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CentOS 7.0 - man page for dbus-daemon (centos section 1)

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 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 mes-
       sage 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-pur-
       pose message bus daemons could be created.

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

       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 appli-
       cations (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.	--nofork Force the message bus not to fork and	become	a
	      daemon, even if the configuration file specifies that it should.

	      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 interfaces.

	      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  sys-
	      temd 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 appli-
       cation. For example, one configuration file might set up the message  bus  to  be  a  sys-
       temwide 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

       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/system.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 names-
       paced.  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 mes-
       sage 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>


       Include a file <include>filename.conf</include> at this point.  If the filename	is  rela-
       tive, 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.


       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 /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 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.


       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_SESSION_BUS_ADDRESS is

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

       tcp addresses also allow a bind=hostname option, which will override the host option spec-
       ifying 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 particu-
       lar 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/> ele-
       ments 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/Stan-
       dards/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.


       <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/sys-

       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 prob-
       ably be nonsense.


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


       <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
	     "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  con-
       nections to the bus. A policy is made up of <allow> and <deny> elements. Policies are nor-
       mally 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_destina-
       tion" 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.

       <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_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 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.  Eaves-
       dropping  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

       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.ConnectionMan-
       ager.(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_destina-
       tion="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"


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


       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 con-
       figuration file will be used.

       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  con-
	       text. 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  effi-
	       ciency,	the policy enforcement code of SELinux typically handles security identi-
	       fiers (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

       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  configura-
       tion 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 several things you can try.

       Remember that the system bus is heavily locked down and if you haven't installed  a  secu-
       rity  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 pro-
       gram,  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 debug-
       ger, or run it with verbose output, without messing up your real session and  system  dae-

       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  envi-
       ronment	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

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

       Please send bug reports to the D-Bus mailing list or bug tracker, see http://www.freedesk-


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