perror函数进行错误检查，以查看特定库调用失败的原因。