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dbus-daemon(1)                                                  dbus-daemon(1)

       dbus-daemon - Message bus daemon

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

       dbus-daemon is the D-Bus message bus daemon. See for more information
       about the big picture. D-Bus is first a library that provides one-to-one communication between any two applica-
       tions; dbus-daemon is an application that uses this library to implement a message bus  daemon.  Multiple  pro-
       grams 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=/etc/dbus-1/session.conf" and the --system option is
       equivalent to "--config-file=/etc/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.

       The following options are supported:

              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.

              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.

       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 "/etc/dbus-1/sys-
       tem.conf" and "/etc/dbus-1/session.conf".  These files  normally  <include>  a  system-local.conf  or  session-
       local.conf; 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.


       Root element.


       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  con-
       trolled 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 environ-
       ment 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>


       Include a file <include>filename.conf</include> at this point.  If the filename is relative, it is located rel-
       ative 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.


       Include all files in <includedir>foo.d</includedir> at this point. Files in the directory are included in unde-
       fined 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 /etc/dbus-1/system.d that
       allowed all apps to receive this message and allowed the printer daemon user to send it.


       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

       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  chang-
       ing 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.


       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.


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


       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.

       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_SES-
       SION_BUS_ADDRESS is set.

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

       tcp addresses also allow a bind=hostname option, which will override the host option specifying what address to
       bind  to, without changing the address reported by the bus. The bind option can also take a special name '*' to
       cause the bus to listen on all local address (INADDR_ANY). The specified host should be a  valid  name  of  the
       local machine or weird stuff will happen.

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


       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>


       Adds  a  directory  to scan for .service files. Directories are scanned starting with the last 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 bus.


       <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 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.


       <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
       /etc/dbus-1/system.conf. Putting it in any other configuration file would probably be nonsense.


       <servicehelper/> specifies the setuid helper that is used to launch system daemons with an alternate user. Typ-
       ically 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  /etc/dbus-1/system.conf.
       Putting it in any other configuration file would probably be nonsense.


       <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_outgoing_bytes"         : total size in bytes of messages
                                            queued up for a single connection
             "max_message_size"           : max size of a single message in
             "service_start_timeout"      : milliseconds (thousandths) until
                                            a started service has to connect
             "auth_timeout"               : milliseconds (thousandths) a
                                            connection is given to
             "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_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.


       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  ser-

       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.

       <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_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_interface="org.freedesktop.System" send_member="Reboot"/>
          <deny receive_interface="org.freedesktop.System" receive_member="Reboot"/>
          <deny own="org.freedesktop.System"/>
          <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 either.

       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>,  eaves-
       drop="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

       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 unex-

       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 "*" aren't allowed for now because they'd be work to implement and maybe encourage sloppy security

       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=""  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 par-
       ticular, do NOT specify <deny send_interface=""/>!  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="" send_destination=""/>


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


       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

       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  con-
       text of the bus itself.

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

       See 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

               In order to better encapsulate security contexts and to provide greater efficiency, the policy enforce-
               ment 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  per-
       missions 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 con-
       text 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 docu-
       ment.  If a name has no security context associated in the configuration file, the security context of the  bus
       daemon itself will be used.

       If you're trying to figure out where your messages are going or why you aren't getting messages, there are sev-
       eral 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_SES-
       SION_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.


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