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dbus/doc/dbus-specification.xml
Simon McVittie 7bf5b2b5d6 spec: Document a recommendation for IDNs in reversed domain names 
D-Bus names are in an ASCII subset, so IDNs have to be in their ACE
encoding (Punycode). ACE-encoded labels contain hyphen/minus characters,
and like any other label containing a hyphen/minus, we recommend
replacing those with underscores, to improve interoperability with
contexts that allow underscores but not hyphen/minus, such as
D-Bus object paths and Flatpak app-IDs.

Prompted by <https://github.com/flatpak/flatpak/issues/4974>.

Signed-off-by: Simon McVittie <smcv@collabora.com>
2022-07-13 15:55:42 +01:00

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<?xml version="1.0" standalone="no" ?>
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
"http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd"
[
]>
<article id="index">
<articleinfo>
<title>D-Bus Specification</title>
<releaseinfo>Version 0.39</releaseinfo>
<date>(not yet finalized)</date>
<authorgroup>
<author>
<firstname>Havoc</firstname>
<surname>Pennington</surname>
<affiliation>
<orgname>Red Hat, Inc.</orgname>
<address>
<email>hp@pobox.com</email>
</address>
</affiliation>
</author>
<author>
<firstname>Anders</firstname>
<surname>Carlsson</surname>
<affiliation>
<orgname>CodeFactory AB</orgname>
<address>
<email>andersca@codefactory.se</email>
</address>
</affiliation>
</author>
<author>
<firstname>Alexander</firstname>
<surname>Larsson</surname>
<affiliation>
<orgname>Red Hat, Inc.</orgname>
<address>
<email>alexl@redhat.com</email>
</address>
</affiliation>
</author>
<author>
<firstname>Sven</firstname>
<surname>Herzberg</surname>
<affiliation>
<orgname>Imendio AB</orgname>
<address>
<email>sven@imendio.com</email>
</address>
</affiliation>
</author>
<author>
<firstname>Simon</firstname>
<surname>McVittie</surname>
<affiliation>
<orgname>Collabora Ltd.</orgname>
<address>
<email>smcv@collabora.com</email>
</address>
</affiliation>
</author>
<author>
<firstname>David</firstname>
<surname>Zeuthen</surname>
<affiliation>
<address>
<email>zeuthen@gmail.com</email>
</address>
</affiliation>
</author>
</authorgroup>
<revhistory>
<revision>
<revnumber>latest</revnumber>
<date></date>
<authorinitials></authorinitials>
<revremark>
See <ulink url='http://cgit.freedesktop.org/dbus/dbus/log/doc/dbus-specification.xml'>commit log</ulink>
</revremark>
</revision>
<revision>
<revnumber>0.38</revnumber>
<date>2022-02-23</date>
<authorinitials></authorinitials>
<revdescription>
<itemizedlist>
<listitem><simpara>Add ActivatableServicesChanged signal and feature flag</simpara></listitem>
<listitem><simpara><literal>*</literal> is optionally-escaped in addresses</simpara></listitem>
</itemizedlist>
</revdescription>
</revision>
<revision>
<revnumber>0.37</revnumber>
<date>2021-12-17</date>
<authorinitials></authorinitials>
<revdescription>
<itemizedlist>
<listitem><simpara>Update recommendations for interoperable DBUS_COOKIE_SHA1 timeouts</simpara></listitem>
<listitem><simpara>Clarify padding requirements for arrays and variants</simpara></listitem>
<listitem><simpara>Describe where the interoperable machine ID comes from</simpara></listitem>
<listitem><simpara>Clarify use of dictionary (array of dict-entry) types</simpara></listitem>
</itemizedlist>
</revdescription>
</revision>
<revision>
<revnumber>0.36</revnumber>
<date>2020-04-21</date>
<authorinitials></authorinitials>
<revdescription>
<itemizedlist>
<listitem><simpara>Fix a typo in an annotated hexdump of part of a message</simpara></listitem>
</itemizedlist>
</revdescription>
</revision>
<revision>
<revnumber>0.35</revnumber>
<date>2019-05-13</date>
<authorinitials></authorinitials>
<revdescription>
<itemizedlist>
<listitem><simpara>Add UnixGroupIDs to GetConnectionCredentials</simpara></listitem>
<listitem><simpara>Avoid redundancy in defining interface name syntax</simpara></listitem>
</itemizedlist>
</revdescription>
</revision>
<revision>
<revnumber>0.34</revnumber>
<date>2018-12-04</date>
<authorinitials>pwithnall</authorinitials>
<revdescription>
<itemizedlist>
<listitem><simpara>Correct ObjectManager example AddMatch rule</simpara></listitem>
</itemizedlist>
</revdescription>
</revision>
<revision>
<revnumber>0.33</revnumber>
<date>2018-04-27</date>
<authorinitials>smcv</authorinitials>
<revdescription>
<itemizedlist>
<listitem><simpara>Deprecate TCP on Unix</simpara></listitem>
<listitem><simpara>Deprecate non-local TCP everywhere</simpara></listitem>
</itemizedlist>
</revdescription>
</revision>
<revision>
<revnumber>0.32</revnumber>
<date>2018-01-30</date>
<authorinitials>smcv</authorinitials>
<revdescription>
<itemizedlist>
<listitem><simpara>Deprecate hyphen/minus in bus names, with
underscore as the recommended replacement</simpara></listitem>
<listitem><simpara>Document the convention for escaping
leading digits in interface and bus names
(org._7_zip)</simpara></listitem>
<listitem><simpara>Recommend using SASL EXTERNAL where
possible, or DBUS_COOKIE_SHA1 otherwise</simpara></listitem>
<listitem><simpara>Message buses should not accept SASL
ANONYMOUS</simpara></listitem>
<listitem><simpara>Document the meaning of non-empty SASL
authorization identity strings</simpara></listitem>
<listitem><simpara>Document the optional argument to SASL
ERROR</simpara></listitem>
<listitem><simpara>Document who sends each SASL command, and
the possible replies</simpara></listitem>
<listitem><simpara>Document the authentication states used to
negotiate Unix fd-passing</simpara></listitem>
<listitem><simpara>Servers that relay messages should remove
header fields they do not understand</simpara></listitem>
<listitem><simpara>Clarify who controls each header
field</simpara></listitem>
<listitem><simpara>Document the HeaderFiltering message bus
feature flag</simpara></listitem>
<listitem><simpara>Non-message-bus servers may use the SENDER
and DESTINATION fields</simpara></listitem>
</itemizedlist>
</revdescription>
</revision>
<revision>
<revnumber>0.31</revnumber>
<date>2017-06-29</date>
<authorinitials>smcv, TG</authorinitials>
<revdescription>
<itemizedlist>
<listitem><simpara>Don't require implementation-specific search
paths to be lowest priority</simpara></listitem>
<listitem><simpara>Correct regex syntax for optionally-escaped
bytes in addresses so it includes hyphen-minus, forward slash
and underscore as intended</simpara></listitem>
<listitem><simpara>Describe all message bus methods in the same
section</simpara></listitem>
<listitem><simpara>Clarify the correct object path for method calls
to the message bus</simpara></listitem>
<listitem><simpara>Document that the message bus implements
Introspectable, Peer and Properties</simpara></listitem>
<listitem><simpara>Add new Features and Interfaces properties for
message bus feature-discovery</simpara></listitem>
<listitem><simpara>Add unix:dir=..., which resembles
unix:tmpdir=... but never uses abstract
sockets</simpara></listitem>
<listitem><simpara>Don't require eavesdrop='true' to be accepted
from connections not sufficiently privileged to use it
successfully</simpara></listitem>
<listitem><simpara>Formally deprecate eavesdropping in favour of
BecomeMonitor</simpara></listitem>
</itemizedlist>
</revdescription>
</revision>
<revision>
<revnumber>0.30</revnumber>
<date>2016-11-28</date>
<authorinitials>smcv, PW</authorinitials>
<revremark>
Define the jargon terms service activation and auto-starting more
clearly. Document the SystemdService key in service files.
