How to use getaddrinfo to connect to a server using the external IP?

Question

I'm writing a small C client/server application, but I cannot make the connection work when using the external IP address. The code for both client and server is taken from here, in particular the clients do:

    char *default_server_name = "localhost";
    char *server_name = NULL;
    int nport = DEFAULT_DAMA_PORT;
    char port[15];

    // Parse the command line options
    if (parse_options(argc, argv, &server_name, &nport) < 0) {
        return -1;
    }

    if (server_name == NULL) {
        server_name = default_server_name;
    }

    snprintf(port, 15, "%d", nport);

    // Connect to the server
    int client_socket;
    struct addrinfo hints, *servinfo, *p;
    int rv;

    memset(&hints, 0, sizeof(hints));
    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;
    if ((rv = getaddrinfo(server_name, port, &hints, &servinfo)) != 0) {
        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv));
        exit(1);
    }

    for (p=servinfo; p != NULL; p = p->ai_next) {
        if ((client_socket = socket(p->ai_family, p->ai_socktype, p->ai_protocol)) == -1) {
#ifdef DEBUG
            perror("socket");
#endif
            continue;
        }

        if (connect(client_socket, p->ai_addr, p->ai_addrlen) == -1) {
            close(client_socket);
#ifdef DEBUG
            perror("connect");
#endif
            continue;
        }

        // Connected succesfully!
        break;
    }

    if (p == NULL) {
        // The loop wasn't able to connect to the server
        fprintf(stderr, "Couldn't connect to the server\n.");
        exit(1);
    }

While the server:

    int nport;
    char port[15];
    if (parse_options(argc, argv, &nport) < 0) {
        return -1;
    }

    snprintf(port, 15, "%d", nport);

    int server_socket;
    struct addrinfo hints, *servinfo, *p;
    int rv;

    memset(&hints, 0, sizeof(hints));
    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_flags = AI_PASSIVE;

    if ((rv = getaddrinfo(NULL, port, &hints, &servinfo)) != 0) {
        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv));
        exit(1);
    }

    for (p=servinfo; p != NULL; p = p->ai_next) {
        if ((server_socket = socket(p->ai_family, p->ai_socktype, p->ai_protocol)) == -1) {
#ifdef DEBUG
            perror("socket");
#endif
            continue;
        }

        if (bind(server_socket, p->ai_addr, p->ai_addrlen) == -1) {
            close(server_socket);
#ifdef DEBUG
            perror("bind");
#endif
            continue;
        }

        // We binded successfully!
        break;
    }

    if (p == NULL) {
        fprintf(stderr, "failed to bind socket\n");
        exit(2);
    }

    int pl_one, pl_two;
    socklen_t pl_one_len, pl_two_len;
    struct sockaddr_in pl_one_addr, pl_two_addr;

    if (listen(server_socket, 2) < 0) {
        fatal_error("Error in syscall listen.", 2);
    }

    // Get the two clients connections.
    pl_one_len = sizeof(pl_one_addr);
    pl_one = accept(server_socket,
                    (struct sockaddr *)&pl_one_addr,
                    &pl_one_len);
    if (pl_one < 0) {
        fatal_error("Error in syscall accept.", 3);
    }

    pl_two_len = sizeof(pl_two_addr);
    pl_two = accept(server_socket,
                    (struct sockaddr *)&pl_two_addr,
                    &pl_two_len);
    if (pl_two < 0) {
        fatal_error("Error in syscall accept.", 3);
    }

If I specify the IP of my machine in the command line, and hence the server_name in the clients is set to a string like 151.51.xxx.xxx, then the sockets cannot connect to the server. Also using 127.0.0.1 shows the same behaviour, which makes me think that when the documentation states:

The host name that you're interested in goes in the nodename parameter. The address can be either a host name, like "www.example.com", or an IPv4 or IPv6 address (passed as a string).

it's just joking.

Am I doing something incorrectly? Could there be some problem with firewall etc. that is preventing the clients to connect using the IP addresses?

Note: I've already searched a lot for this problem and some people say to avoid using getaddrinfo at all and directly fill with INADDR_ANY, but the getaddrinfo documentation states that passing NULL as nodename should already fill the address with INADDR_ANY, so I don't see why should I use the old method when the new one does this automatically.

Answer 1

As written in comments you are trying to connect to a server that is located in a network behind a router.

127.0.0.1 or localhost are redirections of the operating system and will not go over the router. That's why it worked for you.

If you specify an external ip address, the connection is made via the router and the ports that you use need to be forwarded in the router configuration. Any normal home internet end user router blocks incoming connections from any port by default.