环回接口上的客户端服务器连接

Question

我正在尝试测试一个使用本地主机地址的客户端-服务器简单实现。这是代码。

服务器：

/*
 * Sequential busy-waiting
 */
int main(int argc, char** argv) {

    int opt, client_addr_l, errsv;
    unsigned short port;
    struct sockaddr_in server_addr, client_addr;

    /* ... */

    printf("Port number retrieved (%d), server is starting ...\n", port);

    /*TCP Socket creation*/
    sock_ds = socket(AF_INET, SOCK_STREAM, 0); 
    if(sock_ds == -1){
        fprintf(stderr, "Socket creation error: %s\n", strerror(errno));
        exit(EXIT_FAILURE);
    }

    /*Server address binding*/
    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(port);
    server_addr.sin_addr.s_addr = INADDR_ANY;

   /*!!!! */
   int optval = 1;                                      
   if( (setsockopt(sock_ds,SOL_SOCKET,SO_REUSEADDR,&optval,sizeof(optval))) == -1 ) {                
       printf("Error on setsockopt\n");                         
       exit(EXIT_FAILURE);                                  
   }
  /*????*/

   if(bind(sock_ds, (struct sockaddr*)&server_addr, sizeof(server_addr)) == -1){
       fprintf(stderr, "Address binding error\n");
       exit(EXIT_FAILURE);
   }

   /*Server with passive socket*/
   if(listen(sock_ds, SOMAXCONN) == -1){
       fprintf(stderr, "Listen call error: %s\n", strerror(errno));
       exit(EXIT_FAILURE);
   }

   while(1){
       memset(&client_addr, 0, sizeof(client_addr));
       acc_sock_ds = accept(sock_ds, (struct sockaddr *)&client_addr, &client_addr_l);
       printf("DEBUG: LINE201, acc_sock_ds = %d\n", acc_sock_ds);
       /*Connect error management*/
       if(acc_sock_ds == -1){
           fprintf(stderr, "Fatal error on accept %d(%s)\n"
                   , errsv, strerror(errsv));
            exit(EXIT_FAILURE);
       }

       //sin_addr to ASCII (string) );
       printf("Connected with: %s\n", inet_ntoa(client_addr.sin_addr)); 

       /*...*/     

       close(acc_sock_ds);

      /*...*/
    }

/*...*/

}

客户端：

 int main(){

    int sock_ds;
    struct sockaddr_in remote_addr;
    struct hostent *hp;

    /*TCP Socket creation*/
    sock_ds = socket(AF_INET, SOCK_STREAM, 0); 
    if(sock_ds == -1){
        fprintf(stderr, "Socket creation error\n");
        exit(EXIT_FAILURE);
    }

    remote_addr.sin_family = AF_INET;
    remote_addr.sin_port = htons(25556);

    hp = gethostbyname("localhost");
    bcopy(hp -> h_addr, &remote_addr.sin_addr, hp -> h_length); //fills address entry

   if(connect(sock_ds, (struct sockaddr*)&remote_addr, sizeof(remote_addr)) == -1){ //connection attempt
       fprintf(stderr, "Connect failure(%s)\n", strerror(errno));
       exit(EXIT_FAILURE);        
   }

   /*...*/

}

当我在两个不同的终端上运行它们时，服务器返回给我：

Port number retrieved (25556), server is starting ...
Server is ready. Waiting for client connections.
DEBUG: LINE201, acc_sock_ds = 4
Connected with: 0.0.0.0

我的问题是:为什么服务器检索到的客户端地址是0.0.0.0。它不应该是127.0.0.1？

Answer 1

看起来您传递的是第三个参数接受()未初始化，它应该设置为第二个参数的大小。除此之外，它应该是一个整数，而不是一个socklen_t，请参见http://pubs.opengroup.org/onlinepubs/009695399/functions/accept.html

可以尝试将client_addr_l声明为socklen_t，然后将其设置为sizeof(结构sockaddr_in)，然后传递给accept()？

我猜它的统一化值是零，因此accept()不能将远程地址设置为您的client_addr，因为它的大小为零。因此，client_addr是原封不动的，正如您之前将其置零一样，您得到的是0.0.0.0。

Answer 2

0.0.0.0表示您的服务器接受来自设备中任何接口的连接

因此，地址为127.0..0.1的环回接口也包括在内

Answer 3

这似乎是一种特殊的情况。所有可能的地址都在侦听您的连接。Here就是这方面的一个线索。

引用：

0.0.0.0, in this context, means "all IP addresses on the local machine" 
(in fact probably, "all IPv4 addresses on the local machine"). 
So, if your webserver machine has two ip addresses, 192.168.1.1 and 10.1.2.1, 
and you allow a webserver daemon like apache to listen on 0.0.0.0, 
it will be reachable at both of those IPs. 
But only to what can contact those IPs and the web port(s).