Connecting to MQTT broker using NRF5340 with NRF9161 setup as SLM

[AdrianT]

I am planning to modify the SLM application (NRF9161) to load its own certificates for TLS upon startup. This should be doable via modem_key_mgmt functions.

However, NRF5340 will still manage the MQTT connection to the broker. But in this case, it has no information on the certificates since it is only stored in NRF9161. Is my assumption correct that the MQTT transport type selected should be MQTT_TRANSPORT_NON_SECURE?

[Charlie]

Hi AdrianT,

You can still use secure connection, as long as your nRF9161 has certificates in the pointed sec_tag. It does not matter which command host you are using.

See MQTT client AT commands (nordicsemi.com):

AT#XMQTTCON=<op>[,<username>,<password>,<url>,<port>[,<sec_tag>]]

• The <op> parameter is an integer. It can accept one of the following values:

  • 0 - Disconnect from the MQTT broker.

  • 1 - Connect to the MQTT broker using IP protocol family version 4.

  • 2 - Connect to the MQTT broker using IP protocol family version 6.

• The <username> parameter is a string. It indicates the MQTT client username.

• The <password> parameter is a string. It indicates the MQTT client password in cleartext.

• The <url> parameter is a string. It indicates the MQTT broker hostname.

• The <port> parameter is an unsigned 16-bit integer (0 - 65535). It indicates the MQTT broker port.

• The <sec_tag> parameter is an integer. It indicates the credential of the security tag used for establishing a secure connection.

Try to play this sample with Serial Terminal first to try MQTT TLS connection and you can refer to the following discussion. 

(+) Are there any examples out there for a host application in C that is using AT commands via serial to the nRF9160 running the Serial LTE Modem for a MQTT TLS connection? - Nordic Q&A - Nordic DevZone - Nordic DevZone (nordicsemi.com)

Best regards,

Charlie

[AdrianT]

Thanks for the response.

I am using the mqtt subystem in zephyr - thus, I am not directly using the AT Commands that you gave. Instead, I am calling mqtt_connect() function with the corresponding configurations populated in mqtt_client struct (in mqtt.h file).

Is this still applicable?

[Charlie]

Never tried, but I believe it is possible.

I assume you want to run MQTT protocol stack on nRF5340, then you can use TCP socket from SLM with TLS supported.

[AdrianT]

Yes that is correct.