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575 lines
19 KiB
Groff
575 lines
19 KiB
Groff
.\"
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.\" dbus-daemon manual page.
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.\" Copyright (C) 2003 Red Hat, Inc.
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.\"
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.TH dbus-daemon 1
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.SH NAME
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dbus-daemon \- Message bus daemon
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.SH SYNOPSIS
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.PP
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.B dbus-daemon
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dbus-daemon [\-\-version] [\-\-session] [\-\-system] [\-\-config-file=FILE]
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[\-\-print-address[=DESCRIPTOR]] [\-\-print-pid[=DESCRIPTOR]] [\-\-fork]
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.SH DESCRIPTION
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\fIdbus-daemon\fP is the D-BUS message bus daemon. See
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http://www.freedesktop.org/software/dbus/ for more information about
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the big picture. D-BUS is first a library that provides one-to-one
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communication between any two applications; \fIdbus-daemon\fP is an
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application that uses this library to implement a message bus
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daemon. Multiple programs connect to the message bus daemon and can
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exchange messages with one another.
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.PP
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There are two standard message bus instances: the systemwide message bus
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(installed on many systems as the "messagebus" init service) and the
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per-user-login-session message bus (started each time a user logs in).
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\fIdbus-daemon\fP is used for both of these instances, but with
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a different configuration file.
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.PP
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The \-\-session option is equivalent to
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"\-\-config-file=@EXPANDED_SYSCONFDIR@/dbus-1/session.conf" and the \-\-system
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option is equivalent to
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"\-\-config-file=@EXPANDED_SYSCONFDIR@/dbus-1/system.conf". By creating
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additional configuration files and using the \-\-config-file option,
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additional special-purpose message bus daemons could be created.
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.PP
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The systemwide daemon is normally launched by an init script,
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standardly called simply "messagebus".
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.PP
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The systemwide daemon is largely used for broadcasting system events,
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such as changes to the printer queue, or adding/removing devices.
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.PP
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The per-session daemon is used for various interprocess communication
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among desktop applications (however, it is not tied to X or the GUI
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in any way).
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.PP
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SIGHUP will cause the D-BUS daemon to PARTIALLY reload its
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configuration file. Some configuration changes would require kicking
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all apps off the bus; so they will only take effect if you restart the
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daemon. Policy changes should take effect with SIGHUP.
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.SH OPTIONS
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The following options are supported:
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.TP
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.I "--config-file=FILE"
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Use the given configuration file.
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.TP
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.I "--fork"
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Force the message bus to fork and become a daemon, even if
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the configuration file does not specify that it should.
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In most contexts the configuration file already gets this
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right, though.
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.TP
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.I "--print-address[=DESCRIPTOR]"
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Print the address of the message bus to standard output, or
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to the given file descriptor. This is used by programs that
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launch the message bus.
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.TP
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.I "--print-pid[=DESCRIPTOR]"
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Print the process ID of the message bus to standard output, or
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to the given file descriptor. This is used by programs that
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launch the message bus.
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.TP
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.I "--session"
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Use the standard configuration file for the per-login-session message
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bus.
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.TP
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.I "--system"
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Use the standard configuration file for the systemwide message bus.
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.TP
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.I "--version"
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Print the version of the daemon.
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.SH CONFIGURATION FILE
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A message bus daemon has a configuration file that specializes it
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for a particular application. For example, one configuration
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file might set up the message bus to be a systemwide message bus,
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while another might set it up to be a per-user-login-session bus.
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.PP
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The configuration file also establishes resource limits, security
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parameters, and so forth.
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.PP
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The configuration file is not part of any interoperability
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specification and its backward compatibility is not guaranteed; this
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document is documentation, not specification.
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.PP
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The standard systemwide and per-session message bus setups are
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configured in the files "@EXPANDED_SYSCONFDIR@/dbus-1/system.conf" and
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"@EXPANDED_SYSCONFDIR@/dbus-1/session.conf". These files normally
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<include> a system-local.conf or session-local.conf; you can put local
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overrides in those files to avoid modifying the primary configuration
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files.
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.PP
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The configuration file is an XML document. It must have the following
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doctype declaration:
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.nf
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<!DOCTYPE busconfig PUBLIC "-//freedesktop//DTD D-BUS Bus Configuration 1.0//EN"
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"http://www.freedesktop.org/standards/dbus/1.0/busconfig.dtd">
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.fi
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.PP
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The following elements may be present in the configuration file.
