# MQTT Preread Enables extracting client IDs and usernames from `CONNECT` packets for Message Queuing Telemetry Transport (MQTT) versions [3.1.1](http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Toc398718028) and [5.0](https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901033). When [building from the source code](https://en.angie.software//angie/docs/installation/sourcebuild.md#sourcebuild), the module must be enabled with the [build parameter](https://en.angie.software//angie/docs/installation/sourcebuild.md#configure) `--with-stream_mqtt_preread_module`. In packages and images from [our repositories](https://en.angie.software//angie/docs/installation/index.md#install-packages), the module is included in the build. ## Configuration Example ### Choosing a server in a group by client ID: ```nginx stream { mqtt_preread on; upstream mqtt { hash $mqtt_preread_clientid; # ... } } ``` ## Directives ### mqtt_preread | [Syntax](https://en.angie.software//angie/docs/configuration/configfile.md#configfile) | `mqtt_preread` `on` | `off`; | |------------------------------------------------------------------------------------------|--------------------------------| | Default | `mqtt_preread off;` | | [Context](https://en.angie.software//angie/docs/configuration/configfile.md#configfile) | stream, server | Controls extracting information from `CONNECT` packets during the [preread phase](https://en.angie.software//angie/docs/configuration/processing.md#stream-sessions). If the parameter is enabled (`on`), the variables listed below are populated in the context where it is specified. ## Built-in Variables For detailed description of value semantics, see the MQTT protocol specification versions [3.1.1](http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Toc398718031) and [5.0](https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901033). ### `$mqtt_preread_clientid` Unique client identifier. ### `$mqtt_preread_username` Optional username.