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Verification tools

Most MQTT problems come down to one question: who sent what to which topic, and whom did the broker forward it to? This page lists the tools that answer it.

Watch traffic with mosquitto_sub

Section titled “Watch traffic with mosquitto_sub”

Run mosquitto_sub with a wildcard filter to see the messages that pass through the broker. Always pass -V 5 to see MQTT 5.0 properties.

터미널 창
mosquitto_sub -V 5 -t 'opcua/#' -F '%t qos=%q retain=%r content-type=%C user-properties=%P %l bytes'
-F code Prints
%t The topic
%q, %r The QoS and RETAIN flag as received
%C The Content Type property
%P User properties (name:value, comma-separated)
%l The payload length in bytes
%x The payload in hex
%p The raw payload
%I The time of receipt (ISO 8601)
%j The whole message as one JSON line

-F '%j' includes the properties in JSON:

{"tst":"2026-10-15T02:07:35.001558+0900","topic":"demo/a","qos":0,"retain":false,"payloadlen":2,"properties":{"content-type":"text/plain","user-properties":[{"k":"v"}]},"payload":"hi"}

Other useful options:

Option Meaning
-q 1 Subscribe at QoS 1 instead of the default QoS 0.
-C <count> Exit after that many messages.
-W <seconds> Print Timed out and exit with code 27 if the client is still running after that time.
-N Don’t add a newline after the payload; use it to save binary payloads to a file.
-d Print the packets the client sends and receives.

Inject messages with mosquitto_pub

Section titled “Inject messages with mosquitto_pub”

To test whether a simulator subscriber accepts a message, publish it with the OPC UA PubSub properties:

터미널 창
mosquitto_pub -V 5 -q 1 -t opcua/uadp/data/77/demo \
  -D publish content-type application/opcua+uadp \
  -D publish user-property UAMessageType ua-data \
  -f dataset.bin

-f sends the file content as the payload. Publish and subscribe with the simulator shows how to capture a UADP payload.

Read the broker log

Section titled “Read the broker log”

Mosquitto started with mosquitto -v or log_type all writes one line per packet:

New client connected from 127.0.0.1:64658 as pdcomm-opcua-sim-79619-17 (p5, c1, k30).
Received SUBSCRIBE from pdcomm-opcua-sim-79619-17
  opcua/uadp/data/77/quickstart (QoS 1)
Received PUBLISH from pdcomm-opcua-sim-79619-17 (d0, q1, r0, m2, 'opcua/uadp/data/77/quickstart', ... (48 bytes))
Sending PUBLISH to pdcomm-opcua-sim-79619-17 (d0, q1, r0, m1, 'opcua/uadp/data/77/quickstart', ... (48 bytes))
Sending PUBACK to pdcomm-opcua-sim-79619-17 (m2, rc0)
Notation Meaning
p5, c1, k30 MQTT 5.0, Clean Start 1, keep alive 30 seconds. A certificate or user name adds u'…'.
d0, q1, r0, m2 DUP 0, QoS 1, RETAIN 0, packet identifier 2
rc0 Reason code 0x00 (Success); rc135 is 0x87 (Not authorized)
Denied PUBLISH from … An ACL refused the publication.
disconnected: not authorised. Authentication failed and the broker closed the connection.

The simulator’s client identifier is pdcomm-opcua-sim-<process ID>-<DataSetWriterId>, which tells you which simulator a log line belongs to.

$SYS topics

Section titled “$SYS topics”

Mosquitto publishes broker information under $SYS/broker/ and refreshes it every sys_interval seconds (10 by default):

터미널 창
mosquitto_sub -V 5 -t '$SYS/broker/version' -t '$SYS/broker/clients/connected' -t '$SYS/broker/messages/received' -v
$SYS/broker/version mosquitto version 2.1.2

Filters that start with # don’t match $SYS, so subscribe to $SYS topics explicitly.

Read the simulator status

Section titled “Read the simulator status”

The PubSub status of the simulator (the GUI status line or headless pubsub_status) tells you a lot on its own:

You see It means Check next
sent stays above acknowledged The broker didn’t send a PUBACK, or answered with a failure. MQTT publish refused: 0x.. in error, Denied PUBLISH in the broker log
rejected grows Messages arrived but their format or properties didn’t match. The error value and Delivery problems
The observer sees messages but received doesn’t change The topic differs, or the broker withholds messages the simulator may not read. The simulator topic and the ACL
running is false and error starts with MQTT: The broker connection dropped. The broker log and Connection problems
dropped grows Nobody took the received messages, and the queue of 256 overflowed. Run pubsub_received or click Show received more often.

The simulator trace

Section titled “The simulator trace”

The simulator records every action in its trace. In the GUI, click Refresh trace on the Trace & Diagnostics tab; in headless mode, send {"action":"trace"}. Failed actions keep their error message:

{"action":"pubsub_start","elapsed_us":112,"error":"MQTT broker connection failed: MQTT transport I/O: ConnectionRefused","ok":false,"sequence":2,"time_ms":0,"workspace":"pubsub"}

Turn on the diagnostic log file with Diagnostic log (off by default) and Apply logging on the same tab, or with logging_configure in headless mode:

{"action":"logging_configure","config":{"enabled":true,"file":"logs/opcua-simulator.jsonl"}}

The log format and levels are described with the diagnostic logging docs in the Tools section.

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