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.TP
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.I "<busconfig>"
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.PP
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Root element.
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.TP
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.I "<type>"
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.PP
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The well-known type of the message bus. Currently known values are
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"system" and "session"; if other values are set, they should be
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either added to the D-BUS specification, or namespaced. The last
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<type> element "wins" (previous values are ignored).
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.PP
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Example: <type>session</type>
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.TP
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.I "<include>"
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.PP
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Include a file <include>filename.conf</include> at this point. If the
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filename is relative, it is located relative to the configuration file
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doing the including.
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.PP
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<include> has an optional attribute "ignore_missing=(yes|no)"
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which defaults to "no" if not provided. This attribute
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controls whether it's a fatal error for the included file
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to be absent.
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.TP
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.I "<includedir>"
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.PP
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Include all files in <includedir>foo.d</includedir> at this
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point. Files in the directory are included in undefined order.
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Only files ending in ".conf" are included.
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.PP
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This is intended to allow extension of the system bus by particular
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packages. For example, if CUPS wants to be able to send out
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notification of printer queue changes, it could install a file to
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@EXPANDED_SYSCONFDIR@/dbus-1/system.d that allowed all apps to receive
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this message and allowed the printer daemon user to send it.
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.TP
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.I "<user>"
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.PP
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The user account the daemon should run as, as either a username or a
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UID. If the daemon cannot change to this UID on startup, it will exit.
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If this element is not present, the daemon will not change or care
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about its UID.
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.PP
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The last <user> entry in the file "wins", the others are ignored.
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.PP
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The user is changed after the bus has completed initialization. So
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sockets etc. will be created before changing user, but no data will be
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read from clients before changing user. This means that sockets
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and PID files can be created in a location that requires root
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privileges for writing.
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.TP
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.I "<fork>"
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.PP
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If present, the bus daemon becomes a real daemon (forks
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into the background, etc.). This is generally used
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rather than the \-\-fork command line option.
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.TP
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.I "<listen>"
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.PP
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Add an address that the bus should listen on. The
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address is in the standard D-BUS format that contains
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a transport name plus possible parameters/options.
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.PP
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Example: <listen>unix:path=/tmp/foo</listen>
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.PP
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If there are multiple <listen> elements, then the bus listens
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on multiple addresses. The bus will pass its address to
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started services or other interested parties with
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the last address given in <listen> first. That is,
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apps will try to connect to the last <listen> address first.
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.TP
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.I "<auth>"
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.PP
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Lists permitted authorization mechanisms. If this element doesn't
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exist, then all known mechanisms are allowed. If there are multiple
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<auth> elements, all the listed mechanisms are allowed. The order in
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which mechanisms are listed is not meaningful.
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.PP
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Example: <auth>EXTERNAL</auth>
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.PP
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Example: <auth>DBUS_COOKIE_SHA1</auth>
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.TP
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.I "<servicedir>"
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.PP
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Adds a directory to scan for .service files. Directories are
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scanned starting with the last to appear in the config file
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(the first .service file found that provides a particular
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service will be used).
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.PP
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Service files tell the bus how to automatically start a program.
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They are primarily used with the per-user-session bus,
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not the systemwide bus.
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.TP
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.I "<limit>"
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.PP
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<limit> establishes a resource limit. For example:
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.nf
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<limit name="max_message_size">64</limit>
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<limit name="max_completed_connections">512</limit>
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.fi
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.PP
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The name attribute is mandatory.
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Available limit names are:
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.nf
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"max_incoming_bytes" : total size in bytes of messages
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incoming from a single connection
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"max_outgoing_bytes" : total size in bytes of messages
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queued up for a single connection
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"max_message_size" : max size of a single message in
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bytes
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"service_start_timeout" : milliseconds (thousandths) until
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a started service has to connect
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"auth_timeout" : milliseconds (thousandths) a
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connection is given to
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authenticate
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"max_completed_connections" : max number of authenticated connections
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"max_incomplete_connections" : max number of unauthenticated
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connections
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"max_connections_per_user" : max number of completed connections from
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the same user
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"max_pending_service_starts" : max number of service launches in
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progress at the same time
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"max_names_per_connection" : max number of names a single
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connection can own
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"max_match_rules_per_connection": max number of match rules for a single
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connection
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"max_replies_per_connection" : max number of pending method
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replies per connection
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(number of calls-in-progress)
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"reply_timeout" : milliseconds (thousandths)
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until a method call times out
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.fi
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.PP
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The max incoming/outgoing queue sizes allow a new message to be queued
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if one byte remains below the max. So you can in fact exceed the max
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by max_message_size.
