java udp 接收16进制,奇怪的多播UDP数据包接收（recvfrom）

EDIT: Problem explanation modified to make it clearer.

我写了两个简单的程序：

客户端程序：将UDP组播数据包发送到组播IP地址和端口 .

服务器程序：请求成员IP的成员资格，绑定到同一端口并使用'recvfrom'接收这些数据包 . 它还将这些数据包的内容与源地址和端口一起打印到控制台上 .

现在，我在Machine_S上启动'server'程序，在Machine_C上启动'client'程序 . 两台机器都在同一个局域网中 .

客户端's multicast packets reach Machine_S, which I am able to see in Wireshark. However, '服务器程序无法读取它们，即对recvfrom的调用不会返回 .

现在，我在Machine_S上启动了另一个'client'实例 . 一旦完成，'服务器'程序就开始从两个客户端(从两台机器)接收数据包并将它们打印到控制台！

此时，即使我关闭Machine_S上的本地“客户端”实例，“服务器”仍将继续从Machine_C上的远程“客户端”接收数据包 .

简而言之，只有当本地客户端也将数据包发送到同一个多播地址和端口时，服务器程序似乎才会从远程客户端接收数据包 .

Client Code:

// COMMON MACROS FOR BOTH CLIENT AND SERVER

#define HELLO_PORT 12345

#define HELLO_GROUP "239.0.0.50"

// after initializing winsock

// create UDP socket

if ((sockid = socket (AF_INET, SOCK_DGRAM, 0)) < 0) {

printf("SOCKET Error Code: %d\n", WSAGetLastError());

WSACleanup();

return 1;

}

// set up destination address

memset(&addr,0,sizeof(addr));

addr.sin_family = AF_INET;

addr.sin_addr.s_addr = inet_addr(HELLO_GROUP);

addr.sin_port = htons(HELLO_PORT);

// now just sendto() our destination!

while (times--) {

printf("Sent To : %d\n", HELLO_PORT);

if (sendto (sockid, message, strlen(message)+1, 0, (struct sockaddr *)&addr, sizeof(addr)) < 0) {

printf("SENDTO Error Code: %d\n", WSAGetLastError());

break;

}

Sleep(500);

}

closesocket(sockid);

WSACleanup();

Server Code:

// create UDP socket

if ((sockid = socket (AF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0) {

printf("SOCKET Error Code: %d\n", WSAGetLastError());

WSACleanup();

return 1;

}

// set up local address

memset(&addr,0,sizeof(addr));

addr.sin_family = AF_INET;

addr.sin_addr.s_addr = htonl (INADDR_ANY);

addr.sin_port = htons (HELLO_PORT);

// bind to local address

if (bind(sockid, (struct sockaddr *)&addr, sizeof(addr)) < 0) {

printf("BIND Error Code: %d\n", WSAGetLastError());

closesocket(sockid);

WSACleanup();

return 1;

}

// join a multicast group

mreq.imr_multiaddr.s_addr = inet_addr (HELLO_GROUP);

mreq.imr_interface.s_addr = htonl (INADDR_ANY);

if (setsockopt(sockid, IPPROTO_IP, IP_ADD_MEMBERSHIP, (const char *)&mreq, sizeof(mreq)) < 0) {

printf("SETSOCKETOPT Error Code: %d\n", WSAGetLastError());

closesocket(sockid);

WSACleanup();

return 1;

}

memset(&addr,0,sizeof(addr));

// now just enter a read-print loop

while (times--) {

addrlen = sizeof(addr);

if ((nbytes = recvfrom(sockid, msgbuf, MSGBUFSIZE, 0, (struct sockaddr *)&addr, &addrlen)) < 0) {

printf("RECVFROM Error Code: %d\n", WSAGetLastError());

break;

}

inet_ntop(AF_INET, &(addr.sin_addr), (PSTR)&address, MSGBUFSIZE);

srcport = ntohs(addr.sin_port);

printf("From\t%s\t%d\t", address, srcport);

puts(msgbuf);

Sleep(1000);

}

closesocket(sockid);

WSACleanup();

几点需要注意：

每台机器只有1个网络接口

多播IP为239.0.0.50，端口为12345

这可能是什么原因？