Common mosquitto_sub errors when connecting to your MQTT broker and how to fix them

[francisco.lopez]

Here is a list of known errors that you can get while connecting with mosquitto_sub. The following indications use -d (–debug) option to get more verbose output.

Consider the following indications.

1. Firewall in place. If you run mosquitto_sub with -d option (–debug) and get no single
   line of output like this:

   >> mosquitto_sub -d -h node02.myqtthub.com  -i myHubxxxxIdMosq -u myHubxxxxIdMosqUsr -P xxxx -t message/topic/to/subscribe -q 2
   (command issued, waiting and without showing no input)
   

   Then check your connection see if some firewall is blocking 1883/tcp or 8883/tcp.

2. DNS problem. If you run your mosquitto_sub with -d option (–debug) and there is a DNS
   o route gateway problem, you will get something like:

   >> mosquitto_sub -d -h node02.myqtthub.com  -i myHubxxxxIdMosq -u myHubxxxxIdMosqUsr -P xxxx -t message/topic/to/subscribe -q 2 
   Unable to connect (Lookup error.).
   

3. Missing default route gateway. If you run your mosquitto_sub with -d option (–debug) and without
   a default route gateway, you will get the following error:

   >>> mosquitto_sub -d -h node02.myqtthub.com  -i myHubxxxxxxIdMosq -u myHubxxxxxxIdMosqUsr -P xxxxx -t message/topic/to/subscribe -q 2 
   Error: Network is unreachable
   

4. Wrong clientId, userName or password. If you run your mosquitto_sub with -d option (–debug) and using wrong clientId, userName or password,
   you will get:

   >> mosquitto_sub -d -h node02.myqtthub.com  -i myHubxxxxxxIdMosq -u myHubxxxxxxIdMosqUsr -P test1234f -t message/topic/to/subscribe -q 2 
   Client myHubxxxxxxIdMosq sending CONNECT
   Client myHubxxxxxxIdMosq received CONNACK (3)
   Connection Refused: broker unavailable.
   Client myHubxxxxxxIdMosq sending DISCONNECT
   
   

5. Output expected. If you run your mosquitto_sub with --debug, you should get the following
   if you connect without any issue:

   >> mosquitto_sub -d -h node02.myqtthub.com  -i myHubxxxxIdMosq -u myHubxxxxIdMosqUsr -P xxx -t message/topic/to/subscribe -q 2 
   Client myHubxxxxxxIdMosq sending CONNECT
   Client myHubxxxxxxIdMosq received CONNACK (0)
   Client myHubxxxxxxIdMosq sending SUBSCRIBE (Mid: 1, Topic: message/topic/to/subscribe, QoS: 2)
   Client myHubxxxxxxIdMosq received SUBACK
   Subscribed (mid: 1): 2
   Client myHubxxxxxxIdMosq sending PINGREQ
   Client myHubxxxxxxIdMosq received PINGRESP
   Client myHubxxxxxxIdMosq sending PINGREQ
   Client myHubxxxxxxIdMosq received PINGRESP