MOSQUITTO.CONF(5) File formats and conventions MOSQUITTO.CONF(5)

mosquitto.conf - the configuration file for mosquitto

mosquitto.conf

mosquitto.conf is the configuration file for mosquitto. This file can reside anywhere as long as mosquitto can read it. By default, mosquitto does not need a configuration file and will use the default values listed below. See mosquitto(8) for information on how to load a configuration file.

Mosquitto can be instructed to reload the configuration file by sending a SIGHUP signal as described in the Signals section of mosquitto(8). Not all configuration options can be reloaded, as detailed in the options below.

The rest of this man page is divided into the following sections:

File Format a description of the configuration file syntax.

Authentication a discussion of the authentication options available.

General Options the general options for configuring Mosquitto.

Listeners general options for configuring listeners, which are the ways that MQTT clients can connect to Mosquitto, as well as certificate and pre-shared-key based SSL/TLS options.

Configuring Bridges general options for configuring bridges, which are a way of connecting multiple brokers together, as well as certificate and pre-shared-key based SSL/TLS options.

Other The Files, Bugs, See Also sections.

All lines with a # as the very first character are treated as a comment.

Configuration lines start with a variable name. The variable value is separated from the name by a single space.

The authentication options described below allow a wide range of possibilities in conjunction with the listener options. This section aims to clarify the possibilities. An overview is also available at https://mosquitto.org/documentation/authentication-methods/

The simplest option is to have no authentication at all. This is the default if no other options are given. Unauthenticated encrypted support is provided by using the certificate based SSL/TLS based options certfile and keyfile.

MQTT provides username/password authentication as part of the protocol. Use the password_file option to define the valid usernames and passwords. Be sure to use network encryption if you are using this option otherwise the username and password will be vulnerable to interception. Use the per_listener_settings to control whether passwords are required globally or on a per-listener basis.

Mosquitto provides the Dynamic Security plugin which handles username/password authentication and access control in a much more flexible way than a password file. See https://mosquitto.org/documentation/dynamic-security/

When using certificate based encryption there are three options that affect authentication. The first is require_certificate, which may be set to true or false. If false, the SSL/TLS component of the client will verify the server but there is no requirement for the client to provide anything for the server: authentication is limited to the MQTT built in username/password. If require_certificate is true, the client must provide a valid certificate in order to connect successfully. In this case, the second and third options, use_identity_as_username and use_subject_as_username, become relevant. If set to true, use_identity_as_username causes the Common Name (CN) from the client certificate to be used instead of the MQTT username for access control purposes. The password is not used because it is assumed that only authenticated clients have valid certificates. This means that any CA certificates you include in cafile or capath will be able to issue client certificates that are valid for connecting to your broker. If use_identity_as_username is false, the client must authenticate as normal (if required by password_file) through the MQTT options. The same principle applies for the use_subject_as_username option, but the entire certificate subject is used as the username instead of just the CN.

When using pre-shared-key based encryption through the psk_hint and psk_file options, the client must provide a valid identity and key in order to connect to the broker before any MQTT communication takes place. If use_identity_as_username is true, the PSK identity is used instead of the MQTT username for access control purposes. If use_identity_as_username is false, the client may still authenticate using the MQTT username/password if using the password_file option.

Both certificate and PSK based encryption are configured on a per-listener basis.

Authentication plugins can be created to augment the password_file, acl_file and psk_file options with e.g. SQL based lookups.

It is possible to support multiple authentication schemes at once. A config could be created that had a listener for all of the different encryption options described above and hence a large number of ways of authenticating.

acl_file file path

Note: It is suggested that you use the mosquitto_acl_file plugin instead of this option. Using plugins for authentication and authorisation allows greater control of what listeners they are applied to, without the need for the deprecated per_listener_settings option.

Set the path to an access control list file. If defined, the contents of the file are used to control client access to topics on the broker.

If this parameter is defined then only the topics listed will have access. Topic access is added with lines of the format:

topic [read|write|readwrite|deny] <topic>

The access type is controlled using "read", "write", "readwrite" or "deny". This parameter is optional (unless <topic> includes a space character) - if not given then the access is read/write. <topic> can contain the + or # wildcards as in subscriptions. The "deny" option can used to explicitly deny access to a topic that would otherwise be granted by a broader read/write/readwrite statement. Any "deny" topics are handled before topics that grant read/write access.

The first set of topics are applied to anonymous clients, assuming allow_anonymous is true. User specific topic ACLs are added after a user line as follows:

user <username>

The username referred to here is the same as in password_file. It is not the clientid.

It is also possible to define ACLs based on pattern substitution within the topic. The form is the same as for the topic keyword, but using pattern as the keyword.

pattern [read|write|readwrite|deny] <topic>

The patterns available for substitution are:

•%c to match the client id of the client
•%u to match the username of the client

The substitution pattern must be the only text for that level of hierarchy. Pattern ACLs apply to all users even if the "user" keyword has previously been given.

Example:

pattern write sensor/%u/data

Allow access for bridge connection messages:

pattern write $SYS/broker/connection/%c/state

If the first character of a line of the ACL file is a # it is treated as a comment.

If per_listener_settings is true, this option applies to the current listener being configured only. If per_listener_settings is false, this option applies to all listeners.

Note: In general it is not possible to grant write access to topics in the $SYS topic tree. The exception is $SYS/broker/connection/+/state.

Reloaded on reload signal. The currently loaded ACLs will be freed and reloaded. Existing subscriptions will be affected after the reload.

See also https://mosquitto.org/documentation/dynamic-security/

allow_anonymous [ true | false ]

Global boolean value that determines whether clients that connect without providing a username are allowed to connect. If set to false then another means of connection should be created to control authenticated client access.

Defaults to false, unless no listeners are defined in the configuration file, in which case it set to true, but connections are only allowed from the local machine.

If you want to control this setting for each listener individually, use the listener_allow_anonymous option.

(Note: Deprecated) If per_listener_settings is true, this option applies to the current listener being configured only. If per_listener_settings is false, this option applies to all listeners.


Important
In version 1.6.x and earlier, this option defaulted to true unless there was another security option set.
Reloaded on reload signal.

allow_duplicate_messages [ true | false ]

This option is deprecated and will be removed in a future version. The behaviour will default to true.

If a client is subscribed to multiple subscriptions that overlap, e.g. foo/# and foo/+/baz , then MQTT expects that when the broker receives a message on a topic that matches both subscriptions, such as foo/bar/baz, then the client should only receive the message once.

Mosquitto keeps track of which clients a message has been sent to in order to meet this requirement. This option allows this behaviour to be disabled, which may be useful if you have a large number of clients subscribed to the same set of topics and want to minimise memory usage.

It can be safely set to true if you know in advance that your clients will never have overlapping subscriptions, otherwise your clients must be able to correctly deal with duplicate messages even when then have QoS=2.

Defaults to true.

This option applies globally.

Reloaded on reload signal.

allow_zero_length_clientid [ true | false ]

MQTT 3.1.1 and MQTT 5 allow clients to connect with a zero length client id and have the broker generate a client id for them. Use this option to allow/disallow this behaviour. Defaults to true.

See also the auto_id_prefix option.

If per_listener_settings is true, this option applies to the current listener being configured only. If per_listener_settings is false, this option applies to all listeners.

