Provided by: mosquitto_2.0.18-1build3_amd64 
NAME
mosquitto - an MQTT broker
SYNOPSIS
mosquitto [-c config file] [-d | --daemon] [-p port number] [-v]
DESCRIPTION
mosquitto is a broker for the MQTT protocol version 5.0/3.1.1/3.1.
OPTIONS
-c, --config-file
Load configuration from a file. If not given, then the broker will listen on port 1883 bound to the
loopback interface, and the default values as described in mosquitto.conf(5) are used.
Important
See the -p option for a description of changes in behaviour from 1.6.x to 2.0.
-d, --daemon
Run mosquitto in the background as a daemon. All other behaviour remains the same.
-p, --port
Listen on the port specified. May be specified up to 10 times to open multiple sockets listening on
different ports.
Important
In version 1.6.x and earlier, the listener defined by -p (or the default port of 1883) would be
bound to all interfaces and so be accessible from any network. It could also be used in
combination with -c.
From version 2.0 onwards, the listeners defined with -p are bound to the loopback interface only,
and so can only be connected to from the local machine. If both -p is used and a listener is
defined in a configuration file, then the -p options are IGNORED.
-v, --verbose
Use verbose logging. This is equivalent to setting log_type to all in the configuration file. This
overrides and logging options given in the configuration file.
CONFIGURATION
The broker can be configured using a configuration file as described in mosquitto.conf(5) and this is the
main point of information for mosquitto. The files required for SSL/TLS support are described in
mosquitto-tls(7).
PLATFORM LIMITATIONS
Some versions of Windows have limitations on the number of concurrent connections due to the Windows API
being used. In modern versions of Windows, e.g. Windows 10 or Windows Server 2019, this is approximately
8192 connections. In earlier versions of Windows, this limit is 2048 connections.
MQTT SUPPORT
Mosquitto supports MQTT v5.0, v3.1.1, and v3.1.
MQTT v5.0
Mosquitto provides full MQTT v5.0 support, but some features are not used directly. The following
sections describe the new features and explain where Mosquitto does not make use of a feature.
Features
Enhanced authentication
Basic MQTT authentication uses username/password checks. Enhanced authentication allows different
authentication schemes to be integrated into MQTT, and even those schemes with multiple step
processes. Clients request a particular type of authentication and if the broker is configured
for that scheme the authentication continues. Mosquitto supports enhanced authentication through
plugins.
Error handling
Most MQTT packets now have the concept of a reason code which indicates success or failure, and
what the failure was. Mosquitto provides full support for reason codes, but does not make use of
the reason string feature which can be used to provide a human readable error string to explain
the reason code.
Flow control
The number of "in flight" messages for QoS 1 and QoS 2 can be controlled by both the client and
the broker.
Request / response
MQTT v5.0 adds a request/response pattern that allows a client to publish a message and instruct
the subscribers of that message where to publish a response.
Server redirection
Server redirection is the concept of telling a client to connect to a different MQTT broker,
either on CONNECT or with a broker initiated DISCONNECT. Mosquitto does not currently make use of
this feature.
Shared subscriptions
When multiple clients subscribe to the same shared subscription, only one client out of the group
will receive each message which allows for distributing work loads.
Packet properties
MQTT v5.0 allows properties to be added to packets to control certain behaviour. Unless noted,
Mosquitto support the properties listed below.
CONNECT
• Authentication data
• Authentication method
• Maximum packet size
• Receive maximum
• Request problem information - supported but not used
• Request response information - supported but not used
• Session expiry interval
• Topic alias maximum
• User property
Last will and testament
• Content type
• Correlation data
• Message expiry interval
• Payload format indicator
• Response topic
• User property
• Will delay interval
CONNACK
• Assigned client identifier
• Authentication data
• Authentication method
• Maximum packet size
• Maximum qos
• Reason string - supported but not used
• Receive maximum
• Response information - supported but not used
• Retain available
• Server keep alive
• Server reference - supported but not used
• Session expiry interval
• Shared subscription available
• Subscription identifiers available
• Topic alias maximum
• User property
• Wildcard subscription available
PUBLISH
• Content type
• Correlation data
• Message expiry interval
• Payload format indicator
• Response topic
• Subscription identifier
• Topic alias
• User property
PUBACK / PUBREC / PUBREL / PUBCOMP / SUBACK / SUBSCRIBE / SUBACK
• Reason string - supported but not used
• User property
SUBSCRIBE
• Subscription identifier
• User property
DISCONNECT
• Reason string - supported but not used
• Server reference - supported but not used
• Session expiry interval
• User property
AUTH
• Authentication method
• Authentication data
• Reason string - supported but not used
• User property
MQTT v3.1.1
Mosquitto provides full MQTT v3.1.1 support.