Document how AppArmor interacts with service activation, and the
new AssumedAppArmorLabel key in service files (dbus-daemon 1.11.8).
Clarify intended behaviour of Properties.GetAll.
Use versioned interface and bus names in most examples.
</revremark>
</revision>
<revision>
<revnumber>0.29</revnumber>
<date>2016-10-10</date>
<authorinitials>PW</authorinitials>
<revremark>
Introspection arguments may contain annotations; recommend against
using the object path '/'
</revremark>
</revision>
<revision>
<revnumber>0.28</revnumber>
<date>2016-08-15</date>
<authorinitials>PW</authorinitials>
<revremark>Clarify serialization</revremark>
</revision>
<revision>
<revnumber>0.27</revnumber>
<date>2015-12-02</date>
<authorinitials>LU</authorinitials>
<revremark>Services should not send unwanted replies</revremark>
</revision>
<revision>
<revnumber>0.26</revnumber>
<date>2015-02-19</date>
<authorinitials>smcv, rh</authorinitials>
<revremark>
GetConnectionCredentials can return LinuxSecurityLabel or
WindowsSID; add privileged BecomeMonitor method
</revremark>
</revision>
<revision>
<revnumber>0.25</revnumber>
<date>2014-11-10</date>
<authorinitials>smcv, lennart</authorinitials>
<revremark>
ALLOW_INTERACTIVE_AUTHORIZATION flag, EmitsChangedSignal=const
</revremark>
</revision>
<revision>
<revnumber>0.24</revnumber>
<date>2014-10-01</date>
<authorinitials>SMcV</authorinitials>
<revremark>
non-method-calls never expect a reply even without NO_REPLY_EXPECTED;
document how to quote match rules
</revremark>
</revision>
<revision>
<revnumber>0.23</revnumber>
<date>2014-01-06</date>
<authorinitials>SMcV, CY</authorinitials>
<revremark>
method call messages with no INTERFACE may be considered an error;
document tcp:bind=... and nonce-tcp:bind=...; define listenable
and connectable addresses
</revremark>
</revision>
<revision>
<revnumber>0.22</revnumber>
<date>2013-10-09</date>
<authorinitials></authorinitials>
<revremark>add GetConnectionCredentials, document
GetAtdAuditSessionData, document GetConnectionSELinuxSecurityContext,
document and correct .service file syntax and naming
</revremark>
</revision>
<revision>
<revnumber>0.21</revnumber>
<date>2013-04-25</date>
<authorinitials>smcv</authorinitials>
<revremark>allow Unicode noncharacters in UTF-8 (Unicode
Corrigendum #9)</revremark>
</revision>
<revision>
<revnumber>0.20</revnumber>
<date>22 February 2013</date>
<authorinitials>smcv, walters</authorinitials>
<revremark>reorganise for clarity, remove false claims about
basic types, mention /o/fd/DBus</revremark>
</revision>
<revision>
<revnumber>0.19</revnumber>
<date>20 February 2012</date>
<authorinitials>smcv/lp</authorinitials>
<revremark>formally define unique connection names and well-known
bus names; document best practices for interface, bus, member and
error names, and object paths; document the search path for session
and system services on Unix; document the systemd transport</revremark>
</revision>
<revision>
<revnumber>0.18</revnumber>
<date>29 July 2011</date>
<authorinitials>smcv</authorinitials>
<revremark>define eavesdropping, unicast, broadcast; add eavesdrop
match keyword; promote type system to a top-level section</revremark>
</revision>
<revision>
<revnumber>0.17</revnumber>
<date>1 June 2011</date>
<authorinitials>smcv/davidz</authorinitials>
<revremark>define ObjectManager; reserve extra pseudo-type-codes used
by GVariant</revremark>
</revision>
<revision>
<revnumber>0.16</revnumber>
<date>11 April 2011</date>
<authorinitials></authorinitials>
<revremark>add path_namespace, arg0namespace; argNpath matches object
paths</revremark>
</revision>
<revision>
<revnumber>0.15</revnumber>
<date>3 November 2010</date>
<authorinitials></authorinitials>
<revremark></revremark>
</revision>
<revision>
<revnumber>0.14</revnumber>
<date>12 May 2010</date>
<authorinitials></authorinitials>
<revremark></revremark>
</revision>
<revision>
<revnumber>0.13</revnumber>
<date>23 Dezember 2009</date>
<authorinitials></authorinitials>
<revremark></revremark>
</revision>
<revision>
<revnumber>0.12</revnumber>
<date>7 November, 2006</date>
<authorinitials></authorinitials>
<revremark></revremark>
</revision>
<revision>
<revnumber>0.11</revnumber>
<date>6 February 2005</date>
<authorinitials></authorinitials>
<revremark></revremark>
</revision>
<revision>
<revnumber>0.10</revnumber>
<date>28 January 2005</date>
<authorinitials></authorinitials>
<revremark></revremark>
</revision>
<revision>
<revnumber>0.9</revnumber>
<date>7 Januar 2005</date>
<authorinitials></authorinitials>
<revremark></revremark>
</revision>
<revision>
<revnumber>0.8</revnumber>
<date>06 September 2003</date>
<authorinitials></authorinitials>
<revremark>First released document.</revremark>
</revision>
</revhistory>
</articleinfo>
<sect1 id="introduction">
<title>Introduction</title>
<para>
D-Bus is a system for low-overhead, easy to use
interprocess communication (IPC). In more detail:
<itemizedlist>
<listitem>
<para>
D-Bus is <emphasis>low-overhead</emphasis> because it uses a
binary protocol, and does not have to convert to and from a text
format such as XML. Because D-Bus is intended for potentially
high-resolution same-machine IPC, not primarily for Internet IPC,
this is an interesting optimization. D-Bus is also designed to
avoid round trips and allow asynchronous operation, much like
the X protocol.
</para>
</listitem>
<listitem>
<para>
D-Bus is <emphasis>easy to use</emphasis> because it works in terms
of <firstterm>messages</firstterm> rather than byte streams, and
automatically handles a lot of the hard IPC issues. Also, the D-Bus
library is designed to be wrapped in a way that lets developers use
their framework's existing object/type system, rather than learning
a new one specifically for IPC.
</para>
</listitem>
</itemizedlist>
</para>
<para>
The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
protocol, specified in <xref linkend="message-protocol"/>. That is, it is
a system for one application to talk to a single other
application. However, the primary intended application of the protocol is the
D-Bus <firstterm>message bus</firstterm>, specified in <xref
linkend="message-bus"/>. The message bus is a special application that
accepts connections from multiple other applications, and forwards
messages among them.
</para>
<para>
Uses of D-Bus include notification of system changes (notification of when
a camera is plugged in to a computer, or a new version of some software
has been installed), or desktop interoperability, for example a file
monitoring service or a configuration service.
