---
title: "NanoMQTT"
source_url: https://dev.digicert.com/trustcore-sdk/nanomqtt.html
---

NanoMQTT is the MQTT client included in TrustCore SDK (beginning with the **[U6](https://dev.digicert.com/md/trustcore-sdk/release-notes.md)** release). It supports **MQTT 3.1.1** and **MQTT 5**, allowing your application to publish and subscribe over plain TCP (`port 1883`) or TLS (`port 8883`) by re-using TrustCore SDK NanoSSL transport.


> **Info**
>
> NanoMQTT is a *client*. You still need an MQTT broker (for example, Mosquitto, AWS IoT Core, Azure Event Grid MQTT) for testing and production.


NanoMQTT is available in two models:

## License

This project is available under a **dual-license model**:

- **Open Source License**:
  [GNU Affero General Public License v3 (AGPL v3)](https://github.com/digicert/trustcore/blob/main/LICENSE.md): This license allows you to use, modify, and distribute the code for free in accordance with AGPL terms.
- **Commercial License**:
  If you wish to use TrustCore SDK in a proprietary or commercial product (e.g., embedded in closed-source firmware or commercial SaaS applications), a commercial license is available under DigiCert’s [Master Services Agreement](https://www.digicert.com/master-services-agreement/) (MSA). Contact us at [sales@digicert.com](mailto:sales@digicert.com) for commercial licensing details.

## Key features

- QoS 0, 1, 2 publish/subscribe with retain flag, wildcard topics, and last-will messages.
- MQTT 5 properties on *CONNECT*, *PUBLISH*, *SUBSCRIBE*, *UNSUBSCRIBE*, *WILL*, and *DISCONNECT* packets.
- Session control using *clean start* or a *session-expiry* interval (0 - 4 294 967 295 s).
- Transport flexibility by using plain TCP or TLS through NanoSSL.
- Proxy support via the `--proxy` flag (HTTP or HTTPS forward proxies).

## Configurable limits

| Item | Default | Notes |
| --- | --- | --- |
| Topic length | 65 535 bytes | MQTT spec limit |
| Publish QoS level (0 / 1 / 2) | 0 | Select per message via `--mqtt_pub_qos` |
| Maximum packet size accepted | Unlimited | Limit with MQTT 5 *CONNECT* property `max_packet_size` |
| Receive-maximum (parallel QoS 1/2) | Unlimited | Limit with MQTT 5 *CONNECT* property `receive_maximum` |

## Protocol compliance

| Control-packet family | MQTT 3.1.1 | MQTT 5 (extensions) |
| --- | --- | --- |
| CONNECT / CONNACK | ✔ | ✔ (reason codes, user properties) |
| PUBLISH flow (ACK, REC, REL, COMP) | ✔ | ✔ |
| SUBSCRIBE / SUBACK | ✔ | ✔ (retain-handling, retain-as-published, no-local) |
| UNSUBSCRIBE / UNSUBACK | ✔ | ✔ |
| DISCONNECT | ✔ | ✔ (reason codes, session-expiry) |
| AUTH | — | ✔ |

## Usage with other TrustCore SDK modules

| Component | Usage in NanoMQTT |
| --- | --- |
| **NanoSSL** | Provides TLS record protection. Call `MQTT_setTransportSSL()` with an active NanoSSL connection to secure the session. |
| **NanoCrypto** | Supplies hashing and cipher routines — no duplicate crypto code. |
| **Common socket layer** | NanoMQTT uses the same non-blocking socket helpers as other TrustCore modules, enabling a single event loop. |

## Quick start

```bash
# Build the sample client (adds TLS support)
./scripts/nanomqtt/mqtt_client/build_mqtt_client.sh --ssl

# Terminal 1 — subscribe (plain TCP, port 1883)
./bin/mqtt_client_sample \
--mqtt_servername https://test.mosquitto.org/
--mqtt_port 1883 \
--mqtt_sub_topic demo/led \
--mqtt_clean_start

# Terminal 2 — publish (TLS, port 8883)
./bin/mqtt_client_sample \
--mqtt_servername https://test.mosquitto.org/
--mqtt_port 8883 \
--mqtt_transport SSL \
--ssl_ca_file /etc/ssl/certs/ca-bundle.crt \
--mqtt_pub_topic demo/led \
--mqtt_pub_message "ON" \
--mqtt_clean_start
```

The subscriber prints:

```
demo/led  ON
```


> **Tip**
>
> During early tests, you can add `--ssl_allow_untrusted` to skip certificate validation. **Do not use this flag in production.**

## In this section

- [Get started](https://dev.digicert.com/md/trustcore-sdk/nanomqtt/get-started.md)
- [Embed NanoMQTT in C](https://dev.digicert.com/md/trustcore-sdk/nanomqtt/embed-nanomqtt-in-c.md)
- [Command-line reference](https://dev.digicert.com/md/trustcore-sdk/nanomqtt/command-line-reference.md)
- [Integration notes](https://dev.digicert.com/md/trustcore-sdk/nanomqtt/integration-notes.md)
- [Troubleshoot](https://dev.digicert.com/md/trustcore-sdk/nanomqtt/troubleshoot.md)