dbus-daemon dbus-daemon [--version] [--session] [--system] [--config-
file=FILE] [--print-address[=DESCRIPTOR]] [--print-pid[=DESCRIPTOR]]
dbus-daemon is the D-Bus message bus daemon. See http://www.freedesk-
top.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 dif-
ferent configuration file.
The --session option is equivalent to "--config-file=/etc/dbus-1/ses-
sion.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, stan-
dardly 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
SIGHUP will cause the D-Bus daemon to PARTIALLY reload its configura-
tion file and to flush its user/group information caches. Some configu-
ration 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:
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
Use the standard configuration file for the systemwide message
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 specifica-
tion and its backward compatibility is not guaranteed; this document is
documentation, not specification.
The standard systemwide and per-session message bus setups are config-
ured in the files "/etc/dbus-1/system.conf" and "/etc/dbus-1/ses-
sion.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 busconfig PUBLIC "-//freedesktop//DTD D-Bus Bus Configuration 1.0//EN"
The following elements may be present in the configuration file.
If the well-known type of the message bus is "session", then the
DBUS_STARTER_BUS_TYPE environment variable will be set to "session" and
the DBUS_SESSION_BUS_ADDRESS environment variable will be set to the
address of the session bus. Likewise, if the type of the message bus
is "system", then the DBUS_STARTER_BUS_TYPE environment variable will
be set to "system" and the DBUS_SESSION_BUS_ADDRESS environment vari-
able will be set to the address of the system bus (which is normally
well known anyway).
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.
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 notifica-
tion 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.
If present, the bus daemon becomes a real daemon (forks into the back-
ground, etc.). This is generally used rather than the --fork command
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
If there are multiple <listen> elements, then the bus listens on multi-
ple 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
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 sys-
tem. 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
tcp addresses also allow a bind=hostname option, which will override
the host option specifying what address to bind to, without changing
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.
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
<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 directo-
The "XDG Base Directory Specification" can be found at http://freedesk-
top.org/wiki/Standards/basedir-spec if it hasn't moved, otherwise try
your favorite search engine.
The <standard_session_servicedirs/> option is only relevant to the per-
user-session bus daemon defined in /etc/dbus-1/session.conf. Putting it
in any other configuration file would probably be nonsense.
<standard_system_servicedirs/> specifies the standard system-wide acti-
vation 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-
The <servicehelper/> option is only relevant to the per-system bus dae-
mon defined in /etc/dbus-1/system.conf. Putting it in any other config-
uration file would probably be nonsense.
<limit> establishes a resource limit. For example:
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_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
Currently, the system bus has a default-deny policy for sending method
calls and owning bus names. Everything else, in particular reply mes-
sages, receive checks, and signals has a default allow policy.
In general, it is best to keep system services as small, targeted pro-
grams 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: daemon.1.in
user="username or userid"
group="group name or gid"
Policies are applied to a connection as follows:
- all context="default" policies are applied
- all group="connection's user's group" policies are applied
in undefined order
- all user="connection's auth user" policies are applied
in undefined order
- all at_console="true" policies are applied
- all at_console="false" policies are applied
- all context="mandatory" policies are applied
Policies applied later will override those applied earlier, when the
policies overlap. Multiple policies with the same user/group/context
are applied in the order they appear in the config file.
A <deny> element appears below a <policy> element and prohibits some
action. The <allow> element makes an exception to previous <deny>
statements, and works just like <deny> but with the inverse meaning.
The possible attributes of these elements are:
send_type="method_call" | "method_return" | "signal" | "error"
<deny send_interface="org.freedesktop.System" send_member="Reboot"/>
<deny receive_interface="org.freedesktop.System" receive_member="Reboot"/>
The <deny> element's attributes determine whether the deny "matches" a
particular action. If it matches, the action is denied (unless later
rules in the config file allow it).
send_destination and receive_sender rules mean that messages may not be
sent to or received from the *owner* of the given name, not that they
may not be sent *to that name*. That is, if a connection owns services
A, B, C, and sending to A is denied, sending to B or C will not work
The other send_* and receive_* attributes are purely textual/by-value
matches against the given field in the message header.
"Eavesdropping" occurs when an application receives a message that was
explicitly addressed to a name the application does not own, or is a
reply to such a message. Eavesdropping thus only applies to messages
that are addressed to services and replies to such messages (i.e. it
does not apply to signals).
For <allow>, eavesdrop="true" indicates that the rule matches even when
eavesdropping. eavesdrop="false" is the default and means that the rule
only allows messages to go to their specified recipient. For <deny>,
eavesdrop="true" indicates that the rule matches only when eavesdrop-
ping. 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 mes-
sage). This attribute only makes sense for reply messages (errors and
method returns), and is ignored for other message types.
user and group denials mean that the given user or group may not con-
nect to the message bus.
For "name", "username", "groupname", etc. the character "*" can be
substituted, meaning "any." Complex globs like "foo.bar.*" aren't
allowed for now because they'd be work to implement and maybe encourage
sloppy security anyway.
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_inter-
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.freedesk-
top.Foobar" then the source context will be the context of the connec-
tion and the target context will be "foo_t" - see the short discussion
If two <associate> elements specify the same name, the element appear-
ing later in the configuration file will be used.
See http://www.nsa.gov/selinux/ for full details on SELinux. Some use-
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 par-
ticular subject or object that are relevant to the security
In order to better encapsulate security contexts and to provide
greater efficiency, the policy enforcement code of SELinux typ-
ically 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 secu-
rity server. The object security class indicates the kind of
object, e.g. a process, a regular file, a directory, a TCP
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 con-
nection, the bus daemon will check permissions with the security con-
text of the first connection as source, security context of the second
connection as target, object class "dbus" and requested permission
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
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.
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
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 com-
piled 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.
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