int main() {
    int udp_socket_fd = socket(AF_INET, SOCK_DGRAM, 0);
    if (udp_socket_fd == -1) {
        printf("socket failed!\\n");
        return -1;
    }

    // Set the destination IP address.
    struct sockaddr_in bind_addr = { 0 };
    bind_addr.sin_family = AF_INET;
    bind_addr.sin_port = htons(xxxx); // The recommended port range in htons(xxxx) is 10000–20000.
    bind_addr.sin_addr.s_addr = inet_addr("0.0.0.0"); // Bind the IP

    // Bind the port.
    int ret = bind(udp_socket_fd, (struct sockaddr *)&bind_addr, sizeof(bind_addr));
    if (ret < 0) {
        perror("bind fail:");
        close(udp_socket_fd);
        return -1;
    }

    // Start to wait for a client message.
    struct sockaddr_in  upstream_addr = { 0 };  // Address used to store the CAR proxy.
    int len = sizeof(upstream_addr);
    char buf[1024] = { 0 }; // Receive buffer.
    while (true) {
        ret = recvfrom(udp_socket_fd, buf, sizeof(buf), 0, (struct sockaddr *)&upstream_addr, &len);
        if (ret == -1) {
            break;
        }
        // buf is the message "test" sent by the frontend.
        // upstream_addr can be subsequently used to send messages to the frontend.
        const char* response = "response";
        sendto(udp_socket_fd, response, strlen(response), 0, (struct sockaddr *)&upstream_addr, sizeof(upstream_addr));
    }
    return 0;
}