CODEWITHSUNDEEP
csocketstcpip
f4lco
f4lco

Reputation: 3824

TCP: Establish a TCP connection

I've read many examples on networking with C and I'm stuck. I can see that the TCP packets with the SYN flag are on the wire (with wireshark), but the receiving end (I've set up a virtual machine for testing purposes) sends nothing back, not even RST.

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <strings.h>


int establish_tcp(int port) {
    char *ip = "10.0.2.15"; /* Virtual Machine */
    struct sockaddr_in sockaddrin;
    struct hostent *host;
    int sock;

    sock = socket(AF_INET, SOCK_STREAM, 0);
    if(sock == -1) {
        fprintf(stderr, "Socket Error\n");
        exit(1);
    }

    sockaddrin.sin_family = AF_INET;

    host = gethostbyname(ip);
    if(host == NULL) {
        fprintf(stderr, "%s unknown host.\n", ip);
        exit(2);
    }

    /* copies the internet address into the struct */
    bcopy(host->h_addr, &sockaddrin.sin_addr, host->h_length);

    /* specify port (used by TCP a layer above */
    sockaddrin.sin_port = port;

    /* try to connect */
    return connect(sock, (struct sockaddr*) &sockaddrin, sizeof(struct sockaddr_in));
}

int main(void) {
    printf("status: %i\n", establish_tcp(80));
    return 0;
}

It takes a while until the packets are timed out and -1 is returned as status code. How can it happen that the target machine doesn't send a reply? What have I overlooked?

I think I've figured out that it is not a setup problem. I'm running Ubuntu 12.04. The virtual machine is a Debian Wheezy, I check its IP with ifconfig. I tried if the machine is reachable with telnet.
To escape issues which might be related to Virtual Box I tried to replace the IP with the one of Google, yielding the same results.

Upvotes: 1

Views: 6058

Answers (1)

Some programmer dude
Some programmer dude

Reputation: 409442

The problem most likely is this:

sockaddrin.sin_port = port;

The port number has to be in network byte order, which is different from the native byte order in x86 and x86_64 machines.

Change to:

sockaddrin.sin_port = htons(port);

Upvotes: 4

Related Questions