多播的測試程式碼如下：

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>

int main(int argc, char*argv[])  
{     
	int sockfd; // 套接字檔案描述符  
	struct sockaddr_in local_addr; // 本地地址 
	int err = -1; 
	char group[16] = "224.0.0.88"; // 多播組 IP
	  
	sockfd = socket(AF_INET, SOCK_DGRAM, 0);  //建立套接字
	if (sockfd == -1)  
	{  
		perror("socket()");  
		return -1;  
	}    
	  
	// 初始化地址 
	memset(&local_addr, 0, sizeof(local_addr));  
	local_addr.sin_family = AF_INET;  
	local_addr.sin_addr.s_addr = htonl(INADDR_ANY);  
	local_addr.sin_port = htons(8080);
	  
	// 繫結socket  
	err = bind(sockfd,(struct sockaddr*)&local_addr, sizeof(local_addr));  
	if(err < 0)  
	{  
		perror("bind()");  
		return -2;  
	}    

	struct ip_mreq mreq; // 多播地址結構體                                 

	// 加入多播組，相當於建立一個QQ群，某人加入此群
	mreq.imr_multiaddr.s_addr = inet_addr(group); // 多播地址，類似於 QQ 群號 
	mreq.imr_interface.s_addr = htonl(INADDR_ANY);// 將本機加入多播組，類似於某人加入此群
	
	// 加入多播組
	err = setsockopt(sockfd, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq));  
	if (err < 0)  
	{  
		perror("[50]setsockopt():IP_ADD_MEMBERSHIP");  
		return -4;  
	} 

	int times = 0;  
	int addr_len = 0;  
	char buff[256] = {0};  
	int n = 0; 
	
	printf("加入多播組成功，準備接收資料\n");

	//接收資料
	while(1) 
	{  
		addr_len = sizeof(local_addr);  
		memset(buff, 0, sizeof(buff));     
		
		// 接收資料 
		n = recvfrom(sockfd, buff, sizeof(buff), 0,(struct sockaddr*)&local_addr, &addr_len);  
		if( n == -1)   
		{  
			perror("recvfrom()"); 
			close(sockfd); 
			return -1;
		}  

		printf("Recv message from server: %s\n", buff);  
	}  
	  
	// 退出廣播組 
	err = setsockopt(sockfd, IPPROTO_IP, IP_DROP_MEMBERSHIP,&mreq, sizeof(mreq)); 
	if (err < 0)  
	{  
		perror("[79]setsockopt():IP_DROP_MEMBERSHIP");  
		return -4;  
	}  

		  
	close(sockfd); 

	return 0;  
}
 

查詢相關資料得到的答案如下：

It means that the tool is trying to use multicast but the network interface doesn't support it There are two likely causes:
·Your machine doesn't have multicast support enabled. For example, on Linux and FreeBSD it is possible to compile a kernel which doesn't support multicast.  
·You don't have a route for multicast traffic. Some systems don't add this by default, and you need to run：
route add -net 224.0.0.0 netmask 255.255.255.255 eth0(or similar)

這主要和當前的網路配置有關，因為多播IP地址沒有加入到路由表中。

解決方法：把需要用到的多播地址(如本例的224.0.0.88)加入到路由表中，命令如下：

sudo route add -net 224.0.0.88 netmask 255.255.255.255 eth0

224.0.0.88：為當前使用的多播IP地址

eth0：為當前使用的有效網絡卡

其它輔助命令：

sudo route del -net 224.0.0.88 netmask 255.255.255.255 eth0

route -n //檢視路由表資訊

具體操作過程如下圖：