TCP Self-Connect Problem Example

tcp_self_connect.c

#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>

int main(int argc, char const *argv[])
{
    int port;

    if (argc == 1) {
        port = 44600;
    } else {
        port = atoi(argv[1]);
    }

    int socketfd, connection_status;
    struct sockaddr_in destination, source;
    socklen_t len = sizeof(source);

    bzero(&destination, sizeof(destination));

    socketfd = socket(AF_INET, SOCK_STREAM, 0);

    if (socketfd == -1) {
        perror("socket create: ");
        exit(1);
    }

    destination.sin_family = AF_INET;
    destination.sin_addr.s_addr = inet_addr("127.0.0.1");
    destination.sin_port = htons(port);

    for (;;) {
        bzero(&source, sizeof(source));

        connection_status = connect(socketfd, (struct sockaddr *)&destination, sizeof(destination));
        if (getsockname(socketfd, (struct sockaddr *)&source, &len) == -1) {
            perror("getsockname error: ");
            exit(1);
        }

        printf("Source port: %d\n", ntohs(source.sin_port));
        printf("Destination port: %d\n", port);

        if (connection_status != 0) {
            printf("The app's source port number is not the same as the port the app is listening on.\n");
            printf("--\n");
        } else {
            printf("Connected - The source and destination ports are the same.\n");
            sleep(100);
            exit(0);
        }

    }

    return 0;
}