MQTT v3.1
Mosquitto provides full MQTT v3.1 support.
MQTT v3
MQTT v3 is an obsolete version of the protocol that does not support username/password authentication and
used the clean start flag in the CONNECT packet which applied only to the start of a session. An MQTT v3
client will be able to successfully connect to a Mosquitto instance that does not require authentication.
BROKER STATUS
Clients can find information about the broker by subscribing to topics in the $SYS hierarchy as follows.
Topics marked as static are only sent once per client on subscription. All other topics are updated every
sys_interval seconds. If sys_interval is 0, then updates are not sent.
Note that if you are using a command line client to interact with the $SYS topics and your shell
interprets $ as an environment variable, you need to place the topic in single quotes '$SYS/...' or to
escape the dollar symbol: \$SYS/... otherwise the $SYS will be treated as an environment variable.
$SYS/broker/bytes/received
The total number of bytes received since the broker started.
$SYS/broker/bytes/sent
The total number of bytes sent since the broker started.
$SYS/broker/clients/connected, $SYS/broker/clients/active (deprecated)
The number of currently connected clients.
$SYS/broker/clients/expired
The number of disconnected persistent clients that have been expired and removed through the
persistent_client_expiration option.
$SYS/broker/clients/disconnected, $SYS/broker/clients/inactive (deprecated)
The total number of persistent clients (with clean session disabled) that are registered at the
broker but are currently disconnected.
$SYS/broker/clients/maximum
The maximum number of clients that have been connected to the broker at the same time.
$SYS/broker/clients/total
The total number of active and inactive clients currently connected and registered on the broker.
$SYS/broker/connection/#
When bridges are configured to/from the broker, common practice is to provide a status topic that
indicates the state of the connection. This is provided within $SYS/broker/connection/ by default. If
the value of the topic is 1 the connection is active, if 0 then it is not active. See the Bridges
section below for more information on bridges.
$SYS/broker/heap/current size
The current size of the heap memory in use by mosquitto. Note that this topic may be unavailable
depending on compile time options.
$SYS/broker/heap/maximum size
The largest amount of heap memory used by mosquitto. Note that this topic may be unavailable
depending on compile time options.
$SYS/broker/load/connections/+
The moving average of the number of CONNECT packets received by the broker over different time
intervals. The final "+" of the hierarchy can be 1min, 5min or 15min. The value returned represents
the number of connections received in 1 minute, averaged over 1, 5 or 15 minutes.
$SYS/broker/load/bytes/received/+
The moving average of the number of bytes received by the broker over different time intervals. The
final "+" of the hierarchy can be 1min, 5min or 15min. The value returned represents the number of
bytes received in 1 minute, averaged over 1, 5 or 15 minutes.
$SYS/broker/load/bytes/sent/+
The moving average of the number of bytes sent by the broker over different time intervals. The final
"+" of the hierarchy can be 1min, 5min or 15min. The value returned represents the number of bytes
sent in 1 minute, averaged over 1, 5 or 15 minutes.
$SYS/broker/load/messages/received/+
The moving average of the number of all types of MQTT messages received by the broker over different
time intervals. The final "+" of the hierarchy can be 1min, 5min or 15min. The value returned
represents the number of messages received in 1 minute, averaged over 1, 5 or 15 minutes.
$SYS/broker/load/messages/sent/+
The moving average of the number of all types of MQTT messages sent by the broker over different time
intervals. The final "+" of the hierarchy can be 1min, 5min or 15min. The value returned represents
the number of messages send in 1 minute, averaged over 1, 5 or 15 minutes.
$SYS/broker/load/publish/dropped/+
The moving average of the number of publish messages dropped by the broker over different time
intervals. This shows the rate at which durable clients that are disconnected are losing messages.
The final "+" of the hierarchy can be 1min, 5min or 15min. The value returned represents the number
of messages dropped in 1 minute, averaged over 1, 5 or 15 minutes.
$SYS/broker/load/publish/received/+
The moving average of the number of publish messages received by the broker over different time
intervals. The final "+" of the hierarchy can be 1min, 5min or 15min. The value returned represents
the number of publish messages received in 1 minute, averaged over 1, 5 or 15 minutes.
$SYS/broker/load/publish/sent/+
The moving average of the number of publish messages sent by the broker over different time
intervals. The final "+" of the hierarchy can be 1min, 5min or 15min. The value returned represents
the number of publish messages sent in 1 minute, averaged over 1, 5 or 15 minutes.
$SYS/broker/load/sockets/+
The moving average of the number of socket connections opened to the broker over different time
intervals. The final "+" of the hierarchy can be 1min, 5min or 15min. The value returned represents
the number of socket connections in 1 minute, averaged over 1, 5 or 15 minutes.