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.PP
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max_completed_connections divided by max_connections_per_user is the
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number of users that can work together to DOS all other users by using
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up all connections.
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.TP
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.I "<policy>"
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.PP
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The <policy> element defines a policy to be applied to a particular
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set of connections to the bus. A policy is made up of
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<allow> and <deny> elements.
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.PP
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The <policy> element has one of three attributes:
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.nf
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context="(default|mandatory)"
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user="username or userid"
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group="group name or gid"
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.fi
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.PP
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Policies are applied to a connection as follows:
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.nf
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- all context="default" policies are applied
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- all group="connection's user's group" policies are applied
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in undefined order
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- all user="connection's auth user" policies are applied
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in undefined order
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- all context="mandatory" policies are applied
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.fi
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.PP
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Policies applied later will override those applied earlier,
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when the policies overlap. Multiple policies with the same
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user/group/context are applied in the order they appear
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in the config file.
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.TP
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.I "<deny>"
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.I "<allow>"
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.PP
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A <deny> element appears below a <policy> element and prohibits some
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action. The <allow> element makes an exception to previous <deny>
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statements, and works just like <deny> but with the inverse meaning.
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.PP
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The possible attributes of these elements are:
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.nf
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send_interface="interface_name"
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send_member="method_or_signal_name"
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send_error="error_name"
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send_destination="name"
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send_type="method_call" | "method_return" | "signal" | "error"
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send_path="/path/name"
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receive_interface="interface_name"
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receive_member="method_or_signal_name"
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receive_error="error_name"
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receive_sender="name"
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receive_type="method_call" | "method_return" | "signal" | "error"
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receive_path="/path/name"
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send_requested_reply="true" | "false"
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receive_requested_reply="true" | "false"
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eavesdrop="true" | "false"
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own="name"
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user="username"
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group="groupname"
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.fi
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.PP
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Examples:
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.nf
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<deny send_interface="org.freedesktop.System" send_member="Reboot"/>
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<deny receive_interface="org.freedesktop.System" receive_member="Reboot"/>
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<deny own="org.freedesktop.System"/>
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<deny send_destination="org.freedesktop.System"/>
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<deny receive_sender="org.freedesktop.System"/>
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<deny user="john"/>
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<deny group="enemies"/>
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.fi
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.PP
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The <deny> element's attributes determine whether the deny "matches" a
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particular action. If it matches, the action is denied (unless later
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rules in the config file allow it).
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.PP
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send_destination and receive_sender rules mean that messages may not be
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sent to or received from the *owner* of the given name, not that
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they may not be sent *to that name*. That is, if a connection
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owns services A, B, C, and sending to A is denied, sending to B or C
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will not work either.
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.PP
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The other send_* and receive_* attributes are purely textual/by-value
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matches against the given field in the message header.
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.PP
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"Eavesdropping" occurs when an application receives a message that
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was explicitly addressed to a name the application does not own.
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Eavesdropping thus only applies to messages that are addressed to
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services (i.e. it does not apply to signals).
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.PP
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For <allow>, eavesdrop="true" indicates that the rule matches even
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when eavesdropping. eavesdrop="false" is the default and means that
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the rule only allows messages to go to their specified recipient.
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For <deny>, eavesdrop="true" indicates that the rule matches
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only when eavesdropping. eavesdrop="false" is the default for <deny>
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also, but here it means that the rule applies always, even when
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not eavesdropping. The eavesdrop attribute can only be combined with
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receive rules (with receive_* attributes).
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.PP
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The [send|receive]_requested_reply attribute works similarly to the eavesdrop
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attribute. It controls whether the <deny> or <allow> matches a reply
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that is expected (corresponds to a previous method call message).