Reloaded on reload signal.

auth_plugin_deny_special_chars [ true | false ]

If true then before an ACL check is made, the username/client id of the client needing the check is searched for the presence of either a '+' or '#' character. If either of these characters is found in either the username or client id, then the ACL check is denied before it is sent to the plugin.

This check prevents the case where a malicious user could circumvent an ACL check by using one of these characters as their username or client id. This is the same issue as was reported with mosquitto itself as CVE-2017-7650.

If you are entirely sure that the plugin you are using is not vulnerable to this attack (i.e. if you never use usernames or client ids in topics) then you can disable this extra check and hence have all ACL checks delivered to your plugin by setting this option to false.

Defaults to true.

Applies to the current authentication plugin being configured.

Not currently reloaded on reload signal.

auto_id_prefix prefix

This option is deprecated, please use listener_auto_id_prefix instead.

If allow_zero_length_clientid is true, this option allows you to set a string that will be prefixed to the automatically generated client ids to aid visibility in logs. Defaults to auto-.

If per_listener_settings is true, this option applies to the current listener being configured only. If per_listener_settings is false, this option applies to all listeners.

Reloaded on reload signal.

autosave_interval seconds

The number of seconds that mosquitto will wait between each time it saves the in-memory database to disk. If set to 0, the in-memory database will only be saved when mosquitto exits or when receiving the SIGUSR1 signal. Note that this setting only has an effect if the built-in persistence is enabled. Defaults to 1800 seconds (30 minutes).

This option applies globally.

Reloaded on reload signal.

autosave_on_changes [ true | false ]

If true, mosquitto will count the number of subscription changes, retained messages received and queued messages and if the total exceeds autosave_interval then the in-memory database will be saved to disk. If false, mosquitto will save the in-memory database to disk by treating autosave_interval as a time in seconds.

Applies to built-in persistence only.

This option applies globally.

Reloaded on reload signal.

check_retain_source [ true | false ]

This option affects the scenario when a client subscribes to a topic that has retained messages. It is possible that the client that published the retained message to the topic had access at the time they published, but that access has been subsequently removed. If check_retain_source is set to true, the default, the source of a retained message will be checked for access rights before it is republished. When set to false, no check will be made and the retained message will always be published.

This option applies globally, regardless of the per_listener_settings option.

clientid_prefixes prefix

This option is deprecated and will be removed in a future version.

If defined, only clients that have a clientid with a prefix that matches clientid_prefixes will be allowed to connect to the broker. For example, setting "secure-" here would mean a client "secure-client" could connect but another with clientid "mqtt" couldn't. By default, all client ids are valid.

This option applies globally.

Reloaded on reload signal. Note that currently connected clients will be unaffected by any changes.

connection_messages [ true | false ]

If set to true, the log will include entries when clients connect and disconnect. If set to false, these entries will not appear.

This option applies globally.

Reloaded on reload signal.

enable_control_api [ true | false ]

If set to true, the $CONTROL/broker/v1 will be enabled. This currently allows interrogating which plugins are enabled, but in the future will allow access to configuring general broker options. If you enable this you should ensure you have authentication and appropriate access control configured, i.e. only allowing access to $CONTROL/broker/# for a limited set of users.

This option applies globally.

Reloaded on reload signal.

global_max_clients count

The maximum number of client sessions to allow across the whole broker. In this context a client session means either a client currently connected via the network, or a client that has clean_session = False (MQTT v3.x) and is disconnected, or has disconnected and still hasn't exceeded its session expiry interval (MQTT v5).

See also the max_connections setting, which applies to listeners. If you set global_max_clients to 1000 and max_connections on a listener to 10, then that means only 10 simultaneous connections will be allowed at once, with an overall maximum of 1000 client sessions.

This option applies globally.

Defaults to -1 (unlimited)

Reloaded on reload signal.

global_max_connections count

The maximum number of currently connected clients to allow across the whole broker.

See also the global_max_clients and max_connections settings.

This option applies globally.

Defaults to -1 (unlimited)

Reloaded on reload signal.

global_plugin file path

Load an external module to extend broker features. This loads plugins for use across all listeners, regardless of the value of the per_listener_settings option. This option functions the same as the plugin option in all other ways.

If you set per_listener_settings true, then define both a global_plugin and a plugin (which will be attached to a specific listener), then the global plugin will always be processed first.

If you set per_listener_settings false, then global_plugin behaves identically to plugin.

include_dir dir

External configuration files may be included by using the include_dir option. This defines a directory that will be searched for config files. All files that end in '.conf' will be loaded as a configuration file. It is best to have this as the last option in the main file. This option will only be processed from the main configuration file. The directory specified must not contain the main configuration file.

The configuration files in include_dir are loaded in case sensitive alphabetical order, with the upper case of each letter ordered before the lower case of the same letter.

Example Load Order for include_dir.  Given the files b.conf, A.conf, 01.conf, a.conf, B.conf, and 00.conf inside include_dir, the config files would be loaded in this order:

00.conf
01.conf
A.conf
a.conf
B.conf
b.conf

If this option is used multiple times, then each include_dir option is processed completely in the order that they are written in the main configuration file.

Example Load Order for Multiple include_dir.  Assuming a directory one.d containing files B.conf and C.conf, and a second directory two.d containing files A.conf and D.conf, and a config:

include_dir one.d
include_dir two.d

Then the config files would be loaded in this order:

# files from one.d
B.conf
C.conf
# files from two.d
A.conf
D.conf

log_dest destinations

Send log messages to a particular destination. Possible destinations are: stdout stderr syslog topic file dlt android.

stdout and stderr log to the console on the named output.

syslog uses the userspace syslog facility which usually ends up in /var/log/messages or similar.

topic logs to the broker topic '$SYS/broker/log/<severity>', where severity is one of E, W, N, I, M which are error, warning, notice, information and message. Message type severity is used by the subscribe and unsubscribe log_type options and publishes log messages at $SYS/broker/log/M/subscribe and $SYS/broker/log/M/unsubscribe. Debug messages are never logged on topics.

The file destination requires an additional parameter which is the file to be logged to, e.g. "log_dest file /var/log/mosquitto.log". The file will be closed and reopened when the broker receives a HUP signal. Only a single file destination may be configured.

The dlt destination is for the automotive `Diagnostic Log and Trace` tool. This requires that Mosquitto has been compiled with DLT support.

The android destination is for Android only, and will output to the logd daemon.

Use "log_dest none" if you wish to disable logging. Defaults to stderr. This option may be specified multiple times.

Note that if the broker is running as a Windows service it will default to "log_dest none" and neither stdout nor stderr logging is available.

Reloaded on reload signal.

log_facility local facility

If using syslog logging (not on Windows), messages will be logged to the "daemon" facility by default. Use the log_facility option to choose which of local0 to local7 to log to instead. The option value should be an integer value, e.g. "log_facility 5" to use local5.

log_timestamp [ true | false ]

Boolean value, if set to true a timestamp value will be added to each log entry. The default is true.

Reloaded on reload signal.

log_timestamp_format format

Set the format of the log timestamp. If left unset, this is the number of seconds since the Unix epoch. This option is a free text string which will be passed to the strftime function as the format specifier. To get an ISO 8601 datetime, for example: 
log_timestamp_format %Y-%m-%dT%H:%M:%S

Reloaded on reload signal.

log_type types

Choose types of messages to log. Possible types are: debug, error, warning, notice, information, subscribe, unsubscribe, websockets, none, all.

