Enhancements Core MQTT Functionalities

Enhanced Connection Rate Limiter for Improved System Resilience. Improved system stability and responsiveness under high connection rates: Previously, when the connection rate limit was exceeded, listener acceptors would ignore new connection attempts, potentially resulting in an unrecoverable state if a large number of clients connected or reconnected frequently within a short period. Listeners now accept pending connections but immediately close them if the rate limit is reached. This reduces resource strain and improves system resilience during peak loads.

New listener option nolinger introduced: When set to true, a TCP-RST is sent immediately upon socket closure, helping to mitigate SYN flood attacks and further enhancing connection-handling efficiency.

max_connection configuration for MQTT listeners now capped by system limits: The max_connection value for MQTT listeners is now constrained by the system's limits (e.g., ulimit from the OS and node.process_limit). If configured to infinity or a value greater than the system limit, it will automatically be adjusted to match the system's maximum limit.

SSL listeners' ssl_options now validated before changes: Previously, invalid SSL options (such as unsupported TLS versions) could be accepted, causing client connection failures after a listener reconfiguration. With this update:

The node will fail to boot if a listener is configured with invalid SSL options. Requests to apply invalid SSL options via the Dashboard or config API will now fail with a 400 status code.

Authentication and Authorization Added support for using memberOf syntax in LDAP extensible match filter, for example: (&(objectClass=class)(memberOf:1.2.840.113556.1.4.1941:=CN=GroupName,OU=emqx,DC=WL,DC=com)).

Data Integration Added support for configuring exchange and routing_key as template values in the RabbitMQ producer. This enables dynamic routing based on message payloads. For example, To dynamically set the routing_key based on a field in the payload, configure it as ${payload.akey}.

Note: In batch mode, the exchange and routing_key template values must remain constant for all messages in the batch. This ensures consistent routing and avoids conflicts during batch processing.

Exposed additional metadata for RabbitMQ source actions in the rule engine, including queue, exchange, and routing_key. This allows users to access these fields directly in their rules for enhanced processing and routing logic.

Introduced vhost-style bucket access and improved redirect handling for S3-compatible storage providers. These improvements are now available in S3 Bridges and File Transfer backend configurations.

Configuration Added support for client ID override.

EMQX now provides greater flexibility by allowing custom client ID overrides using the mqtt.clientid_override={Expression} configuration. This introduces a more dynamic approach to client ID management. As part of this update, the use_userid_as_clientid and peer_cert_as_clientid options are deprecated, though they will remain available for compatibility until version 6.0.

MQTT over QUIC Improved QUIC transport, upgrade quicer to 0.1.9. Early release of remote stream resources in the abnormal scenarios. Added more troubleshooting APIs. For more details, see: https://github.com/emqx/quic/compare/0.1.6...0.1.9 .