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dbus-daemon-1(1) dbus-daemon-1(1)
NAME
dbus-daemon-1 - Message bus daemon
SYNOPSIS
dbus-daemon-1 dbus-daemon-1 [--version] [--session] [--system] [--con-
fig-file=FILE] [--print-address[=DESCRIPTOR]] [--print-pid[=DESCRIP-
TOR]] [--fork]
DESCRIPTION
dbus-daemon-1 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-1 is an appli-
cation that uses this library to implement a message bus daemon. Mul-
tiple programs connect to the message bus daemon and can exchange mes-
sages with one another.
There are two standard message bus instances: the systemwide message
bus (installed on many systems as the "messagebus" service) and the
per-user-login-session message bus (started each time a user logs in).
dbus-daemon-1 is used for both of these instances, but with a differ-
ent 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).
There is no way to cause the D-BUS daemon to reload its configuration
file (HUP will not do so). The reason is that changing configuration
would break the semantics expected by applications connected to the
message bus. Thus, changing configuration would require kicking all
apps off the bus; so you may as well just restart the daemon.
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 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:
The following elements may be present in the configuration file.
Root element.
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 ele-
ment "wins" (previous values are ignored).
Example: session
Include a file filename.conf at this point. If the
filename is relative, it is located relative to the configuration file
doing the including.
has an optional attribute "ignore_missing=(yes|no)" which
defaults to "no" if not provided. This attribute controls whether it's
a fatal error for the included file to be absent.
Include all files in foo.d at this point.
Files in the directory are included in undefined order. Only files
ending in ".conf" are included.
This is intended to allow extension of the system bus by particular
packages. For example, if CUPS wants to be able to send out notifica-
tion of printer queue changes, it could install a file to
/etc/dbus-1/system.d that allowed all apps to receive this message and
allowed the printer daemon user to send it.
The user account the daemon should run as, as either a username or a
UID. If the daemon cannot change to this UID on startup, it will exit.
If this element is not present, the daemon will not change or care
about its UID.
The last 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.
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.
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: unix:path=/tmp/foo
If there are multiple elements, then the bus listens on mul-
tiple addresses. The bus will pass its address to activated services
or other interested parties with the last address given in
first. That is, apps will try to connect to the last address
first.
Lists permitted authorization mechanisms. If this element doesn't
exist, then all known mechanisms are allowed. If there are multiple
elements, all the listed mechanisms are allowed. The order in
which mechanisms are listed is not meaningful.
Example: EXTERNAL
Example: DBUS_COOKIE_SHA1
Adds a directory to scan for .service files. Directories are scanned
starting with the last to appear in the config file (the first .ser-
vice file found that provides a particular service will be used).
Service files tell the bus how to automatically start a particular
service. They are primarily used with the per-user-session bus, not
the systemwide bus.
establishes a resource limit. For example:
64
512
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
"activation_timeout" : milliseconds (thousandths) until
an activated 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_activations" : max number of activations in
progress at the same time
"max_services_per_connection": max number of services a single
connection can own
"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 DOS all other users by using
up all connections.
The element defines a policy to be applied to a particular
set of connections to the bus. A policy is made up of and
elements.
The 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.
A element appears below a element and prohibits some
action. The element makes an exception to previous
statements, and works just like 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="service_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="service_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="servicename"
user="username"
group="groupname"
Examples:
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 service, not that
they may not be sent *to that service 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 service the application does not own.
Eavesdropping thus only applies to messages that are addressed to ser-
vices (i.e. it does not apply to signals).
For , 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
, eavesdrop="true" indicates that the rule matches only when
eavesdropping. eavesdrop="false" is the default for also, but
here it means that the rule applies always, even when not eavesdrop-
ping. 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 or 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 , [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 , [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 "service_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 for a
user or group; user/group denials can only be inside context="default"
or context="mandatory" policies.
A single rule may specify combinations of attributes such as
send_service and send_interface and send_type. In this case, the
denial applies only if both attributes match the message being denied.
e.g. 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 inter-
face field in messages is optional.
The element contains settings related to Security Enhanced
Linux. More details below.
An element appears below an element and creates
a mapping. Right now only one kind of association is possible:
elements specify the same service 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 use-
ful excerpts:
Every subject (process) and object (e.g. file, socket, IPC
object, etc) in the system is assigned a collection of secu-
rity attributes, known as a security context. A security con-
text 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 pro-
vide 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 opera-
tions 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 permis-
sion "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 service, the bus daemon
will check permissions with the security context of the connection as
source, the security context specified for the service name with an
element as target, object class "dbus" and requested per-
mission "acquire_svc".
If the service name has no security context associated in the configu-
ration 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(1)
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