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Updated: Wednesday, July 27, 2022

dbus-daemon (1)


dbus-daemon - Message bus daemon



dbus-daemon [--version] [--session] [--system] [--config-file=FILE]
[--print-address [=DESCRIPTOR]] [--print-pid [=DESCRIPTOR]]


DBUS-DAEMON(1)                   User Commands                  DBUS-DAEMON(1)

       dbus-daemon - Message bus daemon


       dbus-daemon [--version] [--session] [--system] [--config-file=FILE]
                   [--print-address [=DESCRIPTOR]] [--print-pid [=DESCRIPTOR]]

       dbus-daemon is the D-Bus message bus daemon. See
       http://www.freedesktop.org/software/dbus/ for more information about
       the big picture. D-Bus is first a library that provides one-to-one
       communication between any two applications; dbus-daemon is an
       application that uses this library to implement a message bus daemon.
       Multiple programs connect to the message bus daemon and can exchange
       messages with one another.

       There are two standard message bus instances: the systemwide message
       bus (installed on many systems as the "messagebus" init service) and
       the per-user-login-session message bus (started each time a user logs
       in).  dbus-daemon is used for both of these instances, but with a
       different configuration file.

       The --session option is equivalent to
       "--config-file=/usr/share/dbus-1/session.conf" and the --system option
       is equivalent to "--config-file=/usr/share/dbus-1/system.conf". By
       creating additional configuration files and using the --config-file
       option, additional special-purpose message bus daemons could be

       The systemwide daemon is normally launched by an init script,
       standardly called simply "messagebus".

       The systemwide daemon is largely used for broadcasting system events,
       such as changes to the printer queue, or adding/removing devices.

       The per-session daemon is used for various interprocess communication
       among desktop applications (however, it is not tied to X or the GUI in
       any way).

       SIGHUP will cause the D-Bus daemon to PARTIALLY reload its
       configuration file and to flush its user/group information caches. Some
       configuration changes would require kicking all apps off the bus; so
       they will only take effect if you restart the daemon. Policy changes
       should take effect with SIGHUP.

       The following options are supported:

           Use the given configuration file.

           Force the message bus to fork and become a daemon, even if the
           configuration file does not specify that it should. In most
           contexts the configuration file already gets this right, though.
           This option is not supported on Windows.

           Force the message bus not to fork and become a daemon, even if the
           configuration file specifies that it should. On Windows, the
           dbus-daemon never forks, so this option is allowed but does

           Print the address of the message bus to standard output, or to the
           given file descriptor. This is used by programs that launch the
           message bus.

           Print the process ID of the message bus to standard output, or to
           the given file descriptor. This is used by programs that launch the
           message bus.

           Use the standard configuration file for the per-login-session
           message bus.

           Use the standard configuration file for the systemwide message bus.

           Print the version of the daemon.

           Print the introspection information for all D-Bus internal

           Set the address to listen on. This option overrides the address
           configured in the configuration file.

           Enable systemd-style service activation. Only useful in conjunction
           with the systemd system and session manager on Linux.

           Don't write a PID file even if one is configured in the
           configuration files.

       A message bus daemon has a configuration file that specializes it for a
       particular application. For example, one configuration file might set
       up the message bus to be a systemwide message bus, while another might
       set it up to be a per-user-login-session bus.

       The configuration file also establishes resource limits, security
       parameters, and so forth.

       The configuration file is not part of any interoperability
       specification and its backward compatibility is not guaranteed; this
       document is documentation, not specification.

       The standard systemwide and per-session message bus setups are
       configured in the files "/usr/share/dbus-1/system.conf" and
       "/usr/share/dbus-1/session.conf". These files normally <include> a
       system-local.conf or session-local.conf in /etc/dbus-1; you can put
       local overrides in those files to avoid modifying the primary
       configuration files.

       The configuration file is an XML document. It must have the following
       doctype declaration:

              <!DOCTYPE busconfig PUBLIC "-//freedesktop//DTD D-Bus Bus Configuration 1.0//EN"

       The following elements may be present in the configuration file.

       o   <busconfig>

       Root element.

       o   <type>

       The well-known type of the message bus. Currently known values are
       "system" and "session"; if other values are set, they should be either
       added to the D-Bus specification, or namespaced. The last <type>
       element "wins" (previous values are ignored). This element only
       controls which message bus specific environment variables are set in
       activated clients. Most of the policy that distinguishes a session bus
       from the system bus is controlled from the other elements in the
       configuration file.