Defaults to error, warning, notice and information. This option may be specified multiple times. Note that the debug type (used for decoding incoming/outgoing network packets) is never logged in topics.

Reloaded on reload signal.

max_inflight_bytes count

Outgoing QoS 1 and 2 messages will be allowed in flight until this byte limit is reached. This allows control of outgoing message rate based on message size rather than message count. If the limit is set to 100, messages of over 100 bytes are still allowed, but only a single message can be in flight at once. Defaults to 0. (No limit).

See also the max_inflight_messages option.

This option applies globally.

Reloaded on reload signal.

max_inflight_messages count

The maximum number of outgoing QoS 1 or 2 messages that can be in the process of being transmitted simultaneously. This includes messages currently going through handshakes and messages that are being retried. Defaults to 20. Set to 0 for no maximum. If set to 1, this will guarantee in-order delivery of messages.

This option applies globally.

Reloaded on reload signal.

max_keepalive value

For MQTT v5 clients, it is possible to have the server send a "server keepalive" value that will override the keepalive value set by the client. This is intended to be used as a mechanism to say that the server will disconnect the client earlier than it anticipated, and that the client should use the new keepalive value. The max_keepalive option allows you to specify that clients may only connect with keepalive less than or equal to this value, otherwise they will be sent a server keepalive telling them to use max_keepalive. This only applies to MQTT v5 clients. The maximum value is 65535. Set to 0 to allow infinite keepalive. Defaults to 0.

Set to 0 to allow clients to set keepalive = 0, which means no keepalive checks are made and the client will never be disconnected by the broker if no messages are received. You should be very sure this is the behaviour that you want.

For MQTT v3.1.1 and v3.1 clients, there is no mechanism to tell the client what keepalive value they should use. If an MQTT v3.1.1 or v3.1 client specifies a keepalive time greater than max_keepalive they will be sent a CONNACK message with the "identifier rejected" reason code, and disconnected.

This option applies globally.

Reloaded on reload signal.

max_packet_size value

For MQTT v5 clients, it is possible to have the server send a "maximum packet size" value that will instruct the client it will not accept MQTT packets with size greater than value bytes. This applies to the full MQTT packet, not just the payload. Setting this option to a positive value will set the maximum packet size to that number of bytes. If a client sends a packet which is larger than this value, it will be disconnected. This applies to all clients regardless of the protocol version they are using, but v3.1.1 and earlier clients will of course not have received the maximum packet size information.

Defaults to 2000000 bytes since 2.1. Earlier versions defaulted to no limit.

This option applies to all clients, not just those using MQTT v5, but it is not possible to notify clients using MQTT v3.1.1 or MQTT v3.1 of the limit.

Setting below 20 bytes is forbidden because it is likely to interfere with normal client operation even with small payloads.

This option applies globally.

Reloaded on reload signal.

max_queued_bytes count

The number of outgoing QoS 1 and 2 messages above those currently in-flight will be queued (per client) by the broker. Once this limit has been reached, subsequent messages will be silently dropped. This is an important option if you are sending messages at a high rate and/or have clients who are slow to respond or may be offline for extended periods of time. Defaults to 0. (No maximum).

See also the max_queued_messages option. If both max_queued_messages and max_queued_bytes are specified, packets will be queued until the first limit is reached.

This option applies globally.

Reloaded on reload signal.

max_queued_messages count

The maximum number of QoS 1 or 2 messages to hold in the queue (per client) above those messages that are currently in flight. Defaults to 1000. Set to 0 for no maximum (not recommended). See also the queue_qos0_messages and max_queued_bytes options.

This option applies globally.

Reloaded on reload signal.

memory_limit limit

This option sets the maximum number of heap memory bytes that the broker will allocate, and hence sets a hard limit on memory use by the broker. Memory requests that exceed this value will be denied. The effect will vary depending on what has been denied. If an incoming message is being processed, then the message will be dropped and the publishing client will be disconnected. If an outgoing message is being sent, then the individual message will be dropped and the receiving client will be disconnected. Defaults to no limit.

This option is only available if memory tracking support is compiled in.

Reloaded on reload signal. Setting to a lower value and reloading will not result in memory being freed.

message_size_limit limit

This option sets the maximum publish payload size that the broker will allow. Received messages that exceed this size will not be accepted by the broker. This means that the message will not be forwarded on to subscribing clients, but the QoS flow will be completed for QoS 1 or QoS 2 messages. MQTT v5 clients using QoS 1 or QoS 2 will receive a PUBACK or PUBREC with the "implementation specific error" reason code.

The default value is 0, which means that all valid MQTT messages are accepted. MQTT imposes a maximum payload size of 268435455 bytes.

This option applies globally.

Reloaded on reload signal.

password_file file path

Note: It is suggested that you use the mosquitto_password_file plugin instead of this option. Using plugins for authentication and authorisation allows greater control of what listeners they are applied to, without the need for the deprecated per_listener_settings option.

Set the path to a password file. If defined, the contents of the file are used to control client access to the broker. The file can be created using the mosquitto_passwd(1) utility. If mosquitto is compiled without TLS support (it is recommended that TLS support is included), then the password file should be a text file with each line in the format "username:password", where the colon and password are optional but recommended. If allow_anonymous is set to false, only users defined in this file will be able to connect. Setting allow_anonymous to true when password_fileis defined is valid and could be used with acl_file to have e.g. read only guest/anonymous accounts and defined users that can publish.

If per_listener_settings is true, this option applies to the current listener being configured only. If per_listener_settings is false, this option applies to all listeners.

Reloaded on reload signal. The currently loaded username and password data will be freed and reloaded. Clients that are already connected will not be affected.

See also mosquitto_passwd(1) and https://mosquitto.org/documentation/dynamic-security/

per_listener_settings [ true | false ]

If true, then authentication and access control settings will be controlled on a per-listener basis. The following options are affected:

password_file, acl_file, psk_file, allow_anonymous, allow_zero_length_clientid, auto_id_prefix. plugin, plugin_opt_*, Note that if set to true, then a durable client (i.e. with clean session set to false) that has disconnected will use the ACL settings defined for the listener that it was most recently connected to.

The default behaviour is for this to be set to false, which maintains the settings behaviour from previous versions of mosquitto.

Reloaded on reload signal.

persistence [ true | false ]

If true, then built-in persistence is enabled. It is recommended that a plugin based persistence is used instead.

If enabled, connection, subscription and message data will be written to disk in mosquitto.db at the location dictated by persistence_location. When mosquitto is restarted, it will reload the information stored in mosquitto.db. The data will be written to disk when mosquitto closes and also at periodic intervals as defined by autosave_interval. Writing of the persistence database may also be forced by sending mosquitto the SIGUSR1 signal. If false, the data will be stored in memory only. Defaults to false.

The persistence file may change its format in a new version. The broker can currently read all old formats, but will only save in the latest format. It should always be safe to upgrade, but cautious users may wish to take a copy of the persistence file before installing a new version so that they can roll back to an earlier version if necessary.

This option applies globally.

