Back to main site | Back to man page index

dbus-daemon(1)                                 General Commands Manual                                 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]


DESCRIPTION
       dbus-daemon is the D-Bus message bus daemon. See http://www.freedesktop.org/software/dbus/ for  more  informa‐
       tion  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=/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.


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

       --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.

       --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.


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


       <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 /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 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.


       <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.


       <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.

       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>


       <auth>


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


       Example: <auth>EXTERNAL</auth>


       Example: <auth>DBUS_COOKIE_SHA1</auth>


       <servicedir>


       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/>



       <standard_system_servicedirs/>


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


       The  <standard_system_servicedirs/>  option  is  only  relevant  to  the  per-system  bus  daemon  defined  in
       /etc/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  /etc/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
             "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

       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.  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‐
       vice.


       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>



          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).

       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>,
       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
       like  "foo.bar.*"  aren't allowed for now because they'd be work to implement and maybe encourage sloppy secu‐
       rity 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.freedesk‐
       top.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:

               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 con‐
               texts. 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  connec‐
       tion 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.


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.

       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.


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/soft‐
       ware/dbus/



                                                                                                       dbus-daemon(1)