Question

I have a very simple server written in C and an equally simple client written in Java. When I run them both on the same computer everything works, but when I try to run the server on computer A and the client on computer B, I get the error IOException connection refused from the java client. I can’t seem to find out whats happening, any thoughts? I’ve even turned off the firewalls but the problem still persists.

server.

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>

#define PORT 3557
#define BUF 256

int main(int argc, char *argv[])
{
    struct sockaddr_in host, remote;
    int host_fd, remote_fd;
    int size = sizeof(struct sockaddr);;

    char data[BUF];

    host.sin_family = AF_INET;
    host.sin_addr.s_addr = htonl(INADDR_ANY);
    host.sin_port = htons(PORT);
    memset(&host.sin_zero, 0, sizeof(host.sin_zero));

    host_fd = socket(AF_INET, SOCK_STREAM, 0);
    if(host_fd == -1) {
        printf("socket error %d\n", host_fd);
        return 1;
    }

    if(bind(host_fd, (struct sockaddr *)&host, size)) {
        printf("bind error\n");
        return 1;
    }

    if(listen(host_fd, 5)) {
        printf("listen error");
        return 1;
    }

    printf("Server setup, waiting for connection...\n");
    remote_fd = accept(host_fd, (struct sockaddr *)&remote, &size);

    printf("connection made\n");

    int read = recv(remote_fd, data, BUF, 0);
    data[read] = '\0';
    printf("read = %d, data = %s\n", read, data);

    shutdown(remote_fd, SHUT_RDWR);
    close(remote_fd);

    return 0;
}

client.

import java.net.*;
import java.io.*;

public class socket {

    public static void main(String[] argv) {
        DataOutputStream os = null;

        try {
            Socket socket = new Socket("192.168.1.103", 3557);
            os = new DataOutputStream(socket.getOutputStream());
            os.writeBytes("phone 12");

            os.close();
            socket.close();

        } catch (UnknownHostException e) {
            System.out.println("Unkonw exception " + e.getMessage());

        } catch (IOException e) {
            System.out.println("IOException caught " + e.getMessage());
        } 
    }
}

Edit:

Thanks everyone for the quick responses (I was shocked). I solved the problem thanks to the suggestions of using telnet and nc. I found out that my firewall was blocking port 3557 on the server machine. A simple thing I should have thought of, thanks!

Answer 1

There are many possible problems and you should start and try pinpointing the problematic part.

It just might be a networking problem – B might not be able to connect to A because of firewalls and the likes.

To check this you can start with running a port scan from B on A (nmap -p 3557 192.168.1.103 to check if the port seems open to computer B).

If it looks open, you can try connecting with telnet/nc from B instead of your client to see if the server works as expected: telnet 192.168.1.103 3557 or nc 192.168.1.103 3557 and once connected write “phone 12” and see what you get.

These should help you understand which part is causing problems, server, client or network.