使用epoll在客戶端偵聽不同的多播套接字

Question

我有一個客戶端加入同一台計算機上的兩個不同的多播組（相同的端口號）。 在客戶端，我正在使用epoll監聽兩個套接字。 服務器嘗試向第一個組發送多播消息。 但是，epoll在兩個套接字上都接收數據。 是因為套接字在同一台計算機上並且使用相同的端口嗎？ 請指教

程式碼片段：

/* Client code to join multicast group */

multicastPort = "4321";                                                                                                                                                          
  /* Resolve the multicast group address */                                                                                                                                      
  hints.ai_family = PF_UNSPEC;                                                                                                                                                   
  hints.ai_flags  = AI_NUMERICHOST;                                                                                                                                              
  if ((status = getaddrinfo(group_ip_address, NULL, &hints, &multicastAddr)) != 0)                                                                                               
    {                                                                                                                                                                            
        perror("\nError g.");                                                                                                                                                    
    }                                                                                                                                                                            

  hints.ai_family   = multicastAddr->ai_family;                                                                                                                                  
  hints.ai_socktype = SOCK_DGRAM;                                                                                                                                                
  hints.ai_flags    = AI_PASSIVE; /* Return an address we can bind to */                                                                                                         
  if ( getaddrinfo(NULL, multicastPort, &hints, &localAddr) != 0 )                                                                                                               
        perror("\nError f.");                                                                                                                                                    

/* Create socket for receiving datagrams */                                                                                                                                      
  if ( (sd = socket(localAddr->ai_family, localAddr->ai_socktype, 0)) < 0 )                                                                                                      
    perror("socket() failed");                                                                                                                                                   

  /* lose the pesky "Address already in use" error message */                                                                                                                    
  if (setsockopt(sd,SOL_SOCKET,SO_REUSEADDR,(char*)&yes,sizeof(int)) == -1)                                                                                                      
    perror("setsockopt");                                                                                                                                                        

  /* Bind to the multicast port */                                                                                                                                               
  if ( bind(sd, localAddr->ai_addr, localAddr->ai_addrlen) != 0 )                                                                                                                
    perror("bind() failed");                                                                                                                                                     

  struct ip_mreq multicastRequest;  /* Multicast address join structure */                                                                                                       

      /* Specify the multicast group */                                                                                                                                          
      memcpy(&multicastRequest.imr_multiaddr,                                                                                                                                    
       &((struct sockaddr_in*)(multicastAddr->ai_addr))->sin_addr,                                                                                                               
       sizeof(multicastRequest.imr_multiaddr));                                                                                                                                  

      /* Accept multicast from any interface */                                                                                                                                  
      multicastRequest.imr_interface.s_addr = htonl(INADDR_ANY);                                                                                                                 

      /* Join the multicast address */                                                                                                                                           
      if ( setsockopt(sd, IPPROTO_IP, IP_ADD_MEMBERSHIP, (char*) &multicastRequest, sizeof(multicastRequest)) != 0 )                                                             
  perror("setsockopt() failed");                                                                                                                                                 
  /* Create a datagram socket on which to receive. */  
==================================================

/* client code to listen on epoll  sockets*/
int fd_id= multicast_join(lo,group_ip);                                                                                                                                       
   //sprintf(display,"Listening to group %s ip address %s\n", grp_name, grp_ip_address);                                                                                         
   sprintf(display,"Listening to group %s and ip %s\n", grp_name, grp_ip_address);                                                                                               
   PRINT(display);                                                                                                                                                               
   if(fd_id > 0){                                                                                                                                                                
     ADD_CLIENT_IN_LL(client_info,grp_name,group_ip,fd_id);                                                                                                                      
     event->data.fd = fd_id;                                                                                                                                                     
     char buf[30];                                                                                                                                                               
     sprintf(buf,"fd_id %d",fd_id);                                                                                                                                              
     PRINT(buf);                                                                                                                                                                 
     event->events = EPOLLIN|EPOLLET;                                                                                                                                            

     status = epoll_ctl(efd, EPOLL_CTL_ADD, fd_id, event);                                                                                                                       

     if ( status == -1)                                                                                                                                                          
     {                                                                                                                                                                           
       perror("\nError while adding FD to epoll event.");                                                                                                                        
       exit(0);                                                                                                                                                                  
     } 

Answer 1

當在相同的IP和端口上打開兩個UDP套接字時，兩個套接字都將接收到達的所有多播數據包。 如果單播包到達，則實現中定義一個還是另一個接收包。

如果您想知道傳入數據包的目標IP地址是什么，則需要設置IP_PKTINFO套接字選項，並使用recvmsg而不是recvfrom來獲取此附加數據。

// sock is bound AF_INET socket, usually SOCK_DGRAM
// include struct in_pktinfo in the message "ancilliary" control data
setsockopt(sock, IPPROTO_IP, IP_PKTINFO, &opt, sizeof(opt));
// the control data is dumped here
char cmbuf[0x100];
// the remote/source sockaddr is put here
struct sockaddr_in peeraddr;
// if you want access to the data you need to init the msg_iovec fields
struct msghdr mh = {
    .msg_name = &peeraddr,
    .msg_namelen = sizeof(peeraddr),
    .msg_control = cmbuf,
    .msg_controllen = sizeof(cmbuf),
};
recvmsg(sock, &mh, 0);
for ( // iterate through all the control headers
    struct cmsghdr *cmsg = CMSG_FIRSTHDR(&mh);
    cmsg != NULL;
    cmsg = CMSG_NXTHDR(&mh, cmsg))
{
    // ignore the control headers that don't match what we want
    if (cmsg->cmsg_level != IPPROTO_IP ||
        cmsg->cmsg_type != IP_PKTINFO)
    {
        continue;
    }
    struct in_pktinfo *pi = CMSG_DATA(cmsg);
    // at this point, peeraddr is the source sockaddr
    // pi->ipi_spec_dst is the destination in_addr
    // pi->ipi_addr is the receiving interface in_addr
}