获取客户端 IP 地址和端口？

Question

I'm writing a file sharing program and the jist of it is that there is one main server which all clients connect to. However, the clients are the machines hosting the files so when one client requests a file from another, it must create a connection with it. When the client initially connects with the main server I get it's IP and port (I think) with:

    int client_len = sizeof(client);
    int new_sd = accept(sd, (struct sockaddr *)&client, &client_len);

    char str[INET_ADDRSTRLEN]; 
    inet_ntop(AF_INET, &(client.sin_addr), str, INET_ADDRSTRLEN);
    //printf("%s\n", str);
    int port = (int) ntohs(client.sin_port);        
    //printf("Port is: %d\n", (int) ntohs(client.sin_port));

The printf's give me 127.0.0.1 and 40XXX. The main server is using 127.0.0.1 and port 3000. So I'm pretty sure I have the correct client IP and port BUT when I try to set up a connection between 2 clients using 127.0.0.1 and 40XXX, it does not work. The connect function keeps returning an error. Here's the code trying to establish a connection between two clients:

    if ((cd = socket(AF_INET, SOCK_STREAM, 0)) == -1) {
    fprintf(stderr, "Can't creat a socket\n");
        exit(1);
    }
    bzero((char *)&cServer, sizeof(struct sockaddr_in));
    cServer.sin_family = AF_INET;
    cServer.sin_port = htons(cPort);
    if (inet_pton(AF_INET, cAddress, &cServer.sin_addr) == -1) {
        printf("inet_pton error occured\n");
        exit(1);
    } 

    /* Connecting to the server */
    if (connect(cd, (struct sockaddr *)&cServer, sizeof(cServer)) == -1) {
        fprintf(stderr, "Can't connect to server\n");
        exit(1);
    }

cPort is an int while cAddress is a char array.

Here's the code from the client attempting to create a connection for incoming connections. The code to get the IP Address and port return 0.0.0.0 and 0 respectively.

    if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1) {
    fprintf(stderr, "Can't creat a socket\n");
    exit(1);
}

/* Bind an address to the socket    */
bzero((char *)&clientServer, sizeof(struct sockaddr_in));
clientServer.sin_family = AF_INET;
clientServer.sin_port = 0;
clientServer.sin_addr.s_addr = htonl(INADDR_ANY);
if (bind(sockfd, (struct sockaddr *)&clientServer, sizeof(clientServer)) == -1){
    fprintf(stderr, "Can't bind name to socket\n");
    exit(1);
}

char str[INET_ADDRSTRLEN]; 
inet_ntop(AF_INET, &(clientServer.sin_addr), str, INET_ADDRSTRLEN);
printf("%s\n", str);
int clientServerPort = (int) ntohs(clientServer.sin_port);      
printf("Port is: %d\n", (int) ntohs(client.sin_port));

/* queue up to 10 connect requests  */
listen(sockfd, 10);

All help is greatly appreciated!

Answer 1

You can't use the client port for a new connection. That port is only being used to connect to your server, it's not listening for incoming connections. The client needs to bind another port, listen on it, and send that port number to you. You can then send it to another client, and it can connect to the port.

Note also that this may not work at all if the first client is behind a NAT router. The router won't know that it should forward this new port to the client. Peer-to-peer applications with dynamic port numbers are impractical when NAT is involved. Instead, you should define a dedicated port for this purpose; then users can configure port forwarding on their routers for this port.

Answer 2

That's what I initially tried. Using the same code above to get the IP Address and port number after a bind but it returned 0.0.0.0 and 0. I updated my main post. Also would a NAT router affect me even if I'm sticking strictly to local IP's?

Multiple interface issue. Do ifconfig -a, find the right interface.