MQTT: update docs (#16152)

filebeat/docs/filebeat-options.asciidoc

@@ -51,6 +51,7 @@ You can configure {beatname_uc} to use the following inputs:
 * <<{beatname_lc}-input-docker>>
 * <<{beatname_lc}-input-tcp>>
 * <<{beatname_lc}-input-syslog>>
+* <<{beatname_lc}-input-mqtt>>
 * <<{beatname_lc}-input-s3>>
 * <<{beatname_lc}-input-netflow>>
 * <<{beatname_lc}-input-google-pubsub>>
@@ -74,6 +75,8 @@ include::inputs/input-tcp.asciidoc[]
 
 include::inputs/input-syslog.asciidoc[]
 
+include::inputs/input-mqtt.asciidoc[]
+
 include::../../x-pack/filebeat/docs/inputs/input-aws-s3.asciidoc[]
 
 include::../../x-pack/filebeat/docs/inputs/input-netflow.asciidoc[]

filebeat/docs/inputs/input-mqtt.asciidoc

@@ -0,0 +1,81 @@
+:type: mqtt
+
+[id="{beatname_lc}-input-{type}"]
+=== MQTT input
+
+++++
+<titleabbrev>MQTT</titleabbrev>
+++++
+
+Use the `MQTT` input to read data transmitted using lightweight messaging protocol
+for small and mobile devices, optimized for high-latency or unreliable networks.
+
+This input connects to the MQTT broker, subscribes to selected topics and parses data
+into common message lines. Everything happens before line filtering, multiline, and JSON decoding,
+so this input can be used in combination with those settings.
+
+Example configuration:
+
+["source","yaml",subs="attributes"]
+----
+{beatname_lc}.inputs:
+- type: mqtt
+  hosts: <1>
+    - tcp://broker:1883
+    - ssl://secure_broker:8883
+  topics: <2>
+    - sample_topic
+----
+
+<1> `hosts` are required.
+
+<2> `paths` are required.
+
+All other settings are optional.
+
+==== Configuration options
+
+The `mqtt` input supports the following configuration options plus the
+<<{beatname_lc}-input-{type}-common-options>> described later.
+
+===== `hosts`
+
+A list of MQTT brokers to connect to.
+
+===== `topics`
+
+A list of topics to subscribe to and read from.
+
+===== `qos`
+
+An agreement level between the sender of a message and the receiver of a message that defines the guarantee of delivery.
+
+There are 3 QoS levels in MQTT:
+
+* At most once (`0`),
+* At least once (`1`),
+* Exactly once (`2`).
+
+===== `client_id`
+
+A unique identifier of each MQTT client connecting to a MQTT broker.
+
+===== `username`
+
+A client username used for authentication provided on the application level by the MQTT protocol.
+
+===== `password`
+
+A client password used for authentication provided on the application level by the MQTT protocol.
+
+===== `ssl`
+
+Configuration options for SSL parameters like the certificate, key and the certificate authorities
+to use.
+
+See <<configuration-ssl>> for more information.
+
+[id="{beatname_lc}-input-{type}-common-options"]
+include::../inputs/input-common-options.asciidoc[]
+
+:type!:

filebeat/input/mqtt/config.go

@@ -28,8 +28,8 @@ type mqttInputConfig struct {
 	Topics []string `config:"topics" validate:"required,min=1"`
 	QoS    int      `config:"qos" validate:"nonzero,min=0,max=2"`
 
-	ClientID string `config:"clientID" validate:"nonzero"`
-	Username string `config:"user"`
+	ClientID string `config:"client_id" validate:"nonzero"`
+	Username string `config:"username"`
 	Password string `config:"password"`
 
 	TLS *tlscommon.Config `config:"ssl"`