Serial lte modem mqtt connect timeout problem

RE: Serial lte modem mqtt connect timeout problem

Simon D-M — Thu, 15 Jan 2026 12:39:03 GMT

Hi Niuniu,

in NCS, our MQTT implementation is working with BSD socket under the hood.

So the timeout delay should be able to be modified with the "CONFIG_NET_SOCKETS_CONNECT_TIMEOUT" Kconfig (link). By default, this value is at 3000ms, and has a range from 0ms up to 60000ms.

Best regards,

Simon D-M

RE: Serial lte modem mqtt connect timeout problem

brave_niuniu — Thu, 15 Jan 2026 08:27:14 GMT

Hi simonm

I'm sorry for my late reply. The version of SLM is built based on ncs2.9.2.

I connected to the correct broker IP and port, but added an incorrect SSL certificate, and mqtt_connect() waited for a while before returning the result. I just want to know if Zephyr has a connection timeout handling for the interior of mqtt_connect, and if so, what the timeout period is.

Best regards,

Niuniu

RE: Serial lte modem mqtt connect timeout problem

Simon D-M — Tue, 13 Jan 2026 14:22:05 GMT

Hi,

Sorry, I misunderstood your problem.

I wasn't able to reproduce your delay, what version of the SDK / SLM are you using ? I believe some changes to MQTT were made in the latest versions.

Best regards,

Simon D-M

RE: Serial lte modem mqtt connect timeout problem

brave_niuniu — Tue, 13 Jan 2026 06:39:35 GMT

Hi simon,

I monitored the logs of nrf9160, and it really has nothing to do with semaphores; it's an error returned by mqtt_connect();

I want to know the timeout period returned by mqtt_connect() when the MQTT broker parameters are configured incorrectly, because I need to set a time limit in my application.

Best regards,

Niuniu

RE: Serial lte modem mqtt connect timeout problem

Simon D-M — Mon, 12 Jan 2026 12:07:28 GMT

Hi,

This is strange, because modem_cmd_send() uses default Zephyr semaphores, which should be working fine. So it doesn't make sense that it returns after 170 seconds.

What I believe is most likely to be happening, is that maybe after 170 seconds there is something sent to the UART. Can you maybe monitor the UART between the 2 chips to see if there is something happening on it while the function is waiting for an answer?

Best regards,
Simon D-M