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This attribute only makes sense for reply messages (errors and method
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returns), and is ignored for other message types.
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.PP
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For <allow>, [send|receive]_requested_reply="true" is the default and indicates that
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only requested replies are allowed by the
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rule. [send|receive]_requested_reply="false" means that the rule allows any reply
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even if unexpected.
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.PP
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For <deny>, [send|receive]_requested_reply="false" is the default but indicates that
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the rule matches only when the reply was not
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requested. [send|receive]_requested_reply="true" indicates that the rule applies
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always, regardless of pending reply state.
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.PP
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user and group denials mean that the given user or group may
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not connect to the message bus.
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.PP
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For "name", "username", "groupname", etc.
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the character "*" can be substituted, meaning "any." Complex globs
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like "foo.bar.*" aren't allowed for now because they'd be work to
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implement and maybe encourage sloppy security anyway.
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.PP
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It does not make sense to deny a user or group inside a <policy>
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for a user or group; user/group denials can only be inside
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context="default" or context="mandatory" policies.
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.PP
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A single <deny> rule may specify combinations of attributes such as
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send_destination and send_interface and send_type. In this case, the
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denial applies only if both attributes match the message being denied.
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e.g. <deny send_interface="foo.bar" send_destination="foo.blah"/> would
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deny messages with the given interface AND the given bus name.
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To get an OR effect you specify multiple <deny> rules.
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.PP
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You can't include both send_ and receive_ attributes on the same
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rule, since "whether the message can be sent" and "whether it can be
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received" are evaluated separately.
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.PP
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Be careful with send_interface/receive_interface, because the
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interface field in messages is optional.
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.TP
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.I "<selinux>"
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.PP
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The <selinux> element contains settings related to Security Enhanced Linux.
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More details below.
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.TP
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.I "<associate>"
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.PP
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An <associate> element appears below an <selinux> element and
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creates a mapping. Right now only one kind of association is possible:
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.nf
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<associate own="org.freedesktop.Foobar" context="foo_t"/>
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.fi
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.PP
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This means that if a connection asks to own the name
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"org.freedesktop.Foobar" then the source context will be the context
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of the connection and the target context will be "foo_t" - see the
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short discussion of SELinux below.
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.PP
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Note, the context here is the target context when requesting a name,
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NOT the context of the connection owning the name.
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.PP
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There's currently no way to set a default for owning any name, if
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we add this syntax it will look like:
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.nf
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<associate own="*" context="foo_t"/>
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.fi
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If you find a reason this is useful, let the developers know.
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Right now the default will be the security context of the bus itself.
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.PP
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If two <associate> elements specify the same name, the element
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appearing later in the configuration file will be used.
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.SH SELinux
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.PP
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See http://www.nsa.gov/selinux/ for full details on SELinux. Some useful excerpts:
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.IP "" 8
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Every subject (process) and object (e.g. file, socket, IPC object,
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etc) in the system is assigned a collection of security attributes,
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known as a security context. A security context contains all of the
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security attributes associated with a particular subject or object
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that are relevant to the security policy.
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.IP "" 8
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In order to better encapsulate security contexts and to provide
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greater efficiency, the policy enforcement code of SELinux typically
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handles security identifiers (SIDs) rather than security contexts. A
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SID is an integer that is mapped by the security server to a security
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context at runtime.
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.IP "" 8
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When a security decision is required, the policy enforcement code
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passes a pair of SIDs (typically the SID of a subject and the SID of
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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.
|
|
|
|
.IP "" 8
|
|
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.
|
|
|
|
.PP
|
|
D-BUS performs SELinux security checks in two places.
|
|
|
|
.PP
|
|
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".
|
|
|
|
.PP
|
|
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.
|
|
|
|
.PP
|
|
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 with an <associate> element as target, object
|
|
class "dbus" and requested permission "acquire_svc".
|
|
|
|
.PP
|
|
If the name has no security context associated in the
|
|
configuration file, the security context of the bus daemon
|
|
itself will be used.
|
|
|
|
.SH AUTHOR
|
|
See http://www.freedesktop.org/software/dbus/doc/AUTHORS
|
|
|
|
.SH BUGS
|
|
Please send bug reports to the D-BUS mailing list or bug tracker,
|
|
see http://www.freedesktop.org/software/dbus/
|