Reloaded on reload signal.

persistence_file file name

The filename to use for the built-in persistent database. Defaults to mosquitto.db.

This option applies globally.

Reloaded on reload signal.

persistence_location path

The path where plugins should store any persistence data, and the path where the built-in persistence will store its data. If not given, then the current directory is used.

This option applies globally.

Reloaded on reload signal.

persistent_client_expiration duration

This option allows the session of persistent clients (those with clean session set to false) that are not currently connected to be removed if they do not reconnect within a certain time frame. This is a non-standard option in MQTT v3.1. MQTT v3.1.1 and v5.0 allow brokers to remove client sessions.

Badly designed clients may set clean session to false whilst using a randomly generated client id. This leads to persistent clients that connect once and never reconnect. This option allows these clients to be removed. This option allows persistent clients (those with clean session set to false) to be removed if they do not reconnect within a certain time frame.

The expiration period should be an integer followed by one of s h d w m y for second, hour, day, week, month and year respectively. For example:

•persistent_client_expiration 2m
•persistent_client_expiration 14d
•persistent_client_expiration 1y

Although it is possible to specify the expiration time in seconds, this is not a suggestion that you should use a short expiration interval. If you have a system where you think the clients should be automatically expired in a short time you should see about fixing the clients instead where possible.

As this is a non-standard option, the default if not set is to never expire persistent clients.

This option applies globally.

Reloaded on reload signal.

pid_file file path

Write a pid file to the file specified. If not given (the default), no pid file will be written. If the pid file cannot be written, mosquitto will exit.

If mosquitto is being automatically started by an init script it will usually be required to write a pid file. This should then be configured as e.g. /var/run/mosquitto/mosquitto.pid

Not reloaded on reload signal.

plugin_opt_* value

Options to be passed to the most recent plugin defined in the configuration file. See the specific plugin instructions for details of what options are available.

Applies to the current plugin/global_plugin being configured.

This is also available as the auth_opt_* option, but this use is deprecated and will be removed in a future version.

plugin file path

Specify an external module to use for authentication, access control, and other features. This allows custom username/password and access control functions to be created and other behaviour to be modified.

Can be specified multiple times to load multiple plugins. The plugins will be processed in the order that they are specified.

If password_file, or acl_file are used in the config file alongsize plugin, the plugin checks will run before the built in checks.

Not currently reloaded on reload signal.

If per_listener_settings is set to true, this plugin will be loaded for the current listener only.

See also https://mosquitto.org/documentation/dynamic-security/ and the global_plugin option.

This is also available as the auth_plugin option, but this use is deprecated and will be removed in a future version.

psk_file file path

Set the path to a pre-shared-key file. This option requires a listener to be have PSK support enabled. If defined, the contents of the file are used to control client access to the broker. Each line should be in the format "identity:key", where the key is a hexadecimal string with no leading "0x". A client connecting to a listener that has PSK support enabled must provide a matching identity and PSK to allow the encrypted connection to proceed.

If per_listener_settings is true, this option applies to the current listener being configured only. If per_listener_settings is false, this option applies to all listeners.

Reloaded on reload signal. The currently loaded identity and key data will be freed and reloaded. Clients that are already connected will not be affected.

queue_qos0_messages [ true | false ]

Set to true to queue messages with QoS 0 when a persistent client is disconnected. When bridges topics are configured with QoS level 1 or 2 incoming QoS 0 messages for these topics are also queued. These messages are included in the limit imposed by max_queued_messages. Defaults to false.

Note that the MQTT v3.1.1 spec states that only QoS 1 and 2 messages should be saved in this situation so this is a non-standard option.

This option applies globally.

Reloaded on reload signal.

retain_available [ true | false ]

If set to false, then retained messages are not supported. Clients that send a message with the retain bit will be disconnected if this option is set to false. Defaults to true.

This option applies globally.

Reloaded on reload signal.

retain_expiry_interval minutes

The default behaviour of mosquitto is to remove retained messages that have reached their message-expiry-interval property the next time that that message is accessed - either by being replaced by a new message, or on the next subscription that matches the message. If you have a pattern of publishing many retained messages with a message-expiry-interval, but that are not subscribed to, then the expired retained messages will remain in memory. This option configures the broker to periodically check the retained tree for expired messages.

Defaults to off. Setting to a value greater than zero means the broker will make a check at an interval of that number of minutes.

This option applies globally.

Reloaded on reload signal.

set_tcp_nodelay [ true | false ]

If set to true, the TCP_NODELAY option will be set on client sockets to disable Nagle's algorithm. This has the effect of reducing latency of some messages at potentially increasing the number of TCP packets being sent. Defaults to false.

This option applies globally.

Reloaded on reload signal.

sys_interval seconds

The integer number of seconds between updates of the $SYS subscription hierarchy, which provides status information about the broker. If unset, defaults to 10 seconds.

Set to 0 to disable publishing the $SYS hierarchy completely.

This option applies globally.

Reloaded on reload signal.

upgrade_outgoing_qos [ true | false ]

The MQTT specification requires that the QoS of a message delivered to a subscriber is never upgraded to match the QoS of the subscription. Enabling this option changes this behaviour. If upgrade_outgoing_qos is set true, messages sent to a subscriber will always match the QoS of its subscription. This is a non-standard option not provided for by the spec. Defaults to false.

This option applies globally.

Reloaded on reload signal.

user username

When run as root, change to this user and its primary group on startup. If set to "mosquitto" or left unset, and if the "mosquitto" user does not exist, then mosquitto will change to the "nobody" user instead. If this is set to another value and mosquitto is unable to change to this user and group, it will exit with an error. The user specified must have read/write access to the persistence database if it is to be written. If run as a non-root user, this setting has no effect. Defaults to mosquitto.

This setting has no effect on Windows and so you should run mosquitto as the user you wish it to run as.

Not reloaded on reload signal.

The network ports that mosquitto listens on can be controlled using listeners. The default listener options can be overridden and further listeners can be created.

accept_protocol_versions versions

Accepted protocol versions. This sets what versions of the MQTT protocol will be accepted on this listener. Can be any combination of 3, 4, 5 in a comma separated list, e.g. 
# Allow v5.0 only:
listener 1883
accept_protocol_versions 5
# Allow v3.1 and v3.1.1:
listener 1884
accept_protocol_versions 3, 4

Defaults to allowing all versions.

Reloaded on reload signal.

bind_address address

This option is deprecated and will be removed in a future version. Use the listener instead.

Listen for incoming network connections on the specified IP address/hostname only. This is useful to restrict access to certain network interfaces. To restrict access to mosquitto to the local host only, use "bind_address localhost". This only applies to the default listener. Use the listener option to control other listeners.

It is recommended to use an explicit listener rather than rely on the implicit default listener options like this.

Not reloaded on reload signal.

bind_interface device

Listen for incoming network connections only on the specified interface. This is similar to the bind_address option but is useful when an interface has multiple addresses or the address may change.

If used at the same time as the bind_address for the default listener, or the bind address/host part of the listener, then bind_interface will take priority.

This option is not available on Windows and AIX.

Not reloaded on reload signal.

enable_proxy_protocol [ 2 | 1 ]

Enable PROXY protocol support for this listener. Versions 1 and 2 are supported, if you have the choice then version 2 is recommended because it gives you access to TLS information, and support Unix sockets.

