dbus-daemon(1)dbus-daemon(1)NAMEdbus-daemon - Message bus daemon
SYNOPSISdbus-daemon dbus-daemon [--version] [--session] [--system] [--config-
file=FILE] [--print-address[=DESCRIPTOR]] [--print-pid[=DESCRIPTOR]]
[--fork]
DESCRIPTIONdbus-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
any way).
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.
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.
--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.
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 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 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> ele‐
ment "wins" (previous values are ignored).
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 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.
<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 back‐
ground, etc.). This is generally used rather than the --fork command
line option.
<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>
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.
<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 directo‐
ries searched.
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.
<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_outgoing_bytes" : total size in bytes of messages
queued up for a single connection
"max_message_size" : max size of a single message in
bytes
"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 par‐
ticular 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 traf‐
fic and prevent unexpected traffic.
The <policy> element has one of three attributes:
context="(default|mandatory)"
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 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"
user="username"
group="groupname"
Examples:
<deny send_interface="org.freedesktop.System" send_member="Reboot"/>
<deny receive_interface="org.freedesktop.System" receive_member="Reboot"/>
<deny own="org.freedesktop.System"/>
<deny send_destination="org.freedesktop.System"/>
<deny receive_sender="org.freedesktop.System"/>
<deny user="john"/>
<deny group="enemies"/>
The <deny> element's attributes determine whether the deny "matches" a
particular action. If it matches, the action is denied (unless later
rules in the config file allow it).
send_destination and receive_sender rules mean that messages may not be
sent to or received from the *owner* of the given name, not that they
may not be sent *to that name*. That is, if a connection owns services
A, B, C, and sending to A is denied, sending to B or C will not work
either.
The other send_* and receive_* attributes are purely textual/by-value
matches against the given field in the message header.
"Eavesdropping" occurs when an application receives a message that was
explicitly addressed to a name the application does not own. Eaves‐
dropping thus only applies to messages that are addressed to services
(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 receive rules (with
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.
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 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.
<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.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
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 appear‐
ing later in the configuration file will be used.
SELinux
See http://www.nsa.gov/selinux/ for full details on SELinux. Some use‐
ful 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 par‐
ticular 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 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
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 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
"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 proba‐
bly 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.
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/software/dbus/
dbus-daemon(1)
