使用epoll在客戶端監聽不同的多播套接字

Question

我有一臺客戶端在同一臺機器上連接兩個不同的多播組（相同的端口號）。在客戶端，我使用epoll來監聽兩個套接字。 服務器嘗試向第一組發送組播消息。但是，epoll會在這兩個套接字上接收數據。是否因爲套接字在同一臺機器上並使用相同的端口？請指點

代碼片段：

/* 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

當你有相同的IP和端口打開兩個UDP套接字，即到達任何組播報文將由兩個插座接收。如果一個單播數據包到達，是否一個或另一個接收數據包是實現定義的。

如果要知道傳入數據包的目標IP地址是什麼，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 
}