NAME¶
dbus-daemon - Message bus daemon
SYNOPSIS¶
dbus-daemon dbus-daemon [--version] [--session] [--system]
[--config-file=FILE] [--print-address[=DESCRIPTOR]] [--print-pid[=DESCRIPTOR]]
[--fork]
DESCRIPTION¶
dbus-daemon is the D-Bus message bus daemon. See
http://www.freedesktop.org/software/dbus/ for more 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.
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.
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/