dbus-daemon
Message bus daemon
Synopsis
dbus-daemon
dbus-daemon [--version]
[--session] [--system]
[--config-file=FILE]
[--print-address[=DESCRIPTOR]]
[--print-pid[=DESCRIPTOR]]
[--fork]
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description
dbus-daemon
is the D-Bus message bus daemon. See
http://www.freedesktop.org/software/dbus/ for more
information about the big picture. D-Bus is first a
library that provides one-to-one communication
between any two applications; dbus-daemon is an
application that uses this library to implement a message
bus daemon. Multiple programs connect to the message bus
daemon and can exchange messages with one another.
There are two
standard message bus instances: the systemwide message bus
(installed on many systems as the "messagebus"
init service) and the
per-user-login-session message bus
(started each time a user logs in). dbus-daemon
is used for both of these instances, but with a different
configuration file.
The
--session option is equivalent to
"--config-file=/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.
--system
Use the standard configuration
file for the systemwide message bus.
--version
Print the version of the
daemon.
--introspect
Print the introspection
information for all D-Bus internal interfaces.
--address[=ADDRESS]
Set the address to listen on.
This option overrides the address configured in the
configuration file.
--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/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"
"http://www.freedesktop.org/standards/dbus/1.0/busconfig.dtd">
The following elements may be present in the configuration file.
<busconfig>
Root element.
<type>
The well-known type of the message bus. Currently known values
are "system" and "session"; if other values are set, they should
be either added to the D-Bus specification, or namespaced. The
last <type> element "wins" (previous values are ignored).
This element only controls which message bus specific environment
variables are set in activated clients. Most of the policy that
distinguishes a session bus from the system bus is controlled
from the other elements in the configuration file.
If the well-known type of the message bus is "session", then the
DBUS_STARTER_BUS_TYPE environment variable will be set to
"session" and the DBUS_SESSION_BUS_ADDRESS environment variable
will be set to the address of the session bus. Likewise, if the
type of the message bus is "system", then the
DBUS_STARTER_BUS_TYPE environment variable will be set to
"system" and the DBUS_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>
Include a file <include>filename.conf</include> at
this point. If the filename is relative, it is located relative
to the configuration file doing the including.
<include> has an optional attribute
"ignore_missing=(yes|no)" which defaults to "no" if not provided.
This attribute controls whether it’s a fatal error for the
included file to be absent.
<includedir>
Include all files in <includedir>foo.d</includedir>
at this point. Files in the directory are included in undefined
order. Only files ending in ".conf" are included.
This is intended to allow extension of the system bus by
particular packages. For example, if CUPS wants to be able to
send out notification of printer queue changes, it could install
a file to /etc/dbus-1/system.d that allowed all apps to receive
this message and allowed the printer daemon user to send it.
<user>
The user account the daemon should run as, as either a username
or a UID. If the daemon cannot change to this UID on startup, it
will exit. If this element is not present, the daemon will not
change or care about its UID.
The last <user> entry in the file "wins", the others are
ignored.
The user is changed after the bus has completed initialization.
So sockets etc. will be created before changing user, but no data
will be read from clients before changing user. This means that
sockets and PID files can be created in a location that requires
root privileges for writing.
<fork>
If present, the bus daemon becomes a real daemon (forks into the
background, etc.). This is generally used rather than the --fork
command line option.
<keep_umask>
If present, the bus daemon keeps its original umask when forking.
This may be useful to avoid affecting the behavior of child
processes.
<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.
Example: <listen>unix:path=/tmp/foo</listen>
Example:
<listen>tcp:host=localhost,port=1234</listen>
If there are multiple <listen> elements, then the bus
listens on multiple addresses. The bus will pass its address to
started services or other interested parties with the last
address given in <listen> first. That is, apps will try to
connect to the last <listen> address first.
tcp sockets can accept IPv4 addresses, IPv6 addresses or
hostnames. If a hostname resolves to multiple addresses, the
server will bind to all of them. The family=ipv4 or family=ipv6
options can be used to force it to bind to a subset of addresses
Example:
<listen>tcp:host=localhost,port=0,family=ipv4</listen>
A special case is using a port number of zero (or omitting the
port), which means to choose an available port selected by the
operating system. The port number chosen can be obtained with the
--print-address command line parameter and will be present in
other cases where the server reports its own address, such as
when DBUS_SESSION_BUS_ADDRESS is set.
