How can I detect problems with user/password in the connection to MQTT broker at connection time using the libmosquitto library?

Question

I'm testing MQTT mosquitto library with this small program:

/*
  compile using:
  $ gcc -o libmosq libmosq.c -lmosquitto
*/
#include <stdio.h>
#include <mosquitto.h>
#include <stdlib.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
  struct mosquitto *mosq = NULL;
 
  mosquitto_lib_init();
  mosq = mosquitto_new(NULL, true, NULL);
  if(!mosq) {
     fprintf(stderr, "Error: Out of memory.\n");
     exit(1);
  } 
    
  mosquitto_username_pw_set(mosq, "user1", "passwd1");

  int resultCode = mosquitto_connect(mosq, "localhost", 1883, 60);
  if (resultCode != MOSQ_ERR_SUCCESS) {
    fprintf(stderr, "connection error\n");
    exit(1);
  }
  else {
    printf("connection success\n");
  }
  // wait until control+C is done
  sleep(1000000);
}

I'm running a MQTT broker (mosquitto broker version 1.6.10) in localhost at port 1883.

When I run the program I get "connection success" and I see in the mosquitto log:

iot-mosquitto    | 2021-10-06T10:16:11: New connection from 172.17.0.1 on port 1883.
iot-mosquitto    | 2021-10-06T10:16:11: New client connected from 172.17.0.1 as auto-51085B64-A53B-DBE1-DBFB-A6D9D702B69C (p2, c1, k60, u'user1').

I understand that in this case the connection is correct. So far so good.

However, if I use a wrong user/pass (e.g. mosquitto_username_pw_set(mosq, "user1", "xxxxx")) or if don't use user/pass (i.e. removing the mosquitto_username_pw_set() map) I get in the mosquitto broker log:

iot-mosquitto    | 2021-10-06T10:27:58: New connection from 172.17.0.1 on port 1883.
iot-mosquitto    | 2021-10-06T10:27:58: Socket error on client <unknown>, disconnecting.

which is fine. The problem is that in my program I get "connection success" instead of "connection error". In other words, I'm getting MOSQ_ERR_SUCCESS as return value for mosquitto_connect() instead of MOSQ_ERR_ERRNO.

Looking to the MQTT broker traces, it is like my program gets connected (which would explain the MOSQ_ERR_SUCCESS) but instanstanely it's disconnected...

How can I detect problems with user/password in the connection at connection time using the libmosquitto library, please?

Thanks in advance!

EDIT: I understand that there is some way to solve this, as mosquitto_sub (which I understand is based in the same library) is able to detect. Eg:

$ mosquitto_sub -p 1883 -t '#' -u user1 -P xxxxxx
Connection error: Connection Refused: not authorised.

This seems to be the source code for mosquito_sub: github.com/eclipse/mosquitto/blob/master/client/sub_client.c — fgalan
– fgalan, Commented Oct 6, 2021 at 17:59

fgalan · Accepted Answer · 2021-10-13 08:18:57Z

I finally solved it using the following program:

/*
  compile using:
  $ gcc -o libmosq libmosq.c -lmosquitto
*/
#include <stdio.h>
#include <mosquitto.h>
#include <stdlib.h>
#include <unistd.h>

void connection_callback(struct mosquitto* mosq, void *obj, int rc)
{
  if (rc) {
    printf("connection error: %d (%s)\n", rc, mosquitto_connack_string(rc));
    exit(1);
  }
  else {
    printf("connection success\n");
  }
}

int main(int argc, char *argv[])
{
  struct mosquitto *mosq = NULL;
  
  mosquitto_lib_init();
  mosq = mosquitto_new(NULL, true, NULL);
  if(!mosq) {
     fprintf(stderr, "Error: Out of memory.\n");
     exit(1);
  }

  mosquitto_connect_callback_set(mosq, connection_callback);
  mosquitto_username_pw_set(mosq, "user1", "passwd1");

  int resultCode = mosquitto_connect(mosq, "localhost", 1883, 60);
  if (resultCode != MOSQ_ERR_SUCCESS) {
    fprintf(stderr, "error calling mosquitto_connect\n");
    exit(1);
  }

  int loop = mosquitto_loop_start(mosq);
  if(loop != MOSQ_ERR_SUCCESS){
    fprintf(stderr, "Unable to start loop: %i\n", loop);
    exit(1);
  }

  // hang until control+C is done
  sleep(1000000);
}

Main differences compared with the first version:

Using mosquitto_connect_callback_set() to set a callback function for the connection event
Using mosquitto_loop_start(). If I don't add this statement, the callback is not called upon connection

With this program I get the following message when connection is successful:

connection success

and this one when password is wrong or if I remove the mosquitto_username_pw_set() statement:

connection error: 5 (Connection Refused: not authorised.)

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