This option requires the use of a load balancer/proxy such as HAProxy in front of Mosquitto, with the proxy configured to use the PROXY protocol.

The PROXY protocol is used to send information about client socket connections from the proxy to the broker. Without this, the connection information and logs in the broker will contain the IP address and port of the proxy itself, rather than the client.

Enabling this option should only be done when a trusted proxy is placed in front of Mosquitto, and when no other external access to Mosquitto is possible. Giving external access to a listener with this option enabled is a security risk, particularly if you are using client certificates on the proxy.

When using PROXY version 2, this option can be used with TLS termination on the proxy. In this case, TLS information will be passed to the broker as well. Use proxy_protocol_v2_require_tls to reject clients that do not use TLS - this is strongly recommended. Enabling require_certificate on the listener will result in the broker checking the TLS information provided to ensure that the client has provided a valid certificate. Enabling use_identity_as_username as well will result in the commonName value from the certificate being used as the client username instead of any provided in the CONNECT packet. In both cases, the listener must not have TLS configured.

It is not possible to use the use_subject_as_username, certfile, or keyfile options in conjunction with enable_proxy_protocol.

Not reloaded on reload signal.

http_dir directory

When a listener is using the http_api protocol, it is possible to serve http data as well. Set http_dir to a directory which contains the files you wish to serve. If this option is not specified, then no normal http connections will be possible.

This option is also available for websockets listeners if Mosquitto is compiled with websockets support provided by libwebsockets. This is not the default, and will be removed in version 3.0.

Not reloaded on reload signal.

listener port [bind address/host/unix socket path]

Listen for incoming network connection on the specified port. A second optional argument allows the listener to be bound to a specific ip address/hostname. If this variable is used and neither the global bind_address nor port options are used then the default listener will not be started.

The bind address/host option allows this listener to be bound to a specific IP address by passing an IP address or hostname. For websockets listeners, it is only possible to pass an IP address here.

On systems that support Unix Domain Sockets, this option can also be used to create a Unix socket rather than opening a TCP socket. In this case, the port must be set to 0, and the unix socket path must be given.

This option may be specified multiple times. See also the mount_point option.

Not reloaded on reload signal.

listener_allow_anonymous [ true | false ]

Boolean value that determines whether clients that connect without providing a username are allowed to connect to this specific listener. If set, this overrides the value set by allow_anonymous If set to false then another means of connection should be created to control authenticated client access.

If not explicitly set, the value from allow_anonymous will be used.

Not reloaded on reload signal.

auto_id_prefix prefix

This option allows you to set a string that will be prefixed to the automatically generated client ids (i.e. for when a client connects without providing a client id) to aid visibility in logs. Defaults to auto-.

Not reloaded on reload signal.

max_connections count

Limit the total number of clients connected for the current listener. Set to -1 to have "unlimited" connections. Note that other limits may be imposed that are outside the control of mosquitto. See e.g. limits.conf().

Not reloaded on reload signal.

max_qos value

Limit the QoS value allowed for clients connecting to this listener. Defaults to 2, which means any QoS can be used. Set to 0 or 1 to limit to those QoS values. This makes use of an MQTT v5 feature to notify clients of the limitation. MQTT v3.1.1 clients will not be aware of the limitation. Clients publishing to this listener with a too-high QoS will be disconnected.

Not reloaded on reload signal.

max_topic_alias_broker number

When publishing to MQTT v5 clients, Mosquitto can create topic aliases on a first come first serve basis, i.e. the topics that are published to a client first have aliases created. This option controls the number of aliases per client. It applies per listener.

Note that this behaviour is not guaranteed to remain the same. It is possible that future versions introduce mechanisms for controlling which topics receive aliases.

This option sets the maximum number topic aliases that Mosquitto will create for an MQTT v5 client, even if the client allows more.

For example, if the client sets topic-alias-maximum to 100 and this option is set to 10, the broker will create at most 10 topic aliases. Likewise, if the client sets topic-alias-maximum to 20 and this option is set to 100, then the broker will create at most 20 topic aliases.

Defaults to 10. Maximum of 65535. Set to 0 to disable broker to client topic aliases completely.

Not reloaded on reload signal.

max_topic_alias number

This option sets the maximum number topic aliases that an MQTT v5 client is allowed to create. This option applies per listener. Defaults to 10. Set to 0 to disallow topic aliases from clients. The maximum value possible is 65535.

Not reloaded on reload signal.

mount_point topic prefix

This option is used with the listener option to isolate groups of clients. When a client connects to a listener which uses this option, the string argument is attached to the start of all topics for this client. This prefix is removed when any messages are sent to the client. This means a client connected to a listener with mount point example can only see messages that are published in the topic hierarchy example and below.

Not reloaded on reload signal.

port port number

This option is deprecated and will be removed in a future version. Use the listener instead.

Set the network port for the default listener to listen on. Defaults to 1883.

Not reloaded on reload signal.

It is recommended to use an explicit listener rather than rely on the implicit default listener options like this.

protocol value

Set the protocol to accept for the current listener. Can be mqtt, the default, websockets, or http_api.

mqtt: the standard MQTT listener.

websockets: MQTT tunnelled over WebSockets. If the legacy websockets support using libwebsockets is used, then only a reduced TLS feature set is available, namely cafile, certfile, keyfile, ciphers, and ciphers_tls1.3.

http_api: This starts the listener as a very simple webserver (see the http_dir option) that can also serve some HTTP API requests. TLS is supported for this listener, however only the certfile and keyfile options are allowed. Authentication is not currently possible - to use this listener it is strongly recommended to bind the listener to the loopback interface and then configure a reverse proxy with the appropriate encryption and authentication.

Not reloaded on reload signal.

proxy_protocol_v2_require_tls [ true | false ]

When a listener is using the PROXY protocol, this option can be used to require that the client connection is using TLS. If set to true, then the PROXY protocol header must contain a TLS indicator. If it does not, the connection will be closed. Defaults to false.

Not reloaded on reload signal.

socket_domain [ ipv4 | ipv6 ]

By default, a listener will attempt to listen on all supported IP protocol versions. If you do not have an IPv4 or IPv6 interface you may wish to disable support for either of those protocol versions. In particular, note that due to the limitations of the websockets library, it will only ever attempt to open IPv6 sockets if IPv6 support is compiled in, and so will fail if IPv6 is not available.

Set to ipv4 to force the listener to only use IPv4, or set to ipv6 to force the listener to only use IPv6. If you want support for both IPv4 and IPv6, then do not use the socket_domain option.

Not reloaded on reload signal.

use_username_as_clientid [ true | false ]

Set use_username_as_clientid to true to replace the clientid that a client connected with its username. This allows authentication to be tied to the clientid, which means that it is possible to prevent one client disconnecting another by using the same clientid. Defaults to false.

If a client connects with no username it will be disconnected as not authorised when this option is set to true. Do not use in conjunction with clientid_prefixes.

This does not apply globally, but on a per-listener basis.

See also use_identity_as_username.

Not reloaded on reload signal.

packet_buffer_size size, websockets_headers_size size

Change the size of the buffer used when reading from the network before the size of the MQTT packet is known. Defaults to 4096. Packets received that are smaller than this value in principle only need a single read() call, making reading packets more efficient.