</para>
<para>
D-Bus is designed for two specific use cases:
<itemizedlist>
<listitem>
<para>
A "system bus" for notifications from the system to user sessions,
and to allow the system to request input from user sessions.
</para>
</listitem>
<listitem>
<para>
A "session bus" used to implement desktop environments such as
GNOME and KDE.
</para>
</listitem>
</itemizedlist>
D-Bus is not intended to be a generic IPC system for any possible
application, and intentionally omits many features found in other
IPC systems for this reason.
</para>
<para>
At the same time, the bus daemons offer a number of features not found in
other IPC systems, such as single-owner "bus names" (similar to X
selections), on-demand startup of services, and security policies.
In many ways, these features are the primary motivation for developing
D-Bus; other systems would have sufficed if IPC were the only goal.
</para>
<para>
D-Bus may turn out to be useful in unanticipated applications, but future
versions of this spec and the reference implementation probably will not
incorporate features that interfere with the core use cases.
</para>
<para>
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119. However, the
document could use a serious audit to be sure it makes sense to do
so. Also, they are not capitalized.
</para>
<sect2 id="stability">
<title>Protocol and Specification Stability</title>
<para>
The D-Bus protocol is frozen (only compatible extensions are allowed) as
of November 8, 2006. However, this specification could still use a fair
bit of work to make interoperable reimplementation possible without
reference to the D-Bus reference implementation. Thus, this
specification is not marked 1.0. To mark it 1.0, we'd like to see
someone invest significant effort in clarifying the specification
language, and growing the specification to cover more aspects of the
reference implementation's behavior.
</para>
<para>
Until this work is complete, any attempt to reimplement D-Bus will
probably require looking at the reference implementation and/or asking
questions on the D-Bus mailing list about intended behavior.
Questions on the list are very welcome.
</para>
<para>
Nonetheless, this document should be a useful starting point and is
to our knowledge accurate, though incomplete.
</para>
</sect2>
</sect1>
<sect1 id="type-system">
<title>Type System</title>
<para>
D-Bus has a type system, in which values of various types can be
serialized into a sequence of bytes referred to as the
<firstterm>wire format</firstterm> in a standard way.
Converting a value from some other representation into the wire
format is called <firstterm>marshaling</firstterm> and converting
it back from the wire format is <firstterm>unmarshaling</firstterm>.
</para>
<para>
The D-Bus protocol does not include type tags in the marshaled data; a
block of marshaled values must have a known <firstterm>type
signature</firstterm>. The type signature is made up of zero or more
<firstterm id="term-single-complete-type">single complete
types</firstterm>, each made up of one or more
<firstterm>type codes</firstterm>.
</para>
<para>
A type code is an ASCII character representing the
type of a value. Because ASCII characters are used, the type signature
will always form a valid ASCII string. A simple string compare
determines whether two type signatures are equivalent.
</para>
<para>
A single complete type is a sequence of type codes that fully describes
one type: either a basic type, or a single fully-described container type.
A single complete type is a basic type code, a variant type code,
an array with its element type, or a struct with its fields (all of which
are defined below). So the following signatures are not single complete
types:
<programlisting>
"aa"
</programlisting>
<programlisting>
"(ii"
</programlisting>
<programlisting>
"ii)"
</programlisting>
And the following signatures contain multiple complete types:
<programlisting>
"ii"
</programlisting>
<programlisting>
"aiai"
</programlisting>
<programlisting>
"(ii)(ii)"
</programlisting>
Note however that a single complete type may <emphasis>contain</emphasis>
multiple other single complete types, by containing a struct or dict
entry.
</para>
<sect2 id="basic-types">
<title>Basic types</title>
<para>
The simplest type codes are the <firstterm id="term-basic-type">basic
types</firstterm>, which are the types whose structure is entirely
defined by their 1-character type code. Basic types consist of
fixed types and string-like types.
</para>
<para>
The <firstterm id="term-fixed-type">fixed types</firstterm>
are basic types whose values have a fixed length, namely BYTE,
BOOLEAN, DOUBLE, UNIX_FD, and signed or unsigned integers of length
16, 32 or 64 bits.
</para>
<para>
As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
the ASCII character 'i'. So the signature for a block of values
containing a single <literal>INT32</literal> would be:
<programlisting>
"i"
</programlisting>
A block of values containing two <literal>INT32</literal> would have this signature:
<programlisting>
"ii"
</programlisting>
</para>
<para>
The characteristics of the fixed types are listed in this table.
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Conventional name</entry>
<entry>ASCII type-code</entry>
<entry>Encoding</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>BYTE</literal></entry>
<entry><literal>y</literal> (121)</entry>
<entry>Unsigned 8-bit integer</entry>
</row>
<row>
<entry><literal>BOOLEAN</literal></entry>
<entry><literal>b</literal> (98)</entry>
<entry>Boolean value: 0 is false, 1 is true, any other value
allowed by the marshalling format is invalid</entry>
</row>
<row>
<entry><literal>INT16</literal></entry>
<entry><literal>n</literal> (110)</entry>
<entry>Signed (two's complement) 16-bit integer</entry>
</row>
<row>
<entry><literal>UINT16</literal></entry>
<entry><literal>q</literal> (113)</entry>
<entry>Unsigned 16-bit integer</entry>
</row>
<row>
<entry><literal>INT32</literal></entry>
<entry><literal>i</literal> (105)</entry>
<entry>Signed (two's complement) 32-bit integer</entry>
</row>
<row>
<entry><literal>UINT32</literal></entry>
<entry><literal>u</literal> (117)</entry>
<entry>Unsigned 32-bit integer</entry>
</row>
<row>
<entry><literal>INT64</literal></entry>
<entry><literal>x</literal> (120)</entry>
<entry>Signed (two's complement) 64-bit integer
(mnemonic: x and t are the first characters in "sixty" not
already used for something more common)</entry>
</row>
<row>
<entry><literal>UINT64</literal></entry>
<entry><literal>t</literal> (116)</entry>
<entry>Unsigned 64-bit integer</entry>
</row>
<row>
<entry><literal>DOUBLE</literal></entry>
<entry><literal>d</literal> (100)</entry>
<entry>IEEE 754 double-precision floating point</entry>
</row>
<row>
<entry><literal>UNIX_FD</literal></entry>
<entry><literal>h</literal> (104)</entry>
<entry>Unsigned 32-bit integer representing an index into an
out-of-band array of file descriptors, transferred via some
platform-specific mechanism (mnemonic: h for handle)</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
The <firstterm id="term-string-like-type">string-like types</firstterm>
are basic types with a variable length. The value of any string-like
type is conceptually 0 or more Unicode codepoints encoded in UTF-8,
none of which may be U+0000. The UTF-8 text must be validated
strictly: in particular, it must not contain overlong sequences
or codepoints above U+10FFFF.
</para>
<para>
Since D-Bus Specification version 0.21, in accordance with Unicode
Corrigendum #9, the "noncharacters" U+FDD0..U+FDEF, U+nFFFE and
U+nFFFF are allowed in UTF-8 strings (but note that older versions of
D-Bus rejected these noncharacters).
</para>
<para>
The marshalling formats for the string-like types all end with a
single zero (NUL) byte, but that byte is not considered to be part of
the text.
</para>
<para>
The characteristics of the string-like types are listed in this table.
