How to connect to mqtt broker using libmosquitto and SSL/TLS in C++

Question

I'm trying to subscribe to an mqtt broker which uses SSL/TLS to authenticate clients. I use libmosquitto to do that.

I run this code to perform a subscription

#include <csignal>
#include <iostream>
#include <sys/types.h>
#include <unistd.h>

#include <mosquitto.h>

#define WITH_AUTHENTICATION

#define MQTT_HOST       "exmaple.com"
#define MQTT_PORT       8883
#define TARGET_USER     "use"
#define TARGET_PW       "password"
#define TARGET_TOPIC    "/example-topic"
#define CERTIFICATE     "/home/luca/TRIALS/tryMqttS/cert.pem"

using namespace std;

static int run  = 1;




void signalHandler (int s) {
    run = 0;
}




void messageCallback (struct mosquitto *mosq, void *obj, const struct mosquitto_message *message) {
    bool match  = 0;
    cout << "got message  " << (char *) message->payload << "  from topic  " << message->topic << endl;
}




int main(int argc, char *argv[])
{
    uint8_t reconnect       = true;
    string clientID         = "mosquitto_client_" + to_string (getpid());
    struct mosquitto *mosq  = nullptr;
    int resCode             = 0;

    signal (SIGINT, signalHandler);
    signal (SIGTERM, signalHandler);

    mosquitto_lib_init ();

    mosq    = mosquitto_new (clientID.c_str(), true, 0);

    if(mosq){
        mosquitto_message_callback_set (mosq, messageCallback);

#ifdef WITH_AUTHENTICATION
        cout << "Pw set result:             " << mosquitto_strerror (mosquitto_username_pw_set  (mosq, TARGET_USER, TARGET_PW)) << endl;
        cout << "Tls insecure set result:   " << mosquitto_strerror (mosquitto_tls_insecure_set (mosq, false)) << endl;
        cout << "Tls opts set result:       " << mosquitto_strerror (mosquitto_tls_opts_set     (mosq, 1, NULL, NULL)) << endl;
        cout << "Tls set result:            " << mosquitto_strerror (mosquitto_tls_set (mosq, CERTIFICATE, nullptr, nullptr, nullptr, /*pw_cb * */ nullptr)) << endl;
#endif


        cout << "Connection result:         " << mosquitto_strerror (mosquitto_connect (mosq, MQTT_HOST, MQTT_PORT, 60)) << endl;
        cout << "Subscription result:       " << mosquitto_strerror (mosquitto_subscribe (mosq, NULL, TARGET_TOPIC, 0)) << endl;


        while (run) {
            resCode = mosquitto_loop (mosq, 20, 1);
            if (resCode) {
                cout << "ERROR:  " << mosquitto_strerror (resCode) << "  (" << resCode << ")\n";
                sleep(1);
                mosquitto_reconnect (mosq);
            }
        }

        mosquitto_destroy (mosq);
    }

    mosquitto_lib_cleanup ();

    return 0;
}

but the output is every time the same:

Connection result:    0
Subscription result:  0
ERROR:  The connection was lost.  (7)
ERROR:  The connection was lost.  (7)
ERROR:  The connection was lost.  (7)
ERROR:  The connection was lost.  (7)
ERROR:  The connection was lost.  (7)
ERROR:  The connection was lost.  (7)
ERROR:  The connection was lost.  (7)
ERROR:  The connection was lost.  (7)
ERROR:  The connection was lost.  (7)
ERROR:  The connection was lost.  (7)
ERROR:  The connection was lost.  (7)

Using an external tool (e.g. mqttfx) and using the same authentication credentials, the subscription is good and I can receive messages published on topic.

How can I perform the subscription correctly? Are there some missing settings?

Unfortunately I haven't access to machine where broker is running — Luca, Mar 18 '19 at 20:46
Your code says `localhost` so why don't you have access to to the broker logs? — hardillb, Mar 18 '19 at 20:48
Fine then use something like `example.com` as `localhost` has very specific meaning (it behaves differently to any external device) and implies a bunch of things which are not the case here. — hardillb, Mar 18 '19 at 22:13
The posted code is C++, not C. Please remove the 'c' tag from the question — user3629249, Mar 19 '19 at 18:38

score 0 · Answer 1 · answered Aug 27 '19 at 09:45

For subscribing MQTT Brokerneeds to be running at your specified host and port. In your code you're using exmaple.com(I think you meant example.com) as MQTT Broker host. There is no Broker running on this host. So it fails to connect. Use test.mosquitto.org which hosts publicly available MQTT Broker.

Or you can run your own MQTT Broker on your computer at port 8883. Then you can connect using localhost

How to connect to mqtt broker using libmosquitto and SSL/TLS in C++

1 Answers1