       If the well-known type of the message bus is "session", then the
       DBUS_STARTER_BUS_TYPE environment variable will be set to "session" and
       the DBUS_SESSION_BUS_ADDRESS environment variable will be set to the
       address of the session bus. Likewise, if the type of the message bus is
       "system", then the DBUS_STARTER_BUS_TYPE environment variable will be
       set to "system" and the DBUS_SESSION_BUS_ADDRESS environment variable
       will be set to the address of the system bus (which is normally well
       known anyway).

       Example: <type>session</type>

       o   <include>

       Include a file <include>filename.conf</include> at this point. If the
       filename is relative, it is located relative to the configuration file
       doing the including.

       <include> has an optional attribute "ignore_missing=(yes|no)" which
       defaults to "no" if not provided. This attribute controls whether it's
       a fatal error for the included file to be absent.

       o   <includedir>

       Include all files in <includedir>foo.d</includedir> at this point.
       Files in the directory are included in undefined order. Only files
       ending in ".conf" are included.

       This is intended to allow extension of the system bus by particular
       packages. For example, if CUPS wants to be able to send out
       notification of printer queue changes, it could install a file to
       /usr/share/dbus-1/system.d or /etc/dbus-1/system.d that allowed all
       apps to receive this message and allowed the printer daemon user to
       send it.

       o   <user>

       The user account the daemon should run as, as either a username or a
       UID. If the daemon cannot change to this UID on startup, it will exit.
       If this element is not present, the daemon will not change or care
       about its UID.

       The last <user> entry in the file "wins", the others are ignored.

       The user is changed after the bus has completed initialization. So
       sockets etc. will be created before changing user, but no data will be
       read from clients before changing user. This means that sockets and PID
       files can be created in a location that requires root privileges for

       o   <fork>

       If present, the bus daemon becomes a real daemon (forks into the
       background, etc.). This is generally used rather than the --fork
       command line option.

       o   <keep_umask>

       If present, the bus daemon keeps its original umask when forking. This
       may be useful to avoid affecting the behavior of child processes.

       o   <syslog>

       If present, the bus daemon will log to syslog.

       o   <pidfile>

       If present, the bus daemon will write its pid to the specified file.
       The --nopidfile command-line option takes precedence over this setting.

       o   <allow_anonymous>

       If present, connections that authenticated using the ANONYMOUS
       mechanism will be authorized to connect. This option has no practical
       effect unless the ANONYMOUS mechanism has also been enabled using the
       <auth> element, described below.

       o   <listen>

       Add an address that the bus should listen on. The address is in the
       standard D-Bus format that contains a transport name plus possible

       Example: <listen>unix:path=/tmp/foo</listen>

       Example: <listen>tcp:host=localhost,port=1234</listen>

       If there are multiple <listen> elements, then the bus listens on
       multiple addresses. The bus will pass its address to started services
       or other interested parties with the last address given in <listen>
       first. That is, apps will try to connect to the last <listen> address

       tcp sockets can accept IPv4 addresses, IPv6 addresses or hostnames. If
       a hostname resolves to multiple addresses, the server will bind to all
       of them. The family=ipv4 or family=ipv6 options can be used to force it
       to bind to a subset of addresses

       Example: <listen>tcp:host=localhost,port=0,family=ipv4</listen>

       A special case is using a port number of zero (or omitting the port),
       which means to choose an available port selected by the operating
       system. The port number chosen can be obtained with the --print-address
       command line parameter and will be present in other cases where the
       server reports its own address, such as when DBUS_SESSION_BUS_ADDRESS
       is set.

       Example: <listen>tcp:host=localhost,port=0</listen>

       tcp/nonce-tcp addresses also allow a bind=hostname option, used in a
       listenable address to configure the interface on which the server will
       listen: either the hostname is the IP address of one of the local
       machine's interfaces (most commonly, a DNS name that
       resolves to one of those IP addresses, '' to listen on all IPv4
       interfaces simultaneously, or '::' to listen on all IPv4 and IPv6
       interfaces simultaneously (if supported by the OS). If not specified,
       the default is the same value as "host".