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Conventional name</entry>
<entry>ASCII type-code</entry>
<entry>Validity constraints</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>STRING</literal></entry>
<entry><literal>s</literal> (115)</entry>
<entry>No extra constraints</entry>
</row>
<row>
<entry><literal>OBJECT_PATH</literal></entry>
<entry><literal>o</literal> (111)</entry>
<entry>Must be
<link linkend="message-protocol-marshaling-object-path">a
syntactically valid object path</link></entry>
</row>
<row>
<entry><literal>SIGNATURE</literal></entry>
<entry><literal>g</literal> (103)</entry>
<entry>Zero or more
<firstterm linkend="term-single-complete-type">single
complete types</firstterm></entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<sect3 id="message-protocol-marshaling-object-path">
<title>Valid Object Paths</title>
<para>
An object path is a name used to refer to an object instance.
Conceptually, each participant in a D-Bus message exchange may have
any number of object instances (think of C++ or Java objects) and each
such instance will have a path. Like a filesystem, the object
instances in an application form a hierarchical tree.
</para>
<para>
Object paths are often namespaced by starting with a reversed
domain name and containing an interface version number, in the
same way as
<link linkend="message-protocol-names-interface">interface
names</link> and
<link linkend="message-protocol-names-bus">well-known
bus names</link>.
This makes it possible to implement more than one service, or
more than one version of a service, in the same process,
even if the services share a connection but cannot otherwise
co-operate (for instance, if they are implemented by different
plugins).
</para>
<para>
Using an object path of <literal>/</literal> is allowed, but
recommended against, as it makes versioning of interfaces hard. Any
signals emitted from a D-Bus object have the service’s unique bus name
associated with them, rather than its well-known name. This means that
receipients of the signals must rely entirely on the signal name and
object path to work out which interface the signal originated from.
</para>
<para>
For instance, if the owner of <literal>example.com</literal> is
developing a D-Bus API for a music player, they might use the
hierarchy of object paths that start with
<literal>/com/example/MusicPlayer1</literal> for its objects.
</para>
<para>
The following rules define a valid object path. Implementations must
not send or accept messages with invalid object paths.
<itemizedlist>
<listitem>
<para>
The path may be of any length.
</para>
</listitem>
<listitem>
<para>
The path must begin with an ASCII '/' (integer 47) character,
and must consist of elements separated by slash characters.
</para>
</listitem>
<listitem>
<para>
Each element must only contain the ASCII characters
"[A-Z][a-z][0-9]_"
</para>
</listitem>
<listitem>
<para>
No element may be the empty string.
</para>
</listitem>
<listitem>
<para>
Multiple '/' characters cannot occur in sequence.
</para>
</listitem>
<listitem>
<para>
A trailing '/' character is not allowed unless the
path is the root path (a single '/' character).
</para>
</listitem>
</itemizedlist>
</para>
</sect3>
<sect3 id="message-protocol-marshaling-signature">
<title>Valid Signatures</title>
<para>
An implementation must not send or accept invalid signatures.
Valid signatures will conform to the following rules:
<itemizedlist>
<listitem>
<para>
The signature is a list of single complete types.
Arrays must have element types, and structs must
have both open and close parentheses.
</para>
</listitem>
<listitem>
<para>
Only type codes, open and close parentheses, and open and
close curly brackets are allowed in the signature. The
<literal>STRUCT</literal> type code
is not allowed in signatures, because parentheses
are used instead. Similarly, the
<literal>DICT_ENTRY</literal> type code is not allowed in
signatures, because curly brackets are used instead.
</para>
</listitem>
<listitem>
<para>
The maximum depth of container type nesting is 32 array type
codes and 32 open parentheses. This implies that the maximum
total depth of recursion is 64, for an "array of array of array
of ... struct of struct of struct of ..." where there are 32
array and 32 struct.
</para>
</listitem>
<listitem>
<para>
The maximum length of a signature is 255.
</para>
</listitem>
</itemizedlist>
</para>
<para>
When signatures appear in messages, the marshalling format
guarantees that they will be followed by a nul byte (which can
be interpreted as either C-style string termination or the INVALID
type-code), but this is not conceptually part of the signature.
</para>
</sect3>
</sect2>
<sect2 id="container-types">
<title>Container types</title>
<para>
In addition to basic types, there are four <firstterm>container</firstterm>
types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
and <literal>DICT_ENTRY</literal>.
</para>
<para>
<literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
code does not appear in signatures. Instead, ASCII characters
'(' and ')' are used to mark the beginning and end of the struct.
So for example, a struct containing two integers would have this
signature:
<programlisting>
"(ii)"
</programlisting>
Structs can be nested, so for example a struct containing
an integer and another struct:
<programlisting>
"(i(ii))"
</programlisting>
The value block storing that struct would contain three integers; the
type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
"(iii)" or "iii".
</para>
<para>
The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
but is useful in code that implements the protocol. This type code
is specified to allow such code to interoperate in non-protocol contexts.
</para>
<para>
Empty structures are not allowed; there must be at least one
type code between the parentheses.
</para>
<para>
<literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
followed by a <firstterm>single complete type</firstterm>. The single
complete type following the array is the type of each array element. So
the simple example is:
<programlisting>
"ai"
</programlisting>
which is an array of 32-bit integers. But an array can be of any type,
such as this array-of-struct-with-two-int32-fields:
<programlisting>
"a(ii)"
</programlisting>
Or this array of array of integer:
<programlisting>
"aai"
</programlisting>
</para>
<para>
<literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
type <literal>VARIANT</literal> will have the signature of a single complete type as part
of the <emphasis>value</emphasis>. This signature will be followed by a
marshaled value of that type.
</para>
<para>
Unlike a message signature, the variant signature can
contain only a single complete type. So "i", "ai"
or "(ii)" is OK, but "ii" is not. Use of variants may not
cause a total message depth to be larger than 64, including
other container types such as structures.
</para>
<para>
A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
than parentheses it uses curly braces, and it has more restrictions.
The restrictions are: it occurs only as an array element type; it has
exactly two single complete types inside the curly braces; the first
single complete type (the "key") must be a basic type rather than a
container type. Implementations must not accept dict entries outside of
arrays, must not accept dict entries with zero, one, or more than two
fields, and must not accept dict entries with non-basic-typed keys. A
dict entry is always a key-value pair.
</para>
<para>
The first field in the <literal>DICT_ENTRY</literal> is always the key.
A message is considered corrupt if the same key occurs twice in the same
array of <literal>DICT_ENTRY</literal>. However, for performance reasons
implementations are not required to reject dicts with duplicate keys.
</para>
<para>
In most languages, an array of dict entry would be represented as a
map, hash table, or dict object.
</para>
</sect2>
<sect2>
<title>Summary of types</title>
<para>
The following table summarizes the D-Bus types.