$SYS/broker/messages/inflight
The number of messages with QoS>0 that are awaiting acknowledgments.
$SYS/broker/messages/received
The total number of messages of any type received since the broker started.
$SYS/broker/messages/sent
The total number of messages of any type sent since the broker started.
$SYS/broker/publish/messages/dropped
The total number of publish messages that have been dropped due to inflight/queuing limits. See the
max_inflight_messages and max_queued_messages options in mosquitto.conf(5) for more information.
$SYS/broker/publish/messages/received
The total number of PUBLISH messages received since the broker started.
$SYS/broker/publish/messages/sent
The total number of PUBLISH messages sent since the broker started.
$SYS/broker/retained messages/count
The total number of retained messages active on the broker.
$SYS/broker/store/messages/count, $SYS/broker/messages/stored (deprecated)
The number of messages currently held in the message store. This includes retained messages and
messages queued for durable clients.
$SYS/broker/store/messages/bytes
The number of bytes currently held by message payloads in the message store. This includes retained
messages and messages queued for durable clients.
$SYS/broker/subscriptions/count
The total number of subscriptions active on the broker.
$SYS/broker/version
The version of the broker. Static.
WILDCARD TOPIC SUBSCRIPTIONS
In addition to allowing clients to subscribe to specific topics, mosquitto also allows the use of two
wildcards in subscriptions. + is the wildcard used to match a single level of hierarchy. For example,
for a topic of "a/b/c/d", the following example subscriptions will match:
• a/b/c/d
• +/b/c/d
• a/+/c/d
• a/+/+/d
• +/+/+/+
The following subscriptions will not match:
• a/b/c
• b/+/c/d
• +/+/+
The second wildcard is # and is used to match all subsequent levels of hierarchy. With a topic of
"a/b/c/d", the following example subscriptions will match:
• a/b/c/d
• #
• a/#
• a/b/#
• a/b/c/#
• +/b/c/#
The $SYS hierarchy does not match a subscription of "#". If you want to observe the entire $SYS
hierarchy, subscribe to $SYS/#.
Note that the wildcards must be only ever used on their own, so a subscription of "a/b+/c" is not valid
use of a wildcard. The # wildcard must only ever be used as the final character of a subscription.
BRIDGES
Multiple brokers can be connected together with the bridging functionality. This is useful where it is
desirable to share information between locations, but where not all of the information needs to be
shared. An example could be where a number of users are running a broker to help record power usage and
for a number of other reasons. The power usage could be shared through bridging all of the user brokers
to a common broker, allowing the power usage of all users to be collected and compared. The other
information would remain local to each broker.
For information on configuring bridges, see mosquitto.conf(5).
SIGNALS
On POSIX systems Mosquitto can receive signals and act on them as described below. To send signals, use
e.g. kill -HUP <process id of mosquitto>
SIGHUP
Upon receiving the SIGHUP signal, mosquitto will attempt to reload configuration file data, assuming
that the -c argument was provided when mosquitto was started. Not all configuration parameters can be
reloaded without restarting. See mosquitto.conf(5) for details.
If TLS certificates are in use, then mosquitto will also reload certificate on receiving a SIGHUP.
SIGUSR1
Upon receiving the SIGUSR1 signal, mosquitto will write the persistence database to disk. This signal
is only acted upon if persistence is enabled.
SIGUSR2
The SIGUSR2 signal causes mosquitto to print out the current subscription tree, along with
information about where retained messages exist. This is intended as a testing feature only and may
be removed at any time.
FILES
/etc/mosquitto/mosquitto.conf
Configuration file. See mosquitto.conf(5).
/var/lib/mosquitto/mosquitto.db
Persistent message data storage location if persist enabled.
/etc/hosts.allow, /etc/hosts.deny
Host access control via tcp-wrappers as described in hosts_access(5).
BUGS
mosquitto bug information can be found at https://github.com/eclipse/mosquitto/issues
SEE ALSO
mqtt(7), mosquitto-tls(7), mosquitto.conf(5), hosts_access(5), mosquitto_ctrl(1), mosquitto_passwd(1),
mosquitto_pub(1), mosquitto_rr(1), mosquitto_sub(1), libmosquitto(3)
THANKS
Thanks to Andy Stanford-Clark for being one of the people who came up with MQTT in the first place.
Thanks to Andy and Nicholas O'Leary for providing clarifications of the protocol.
Thanks also to everybody at the Ubuntu UK Podcast and Linux Outlaws for organising OggCamp, where Andy
gave a talk that inspired mosquitto.
AUTHOR
Roger Light <roger@atchoo.org>