       Example: <listen>tcp:host=localhost,bind=,port=0</listen>

       o   <auth>

       Lists permitted authorization mechanisms. If this element doesn't
       exist, then all known mechanisms are allowed. If there are multiple
       <auth> elements, all the listed mechanisms are allowed. The order in
       which mechanisms are listed is not meaningful.

       Example: <auth>EXTERNAL</auth>

       Example: <auth>DBUS_COOKIE_SHA1</auth>

       o   <servicedir>

       Adds a directory to scan for .service files. Directories are scanned
       starting with the first to appear in the config file (the first
       .service file found that provides a particular service will be used).

       Service files tell the bus how to automatically start a program. They
       are primarily used with the per-user-session bus, not the systemwide

       o   <standard_session_servicedirs/>

       <standard_session_servicedirs/> is equivalent to specifying a series of
       <servicedir/> elements for each of the data directories in the "XDG
       Base Directory Specification" with the subdirectory "dbus-1/services",
       so for example "/usr/share/dbus-1/services" would be among the
       directories searched.

       The "XDG Base Directory Specification" can be found at
       http://freedesktop.org/wiki/Standards/basedir-spec if it hasn't moved,
       otherwise try your favorite search engine.

       The <standard_session_servicedirs/> option is only relevant to the
       per-user-session bus daemon defined in /etc/dbus-1/session.conf.
       Putting it in any other configuration file would probably be nonsense.

       o   <standard_system_servicedirs/>

       <standard_system_servicedirs/> specifies the standard system-wide
       activation directories that should be searched for service files. This
       option defaults to /usr/share/dbus-1/system-services.

       The <standard_system_servicedirs/> option is only relevant to the
       per-system bus daemon defined in /usr/share/dbus-1/system.conf. Putting
       it in any other configuration file would probably be nonsense.

       o   <servicehelper/>

       <servicehelper/> specifies the setuid helper that is used to launch
       system daemons with an alternate user. Typically this should be the
       dbus-daemon-launch-helper executable in located in libexec.

       The <servicehelper/> option is only relevant to the per-system bus
       daemon defined in /usr/share/dbus-1/system.conf. Putting it in any
       other configuration file would probably be nonsense.

       o   <limit>

       <limit> establishes a resource limit. For example:

             <limit name="max_message_size">64</limit>
             <limit name="max_completed_connections">512</limit>

       The name attribute is mandatory. Available limit names are:

                 "max_incoming_bytes"         : total size in bytes of messages
                                                incoming from a single connection
                 "max_incoming_unix_fds"      : total number of unix fds of messages
                                                incoming from a single connection
                 "max_outgoing_bytes"         : total size in bytes of messages
                                                queued up for a single connection
                 "max_outgoing_unix_fds"      : total number of unix fds of messages
                                                queued up for a single connection
                 "max_message_size"           : max size of a single message in
                 "max_message_unix_fds"       : max unix fds of a single message
                 "service_start_timeout"      : milliseconds (thousandths) until
                                                a started service has to connect
                 "auth_timeout"               : milliseconds (thousandths) a
                                                connection is given to
                 "pending_fd_timeout"         : milliseconds (thousandths) a
                                                fd is given to be transmitted to
                                                dbus-daemon before disconnecting the
                 "max_completed_connections"  : max number of authenticated connections
                 "max_incomplete_connections" : max number of unauthenticated
                 "max_connections_per_user"   : max number of completed connections from
                                                the same user
                 "max_pending_service_starts" : max number of service launches in
                                                progress at the same time
                 "max_names_per_connection"   : max number of names a single
                                                connection can own
                 "max_match_rules_per_connection": max number of match rules for a single
                 "max_replies_per_connection" : max number of pending method
                                                replies per connection
                                                (number of calls-in-progress)
                 "reply_timeout"              : milliseconds (thousandths)
                                                until a method call times out

       The max incoming/outgoing queue sizes allow a new message to be queued
       if one byte remains below the max. So you can in fact exceed the max by

       max_completed_connections divided by max_connections_per_user is the
       number of users that can work together to denial-of-service all other
       users by using up all connections on the systemwide bus.

