Send WebSockets message to server

Question

I am trying to work with an API of one device, but it is using a WS interface with enforced Origin header, which is giving me troubles.

In Chrome, I can open the Console while a page with the correct Origin is loaded, create the WS connection, and send/receive messages without difficulties: Note that sent messages (in green) are always acknowledged by the server.

For reference, this is what happens if I create the connection on a different page, which results in an Origin header mismatch, reported as 404:

To sidestep this problem, I turned to C, because the rest of my program is written in that anyway. This is the code I have right now, based mostly on this answer:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <signal.h>
#include <libwebsockets.h>

#define KGRN "\033[0;32;32m"
#define KCYN "\033[0;36m"
#define KRED "\033[0;32;31m"
#define KYEL "\033[1;33m"
#define KBLU "\033[0;32;34m"
#define KCYN_L "\033[1;36m"
#define KBRN "\033[0;33m"
#define RESET "\033[0m"

static int destroy_flag = 0;
static int connection_flag = 0;
static int writeable_flag = 0;

static void INT_HANDLER(int signo) {
    destroy_flag = 1;
}

struct session_data {
    int fd;
};

struct pthread_routine_tool {
    struct lws_context *context;
    struct lws *wsi;
};

static int websocket_write_back(struct lws *wsi_in, char *str, int str_size_in) 
{
    if (str == NULL || wsi_in == NULL)
        return -1;

    int n;
    int len;
    char *out = NULL;

    if (str_size_in < 1) 
        len = strlen(str);
    else
        len = str_size_in;

    out = (char *)malloc(sizeof(char)*(LWS_SEND_BUFFER_PRE_PADDING + len + LWS_SEND_BUFFER_POST_PADDING));
    //* setup the buffer*/
    memcpy (out + LWS_SEND_BUFFER_PRE_PADDING, str, len );
    //* write out*/
    n = lws_write(wsi_in, out + LWS_SEND_BUFFER_PRE_PADDING, len, LWS_WRITE_TEXT);

    printf(KBLU"[websocket_write_back] %s\n"RESET, str);
    //* free the buffer*/
    free(out);

    return n;
}


static int ws_service_callback(
                         struct lws *wsi,
                         enum lws_callback_reasons reason, void *user,
                         void *in, size_t len)
{

    switch (reason) {

        case LWS_CALLBACK_CLIENT_ESTABLISHED:
            printf(KYEL"[Main Service] Connect with server success.\n"RESET);
            connection_flag = 1;
            break;

        case LWS_CALLBACK_CLIENT_CONNECTION_ERROR:
            printf(KRED"[Main Service] Connect with server error.\n"RESET);
            destroy_flag = 1;
            connection_flag = 0;
            break;

        case LWS_CALLBACK_CLOSED:
            printf(KYEL"[Main Service] LWS_CALLBACK_CLOSED\n"RESET);
            destroy_flag = 1;
            connection_flag = 0;
            break;

        case LWS_CALLBACK_CLIENT_RECEIVE:
            printf(KCYN_L"[Main Service] Client recvived:%s\n"RESET, (char *)in);

            if (writeable_flag)
                destroy_flag = 1;

            break;
        case LWS_CALLBACK_CLIENT_WRITEABLE :
            printf(KYEL"[Main Service] On writeable is called. send byebye message\n"RESET);
            websocket_write_back(wsi, "{\"command\":\"subscribe\",\"identifier\":\"{\\\"channel\\\":\\\"DevicesChannel\\\",\\\"share_token\\\":\\\"D0E91\\\"}\"}", -1);
            websocket_write_back(wsi, "{\"command\":\"message\",\"identifier\":\"{\\\"channel\\\":\\\"DevicesChannel\\\",\\\"share_token\\\":\\\"D0E91\\\"}\",\"data\":\"{\\\"value\\\":100,\\\"action\\\":\\\"set_buzz\\\"}\"}", -1);
            writeable_flag = 1;
            break;

        default:
            break;
    }

    return 0;
}

static void *pthread_routine(void *tool_in)
{
    struct pthread_routine_tool *tool = tool_in;

    printf(KBRN"[pthread_routine] Good day. This is pthread_routine.\n"RESET);

    //* waiting for connection with server done.*/
    while(!connection_flag)
        usleep(1000*20);

    //*Send greeting to server*/ 
    lws_callback_on_writable(tool->wsi);

