Issue with MQTT reconnect to Azure IoT hub, device unable to reconnect/publish

Question

Issue with MQTT reconnect to Azure IoT hub, device unable to reconnect/publish

Ben Hoffman 0

Having an issue where the client device is unable to connect back after a network disruption.

Device registers/provisions with DPS successfully
Device generates a SAS token
Device connects to IoT hub with the token successfully
Device subscribes to messages successfully, and receives messages
Device publishes messages successfully, seen in IoT Hub Explorer
We disconnect Wifi, wait for 1.5 x keepalive, see disconnect callback on the client from the MQTT library (Paho C++ v1.4.1, using Paho C v1.3.13), we see the disconnect event in the IoT Hub Explorer on the cloud side.
Device continues to queue publish messages (QOS 1)
We reconnect Wifi
Device reconnects successfully with the same connection string (client ID) to the IoT hub (and the SAS token has not expired)
Device receives subscribed messages
Device publishes a message, we see the message in IoT Hub explorer on the cloud side, and the PUBACK shows on the device side
IoT hub disconnects the device right after the first publish, we see error resultType 400000 with the following content:

{"statusCode":"400","errorMessage":"","deviceId":"E444T03658LDSB","trackingId":"064F8FAD6FC04B3BB099FEA941D617E7-G2:-TimeStamp:2025-03-01T09:32:18.832955579Z","sdkVersion":"Unknown","authType":"{"scope":"device","type":"sas","issuer":"iothub"}","maskedIpAddress":"--.---.---.XXX","protocol":"Mqtt"}

When we run the same scenario against Mosquitto MQTT hosted in the cloud the whole scenario works fine.

We are trying to figure out why the reconnect does not work as we expect, is there an IoT Hub configuration or something we need to do differently on the device around reconnect or publish to make this work?

Ben Hoffman 0 Reputation points

2025-03-03T13:34:17.0233333+00:00

I should also mention that on the CONNECT, we have clean_session = false.
Ben Hoffman 0 Reputation points

2025-03-03T14:33:27.5533333+00:00

And when clean_session = true, everything seems to work fine - however we need the persistence.
Manas R Mohanty 17,270 Reputation points Moderator

2025-03-03T15:03:33.45+00:00

Hi Ben Hoffman

From same client session, it won't be able to possible to use same sas token/connection string (identity)

We have to create a new session then connect. That is why using clean_session = true works as it creates a new session each time device disconnects.

Also, please setup retry logic to connect again to overcome intermittent issues.

Troubleshooting guide for IOT Hub

Thank you
Ben Hoffman 0 Reputation points

2025-03-03T16:14:21.7866667+00:00

Just to be clear, we are not seeing 400027.

I understand that Azure IoT hub only allows one session per device (identity), but if we are allowing the prior session to fully disconnect (we see deviceDisconnected in the IoT Hub Explorer for that device) then it seems like that is not in the way. If we want to resume the session to access the maintained state (e.g. clean_session = false) how can we use a different identity? What is meant by create a new session?

As far as retry logic, that exists in this case but we are intentionally creating the condition for the MQTT connection to disconnect at the protocol level (1.5 x Keepalive) so that we can test message persistence across connections because that is needed for our application.
Sampath 3,885 Reputation points Microsoft External Staff Moderator

2025-03-05T10:57:29.95+00:00

Hello @Ben Hoffman, Just checking in to see if the provided solution was helpful.
Sampath 3,885 Reputation points Microsoft External Staff Moderator

2025-03-06T14:20:47.04+00:00

Hello @Ben Hoffman, We still have not heard back from you. Could you please let us know if the answer provided below was helpful? If it resolves your query, kindly click Accept Answer and Yes, as it could assist other community members reading this thread. Additionally, if you have any further questions, please let us know.

2 answers

Your answer

Ben Hoffman 0 Reputation points

2025-03-03T13:34:17.0233333+00:00

I should also mention that on the CONNECT, we have clean_session = false.
Ben Hoffman 0 Reputation points

2025-03-03T14:33:27.5533333+00:00

And when clean_session = true, everything seems to work fine - however we need the persistence.
Manas R Mohanty 17,270 Reputation points Moderator

2025-03-03T15:03:33.45+00:00

Hi Ben Hoffman

From same client session, it won't be able to possible to use same sas token/connection string (identity)

We have to create a new session then connect. That is why using clean_session = true works as it creates a new session each time device disconnects.

Also, please setup retry logic to connect again to overcome intermittent issues.

Troubleshooting guide for IOT Hub

Thank you
Ben Hoffman 0 Reputation points

2025-03-03T16:14:21.7866667+00:00

Just to be clear, we are not seeing 400027.

I understand that Azure IoT hub only allows one session per device (identity), but if we are allowing the prior session to fully disconnect (we see deviceDisconnected in the IoT Hub Explorer for that device) then it seems like that is not in the way. If we want to resume the session to access the maintained state (e.g. clean_session = false) how can we use a different identity? What is meant by create a new session?

As far as retry logic, that exists in this case but we are intentionally creating the condition for the MQTT connection to disconnect at the protocol level (1.5 x Keepalive) so that we can test message persistence across connections because that is needed for our application.
Sampath 3,885 Reputation points Microsoft External Staff Moderator

2025-03-05T10:57:29.95+00:00

Hello @Ben Hoffman, Just checking in to see if the provided solution was helpful.
Sampath 3,885 Reputation points Microsoft External Staff Moderator

2025-03-06T14:20:47.04+00:00

Hello @Ben Hoffman, We still have not heard back from you. Could you please let us know if the answer provided below was helpful? If it resolves your query, kindly click Accept Answer and Yes, as it could assist other community members reading this thread. Additionally, if you have any further questions, please let us know.

Answer 1

Hello @Ben Hoffman,

welcome to this moderated Azure community forum.

Check out the answer that @Sampath provided regarding the single session per device.

Keep in mind the Azure IoT is not designed as an MQTT broker.

The IoT Hub is a cloud gateway with a registry for device registrations. It support the MQTT protocol with a limited set of topics, a alternative device connectivity strategy, etc.

If you want to communicate using a vanilla MQTT broker, honouring the V3.1,1 and V5 MQTT protocol, please check out the Azure EventGrid Namespace support for MQTT.

This is a public cloud based MQTT broker supporting the MQTT protocol.

It's not a replacement for the IoT Hub but offers separate use cases like device-2-device communication.

Check this blog post on how to connect and use all features.

If the response helped, do "Accept Answer". If it doesn't work, please let us know the progress. All community members with similar issues will benefit by doing so. Your contribution is highly appreciated.

Answer 2

Hello @Ben Hoffman,

You're correct that Azure IoT Hub only allows one session per device identity at a time. If a new connection is made with the same device identity while another session is still active, the previous session is forcibly disconnected.

Thus, if you reconnect with the same identity, the previous session will be disconnected. To resume a session and retain messages, set clean_session = false and reconnect with the same identity. Note that you cannot resume a session with a different identity, because sessions are tied to specific devices.

Creating a new session requires setting clean_session = true, which resets all previous subscriptions and messages.

To test message persistence across disconnections, disconnect at 1.5× Keepalive, ensure clean_session = false, and use QoS 1 for guaranteed delivery.

For more details, refer to this MSDOC for the Azure IoT Client MQTT State Machine.

Alternatively, if you need multiple endpoints connected to the Azure IoT Hub, you can either use unique device IDs per endpoint or use AMQP device multiplexing. Please note that there are currently bugs in this feature in azure-iot-sdk-c.

Hope this helps!

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