       Limits are normally only of interest on the systemwide bus, not the
       user session buses.

       o   <policy>

       The <policy> element defines a security policy to be applied to a
       particular set of connections to the bus. A policy is made up of
       <allow> and <deny> elements. Policies are normally used with the
       systemwide bus; they are analogous to a firewall in that they allow
       expected traffic and prevent unexpected traffic.

       Currently, the system bus has a default-deny policy for sending method
       calls and owning bus names. Everything else, in particular reply
       messages, receive checks, and signals has a default allow policy.

       In general, it is best to keep system services as small, targeted
       programs which run in their own process and provide a single bus name.
       Then, all that is needed is an <allow> rule for the "own" permission to
       let the process claim the bus name, and a "send_destination" rule to
       allow traffic from some or all uids to your service.

       The <policy> element has one of four attributes:

             user="username or userid"
             group="group name or gid"

       Policies are applied to a connection as follows:

              - all context="default" policies are applied
              - all group="connection's user's group" policies are applied
                in undefined order
              - all user="connection's auth user" policies are applied
                in undefined order
              - all at_console="true" policies are applied
              - all at_console="false" policies are applied
              - all context="mandatory" policies are applied

       Policies applied later will override those applied earlier, when the
       policies overlap. Multiple policies with the same user/group/context
       are applied in the order they appear in the config file.


       A <deny> element appears below a <policy> element and prohibits some
       action. The <allow> element makes an exception to previous <deny>
       statements, and works just like <deny> but with the inverse meaning.

       The possible attributes of these elements are:

              send_type="method_call" | "method_return" | "signal" | "error"

              receive_type="method_call" | "method_return" | "signal" | "error"

              send_requested_reply="true" | "false"
              receive_requested_reply="true" | "false"

              eavesdrop="true" | "false"



              <deny send_destination="org.freedesktop.Service" send_interface="org.freedesktop.System" send_member="Reboot"/>
              <deny send_destination="org.freedesktop.System"/>
              <deny receive_sender="org.freedesktop.System"/>
              <deny user="john"/>
              <deny group="enemies"/>

       The <deny> element's attributes determine whether the deny "matches" a
       particular action. If it matches, the action is denied (unless later
       rules in the config file allow it).

       send_destination and receive_sender rules mean that messages may not be
       sent to or received from the *owner* of the given name, not that they
       may not be sent *to that name*. That is, if a connection owns services
       A, B, C, and sending to A is denied, sending to B or C will not work

       The other send_* and receive_* attributes are purely textual/by-value
       matches against the given field in the message header.

       "Eavesdropping" occurs when an application receives a message that was
       explicitly addressed to a name the application does not own, or is a
       reply to such a message. Eavesdropping thus only applies to messages
       that are addressed to services and replies to such messages (i.e. it
       does not apply to signals).

       For <allow>, eavesdrop="true" indicates that the rule matches even when
       eavesdropping. eavesdrop="false" is the default and means that the rule
       only allows messages to go to their specified recipient. For <deny>,
       eavesdrop="true" indicates that the rule matches only when
       eavesdropping. eavesdrop="false" is the default for <deny> also, but
       here it means that the rule applies always, even when not
       eavesdropping. The eavesdrop attribute can only be combined with send
       and receive rules (with send_* and receive_* attributes).

       The [send|receive]_requested_reply attribute works similarly to the
       eavesdrop attribute. It controls whether the <deny> or <allow> matches
       a reply that is expected (corresponds to a previous method call
       message). This attribute only makes sense for reply messages (errors
       and method returns), and is ignored for other message types.

       For <allow>, [send|receive]_requested_reply="true" is the default and
       indicates that only requested replies are allowed by the rule.
       [send|receive]_requested_reply="false" means that the rule allows any
       reply even if unexpected.

       For <deny>, [send|receive]_requested_reply="false" is the default but
       indicates that the rule matches only when the reply was not requested.
       [send|receive]_requested_reply="true" indicates that the rule applies
       always, regardless of pending reply state.

       user and group denials mean that the given user or group may not
       connect to the message bus.