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Category</entry>
<entry>Conventional Name</entry>
<entry>Code</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>reserved</entry>
<entry><literal>INVALID</literal></entry>
<entry>0 (ASCII NUL)</entry>
<entry>Not a valid type code, used to terminate signatures</entry>
</row><row>
<entry>fixed, basic</entry>
<entry><literal>BYTE</literal></entry>
<entry>121 (ASCII 'y')</entry>
<entry>8-bit unsigned integer</entry>
</row><row>
<entry>fixed, basic</entry>
<entry><literal>BOOLEAN</literal></entry>
<entry>98 (ASCII 'b')</entry>
<entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
</row><row>
<entry>fixed, basic</entry>
<entry><literal>INT16</literal></entry>
<entry>110 (ASCII 'n')</entry>
<entry>16-bit signed integer</entry>
</row><row>
<entry>fixed, basic</entry>
<entry><literal>UINT16</literal></entry>
<entry>113 (ASCII 'q')</entry>
<entry>16-bit unsigned integer</entry>
</row><row>
<entry>fixed, basic</entry>
<entry><literal>INT32</literal></entry>
<entry>105 (ASCII 'i')</entry>
<entry>32-bit signed integer</entry>
</row><row>
<entry>fixed, basic</entry>
<entry><literal>UINT32</literal></entry>
<entry>117 (ASCII 'u')</entry>
<entry>32-bit unsigned integer</entry>
</row><row>
<entry>fixed, basic</entry>
<entry><literal>INT64</literal></entry>
<entry>120 (ASCII 'x')</entry>
<entry>64-bit signed integer</entry>
</row><row>
<entry>fixed, basic</entry>
<entry><literal>UINT64</literal></entry>
<entry>116 (ASCII 't')</entry>
<entry>64-bit unsigned integer</entry>
</row><row>
<entry>fixed, basic</entry>
<entry><literal>DOUBLE</literal></entry>
<entry>100 (ASCII 'd')</entry>
<entry>IEEE 754 double</entry>
</row><row>
<entry>string-like, basic</entry>
<entry><literal>STRING</literal></entry>
<entry>115 (ASCII 's')</entry>
<entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
</row><row>
<entry>string-like, basic</entry>
<entry><literal>OBJECT_PATH</literal></entry>
<entry>111 (ASCII 'o')</entry>
<entry>Name of an object instance</entry>
</row><row>
<entry>string-like, basic</entry>
<entry><literal>SIGNATURE</literal></entry>
<entry>103 (ASCII 'g')</entry>
<entry>A type signature</entry>
</row><row>
<entry>container</entry>
<entry><literal>ARRAY</literal></entry>
<entry>97 (ASCII 'a')</entry>
<entry>Array</entry>
</row><row>
<entry>container</entry>
<entry><literal>STRUCT</literal></entry>
<entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
<entry>Struct; type code 114 'r' is reserved for use in
bindings and implementations to represent the general
concept of a struct, and must not appear in signatures
used on D-Bus.</entry>
</row><row>
<entry>container</entry>
<entry><literal>VARIANT</literal></entry>
<entry>118 (ASCII 'v') </entry>
<entry>Variant type (the type of the value is part of the value itself)</entry>
</row><row>
<entry>container</entry>
<entry><literal>DICT_ENTRY</literal></entry>
<entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
<entry>Entry in a dict or map (array of key-value pairs).
Type code 101 'e' is reserved for use in bindings and
implementations to represent the general concept of a
dict or dict-entry, and must not appear in signatures
used on D-Bus.</entry>
</row><row>
<entry>fixed, basic</entry>
<entry><literal>UNIX_FD</literal></entry>
<entry>104 (ASCII 'h')</entry>
<entry>Unix file descriptor</entry>
</row>
<row>
<entry>reserved</entry>
<entry>(reserved)</entry>
<entry>109 (ASCII 'm')</entry>
<entry>Reserved for <ulink
url="https://bugs.freedesktop.org/show_bug.cgi?id=27857">a
'maybe' type compatible with the one in GVariant</ulink>,
and must not appear in signatures used on D-Bus until
specified here</entry>
</row>
<row>
<entry>reserved</entry>
<entry>(reserved)</entry>
<entry>42 (ASCII '*')</entry>
<entry>Reserved for use in bindings/implementations to
represent any <firstterm>single complete type</firstterm>,
and must not appear in signatures used on D-Bus.</entry>
</row>
<row>
<entry>reserved</entry>
<entry>(reserved)</entry>
<entry>63 (ASCII '?')</entry>
<entry>Reserved for use in bindings/implementations to
represent any <firstterm>basic type</firstterm>, and must
not appear in signatures used on D-Bus.</entry>
</row>
<row>
<entry>reserved</entry>
<entry>(reserved)</entry>
<entry>64 (ASCII '@'), 38 (ASCII '&amp;'),
94 (ASCII '^')</entry>
<entry>Reserved for internal use by bindings/implementations,
and must not appear in signatures used on D-Bus.
GVariant uses these type-codes to encode calling
conventions.</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
</sect2>
</sect1>
<sect1 id="message-protocol-marshaling">
<title>Marshaling (Wire Format)</title>
<para>
D-Bus defines a marshalling format for its type system, which is
used in D-Bus messages. This is not the only possible marshalling
format for the type system: for instance, GVariant (part of GLib)
re-uses the D-Bus type system but implements an alternative marshalling
format.
</para>
<sect2>
<title>Byte order and alignment</title>
<para>
Given a type signature, a block of bytes can be converted into typed
values. This section describes the format of the block of bytes. Byte
order and alignment issues are handled uniformly for all D-Bus types.
</para>
<para>
A block of bytes has an associated byte order. The byte order
has to be discovered in some way; for D-Bus messages, the
byte order is part of the message header as described in
<xref linkend="message-protocol-messages"/>. For now, assume
that the byte order is known to be either little endian or big
endian.
</para>
<para>
Each value in a block of bytes is aligned "naturally," for example
4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
8-byte boundary. Boundaries are calculated globally, with respect to
the first byte in the message. To properly align a value,
<firstterm>alignment padding</firstterm> may be necessary before the
value. The alignment padding must always
be the minimum required padding to properly align the following value;
and it must always be made up of nul bytes. The alignment padding must
not be left uninitialized (it can't contain garbage), and more padding
than required must not be used.
</para>
<para>
As an exception to natural alignment, <literal>STRUCT</literal> and
<literal>DICT_ENTRY</literal> values are always aligned to an 8-byte
boundary, regardless of the alignments of their contents.
</para>
</sect2>
<sect2>
<title>Marshalling basic types</title>
<para>
To marshal and unmarshal fixed types, you simply read one value
from the data block corresponding to each type code in the signature.
All signed integer values are encoded in two's complement, DOUBLE
values are IEEE 754 double-precision floating-point, and BOOLEAN
values are encoded in 32 bits (of which only the least significant
bit is used).
</para>
<para>
The string-like types (STRING, OBJECT_PATH and SIGNATURE) are all
marshalled as a
fixed-length unsigned integer <varname>n</varname> giving the
length of the variable part, followed by <varname>n</varname>
nonzero bytes of UTF-8 text, followed by a single zero (nul) byte
which is not considered to be part of the text. The alignment
of the string-like type is the same as the alignment of
<varname>n</varname>: any padding required for <varname>n</varname>
appears immediately before <varname>n</varname> itself. There is never
any alignment padding between <varname>n</varname> and the string text,
or between the string text and the trailing nul. The alignment padding
for the next value in the message (if there is one) starts after the
trailing nul.
</para>
<para>
For the STRING and OBJECT_PATH types, <varname>n</varname> is
encoded in 4 bytes (a <literal>UINT32</literal>), leading to 4-byte
alignment. For the SIGNATURE type, <varname>n</varname> is encoded as a
single byte (a <literal>UINT8</literal>). As a result, alignment
padding is never required before a SIGNATURE.
