DBUS-DAEMON(1) User Commands 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] [--nosyslog] [--syslog] [--syslog-only]
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=/usr/share/dbus-1/session.conf" and the --system option
is equivalent to "--config-file=/usr/share/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.
This option is not supported on Windows.
--nofork
Force the message bus not to fork and become a daemon, even if the
configuration file specifies that it should. On Windows, the
dbus-daemon never forks, so this option is allowed but does
nothing.
--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 via the <listen> directive.
See the documentation of that directive for more details.
--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.
--syslog
Force the message bus to use the system log for messages, in
addition to writing to standard error, even if the configuration
file does not specify that it should. On Unix, this uses the
syslog; on Windows, this uses OutputDebugString().
--syslog-only
Force the message bus to use the system log for messages, and not
duplicate them to standard error. On Unix, this uses the syslog; on
Windows, this uses OutputDebugString().
--nosyslog
Force the message bus to use only standard error for messages, even
if the configuration file specifies that it should use the system
log.
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 "/usr/share/dbus-1/system.conf" and
"/usr/share/dbus-1/session.conf". These files normally <include> a
system-local.conf or session-local.conf in /etc/dbus-1; 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.
o <busconfig>
Root element.
o <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_SYSTEM_BUS_ADDRESS environment variable
will be set to the address of the system bus (which is normally well
known anyway).
Example: <type>session</type>
o <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.
o <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
/usr/share/dbus-1/system.d or /etc/dbus-1/system.d that allowed all
apps to receive this message and allowed the printer daemon user to
send it.
o <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.
o <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.
o <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.
o <syslog>
If present, the bus daemon will log to syslog. The --syslog,
--syslog-only and --nosyslog command-line options take precedence over
this setting.
o <pidfile>
If present, the bus daemon will write its pid to the specified file.
The --nopidfile command-line option takes precedence over this setting.
o <allow_anonymous>
If present, connections that authenticated using the ANONYMOUS
mechanism will be authorized to connect. This option has no practical
effect unless the ANONYMOUS mechanism has also been enabled using the
<auth> element, described below.
Using this directive in the configuration of the well-known system bus
or the well-known session bus will make that bus insecure and should
never be done. Similarly, on custom bus types, using this directive
will usually make the custom bus insecure, unless its configuration has
been specifically designed to prevent anonymous users from causing
damage or escalating privileges.
o <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.
On platforms other than Windows, unix-based transports (unix, systemd,
launchd) are the default for both the well-known system bus and the
well-known session bus, and are strongly recommended.
On Windows, unix-based transports are not available, so TCP-based
transports must be used. Similar to remote X11, the tcp and nonce-tcp
transports have no integrity or confidentiality protection, so they
should normally only be used across the local loopback interface, for
example using an address like tcp:host=127.0.0.1 or
nonce-tcp:host=localhost. In particular, configuring the well-known
system bus or the well-known session bus to listen on a non-loopback
TCP address is insecure.
Developers are sometimes tempted to use remote TCP as a debugging tool.
However, if this functionality is left enabled in finished products,
the result will be dangerously insecure. Instead of using remote TCP,
developers should relay connections via Secure Shell or a similar
protocol[1].
Remote TCP connections were historically sometimes used to share a
single session bus between login sessions of the same user on different
machines within a trusted local area network, in conjunction with
unencrypted remote X11, a NFS-shared home directory and NIS (YP)
authentication. This is insecure against an attacker on the same LAN
and should be considered strongly deprecated; more specifically, it is
insecure in the same ways and for the same reasons as unencrypted
remote X11 and NFSv2/NFSv3. The D-Bus maintainers recommend using a
separate session bus per (user, machine) pair, only accessible from
within that machine.
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/nonce-tcp addresses also allow a bind=hostname option, used in a
listenable address to configure the interface on which the server will
listen: either the hostname is the IP address of one of the local
machine's interfaces (most commonly 127.0.0.1), a DNS name that
resolves to one of those IP addresses, '0.0.0.0' to listen on all IPv4
interfaces simultaneously, or '::' to listen on all IPv4 and IPv6
interfaces simultaneously (if supported by the OS). If not specified,
the default is the same value as "host".
Example: <listen>tcp:host=localhost,bind=0.0.0.0,port=0</listen>
o <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.
On non-Windows operating systems, allowing only the EXTERNAL
authentication mechanism is strongly recommended. This is the default
for the well-known system bus and for the well-known session bus.
Example: <auth>EXTERNAL</auth>
Example: <auth>DBUS_COOKIE_SHA1</auth>
o <servicedir>
Adds a directory to search for .service files, which tell the
dbus-daemon how to start a program to provide a particular well-known
bus name. See the D-Bus Specification for more details about the
contents of .service files.