This also operates as the option that sets the size of the buffer used by websockets when reading the initial header. If you are passing large header data such as cookies then you may need to increase this value.

websockets_log_level level

Change the websockets logging level. This is a global option, it is not possible to set per listener. This is an integer that is interpreted by libwebsockets as a bit mask for its lws_log_levels enum. See the libwebsockets documentation for more details.

To use this option, log_type websockets must also be enabled. Defaults to 0.

websockets_origin level

If set, this will be compared to the http origin header when a connection attempts to upgrade to WebSockets. Only matching origins will be allowed. Use the exact string provided by the browser in the origin header, e.g. http://example.com:8080. Can be specified multiple times per listener.

If not set, connections from any origin will be allowed.

Requires libwebsockets 3.1.0 or later.

The following options are available for all listeners to configure certificate based SSL support. See also "Pre-shared-key based SSL/TLS support".

cafile file path

cafile is used to define the path to a file containing the PEM encoded CA certificates that are trusted when checking incoming client certificates.

capath directory path

capath is used to define a directory that contains PEM encoded CA certificates that are trusted when checking incoming client certificates. For capath to work correctly, the certificates files must have ".pem" as the file ending and you must run "openssl rehash <path to capath>" each time you add/remove a certificate.

capath is not supported for websockets.

certfile file path

Path to the PEM encoded server certificate. This option and keyfile must be present to enable certificate based TLS encryption.

The certificate pointed to by this option will be reloaded when Mosquitto receives a SIGHUP signal. This can be used to load new certificates prior to the existing ones expiring.

ciphers cipher:list

The list of allowed ciphers for this listener, for TLS v1.2 and earlier only, each separated with a colon. Available ciphers can be obtained using the "openssl ciphers" command.

ciphers_tls1.3 cipher:list

The list of allowed ciphersuites for this listener, for TLS v1.3, each separated with a colon.

crlfile file path

If you have require_certificate set to true, you can create a certificate revocation list file to revoke access to particular client certificates. If you have done this, use crlfile to point to the PEM encoded revocation file.

dhparamfile file path

To allow the use of ephemeral DH key exchange, which provides forward security, the listener must load DH parameters. This can be specified with the dhparamfile option. The dhparamfile can be generated with the command e.g. 
openssl dhparam -out dhparam.pem 2048

disable_client_cert_date_checks [ true | false ]

If set true, disable_client_cert_date_checks will change the certificate verification behaviour to allow client certificates that are expired or are not yet valid, when using require_certificate true.

Defaults to false

keyfile file path

If tls_keyform equals "pem" this is the path to the PEM encoded server key. This option and certfile must be present to enable certificate based TLS encryption. If tls_keyform is "engine" this represents the engine handle of the private key.

The private key pointed to by this option will be reloaded when Mosquitto receives a SIGHUP signal. This can be used to load new keys prior to the existing ones expiring.

require_certificate [ true | false ]

By default an SSL/TLS enabled listener will operate in a similar fashion to a https enabled web server, in that the server has a certificate signed by a CA and the client will verify that it is a trusted certificate. The overall aim is encryption of the network traffic. By setting require_certificate to true, a client connecting to this listener must provide a valid certificate in order for the network connection to proceed. This allows access to the broker to be controlled outside of the mechanisms provided by MQTT.

tls_engine engine

A valid openssl engine id. These can be listed with openssl engine command.

tls_engine_kpass_sha1 engine_kpass_sha1

SHA1 of the private key password when using an TLS engine. Some TLS engines such as the TPM engine may require the use of a password in order to be accessed. This option allows a hex encoded SHA1 hash of the password to the engine directly, instead of the user being prompted for the password.

tls_keyform [ pem | engine ]

Specifies the type of private key in use when making TLS connections.. This can be "pem" or "engine". This parameter is useful when a TPM module is being used and the private key has been created with it. Defaults to "pem", which means normal private key files are used.

tls_version version

Configure the minimum version of the TLS protocol to be used for this listener. Possible values are tlsv1.3 and tlsv1.2. If left unset, the default is to allow TLS v1.3 and v1.2.

In Mosquitto version 1.6.x and earlier, this option set the only TLS protocol version that was allowed, rather than the minimum.

use_identity_as_username [ true | false ]

If require_certificate is true, you may set use_identity_as_username to true to use the CN value from the client certificate as a username. If this is true, the password_file option will not be used for this listener.

This takes priority over use_subject_as_username if both are set to true.

See also use_subject_as_username

use_subject_as_username [ true | false ]

If require_certificate is true, you may set use_subject_as_username to true to use the complete subject value from the client certificate as a username. If this is true, the password_file option will not be used for this listener.

The subject will be generated in a form similar to CN=test client,OU=Production,O=Server,L=Nottingham,ST=Nottinghamshire,C=GB.

See also use_identity_as_username

The following options are available for all listeners to configure pre-shared-key based SSL support. See also "Certificate based SSL/TLS support".

ciphers cipher:list

When using PSK, the encryption ciphers used will be chosen from the list of available PSK ciphers. If you want to control which ciphers are available, use this option. The list of available ciphers can be obtained using the "openssl ciphers" command and should be provided in the same format as the output of that command.

psk_hint hint

The psk_hint option enables pre-shared-key support for this listener and also acts as an identifier for this listener. The hint is sent to clients and may be used locally to aid authentication. The hint is a free form string that doesn't have much meaning in itself, so feel free to be creative.

If this option is provided, see psk_file to define the pre-shared keys to be used or create a security plugin to handle them.

tls_version version

Configure the minimum version of the TLS protocol to be used for this listener. Possible values are tlsv1.3 and tlsv1.2. If left unset, the default is to allow TLS v1.3 and v1.2.

In Mosquitto version 1.6.x and earlier, this option set the only TLS protocol version that was allowed, rather than the minimum.

use_identity_as_username [ true | false ]

Set use_identity_as_username to have the psk identity sent by the client used as its username. The username will be checked as normal, so password_file or another means of authentication checking must be used. No password will be used.

Multiple bridges (connections to other brokers) can be configured using the following variables.

Reloaded on reload signal.

address address[:port] [address[:port]], addresses address[:port] [address[:port]]

Specify the address and optionally the port of the bridge to connect to. This must be given for each bridge connection. If the port is not specified, the default of 1883 is used.

If you use an IPv6 address, then the port is not optional.