</para>
<para>
For example, if the current position is a multiple of 8 bytes from the
beginning of a little-endian message, strings ‘foo’, ‘+’ and ‘bar’
would be serialized in sequence as follows:
<screen>
<lineannotation>no padding required, we are already at a multiple of 4</lineannotation>
0x03 0x00 0x00 0x00 <lineannotation>length of ‘foo’ = 3</lineannotation>
0x66 0x6f 0x6f <lineannotation>‘foo’</lineannotation>
0x00 <lineannotation>trailing nul</lineannotation>
<lineannotation>no padding required, we are already at a multiple of 4</lineannotation>
0x01 0x00 0x00 0x00 <lineannotation>length of ‘+’ = 1</lineannotation>
0x2b <lineannotation>‘+’</lineannotation>
0x00 <lineannotation>trailing nul</lineannotation>
0x00 0x00 <lineannotation>2 bytes of padding to reach next multiple of 4</lineannotation>
0x03 0x00 0x00 0x00 <lineannotation>length of ‘bar’ = 3</lineannotation>
0x62 0x61 0x72 <lineannotation>‘bar’</lineannotation>
0x00 <lineannotation>trailing nul</lineannotation>
</screen>
</para>
</sect2>
<sect2>
<title>Marshalling containers</title>
<para>
Arrays are marshalled as a <literal>UINT32</literal>
<varname>n</varname> giving the length of the array data in bytes,
followed by alignment padding to the alignment boundary of the array
element type, followed by the <varname>n</varname> bytes of the
array elements marshalled in sequence. <varname>n</varname> does not
include the padding after the length, or any padding after the
last element. i.e. <varname>n</varname> should be divisible by the
number of elements in the array. Note that the alignment padding for the
first element is required even if there is no first element (an
empty array, where <varname>n</varname> is zero).
</para>
<para>
For instance, if the current position in the message is a multiple
of 8 bytes and the byte-order is big-endian, an array containing only
the 64-bit integer 5 would be marshalled as:
<screen>
00 00 00 08 <lineannotation><varname>n</varname> = 8 bytes of data</lineannotation>
00 00 00 00 <lineannotation>padding to 8-byte boundary</lineannotation>
00 00 00 00 00 00 00 05 <lineannotation>first element = 5</lineannotation>
</screen>
</para>
<para>
Arrays have a maximum length defined to be 2 to the 26th power or
67108864 (64 MiB). Implementations must not send or accept arrays
exceeding this length.
</para>
<para>
Structs and dict entries are marshalled in the same way as their
contents, but their alignment is always to an 8-byte boundary,
even if their contents would normally be less strictly aligned.
</para>
<para>
Variants are marshalled as the <literal>SIGNATURE</literal> of
the contents (which must be a single complete type), followed by a
marshalled value with the type given by that signature. The
variant has the same 1-byte alignment as the signature, which means
that alignment padding before a variant is never needed.
Use of variants must not cause a total message depth to be larger
than 64, including other container types such as structures.
(See <link linkend="message-protocol-marshaling-signature">Valid
Signatures</link>.)
</para>
<para>
It should be noted that while a variant itself does not require any
alignment padding, the contained value does need to be padded
according to the alignment rules of its type.
</para>
<para>
For instance, if the current position in the message is at a multiple of
8 bytes and the byte-order is big-endian, a variant containing a 64-bit
integer 5 would be marshalled as:
<screen>
0x01 0x74 0x00 <lineannotation>signature bytes (length = 1, signature = 't' and trailing nul)</lineannotation>
0x00 0x00 0x00 0x00 0x00 <lineannotation>padding to 8-byte boundary</lineannotation>
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x05 <lineannotation>8 bytes of contained value</lineannotation>
</screen>
</para>
</sect2>
<sect2>
<title>Summary of D-Bus marshalling</title>
<para>
Given all this, the types are marshaled on the wire as follows:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Conventional Name</entry>
<entry>Encoding</entry>
<entry>Alignment</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>INVALID</literal></entry>
<entry>Not applicable; cannot be marshaled.</entry>
<entry>N/A</entry>
</row><row>
<entry><literal>BYTE</literal></entry>
<entry>A single 8-bit byte.</entry>
<entry>1</entry>
</row><row>
<entry><literal>BOOLEAN</literal></entry>
<entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
<entry>4</entry>
</row><row>
<entry><literal>INT16</literal></entry>
<entry>16-bit signed integer in the message's byte order.</entry>
<entry>2</entry>
</row><row>
<entry><literal>UINT16</literal></entry>
<entry>16-bit unsigned integer in the message's byte order.</entry>
<entry>2</entry>
</row><row>
<entry><literal>INT32</literal></entry>
<entry>32-bit signed integer in the message's byte order.</entry>
<entry>4</entry>
</row><row>
<entry><literal>UINT32</literal></entry>
<entry>32-bit unsigned integer in the message's byte order.</entry>
<entry>4</entry>
</row><row>
<entry><literal>INT64</literal></entry>
<entry>64-bit signed integer in the message's byte order.</entry>
<entry>8</entry>
</row><row>
<entry><literal>UINT64</literal></entry>
<entry>64-bit unsigned integer in the message's byte order.</entry>
<entry>8</entry>
</row><row>
<entry><literal>DOUBLE</literal></entry>
<entry>64-bit IEEE 754 double in the message's byte order.</entry>
<entry>8</entry>
</row><row>
<entry><literal>STRING</literal></entry>
<entry>A <literal>UINT32</literal> indicating the string's
length in bytes excluding its terminating nul, followed by
non-nul string data of the given length, followed by a terminating nul
byte.
</entry>
<entry>
4 (for the length)
</entry>
</row><row>
<entry><literal>OBJECT_PATH</literal></entry>
<entry>Exactly the same as <literal>STRING</literal> except the
content must be a valid object path (see above).
</entry>
<entry>
4 (for the length)
</entry>
</row><row>
<entry><literal>SIGNATURE</literal></entry>
<entry>The same as <literal>STRING</literal> except the length is a single
byte (thus signatures have a maximum length of 255)
and the content must be a valid signature (see above).
</entry>
<entry>
1
</entry>
</row><row>
<entry><literal>ARRAY</literal></entry>
<entry>
A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
alignment padding to the alignment boundary of the array element type,
followed by each array element.
</entry>
<entry>
4 (for the length)
</entry>
</row><row>
<entry><literal>STRUCT</literal></entry>
<entry>
A struct must start on an 8-byte boundary regardless of the
type of the struct fields. The struct value consists of each
field marshaled in sequence starting from that 8-byte
alignment boundary.
</entry>
<entry>
8
</entry>
</row><row>
<entry><literal>VARIANT</literal></entry>
<entry>
The marshaled <literal>SIGNATURE</literal> of a single
complete type, followed by a marshaled value with the type
given in the signature.
</entry>
<entry>
1 (alignment of the signature)
</entry>
</row><row>
<entry><literal>DICT_ENTRY</literal></entry>
<entry>
Identical to STRUCT.
</entry>
<entry>
8
</entry>
</row><row>
<entry><literal>UNIX_FD</literal></entry>
<entry>32-bit unsigned integer in the message's byte
order. The actual file descriptors need to be
transferred out-of-band via some platform specific
mechanism. On the wire, values of this type store the index to the
file descriptor in the array of file descriptors that
accompany the message.</entry>
<entry>4</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
</sect2>
</sect1>
<sect1 id="message-protocol">
<title>Message Protocol</title>
<para>
A <firstterm>message</firstterm> consists of a
<firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
think of a message as a package, the header is the address, and the body
contains the package contents. The message delivery system uses the header
information to figure out where to send the message and how to interpret
it; the recipient interprets the body of the message.
</para>
<para>
The body of the message is made up of zero or more
<firstterm>arguments</firstterm>, which are typed values, such as an
integer or a byte array.
</para>
<para>
Both header and body use the D-Bus <link linkend="type-system">type
system</link> and format for serializing data.