If a particular service is found in more than one <servicedir>, the
first directory listed in the configuration file takes precedence. If
two service files providing the same well-known bus name are found in
the same directory, it is arbitrary which one will be chosen (this can
only happen if at least one of the service files does not have the
recommended name, which is its well-known bus name followed by
".service").
o <standard_session_servicedirs/>
<standard_session_servicedirs/> requests a standard set of session
service directories. Its effect is similar to specifying a series of
<servicedir/> elements for each of the data directories, in the order
given here. It is not exactly equivalent, because there is currently no
way to disable directory monitoring or enforce strict service file
naming for a <servicedir/>.
As with <servicedir/> elements, if a particular service is found in
more than one service directory, the first directory takes precedence.
If two service files providing the same well-known bus name are found
in the same directory, it is arbitrary which one will be chosen (this
can only happen if at least one of the service files does not have the
recommended name, which is its well-known bus name followed by
".service").
On Unix, the standard session service directories are:
o $XDG_RUNTIME_DIR/dbus-1/services, if XDG_RUNTIME_DIR is set (see
the XDG Base Directory Specification for details of
XDG_RUNTIME_DIR): this location is suitable for transient services
created at runtime by systemd generators (see
systemd.generator(7)), session managers or other session
infrastructure. It is an extension provided by the reference
implementation of dbus-daemon, and is not standardized in the D-Bus
Specification.
Unlike the other standard session service directories, this
directory enforces strict naming for the service files: the
filename must be exactly the well-known bus name of the service,
followed by ".service".
Also unlike the other standard session service directories, this
directory is never monitored with inotify(7) or similar APIs.
Programs that create service files in this directory while a
dbus-daemon is running are expected to call the dbus-daemon's
ReloadConfig() method after they have made changes.
o $XDG_DATA_HOME/dbus-1/services, where XDG_DATA_HOME defaults to
~/.local/share (see the XDG Base Directory Specification): this
location is specified by the D-Bus Specification, and is suitable
for per-user, locally-installed software.
o directory/dbus-1/services for each directory in XDG_DATA_DIRS,
where XDG_DATA_DIRS defaults to /usr/local/share:/usr/share (see
the XDG Base Directory Specification): these locations are
specified by the D-Bus Specification. The defaults are suitable for
software installed locally by a system administrator
(/usr/local/share) or for software installed from operating system
packages (/usr/share). Per-user or system-wide configuration that
sets the XDG_DATA_DIRS environment variable can extend this search
path to cover installations in other locations, for example
~/.local/share/flatpak/exports/share/ and
/var/lib/flatpak/exports/share/ when flatpak(1) is used.
o ${datadir}/dbus-1/services for the ${datadir} that was specified
when dbus was compiled, typically /usr/share: this location is an
extension provided by the reference dbus-daemon implementation, and
is suitable for software stacks installed alongside dbus-daemon.
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.
On Windows, the standard session service directories are:
o %CommonProgramFiles%/dbus-1/services if %CommonProgramFiles% is
set: this location is suitable for system-wide installed software
packages
o A share/dbus-1/services directory found in the same directory
hierarchy (prefix) as the dbus-daemon: this location is suitable
for software stacks installed alongside dbus-daemon
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.
o <standard_system_servicedirs/>
<standard_system_servicedirs/> specifies the standard system-wide
activation directories that should be searched for service files. As
with session services, the first directory listed has highest
precedence.
On Unix, the standard session service directories are:
o /usr/local/share/dbus-1/system-services: this location is specified
by the D-Bus Specification, and is suitable for software installed
locally by the system administrator
o /usr/share/dbus-1/system-services: this location is specified by
the D-Bus Specification, and is suitable for software installed by
operating system packages
o ${datadir}/dbus-1/system-services for the ${datadir} that was
specified when dbus was compiled, typically /usr/share: this
location is an extension provided by the reference dbus-daemon
implementation, and is suitable for software stacks installed
alongside dbus-daemon
o /lib/dbus-1/system-services: this location is specified by the
D-Bus Specification, and was intended for software installed by
operating system packages and used during early boot (but it should
be considered deprecated, because the reference dbus-daemon is not
designed to be available during early boot)
On Windows, there is no standard system bus, so there are no standard
system bus directories either.
The <standard_system_servicedirs/> option is only relevant to the
per-system bus daemon defined in /usr/share/dbus-1/system.conf. Putting
it in any other configuration file would probably be nonsense.
o <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 /usr/share/dbus-1/system.conf. Putting it in any
other configuration file would probably be nonsense.
o <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
"pending_fd_timeout" : milliseconds (thousandths) a
fd is given to be transmitted to
dbus-daemon before disconnecting the
connection
"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.
o <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, and a default-allow policy for receiving
messages, sending signals, and sending a single success or error reply
for each method call that does not have the NO_REPLY flag. Sending more
than the expected number of replies is not allowed.
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_broadcast="true" | "false"
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).
Rules with one or more of the send_* family of attributes are checked
in order when a connection attempts to send a message. The last rule
that matches the message determines whether it may be sent. The
well-known session bus normally allows sending any message. The
well-known system bus normally allows sending any signal, selected
method calls to the dbus-daemon, and exactly one reply to each
previously-sent method call (either success or an error). Either of
these can be overridden by configuration; on the system bus, services
that will receive method calls must install configuration that allows
them to do so, usually via rules of the form <policy
context="default"><allow send_destination="..."/><policy>.