Multiple host addresses can be specified on the address config. See the round_robin option for more details on the behaviour of bridges with multiple addresses.

bridge_attempt_unsubscribe [ true | false ]

If a bridge has topics that have "out" direction, the default behaviour is to send an unsubscribe request to the remote broker on that topic. This means that changing a topic direction from "in" to "out" will not keep receiving incoming messages. Sending these unsubscribe requests is not always desirable, setting bridge_attempt_unsubscribe to false will disable sending the unsubscribe request. Defaults to true.

bridge_bind_address ip address

If you need to have the bridge connect over a particular network interface, use bridge_bind_address to tell the bridge which local IP address the socket should bind to, e.g. bridge_bind_address 192.168.1.10.

bridge_max_packet_size value

If you wish to restrict the size of messages sent to a remote bridge, use this option. This sets the maximum number of bytes for the total message, including headers and payload. Note that MQTT v5 brokers may provide their own maximum-packet-size property. In this case, the smaller of the two limits will be used. Set to 0 for "unlimited".

bridge_max_topic_alias value

If the bridge is using MQTT v5, this option sets the maximum number of topic aliases that the bridge will allow the remote broker to configure. Defaults to 10, maximum of 65535. Set to 0 to disable incoming topic aliases completely.

bridge_outgoing_retain [ true | false ]

Some MQTT brokers do not allow retained messages. MQTT v5 gives a mechanism for brokers to tell clients that they do not support retained messages, but this is not possible for MQTT v3.1.1 or v3.1. If you need to bridge to a v3.1.1 or v3.1 broker that does not support retained messages, set the bridge_outgoing_retain option to false. This will remove the retain bit on all outgoing messages to that bridge, regardless of any other setting. Defaults to true.

bridge_protocol_version version

Set the version of the MQTT protocol to use with for this bridge. Can be one of mqttv50, mqttv311 or mqttv31. Defaults to mqttv311.

bridge_receive_maximum count

If the bridge is using MQTT v5.0 then use bridge_receive_maximum to limit the number QoS 1 or 2 messages that can be in-flight at once. Must be 1-65535. Defaults to max_inflight_messages, which defaults to 20.

bridge_session_expiry_interval interval

If the bridge is using MQTT v5.0 then use bridge_session_expiry_interval to set the session expiry interval. Set to 0 to have the session expire immediately when the connection drops. Set to 0xFFFFFFFF to have an infinite expiry interval. Otherwise the expiry interval is set to the number of seconds that you specify. Defaults to 0.

bridge_tcp_keepalive idle interval counter

Set TCP keepalive parameters for this bridge connection. Use this option to allow you to set a long MQTT keepalive value, whilst still being able to detect the connection dropping in a reasonable time.

This may be useful when bridging to services that bill for PINGREQ messages.

Disabled by default.

bridge_tcp_user_timeout timeout

It specifies the maximum amount of time (in milliseconds) that transmitted data may remain unacknowledged at TCP level. Popular Linux distributions seem to set this time to "up to 20 minutes with system defaults" (from Linux tcp man page). The default time is related to tcp_retries2. Reducing this value helps detecting dropped connections faster.

Be aware that when used in combination with TCP Keepalive, it might change the latter's behavior.

You can find more details about this setting at: https://blog.cloudflare.com/when-tcp-sockets-refuse-to-die/

Only available on Linux.

cleansession [ true | false ]

Set the clean session option for this bridge. Setting to false (the default), means that all subscriptions on the remote broker are kept in case of the network connection dropping. If set to true, all subscriptions and messages on the remote broker will be cleaned up if the connection drops. Note that setting to true may cause a large amount of retained messages to be sent each time the bridge reconnects.

If you are using bridges with cleansession set to false (the default), then you may get unexpected behaviour from incoming topics if you change what topics you are subscribing to. This is because the remote broker keeps the subscription for the old topic. If you have this problem, connect your bridge with cleansession set to true, then reconnect with cleansession set to false as normal.

local_cleansession [ true | false]

The regular cleansession covers both the local subscriptions and the remote subscriptions. local_cleansession allows splitting this. Setting false will mean that the local connection will preserve subscription, independent of the remote connection.

Defaults to the value of bridge.cleansession unless explicitly specified.

connection name

This variable marks the start of a new bridge connection. It is also used to give the bridge a name which is used as the client id on the remote broker.

keepalive_interval seconds

Set the number of seconds after which the bridge should send a ping if no other traffic has occurred. Defaults to 60. A minimum value of 5 seconds is allowed.

idle_timeout seconds

Set the amount of time a bridge using the lazy start type must be idle before it will be stopped. Defaults to 60 seconds.

local_clientid id

Set the clientid to use on the local broker. If not defined, this defaults to local.<remote_clientid>. If you are bridging a broker to itself, it is important that local_clientid and remote_clientid do not match.

local_password password

Configure the password to be used when connecting this bridge to the local broker. This may be important when authentication and ACLs are being used.

local_username username

Configure the username to be used when connecting this bridge to the local broker. This may be important when authentication and ACLs are being used.

notifications [ true | false ]

If set to true, publish notification messages to the local and remote brokers giving information about the state of the bridge connection. Retained messages are published to the topic $SYS/broker/connection/<remote_clientid>/state unless otherwise set with notification_topics. If the message is 1 then the connection is active, or 0 if the connection has failed. Defaults to true.

This uses the Last Will and Testament (LWT) feature.

notifications_local_only [ true | false ]

If set to true, only publish notification messages to the local broker giving information about the state of the bridge connection. Defaults to false.

notification_topic topic

Choose the topic on which notifications will be published for this bridge. If not set the messages will be sent on the topic $SYS/broker/connection/<remote_clientid>/state.

remote_clientid id

Set the client id for this bridge connection. If not defined, this defaults to 'name.hostname', where name is the connection name and hostname is the hostname of this computer.

This replaces the old "clientid" option to avoid confusion with local/remote sides of the bridge. "clientid" remains valid for the time being.

remote_password value

Configure a password for the bridge. This is used for authentication purposes when connecting to a broker that supports MQTT v3.1 and up and requires a username and/or password to connect. This option is only valid if a remote_username is also supplied.

This replaces the old "password" option to avoid confusion with local/remote sides of the bridge. "password" remains valid for the time being.

remote_username name

Configure a username for the bridge. This is used for authentication purposes when connecting to a broker that supports MQTT v3.1 and up and requires a username and/or password to connect. See also the remote_password option.

This replaces the old "username" option to avoid confusion with local/remote sides of the bridge. "username" remains valid for the time being.

restart_timeout base cap [stable], restart_timeout constant

Set the amount of time a bridge using the automatic start type will wait until attempting to reconnect.

This option can be configured to use a constant delay time in seconds, or to use a backoff mechanism based on "Decorrelated Jitter", which adds a degree of randomness to when the restart occurs, starting at the base and increasing up to the cap. The backoff time will be reset to base after a successful connection. When stable is specified, the backoff time will be reset only if the connection remains open for at least stable seconds. Base has a minimum of 1 second and a maximum of 3600 seconds. Cap has a minimum of base, and a maximum of 7200 seconds.

Set a constant timeout of 20 seconds:

restart_timeout 20

Set backoff with a base (start value) of 10 seconds and a cap (upper limit) of 60 seconds:

restart_timeout 10 60

Same as previous example, but wait for the connection to be stable for at least 30 seconds before resetting the backoff:

restart_timeout 10 60 30

Defaults to jitter with a base of 5 seconds and cap of 30 seconds.

round_robin [ true | false ]

If the bridge has more than one address given in the address/addresses configuration, the round_robin option defines the behaviour of the bridge on a failure of the bridge connection. If round_robin is false, the default value, then the first address is treated as the main bridge connection. If the connection fails, the other secondary addresses will be attempted in turn. Whilst connected to a secondary bridge, the bridge will periodically attempt to reconnect to the main bridge until successful.

If round_robin is true, then all addresses are treated as equals. If a connection fails, the next address will be tried and if successful will remain connected until it fails.

start_type [ automatic | lazy | once ]

Set the start type of the bridge. This controls how the bridge starts and can be one of three types: automatic, lazy and once. Note that RSMB provides a fourth start type "manual" which isn't currently supported by mosquitto.

automatic is the default start type and means that the bridge connection will be started automatically when the broker starts and also restarted after a short delay (30 seconds) if the connection fails.