Example: <listen>tcp:host=localhost,port=0</listen>
tcp 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_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
http://freedesktop.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/>
<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
progress at the same time
"max_names_per_connection" : max number of names a single
connection can own
"max_match_rules_per_connection": max number of match rules for a
single
connection
"max_replies_per_connection" : max number of pending method
replies per connection
(number of calls-in-progress)
"reply_timeout" : milliseconds (thousandths)
until a method call times out
The max incoming/outgoing queue sizes allow a new message to be
queued if one byte remains below the max. So you can in fact
exceed the max by max_message_size.
max_completed_connections divided by max_connections_per_user is
the number of users that can work together to denial-of-service
all other users by using up all connections on the systemwide
bus.
Limits are normally only of interest on the systemwide bus, not
the user session buses.
<policy>
The <policy> element defines a security policy to be
applied to a particular set of connections to the bus. A policy
is made up of <allow> and <deny> elements. Policies
are normally used with the systemwide bus; they are analogous to
a firewall in that they allow expected traffic and prevent
unexpected traffic.
Currently, the system bus has a default-deny policy for sending
method calls and owning bus names. 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 service.
The <policy> element has one of four attributes:
context="(default|mandatory)"
at_console="(true|false)"
user="username or userid"
group="group name or gid"
Policies are applied to a connection as follows:
- all context="default" policies are applied
- all group="connection’s user’s group" policies are applied
in undefined order
- all user="connection’s auth user" policies are applied
in undefined order
- all at_console="true" policies are applied
- all at_console="false" policies are applied
- all context="mandatory" policies are applied
Policies applied later will override those applied earlier, when
the policies overlap. Multiple policies with the same
user/group/context are applied in the order they appear in the
config file.
<deny>
<allow>
A <deny> element appears below a <policy> element and
prohibits some action. The <allow> element makes an
exception to previous <deny> statements, and works just
like <deny> but with the inverse meaning.
The possible attributes of these elements are:
send_interface="interface_name"
send_member="method_or_signal_name"
send_error="error_name"
send_destination="name"
send_type="method_call" | "method_return" | "signal" |
"error"
send_path="/path/name"
receive_interface="interface_name"
receive_member="method_or_signal_name"
receive_error="error_name"
receive_sender="name"
receive_type="method_call" | "method_return" | "signal" |
"error"
receive_path="/path/name"
send_requested_reply="true" | "false"
receive_requested_reply="true" | "false"
eavesdrop="true" | "false"
own="name"
own_prefix="name"
user="username"
group="groupname"
Examples:
<deny send_destination="org.freedesktop.Service"
send_interface="org.freedesktop.System"
send_member="Reboot"/>
<deny send_destination="org.freedesktop.System"/>
<deny receive_sender="org.freedesktop.System"/>
<deny user="john"/>
<deny group="enemies"/>
The <deny> element’s attributes determine whether the deny
"matches" a particular action. If it matches, the action is
denied (unless later rules in the config file allow it).
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 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 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.
<allow own_prefix="a.b"/> allows you to own the name "a.b"
or any name whose first dot-separated elements are "a.b": in
particular, you can own "a.b.c" or "a.b.c.d", but not "a.bc" or
"a.c". This is useful when services like Telepathy and
ReserveDevice define a meaning for subtrees of well-known names,
such as org.freedesktop.Telepathy.ConnectionManager.(anything)
and org.freedesktop.ReserveDevice1.(anything).
It does not make sense to deny a user or group inside a
<policy> for a user or group; user/group denials can only
be inside context="default" or context="mandatory" policies.
A single <deny> rule may specify combinations of attributes
such as send_destination and send_interface and send_type. In
this case, the denial applies only if both attributes match the
message being denied. e.g. <deny send_interface="foo.bar"
send_destination="foo.blah"/> would deny messages with the
given interface AND the given bus name. To get an OR effect you
specify multiple <deny> rules.
You can’t include both send_ and receive_ attributes on the same
rule, since "whether the message can be sent" and "whether it can
be received" are evaluated separately.
Be careful with send_interface/receive_interface, because the
interface field in messages is optional. In particular, do NOT
specify <deny send_interface="org.foo.Bar"/>! This will
cause no-interface messages to be blocked for all services, which
is almost certainly not what you intended. Always use rules of
the form: <deny send_interface="org.foo.Bar"
send_destination="org.foo.Service"/>
<selinux>
The <selinux> element contains settings related to Security
Enhanced Linux. More details below.
