dbus-daemon

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



NAME
       dbus-daemon - Message bus daemon

SYNOPSIS
       dbus-daemon

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


DESCRIPTION
       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 created.

       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.

OPTIONS
       The following options are supported:

       --config-file=FILE
           Use the given configuration file.

       --fork
           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.

       --nofork
           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 nothing.

       --print-address[=DESCRIPTOR]
           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-pid[=DESCRIPTOR]
           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.

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

       --system
           Use the standard configuration file for the systemwide message bus.

       --version
           Print the version of the daemon.

       --introspect
           Print the introspection information for all D-Bus internal
           interfaces.

       --address[=ADDRESS]
           Set the address to listen on. This option overrides the address
           configured in the configuration file via the <listen> directive. See
           the documentation of that directive for more details.

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

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

       --syslog
           Force the message bus to use the system log for messages, in addition
           to writing to standard error, even if the configuration file does not
           specify that it should. On Unix, this uses the syslog; on Windows,
           this uses OutputDebugString().

       --syslog-only
           Force the message bus to use the system log for messages, and not
           duplicate them to standard error. On Unix, this uses the syslog; on
           Windows, this uses OutputDebugString().

       --nosyslog
           Force the message bus to use only standard error for messages, even
           if the configuration file specifies that it should use the system
           log.

CONFIGURATION FILE
       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"
               "http://www.freedesktop.org/standards/dbus/1.0/busconfig.dtd">


       The following elements may be present in the configuration file.

       •   <busconfig>

       Root element.

       •   <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_SYSTEM_BUS_ADDRESS environment variable will be
       set to the address of the system bus (which is normally well known
       anyway).

       Example: <type>session</type>

       •   <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.

       •   <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.

       •   <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
       writing.

       •   <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.

       •   <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.

       •   <syslog>

       If present, the bus daemon will log to syslog. The --syslog,
       --syslog-only and --nosyslog command-line options take precedence over
       this setting.

       •   <pidfile>

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

       •   <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.

       Using this directive in the configuration of the well-known system bus or
       the well-known session bus will make that bus insecure and should never
       be done. Similarly, on custom bus types, using this directive will
       usually make the custom bus insecure, unless its configuration has been
       specifically designed to prevent anonymous users from causing damage or
       escalating privileges.

       •   <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
       parameters/options.

       On platforms other than Windows, unix-based transports (unix, systemd,
       launchd) are the default for both the well-known system bus and the
       well-known session bus, and are strongly recommended.

       On Windows, unix-based transports are not available, so TCP-based
       transports must be used. Similar to remote X11, the tcp and nonce-tcp
       transports have no integrity or confidentiality protection, so they
       should normally only be used across the local loopback interface, for
       example using an address like tcp:host=127.0.0.1 or
       nonce-tcp:host=localhost. In particular, configuring the well-known
       system bus or the well-known session bus to listen on a non-loopback TCP
       address is insecure.

       Developers are sometimes tempted to use remote TCP as a debugging tool.
       However, if this functionality is left enabled in finished products, the
       result will be dangerously insecure. Instead of using remote TCP,
       developers should relay connections via Secure Shell or a similar
       protocol[1].

       Remote TCP connections were historically sometimes used to share a single
       session bus between login sessions of the same user on different machines
       within a trusted local area network, in conjunction with unencrypted
       remote X11, a NFS-shared home directory and NIS (YP) authentication. This
       is insecure against an attacker on the same LAN and should be considered
       strongly deprecated; more specifically, it is insecure in the same ways
       and for the same reasons as unencrypted remote X11 and NFSv2/NFSv3. The
       D-Bus maintainers recommend using a separate session bus per (user,
       machine) pair, only accessible from within that machine.

       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 first.

       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 127.0.0.1), a DNS name that resolves
       to one of those IP addresses, '0.0.0.0' 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=0.0.0.0,port=0</listen>

       •   <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.

       On non-Windows operating systems, allowing only the EXTERNAL
       authentication mechanism is strongly recommended. This is the default for
       the well-known system bus and for the well-known session bus.

       Example: <auth>EXTERNAL</auth>

       Example: <auth>DBUS_COOKIE_SHA1</auth>

       •   <servicedir>

       Adds a directory to search for .service files, which tell the dbus-daemon
       how to start a program to provide a particular well-known bus name. See
       the D-Bus Specification for more details about the contents of .service
       files.

       If a particular service is found in more than one <servicedir>, the first
       directory listed in the configuration file takes precedence. If two
       service files providing the same well-known bus name are found in the
       same directory, it is arbitrary which one will be chosen (this can only
       happen if at least one of the service files does not have the recommended
       name, which is its well-known bus name followed by ".service").