</para>
<sect2 id="message-protocol-messages">
<title>Message Format</title>
<para>
A message consists of a header and a body. The header is a block of
values with a fixed signature and meaning. The body is a separate block
of values, with a signature specified in the header.
</para>
<para>
The length of the header must be a multiple of 8, allowing the body to
begin on an 8-byte boundary when storing the entire message in a single
buffer. If the header does not naturally end on an 8-byte boundary
up to 7 bytes of nul-initialized alignment padding must be added.
</para>
<para>
The message body need not end on an 8-byte boundary.
</para>
<para>
The maximum length of a message, including header, header alignment padding,
and body is 2 to the 27th power or 134217728 (128 MiB).
Implementations must not send or accept messages exceeding this size.
</para>
<para>
The signature of the header is:
<programlisting>
"yyyyuua(yv)"
</programlisting>
Written out more readably, this is:
<programlisting>
BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
</programlisting>
</para>
<para>
These values have the following meanings:
<informaltable>
<tgroup cols="2">
<thead>
<row>
<entry>Value</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>1st <literal>BYTE</literal></entry>
<entry>Endianness flag; ASCII 'l' for little-endian
or ASCII 'B' for big-endian. Both header and body are
in this endianness.</entry>
</row>
<row>
<entry>2nd <literal>BYTE</literal></entry>
<entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
Currently-defined types are described below.
</entry>
</row>
<row>
<entry>3rd <literal>BYTE</literal></entry>
<entry>Bitwise OR of flags. Unknown flags
must be ignored. Currently-defined flags are described below.
</entry>
</row>
<row>
<entry>4th <literal>BYTE</literal></entry>
<entry>Major protocol version of the sending application. If
the major protocol version of the receiving application does not
match, the applications will not be able to communicate and the
D-Bus connection must be disconnected. The major protocol
version for this version of the specification is 1.
</entry>
</row>
<row>
<entry>1st <literal>UINT32</literal></entry>
<entry>Length in bytes of the message body, starting
from the end of the header. The header ends after
its alignment padding to an 8-boundary.
</entry>
</row>
<row>
<entry>2nd <literal>UINT32</literal></entry>
<entry>The serial of this message, used as a cookie
by the sender to identify the reply corresponding
to this request. This must not be zero.
</entry>
</row>
<row>
<entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
<entry>An array of zero or more <firstterm>header
fields</firstterm> where the byte is the field code, and the
variant is the field value. The message type determines
which fields are required.
</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
<firstterm>Message types</firstterm> that can appear in the second byte
of the header are:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Conventional name</entry>
<entry>Decimal value</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>INVALID</literal></entry>
<entry>0</entry>
<entry>This is an invalid type.</entry>
</row>
<row>
<entry><literal>METHOD_CALL</literal></entry>
<entry>1</entry>
<entry>Method call. This message type may prompt a
reply.</entry>
</row>
<row>
<entry><literal>METHOD_RETURN</literal></entry>
<entry>2</entry>
<entry>Method reply with returned data.</entry>
</row>
<row>
<entry><literal>ERROR</literal></entry>
<entry>3</entry>
<entry>Error reply. If the first argument exists and is a
string, it is an error message.</entry>
</row>
<row>
<entry><literal>SIGNAL</literal></entry>
<entry>4</entry>
<entry>Signal emission.</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
Flags that can appear in the third byte of the header:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Conventional name</entry>
<entry>Hex value</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>NO_REPLY_EXPECTED</literal></entry>
<entry>0x1</entry>
<entry>
<para>
This message does not expect method return replies or
error replies, even if it is of a type that can
have a reply; the reply should be omitted.
</para>
<para>
Note that METHOD_CALL is the only message type currently
defined in this specification that can expect a reply,
so the presence or absence of this flag in the other
three message types that are currently
documented is meaningless: replies to those message
types should not be sent, whether this flag is present
or not.
</para>
</entry>
</row>
<row>
<entry><literal>NO_AUTO_START</literal></entry>
<entry>0x2</entry>
<entry>The bus must not launch an owner
for the destination name in response to this message.
</entry>
</row>
<row>
<entry><literal>ALLOW_INTERACTIVE_AUTHORIZATION</literal></entry>
<entry>0x4</entry>
<entry>
<para>
This flag may be set on a method call message to
inform the receiving side that the caller is prepared
to wait for interactive authorization, which might
take a considerable time to complete. For instance,
if this flag is set, it would be appropriate to
query the user for passwords or confirmation via
Polkit or a similar framework.
</para>
<para>
This flag is only useful when
unprivileged code calls a more privileged method call,
and an authorization framework is deployed that allows
possibly interactive authorization. If no such framework
is deployed it has no effect. This flag should not
be set by default by client implementations. If it is
set, the caller should also set a suitably long timeout
on the method call to make sure the user interaction
may complete. This flag is only valid for method call
messages, and shall be ignored otherwise.
</para>
<para>
Interaction that takes place as a part of the
effect of the method being called is outside the scope
of this flag, even if it could also be characterized
as authentication or authorization. For instance, in
a method call that directs a network management service
to attempt to connect to a virtual private network,
this flag should control how the network management
service makes the decision "is this user allowed to
change system network configuration?", but it should
not affect how or whether the network management
service interacts with the user to obtain the credentials
that are required for access to the VPN.
</para>
<para>
If a this flag is not set on a method call, and a
service determines that the requested operation is
not allowed without interactive authorization, but
could be allowed after successful interactive
authorization, it may return the
<literal>org.freedesktop.DBus.Error.InteractiveAuthorizationRequired</literal>
error.
</para>
<para>
The absence of this flag does not guarantee that
interactive authorization will not be applied, since
existing services that pre-date this flag might
already use interactive authorization. However,
existing D-Bus APIs that will use interactive
authorization should document that the call may take
longer than usual, and new D-Bus APIs should avoid
interactive authorization in the absence of this flag.
</para>
</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<sect3 id="message-protocol-header-fields">
<title>Header Fields</title>
<para>
The array at the end of the header contains <firstterm>header
fields</firstterm>, where each field is a 1-byte field code followed
by a field value. A header must contain the required header fields for
its message type, and zero or more of any optional header
fields. Future versions of this protocol specification may add new
fields. Implementations must not invent their own header fields;
only changes to this specification may introduce new header fields.
</para>
<para>
If an implementation sees a header field code that it does not
expect, it must accept and ignore that field, as it will be part
of a new
(but compatible) version of this specification. This also applies
to known header fields appearing in unexpected messages, for
example: if a signal has a reply serial it must be ignored
even though it has no meaning as of this version of the spec.
</para>
<para>
However, implementations must not send or accept known header fields
with the wrong type stored in the field value. So for example a
message with an <literal>INTERFACE</literal> field of type
<literal>UINT32</literal> would be considered corrupt.
</para>
<para>
Server implementations that might relay messages from one
mutually-distrustful client to another, such as the message
bus, should remove header fields that the server does not
recognise. However, a client must assume that the server has
not done so, unless it has evidence to the contrary,
such as having checked for the <literal>HeaderFiltering</literal>
<link linkend="message-bus-properties-features">message bus
feature</link>.
</para>
<para>
New header fields controlled by the message bus
(similar to <literal>SENDER</literal>) might be added to this
specification in future. Such message fields should normally
only be added to messages that are going to be delivered to a
client that specifically requested them (for example by calling
some method), and the message bus should remove those header
fields from all other messages that it relays. This design
principle serves two main purposes. One is to avoid unnecessary
memory and throughput overhead when delivering messages to
clients that are not interested in the new header fields.