Bridges using the lazy start type will be started automatically when the number of queued messages exceeds the number set with the threshold option. It will be stopped automatically after the time set by the idle_timeout parameter. Use this start type if you wish the connection to only be active when it is needed.

A bridge using the once start type will be started automatically when the broker starts but will not be restarted if the connection fails.

threshold count

Set the number of messages that need to be queued for a bridge with lazy start type to be restarted. Defaults to 10 messages.

topic pattern [[[ out | in | both ] qos-level] local-prefix remote-prefix]

Define a topic pattern to be shared between the two brokers. Any topics matching the pattern (which may include wildcards) are shared. The pattern may be enclosed in double quotes in case it contains a space. The second parameter defines the direction that the messages will be shared in, so it is possible to import messages from a remote broker using in, export messages to a remote broker using out or share messages in both directions. If this parameter is not defined, the default of out is used. The QoS level defines the publish/subscribe QoS level used for this topic and defaults to 0.

The local-prefix and remote-prefix options allow topics to be remapped when publishing to and receiving from remote brokers. This allows a topic tree from the local broker to be inserted into the topic tree of the remote broker at an appropriate place.

For incoming topics, the bridge will prepend the pattern with the remote prefix and subscribe to the resulting topic on the remote broker. When a matching incoming message is received, the remote prefix will be removed from the topic and then the local prefix added.

For outgoing topics, the bridge will prepend the pattern with the local prefix and subscribe to the resulting topic on the local broker. When an outgoing message is processed, the local prefix will be removed from the topic then the remote prefix added.

When using topic mapping, an empty prefix can be defined using the place marker "". Using the empty marker for the topic itself is also valid. The table below defines what combination of empty or value is valid. The Full Local Topic and Full Remote Topic show the resulting topics that would be used on the local and remote ends of the bridge. For example, for the first table row if you publish to L/topic on the local broker, then the remote broker will receive a message on the topic R/topic.

Pattern Local Prefix Remote Prefix Validity Full Local Topic Full Remote Topic
pattern L/ R/ valid L/pattern R/pattern
pattern L/ "" valid L/pattern pattern
pattern "" R/ valid pattern R/pattern
pattern "" "" valid (no remapping) pattern pattern
"" local remote valid (remap single local topic to remote) local remote
"" local "" invalid    
"" "" remote invalid    
"" "" "" invalid    

To remap an entire topic tree, use e.g.:

topic # both 2 local/topic/ remote/topic/

This option can be specified multiple times per bridge.

Care must be taken to ensure that loops are not created with this option. If you are experiencing high CPU load from a broker, it is possible that you have a loop where each broker is forever forwarding each other the same messages.

See also the cleansession option if you have messages arriving on unexpected topics when using incoming topics.

Example Bridge Topic Remapping.  The configuration below connects a bridge to the broker at test.mosquitto.org. It subscribes to the remote topic $SYS/broker/clients/total and republishes the messages received to the local topic test/mosquitto/org/clients/total

connection test-mosquitto-org
address test.mosquitto.org
cleansession true
topic clients/total in 0 test/mosquitto/org/ $SYS/broker/

try_private [ true | false ]

If try_private is set to true, the bridge will attempt to indicate to the remote broker that it is a bridge not an ordinary client. If successful, this means that loop detection will be more effective and that retained messages will be propagated correctly. Not all brokers support this feature so it may be necessary to set try_private to false if your bridge does not connect properly.

Defaults to true.

bridge_reload_type [ lazy | immediate ]

If you change bridge options in the configuration file, those configuration changes are applied during a bridge reconnection. The bridge_reload_type option determines when that reconnection happens, and can be set to either lazy or immediate.

lazy is the default, and means that any connected bridge will remain in its current state until a natural reconnection happens, at which point the new configuration will be used.

immediate forces a reconnection and so uses the new configuration straight away.

The following options are available for all bridges to configure SSL/TLS support.

bridge_alpn alpn

Configure the application layer protocol negotiation option for the TLS session. Useful for brokers that support both websockets and MQTT on the same port.

bridge_cafile file path

At least one of bridge_cafile, bridge_capath, or bridge_tls_use_os_certs must be provided to allow SSL/TLS support.

bridge_cafile is used to define the path to a file containing the PEM encoded CA certificates that have signed the certificate for the remote broker.

bridge_capath file path

At least one of bridge_cafile, bridge_capath, or bridge_tls_use_os_certs must be provided to allow SSL/TLS support.

bridge_capath is used to define the path to a directory containing the PEM encoded CA certificates that have signed the certificate for the remote broker. For bridge_capath to work correctly, the certificate files must have ".crt" as the file ending and you must run "openssl rehash <path to bridge_capath>" each time you add/remove a certificate.

bridge_certfile file path

Path to the PEM encoded client certificate for this bridge, if required by the remote broker.

bridge_identity identity

Pre-shared-key encryption provides an alternative to certificate based encryption. A bridge can be configured to use PSK with the bridge_identity and bridge_psk options. This is the client identity used with PSK encryption. Only one of certificate and PSK based encryption can be used on one bridge at once.

bridge_insecure [ true | false ]

When using certificate based TLS, the bridge will attempt to verify the hostname provided in the remote certificate matches the host/address being connected to. This may cause problems in testing scenarios, so bridge_insecure may be set to true to disable the hostname verification.

Setting this option to true means that a malicious third party could potentially impersonate your server, so it should always be set to false in production environments.

bridge_keyfile file path

Path to the PEM encoded private key for this bridge, if required by the remote broker.

bridge_psk key

Pre-shared-key encryption provides an alternative to certificate based encryption. A bridge can be configured to use PSK with the bridge_identity and bridge_psk options. This is the pre-shared-key in hexadecimal format with no "0x". Only one of certificate and PSK based encryption can be used on one bridge at once.

bridge_require_ocsp [ true | false ]

When set to true, the bridge requires OCSP on the TLS connection it opens as client.

bridge_tls_use_os_certs [ true | false ]

At least one of bridge_cafile, bridge_capath, or bridge_tls_use_os_certs must be provided to allow SSL/TLS support.

Set bridge_tls_use_os_certs to true to enable TLS for this bridge, and to configure it to trust the default certificates provided by openssl. This is typically a large number of certificates. Defaults to false.

bridge_tls_version version

Configure the version of the TLS protocol to be used for this bridge. Possible values are tlsv1.3 and tlsv1.2. Defaults to tlsv1.2. The remote broker must support the same version of TLS for the connection to succeed.

bridge_ciphers cipher:list

The list of allowed ciphers for this bridge, for TLS v1.2 and earlier only, each separated with a colon. Available ciphers can be obtained using the "openssl ciphers" command.

bridge_ciphers_tls1.3 cipher:list

The list of allowed ciphersuites for this bridge, for TLS v1.3, each separated with a colon.

mosquitto.conf

mosquitto bug information can be found at https://github.com/eclipse-mosquitto/mosquitto/issues

mosquitto(7), mosquitto(8), mosquitto_passwd(1), mosquitto-tls(7), mqtt(7), limits.conf(5)

Roger Light <roger@atchoo.org>

04/03/2026 Mosquitto Project