Rules with one or more of the receive_* family of attributes, or with
the eavesdrop attribute and no others, are checked for each recipient
of a message (there might be more than one recipient if the message is
a broadcast or a connection is eavesdropping). The last rule that
matches the message determines whether it may be received. The
well-known session bus normally allows receiving any message, including
eavesdropping. The well-known system bus normally allows receiving any
message that was not eavesdropped (any unicast message addressed to the
recipient, and any broadcast message).
The eavesdrop, min_fds and max_fds attributes are modifiers that can be
applied to either send_* or receive_* rules, and are documented below.
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. As a special case, send_destination="*" matches any message
(whether it has a destination specified or not), and receive_sender="*"
similarly matches any message.
Rules with send_broadcast="true" match signal messages with no
destination (broadcasts). Rules with send_broadcast="false" are the
inverse: they match any unicast destination (unicast signals, together
with all method calls, replies and errors) but do not match messages
with no destination (broadcasts). This is not the same as
send_destination="*", which matches any sent message, regardless of
whether it has a destination or not.
The other send_* and receive_* attributes are purely textual/by-value
matches against the given field in the message header, except that for
the attributes where it is allowed, * matches any message (whether it
has the relevant header field or not). For example, send_interface="*"
matches any sent message, even if it does not contain an interface
header field. More complex glob matching such as foo.bar.* is not
allowed.
"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.
The min_fds and max_fds attributes modify either send_* or receive_*
rules. A rule with the min_fds attribute only matches messages if they
have at least that many Unix file descriptors attached. Conversely, a
rule with the max_fds attribute only matches messages if they have no
more than that many file descriptors attached. In practice, rules with
these attributes will most commonly take the form <allow
send_destination="..." max_fds="0"/>, <deny send_destination="..."
min_fds="1"/> or <deny receive_sender="*" min_fds="1"/>.
Rules with the user or group attribute are checked when a new
connection to the message bus is established, and control whether the
connection can continue. Each of these attributes cannot be combined
with any other attribute. As a special case, both user="*" and
group="*" match any connection. If there are no rules of this form, the
default is to allow connections from the same user ID that owns the
dbus-daemon process. The well-known session bus normally uses that
default behaviour, while the well-known system bus normally allows any
connection.
Rules with the own or own_prefix attribute are checked when a
connection attempts to own a well-known bus names. As a special case,
own="*" matches any well-known bus name. The well-known session bus
normally allows any connection to own any name, while the well-known
system bus normally does not allow any connection to own any name,
except where allowed by further configuration. System services that
will own a name must install configuration that allows them to do so,
usually via rules of the form <policy user="some-system-user"><allow
own="..."/></policy>.
<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"/>
o <selinux>
The <selinux> element contains settings related to Security Enhanced
Linux. More details below.
o <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.
o <apparmor>
The <apparmor> element is used to configure AppArmor mediation on the
bus. It can contain one attribute that specifies the mediation mode:
<apparmor mode="(enabled|disabled|required)"/>
The default mode is "enabled". In "enabled" mode, AppArmor mediation
will be performed if AppArmor support is available in the kernel. If it
is not available, dbus-daemon will start but AppArmor mediation will
not occur. In "disabled" mode, AppArmor mediation is disabled. In
"required" mode, AppArmor mediation will be enabled if AppArmor support
is available, otherwise dbus-daemon will refuse to start.
The AppArmor mediation mode of the bus cannot be changed after the bus
starts. Modifying the mode in the configuration file and sending a
SIGHUP signal to the daemon has no effect on the mediation mode.
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.
APPARMOR
The AppArmor confinement context is stored when applications connect to
the bus. The confinement context consists of a label and a confinement
mode. When a security decision is required, the daemon uses the
confinement context to query the AppArmor policy to determine if the
action should be allowed or denied and if the action should be audited.
The daemon performs AppArmor security checks in three places.
First, any time a message is routed from one connection to another
connection, the bus daemon will check permissions with the label of the
first connection as source, label and/or connection name of the second
connection as target, along with the bus name, the path name, the
interface name, and the member name. Reply messages, such as
method_return and error messages, are implicitly allowed if they are in
response to a message that has already been allowed.
Second, any time a connection asks to own a name, the bus daemon will
check permissions with the label of the connection as source, the
requested name as target, along with the bus name.
Third, any time a connection attempts to eavesdrop, the bus daemon will
check permissions with the label of the connection as the source, along
with the bus name.
AppArmor rules for bus mediation are not stored in the bus
configuration files. They are stored in the application's AppArmor
profile. Please see apparmor.d(5) for more details.
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.
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/
NOTES
1. relay connections via Secure Shell or a similar protocol
https://lists.freedesktop.org/archives/dbus/2018-April/017447.html
D-Bus 1.12.20 DBUS-DAEMON(1)