       •   <standard_session_servicedirs/>

       <standard_session_servicedirs/> requests a standard set of session
       service directories. Its effect is similar to specifying a series of
       <servicedir/> elements for each of the data directories, in the order
       given here. It is not exactly equivalent, because there is currently no
       way to disable directory monitoring or enforce strict service file naming
       for a <servicedir/>.

       As with <servicedir/> elements, if a particular service is found in more
       than one service directory, the first directory takes precedence. If two
       service files providing the same well-known bus name are found in the
       same directory, it is arbitrary which one will be chosen (this can only
       happen if at least one of the service files does not have the recommended
       name, which is its well-known bus name followed by ".service").

       On Unix, the standard session service directories are:

       •   $XDG_RUNTIME_DIR/dbus-1/services, if XDG_RUNTIME_DIR is set (see the
           XDG Base Directory Specification for details of XDG_RUNTIME_DIR):
           this location is suitable for transient services created at runtime
           by systemd generators (see systemd.generator(7)), session managers or
           other session infrastructure. It is an extension provided by the
           reference implementation of dbus-daemon, and is not standardized in
           the D-Bus Specification.

           Unlike the other standard session service directories, this directory
           enforces strict naming for the service files: the filename must be
           exactly the well-known bus name of the service, followed by
           ".service".

           Also unlike the other standard session service directories, this
           directory is never monitored with inotify(7) or similar APIs.
           Programs that create service files in this directory while a
           dbus-daemon is running are expected to call the dbus-daemon's
           ReloadConfig() method after they have made changes.

       •   $XDG_DATA_HOME/dbus-1/services, where XDG_DATA_HOME defaults to
           ~/.local/share (see the XDG Base Directory Specification): this
           location is specified by the D-Bus Specification, and is suitable for
           per-user, locally-installed software.

       •   directory/dbus-1/services for each directory in XDG_DATA_DIRS, where
           XDG_DATA_DIRS defaults to /usr/local/share:/usr/share (see the XDG
           Base Directory Specification): these locations are specified by the
           D-Bus Specification. The defaults are suitable for software installed
           locally by a system administrator (/usr/local/share) or for software
           installed from operating system packages (/usr/share). Per-user or
           system-wide configuration that sets the XDG_DATA_DIRS environment
           variable can extend this search path to cover installations in other
           locations, for example ~/.local/share/flatpak/exports/share/ and
           /var/lib/flatpak/exports/share/ when flatpak(1) is used.

       •   ${datadir}/dbus-1/services for the ${datadir} that was specified when
           dbus was compiled, typically /usr/share: this location is an
           extension provided by the reference dbus-daemon implementation, and
           is suitable for software stacks installed alongside dbus-daemon.

       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.

       On Windows, the standard session service directories are:

       •   %CommonProgramFiles%/dbus-1/services if %CommonProgramFiles% is set:
           this location is suitable for system-wide installed software packages

       •   A share/dbus-1/services directory found in the same directory
           hierarchy (prefix) as the dbus-daemon: this location is suitable for
           software stacks installed alongside dbus-daemon

       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.

       •   <standard_system_servicedirs/>

       <standard_system_servicedirs/> specifies the standard system-wide
       activation directories that should be searched for service files. As with
       session services, the first directory listed has highest precedence.

       On Unix, the standard session service directories are:

       •   /usr/local/share/dbus-1/system-services: this location is specified
           by the D-Bus Specification, and is suitable for software installed
           locally by the system administrator

       •   /usr/share/dbus-1/system-services: this location is specified by the
           D-Bus Specification, and is suitable for software installed by
           operating system packages

       •   ${datadir}/dbus-1/system-services for the ${datadir} that was
           specified when dbus was compiled, typically /usr/share: this location
           is an extension provided by the reference dbus-daemon implementation,
           and is suitable for software stacks installed alongside dbus-daemon

       •   /lib/dbus-1/system-services: this location is specified by the D-Bus
           Specification, and was intended for software installed by operating
           system packages and used during early boot (but it should be
           considered deprecated, because the reference dbus-daemon is not
           designed to be available during early boot)

       On Windows, there is no standard system bus, so there are no standard
       system bus directories either.

       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.

       •   <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.

       •   <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
                                                bytes
                 "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
                                                authenticate
                 "pending_fd_timeout"         : milliseconds (thousandths) a
                                                fd is given to be transmitted to
                                                dbus-daemon before disconnecting the
                                                connection
                 "max_completed_connections"  : max number of authenticated connections
                 "max_incomplete_connections" : max number of unauthenticated
                                                connections
                 "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
                                                   connection
                 "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_message_size.

       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.

       •   <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, and a default-allow policy for receiving
       messages, sending signals, and sending a single success or error reply
       for each method call that does not have the NO_REPLY flag. Sending more
       than the expected number of replies is not allowed.