}

int main(void)
{
    //* register the signal SIGINT handler */
    struct sigaction act;
    act.sa_handler = INT_HANDLER;
    act.sa_flags = 0;
    sigemptyset(&act.sa_mask);
    sigaction( SIGINT, &act, 0);


    struct lws_context *context = NULL;
    struct lws_context_creation_info info;
    struct lws *wsi = NULL;
    struct lws_protocols protocol;

    memset(&info, 0, sizeof info);
    info.port = CONTEXT_PORT_NO_LISTEN;
    info.iface = NULL;
    info.protocols = &protocol;
    info.ssl_cert_filepath = NULL;
    info.ssl_private_key_filepath = NULL;
    info.extensions = lws_get_internal_extensions();
    info.gid = -1;
    info.uid = -1;
    info.options = 0;

    protocol.name  = "websockets";
    protocol.callback = &ws_service_callback;
    protocol.per_session_data_size = sizeof(struct session_data);
    protocol.rx_buffer_size = 0;
    protocol.id = 0;
    protocol.user = NULL;

    context = lws_create_context(&info);
    printf(KRED"[Main] context created.\n"RESET);

    if (context == NULL) {
        printf(KRED"[Main] context is NULL.\n"RESET);
        return -1;
    }


    wsi = lws_client_connect(context, "mobu1.herokuapp.com", 443, 1,
        "/cable", "mobu1.herokuapp.com", "link.motorbunny.com",
    if (wsi == NULL) {
        printf(KRED"[Main] wsi create error.\n"RESET);
        return -1;
    }

    printf(KGRN"[Main] wsi create success.\n"RESET);

    struct pthread_routine_tool tool;
    tool.wsi = wsi;
    tool.context = context;

    pthread_t pid;
    pthread_create(&pid, NULL, pthread_routine, &tool);
    pthread_detach(pid);

    while(!destroy_flag)
    {
        lws_service(context, 50);
    }

    lws_context_destroy(context);

    return 0;
}

The result of running the above program is this:

As you can see, the periodic pings from server to my client are being picked up, but the lws_callback_on_writable(wsi); seems to have no effect as the LWS_CALLBACK_CLIENT_WRITEABLE callback never gets called. Additionally, if I call websocket_write_back() directly anywhere else, it doesn't seem to be sending anything to the server, and no acknowledgement is present either.

Is there something obvious I am doing wrong?

EDIT 1: I found this neat wscat, where I can replicate the results from Chrome: Now the question is, how can I interface this with my C program in a way that it can wait for the Welcome message from the server, and then send two messages? And better yet, how to stay connected, so that my program can send multiple commands at different points of time without having to do the handshake all the time?

Answer 1

The reason why the LWS_CALLBACK_CLIENT_WRITEABLE callback never got called was because this particular server uses non-standard handshake. So, to bypass this, I forked a fork of libwsclient and modified the handshake checking function to not fail on mismatch. I also added an optional Origin header.

Now, all I need to do in my original program is

wsclient *client;
char sync_str[6];
void mb_send(int power, char* type)
{
  char cmd[2048];
  sprintf (cmd, "{\"command\":\"message\",\"identifier\":\"{\\\"channel\\\":\\\"DevicesChannel\\\",\\\"share_token\\\":\\\"%s\\\"}\",\"data\":\"{\\\"value\\\":%d,\\\"action\\\":\\\"set_%s\\\"}\"}",sync_str,power,type);
  libwsclient_send(client,cmd);
}
void mb_connect()
{
  char cmd[2048];
  sprintf (cmd, "{\"command\":\"subscribe\",\"identifier\":\"{\\\"channel\\\":\\\"DevicesChannel\\\",\\\"share_token\\\":\\\"%s\\\"}\"}",sync_str);
  libwsclient_send(client,cmd);
  mb_send(0,"buzz");
}
int nop()
{
  return 0;
}
int main()
{
  client = libwsclient_new_extra("wss://mobu1.herokuapp.com/cable","https://link.motorbunny.com");
  if(!client) {
    fprintf(stderr, "Unable to initialize new WS client.\n");
    exit(1);
  }

  libwsclient_onopen(client, &nop);
  libwsclient_onmessage(client, &nop);
  libwsclient_onerror(client, &nop);
  libwsclient_onclose(client, &nop);

  libwsclient_run(client);

  ...
  mb_connect();
  ...  
  mb_send(200,"buzz");
  mb_send(40,"twirl");
  ...
  mb_send(0,"buzz");
  mb_send(0,"twirl");
}