<associate>
An <associate> element appears below an <selinux>
element and creates a mapping. Right now only one kind of
association is possible:
<associate own="org.freedesktop.Foobar" context="foo_t"/>
This means that if a connection asks to own the name
"org.freedesktop.Foobar" then the source context will be the
context of the connection and the target context will be "foo_t"
- see the short discussion of SELinux below.
Note, the context here is the target context when requesting a
name, NOT the context of the connection owning the name.
There’s currently no way to set a default for owning any name, if
we add this syntax it will look like:
<associate own="*" context="foo_t"/>
If you find a reason this is useful, let the developers know.
Right now the default will be the security context of the bus
itself.
If two <associate> elements specify the same name, the
element appearing later in the configuration file will be used.
debugging
If you’re trying to figure out where your messages are going or
why you aren’t getting messages, there are several things you can
try.
Remember that the system bus is heavily locked down and if you
haven’t installed a security policy file to allow your message
through, it won’t work. For the session bus, this is not a
concern.
The simplest way to figure out what’s happening on the bus is to
run the dbus-monitor program, which comes with the D-Bus
package. You can also send test messages with dbus-send.
These programs have their own man pages.
If you want to know what the daemon itself is doing, you might
consider running a separate copy of the daemon to test against.
This will allow you to put the daemon under a debugger, or run it
with verbose output, without messing up your real session and
system daemons.
To run a separate test copy of the daemon, for example you might
open a terminal and type:
DBUS_VERBOSE=1 dbus-daemon --session --print-address
The test daemon address will be printed when the daemon starts.
You will need to copy-and-paste this address and use it as the
value of the DBUS_SESSION_BUS_ADDRESS environment variable when
you launch the applications you want to test. This will cause
those applications to connect to your test bus instead of the
DBUS_SESSION_BUS_ADDRESS of your real session bus.
DBUS_VERBOSE=1 will have NO EFFECT unless your copy of D-Bus was
compiled with verbose mode enabled. This is not recommended in
production builds due to performance impact. You may need to
rebuild D-Bus if your copy was not built with debugging in mind.
(DBUS_VERBOSE also affects the D-Bus library and thus
applications using D-Bus; it may be useful to see verbose output
on both the client side and from the daemon.)
If you want to get fancy, you can create a custom bus
configuration for your test bus (see the session.conf and
system.conf files that define the two default configurations for
example). This would allow you to specify a different directory
for .service files, for example.
selinux
See http://www.nsa.gov/selinux/ for full details on SELinux. Some
useful excerpts:
Every subject (process) and object (e.g. file, socket, IPC
object, etc) in the system is assigned a collection of security
attributes, known as a security context. A security context
contains all of the security attributes associated with a
particular subject or object that are relevant to the security
policy.
In order to better encapsulate security contexts and to provide
greater efficiency, the policy enforcement code of SELinux
typically handles security identifiers (SIDs) rather than
security contexts. A SID is an integer that is mapped by the
security server to a security context at runtime.
When a security decision is required, the policy enforcement code
passes a pair of SIDs (typically the SID of a subject and the SID
of an object, but sometimes a pair of subject SIDs or a pair of
object SIDs), and an object security class to the security
server. The object security class indicates the kind of object,
e.g. a process, a regular file, a directory, a TCP socket, etc.
Access decisions specify whether or not a permission is granted
for a given pair of SIDs and class. Each object class has a set
of associated permissions defined to control operations on
objects with that class.
D-Bus performs SELinux security checks in two places.
First, any time a message is routed from one connection to
another connection, the bus daemon will check permissions with
the security context of the first connection as source, security
context of the second connection as target, object class "dbus"
and requested permission "send_msg".
If a security context is not available for a connection
(impossible when using UNIX domain sockets), then the target
context used is the context of the bus daemon itself. There is
currently no way to change this default, because we’re assuming
that only UNIX domain sockets will be used to connect to the
systemwide bus. If this changes, we’ll probably add a way to set
the default connection context.
Second, any time a connection asks to own a name, the bus daemon
will check permissions with the security context of the
connection as source, the security context specified for the name
in the config file as target, object class "dbus" and requested
permission "acquire_svc".
The security context for a bus name is specified with the
<associate> element described earlier in this document. If
a name has no security context associated in the configuration
file, the security context of the bus daemon itself will be used.
bugs
Please send bug
reports to the D-Bus mailing list or bug tracker, see
http://www.freedesktop.org/software/dbus/
author
See
http://www.freedesktop.org/software/dbus/doc/AUTHORS