       For "name", "username", "groupname", etc. the character "*" can be
       substituted, meaning "any." Complex globs like "foo.bar.*" aren't
       allowed for now because they'd be work to implement and maybe encourage
       sloppy security anyway.

       <allow own_prefix="a.b"/> allows you to own the name "a.b" or any name
       whose first dot-separated elements are "a.b": in particular, you can
       own "a.b.c" or "a.b.c.d", but not "a.bc" or "a.c". This is useful when
       services like Telepathy and ReserveDevice define a meaning for subtrees
       of well-known names, such as
       org.freedesktop.Telepathy.ConnectionManager.(anything) and

       It does not make sense to deny a user or group inside a <policy> for a
       user or group; user/group denials can only be inside context="default"
       or context="mandatory" policies.

       A single <deny> rule may specify combinations of attributes such as
       send_destination and send_interface and send_type. In this case, the
       denial applies only if both attributes match the message being denied.
       e.g. <deny send_interface="foo.bar" send_destination="foo.blah"/> would
       deny messages with the given interface AND the given bus name. To get
       an OR effect you specify multiple <deny> rules.

       You can't include both send_ and receive_ attributes on the same rule,
       since "whether the message can be sent" and "whether it can be
       received" are evaluated separately.

       Be careful with send_interface/receive_interface, because the interface
       field in messages is optional. In particular, do NOT specify <deny
       send_interface="org.foo.Bar"/>! This will cause no-interface messages
       to be blocked for all services, which is almost certainly not what you
       intended. Always use rules of the form: <deny
       send_interface="org.foo.Bar" send_destination="org.foo.Service"/>

       If you're trying to figure out where your messages are going or why you
       aren't getting messages, there are several things you can try.

       Remember that the system bus is heavily locked down and if you haven't
       installed a security policy file to allow your message through, it
       won't work. For the session bus, this is not a concern.

       The simplest way to figure out what's happening on the bus is to run
       the dbus-monitor program, which comes with the D-Bus package. You can
       also send test messages with dbus-send. These programs have their own
       man pages.

       If you want to know what the daemon itself is doing, you might consider
       running a separate copy of the daemon to test against. This will allow
       you to put the daemon under a debugger, or run it with verbose output,
       without messing up your real session and system daemons.

       To run a separate test copy of the daemon, for example you might open a
       terminal and type:

             DBUS_VERBOSE=1 dbus-daemon --session --print-address

       The test daemon address will be printed when the daemon starts. You
       will need to copy-and-paste this address and use it as the value of the
       DBUS_SESSION_BUS_ADDRESS environment variable when you launch the
       applications you want to test. This will cause those applications to
       connect to your test bus instead of the DBUS_SESSION_BUS_ADDRESS of
       your real session bus.

       DBUS_VERBOSE=1 will have NO EFFECT unless your copy of D-Bus was
       compiled with verbose mode enabled. This is not recommended in
       production builds due to performance impact. You may need to rebuild
       D-Bus if your copy was not built with debugging in mind. (DBUS_VERBOSE
       also affects the D-Bus library and thus applications using D-Bus; it
       may be useful to see verbose output on both the client side and from
       the daemon.)

       If you want to get fancy, you can create a custom bus configuration for
       your test bus (see the session.conf and system.conf files that define
       the two default configurations for example). This would allow you to
       specify a different directory for .service files, for example.

       See http://www.freedesktop.org/software/dbus/doc/AUTHORS

       Please send bug reports to the D-Bus mailing list or bug tracker, see

       See attributes(7) for descriptions of the following attributes:

       |Availability   | system/library/dbus |
       |Stability      | Uncommitted         |

       Source code for open source software components in Oracle Solaris can
       be found at https://www.oracle.com/downloads/opensource/solaris-source-

       This software was built from source available at
       https://github.com/oracle/solaris-userland.  The original community
       source was downloaded from

       Further information about this software can be found on the open source
       community website at http://dbus.freedesktop.org.

D-Bus 1.10.32                     06/27/2022                    DBUS-DAEMON(1)