       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:

             context="(default|mandatory)"
             at_console="(true|false)"
             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.

       <deny>
           <allow>

       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_interface="interface_name" | "*"
              send_member="method_or_signal_name" | "*"
              send_error="error_name" | "*"
              send_broadcast="true" | "false"
              send_destination="name" | "*"
              send_type="method_call" | "method_return" | "signal" | "error" | "*"
              send_path="/path/name" | "*"

              receive_interface="interface_name" | "*"
              receive_member="method_or_signal_name" | "*"
              receive_error="error_name" | "*"
              receive_sender="name" | "*"
              receive_type="method_call" | "method_return" | "signal" | "error" | "*"
              receive_path="/path/name" | "*"

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

              eavesdrop="true" | "false"

              own="name" | "*"
              own_prefix="name"
              user="username" | "*"
              group="groupname" | "*"

       Examples:

              <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).

       Rules with one or more of the send_* family of attributes are checked in
       order when a connection attempts to send a message. The last rule that
       matches the message determines whether it may be sent. The well-known
       session bus normally allows sending any message. The well-known system
       bus normally allows sending any signal, selected method calls to the
       dbus-daemon, and exactly one reply to each previously-sent method call
       (either success or an error). Either of these can be overridden by
       configuration; on the system bus, services that will receive method calls
       must install configuration that allows them to do so, usually via rules
       of the form <policy context="default"><allow
       send_destination="..."/><policy>.

       Rules with one or more of the receive_* family of attributes, or with the
       eavesdrop attribute and no others, are checked for each recipient of a
       message (there might be more than one recipient if the message is a
       broadcast or a connection is eavesdropping). The last rule that matches
       the message determines whether it may be received. The well-known session
       bus normally allows receiving any message, including eavesdropping. The
       well-known system bus normally allows receiving any message that was not
       eavesdropped (any unicast message addressed to the recipient, and any
       broadcast message).

       The eavesdrop, min_fds and max_fds attributes are modifiers that can be
       applied to either send_* or receive_* rules, and are documented below.

       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 either. As
       a special case, send_destination="*" matches any message (whether it has
       a destination specified or not), and receive_sender="*" similarly matches
       any message.

       Rules with send_broadcast="true" match signal messages with no
       destination (broadcasts). Rules with send_broadcast="false" are the
       inverse: they match any unicast destination (unicast signals, together
       with all method calls, replies and errors) but do not match messages with
       no destination (broadcasts). This is not the same as
       send_destination="*", which matches any sent message, regardless of
       whether it has a destination or not.

       The other send_* and receive_* attributes are purely textual/by-value
       matches against the given field in the message header, except that for
       the attributes where it is allowed, * matches any message (whether it has
       the relevant header field or not). For example, send_interface="*"
       matches any sent message, even if it does not contain an interface header
       field. More complex glob matching such as foo.bar.*  is not allowed.

       "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.

       The min_fds and max_fds attributes modify either send_* or receive_*
       rules. A rule with the min_fds attribute only matches messages if they
       have at least that many Unix file descriptors attached. Conversely, a
       rule with the max_fds attribute only matches messages if they have no
       more than that many file descriptors attached. In practice, rules with
       these attributes will most commonly take the form <allow
       send_destination="..." max_fds="0"/>, <deny send_destination="..."
       min_fds="1"/> or <deny receive_sender="*" min_fds="1"/>.

       Rules with the user or group attribute are checked when a new connection
       to the message bus is established, and control whether the connection can
       continue. Each of these attributes cannot be combined with any other
       attribute. As a special case, both user="*" and group="*" match any
       connection. If there are no rules of this form, the default is to allow
       connections from the same user ID that owns the dbus-daemon process. The
       well-known session bus normally uses that default behaviour, while the
       well-known system bus normally allows any connection.

       Rules with the own or own_prefix attribute are checked when a connection
       attempts to own a well-known bus names. As a special case, own="*"
       matches any well-known bus name. The well-known session bus normally
       allows any connection to own any name, while the well-known system bus
       normally does not allow any connection to own any name, except where
       allowed by further configuration. System services that will own a name
       must install configuration that allows them to do so, usually via rules
       of the form <policy user="some-system-user"><allow own="..."/></policy>.

       <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
       org.freedesktop.ReserveDevice1.(anything).

       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"/>

       •   <selinux>

       The <selinux> element contains settings related to Security Enhanced
       Linux. More details below.

       •   <associate>

       An <associate> element appears below an <selinux> element and creates a
       mapping. Right now only one kind of association is possible:

              <associate own="org.freedesktop.Foobar" context="foo_t"/>

       This means that if a connection asks to own the name
       "org.freedesktop.Foobar" then the source context will be the context of
       the connection and the target context will be "foo_t" - see the short
       discussion of SELinux below.

