Mqtt forwarder set in Kerlink is not communicating when backhaul is GSM but is ok in Ethernet


We have set into a Kerlink IfemtoCell gateway in AS923 Mhz the ChirpStack MQTT Forwarder so that our chirpstack server which is in EU868 Mhz can receive AS923 mqtt frames.
It is working without any trouble when we use Ethernet as backhaul.

When we disconnect the Ethernet cable, the log files shows that the GSM tries to send the data to the MQTT server, but there is a Timeout and it cannot send data.
To be more precise, we see in the log that it receives the PULL_DATA received, but when sending the PULL_ACK, then it cannot reach the mqtt server and get a timeout.

We are using the latest version of Chirpstack.

Is there any specific configuration to do for GSM backhaul when using the ChirpStack MQTT Forwarder ?

Could you please clarify if the issue is on the UDP or the MQTT connection path? Because the PULL_DATA and PULL_ACK messages are on the UDP connection path and thus on the localhost (assuming the MQTT Forwarder is running on the gateway).

The issue is on the MQTT : in the log, when we are on GSM, we see “MQTT error : time out”
Yes, the MQTT Forwarder is running on the gateway.

Thanks for clarifying. Unfortunately I’m not able to reproduce, but any feedback is welcome. It sounds like MQTT traffic is not allowed when using the GSM backhaul, maybe there are different firewall rules for ethernet vs cellular? Have you tried booting the gateway without ethernet and does the MQTT Forwarder work in that case?

Please let me know if you (or somebody else) has found a solution for this.