       Note, the context here is the target context when requesting a name, NOT
       the context of the connection owning the name.

       There's currently no way to set a default for owning any name, if we add
       this syntax it will look like:

              <associate own="*" context="foo_t"/>

       If you find a reason this is useful, let the developers know. Right now
       the default will be the security context of the bus itself.

       If two <associate> elements specify the same name, the element appearing
       later in the configuration file will be used.

       •   <apparmor>

       The <apparmor> element is used to configure AppArmor mediation on the
       bus. It can contain one attribute that specifies the mediation mode:

              <apparmor mode="(enabled|disabled|required)"/>

       The default mode is "enabled". In "enabled" mode, AppArmor mediation will
       be performed if AppArmor support is available in the kernel. If it is not
       available, dbus-daemon will start but AppArmor mediation will not occur.
       In "disabled" mode, AppArmor mediation is disabled. In "required" mode,
       AppArmor mediation will be enabled if AppArmor support is available,
       otherwise dbus-daemon will refuse to start.

       The AppArmor mediation mode of the bus cannot be changed after the bus
       starts. Modifying the mode in the configuration file and sending a SIGHUP
       signal to the daemon has no effect on the mediation mode.

SELINUX
       See http://www.nsa.gov/selinux/ for full details on SELinux. Some useful
       excerpts:

       Every subject (process) and object (e.g. file, socket, IPC object, etc)
       in the system is assigned a collection of security attributes, known as a
       security context. A security context contains all of the security
       attributes associated with a particular subject or object that are
       relevant to the security policy.

       In order to better encapsulate security contexts and to provide greater
       efficiency, the policy enforcement code of SELinux typically handles
       security identifiers (SIDs) rather than security contexts. A SID is an
       integer that is mapped by the security server to a security context at
       runtime.

       When a security decision is required, the policy enforcement code passes
       a pair of SIDs (typically the SID of a subject and the SID of an object,
       but sometimes a pair of subject SIDs or a pair of object SIDs), and an
       object security class to the security server. The object security class
       indicates the kind of object, e.g. a process, a regular file, a
       directory, a TCP socket, etc.

       Access decisions specify whether or not a permission is granted for a
       given pair of SIDs and class. Each object class has a set of associated
       permissions defined to control operations on objects with that class.

       D-Bus performs SELinux security checks in two places.

       First, any time a message is routed from one connection to another
       connection, the bus daemon will check permissions with the security
       context of the first connection as source, security context of the second
       connection as target, object class "dbus" and requested permission
       "send_msg".

       If a security context is not available for a connection (impossible when
       using UNIX domain sockets), then the target context used is the context
       of the bus daemon itself. There is currently no way to change this
       default, because we're assuming that only UNIX domain sockets will be
       used to connect to the systemwide bus. If this changes, we'll probably
       add a way to set the default connection context.

       Second, any time a connection asks to own a name, the bus daemon will
       check permissions with the security context of the connection as source,
       the security context specified for the name in the config file as target,
       object class "dbus" and requested permission "acquire_svc".

       The security context for a bus name is specified with the <associate>
       element described earlier in this document. If a name has no security
       context associated in the configuration file, the security context of the
       bus daemon itself will be used.

APPARMOR
       The AppArmor confinement context is stored when applications connect to
       the bus. The confinement context consists of a label and a confinement
       mode. When a security decision is required, the daemon uses the
       confinement context to query the AppArmor policy to determine if the
       action should be allowed or denied and if the action should be audited.

       The daemon performs AppArmor security checks in three places.

       First, any time a message is routed from one connection to another
       connection, the bus daemon will check permissions with the label of the
       first connection as source, label and/or connection name of the second
       connection as target, along with the bus name, the path name, the
       interface name, and the member name. Reply messages, such as
       method_return and error messages, are implicitly allowed if they are in
       response to a message that has already been allowed.

       Second, any time a connection asks to own a name, the bus daemon will
       check permissions with the label of the connection as source, the
       requested name as target, along with the bus name.

       Third, any time a connection attempts to eavesdrop, the bus daemon will
       check permissions with the label of the connection as the source, along
       with the bus name.

       AppArmor rules for bus mediation are not stored in the bus configuration
       files. They are stored in the application's AppArmor profile. Please see
       apparmor.d(5) for more details.

DEBUGGING
       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.

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

BUGS
       Please send bug reports to the D-Bus mailing list or bug tracker, see
       http://www.freedesktop.org/software/dbus/

NOTES
        1. relay connections via Secure Shell or a similar protocol
           https://lists.freedesktop.org/archives/dbus/2018-April/017447.html



D-Bus 1.12.20                      07/02/2020                     DBUS-DAEMON(1)