The other is to give clients a reason to call the method that
requests those messages (otherwise, the clients would not work).
This is desirable because looking at the reply to that method
call is a natural way to check that the message bus guarantees
to filter out faked header fields that might have been sent by
malicious peers.
</para>
<para>
Here are the currently-defined header fields:
<informaltable>
<tgroup cols="5">
<thead>
<row>
<entry>Conventional Name</entry>
<entry>Decimal Code</entry>
<entry>Type</entry>
<entry>Required In</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>INVALID</literal></entry>
<entry>0</entry>
<entry>N/A</entry>
<entry>not allowed</entry>
<entry>Not a valid field name (error if it appears in a message)</entry>
</row>
<row>
<entry><literal>PATH</literal></entry>
<entry>1</entry>
<entry><literal>OBJECT_PATH</literal></entry>
<entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
<entry>The object to send a call to,
or the object a signal is emitted from.
The special path
<literal>/org/freedesktop/DBus/Local</literal> is reserved;
implementations should not send messages with this path,
and the reference implementation of the bus daemon will
disconnect any application that attempts to do so.
This header field is controlled by the message sender.
</entry>
</row>
<row>
<entry><literal>INTERFACE</literal></entry>
<entry>2</entry>
<entry><literal>STRING</literal></entry>
<entry><literal>SIGNAL</literal></entry>
<entry>
The interface to invoke a method call on, or
that a signal is emitted from. Optional for
method calls, required for signals.
The special interface
<literal>org.freedesktop.DBus.Local</literal> is reserved;
implementations should not send messages with this
interface, and the reference implementation of the bus
daemon will disconnect any application that attempts to
do so.
This header field is controlled by the message sender.
</entry>
</row>
<row>
<entry><literal>MEMBER</literal></entry>
<entry>3</entry>
<entry><literal>STRING</literal></entry>
<entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
<entry>
The member, either the method name or signal name.
This header field is controlled by the message sender.
</entry>
</row>
<row>
<entry><literal>ERROR_NAME</literal></entry>
<entry>4</entry>
<entry><literal>STRING</literal></entry>
<entry><literal>ERROR</literal></entry>
<entry>The name of the error that occurred, for errors</entry>
</row>
<row>
<entry><literal>REPLY_SERIAL</literal></entry>
<entry>5</entry>
<entry><literal>UINT32</literal></entry>
<entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
<entry>
The serial number of the message this message is a reply
to. (The serial number is the second <literal>UINT32</literal>
in the header.)
This header field is controlled by the message sender.
</entry>
</row>
<row>
<entry><literal>DESTINATION</literal></entry>
<entry>6</entry>
<entry><literal>STRING</literal></entry>
<entry>optional</entry>
<entry>
The name of the connection this message is intended for.
This field is usually only meaningful in combination with
the message bus (see <xref linkend="message-bus"/>),
but other servers may define their own meanings for it.
This header field is controlled by the message sender.
</entry>
</row>
<row>
<entry><literal>SENDER</literal></entry>
<entry>7</entry>
<entry><literal>STRING</literal></entry>
<entry>optional</entry>
<entry>Unique name of the sending connection.
This field is usually only meaningful in combination with
the message bus, but other servers may define their own
meanings for it.
On a message bus, this header field is controlled by
the message bus, so it is as reliable and trustworthy
as the message bus itself.
Otherwise, this header field is controlled by the message
sender, unless there is out-of-band information that
indicates otherwise.
</entry>
</row>
<row>
<entry><literal>SIGNATURE</literal></entry>
<entry>8</entry>
<entry><literal>SIGNATURE</literal></entry>
<entry>optional</entry>
<entry>
The signature of the message body.
If omitted, it is assumed to be the
empty signature "" (i.e. the body must be 0-length).
This header field is controlled by the message sender.
</entry>
</row>
<row>
<entry><literal>UNIX_FDS</literal></entry>
<entry>9</entry>
<entry><literal>UINT32</literal></entry>
<entry>optional</entry>
<entry>The number of Unix file descriptors that
accompany the message. If omitted, it is assumed
that no Unix file descriptors accompany the
message. The actual file descriptors need to be
transferred via platform specific mechanism
out-of-band. They must be sent at the same time as
part of the message itself. They may not be sent
before the first byte of the message itself is
transferred or after the last byte of the message
itself.
This header field is controlled by the message sender.
</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
</sect3>
</sect2>
<sect2 id="message-protocol-names">
<title>Valid Names</title>
<para>
The various names in D-Bus messages have some restrictions.
</para>
<para>
There is a <firstterm>maximum name length</firstterm>
of 255 which applies to bus names, interfaces, and members.
</para>
<sect3 id="message-protocol-names-interface">
<title>Interface names</title>
<para>
Interfaces have names with type <literal>STRING</literal>, meaning that
they must be valid UTF-8. However, there are also some
additional restrictions that apply to interface names
specifically:
<itemizedlist>
<listitem><para>Interface names are composed of 2 or more elements separated by
a period ('.') character. All elements must contain at least
one character.
</para>
</listitem>
<listitem><para>Each element must only contain the ASCII characters
"[A-Z][a-z][0-9]_" and must not begin with a digit.
</para>
</listitem>
<listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
</itemizedlist>
</para>
<para>
Interface names should start with the reversed DNS domain name of
the author of the interface (in lower-case), like interface names
in Java. It is conventional for the rest of the interface name
to consist of words run together, with initial capital letters
on all words ("CamelCase"). Several levels of hierarchy can be used.
It is also a good idea to include the major version of the interface
in the name, and increment it if incompatible changes are made;
this way, a single object can implement several versions of an
interface in parallel, if necessary.
</para>
<para>
For instance, if the owner of <literal>example.com</literal> is
developing a D-Bus API for a music player, they might define
interfaces called <literal>com.example.MusicPlayer1</literal>,
<literal>com.example.MusicPlayer1.Track</literal> and
<literal>com.example.MusicPlayer1.Seekable</literal>.
</para>
<para>
If the author's DNS domain name contains hyphen/minus characters
('-'), which are not allowed in D-Bus interface names, they
should be replaced by underscores. If the DNS domain name
contains a digit immediately following a period ('.'), which
is also not allowed in interface names), the interface name
should add an underscore before that digit. For example, if
the owner of 7-zip.org defined an interface for out-of-process
plugins, it might be named <literal>org._7_zip.Plugin</literal>.
</para>
<para>
If the author's DNS domain name is an internationalized domain
name (IDN) such as <literal>δοκιμή.example</literal>,
the ASCII encoding (known as ACE-encoding or Punycode) such as
<literal>xn--jxalpdlp.example</literal>
should be used as a basis for the reversed-domain-name form.
As with any other name, hyphen/minus characters should be
replaced by underscores in the reversed-domain-name form, for
example <literal>example.xn__jxalpdlp.ExampleService1</literal>.
For more information about internationalized domain names, see
<ulink url="https://datatracker.ietf.org/doc/html/rfc5890">RFC
5890 "Internationalized Domain Names for Applications (IDNA):
Definitions and Document Framework"</ulink>.
</para>
<para>
D-Bus does not distinguish between the concepts that would be
called classes and interfaces in Java: either can be identified on
D-Bus by an interface name.
</para>
</sect3>
<sect3 id="message-protocol-names-bus">
<title>Bus names</title>
<para>
Connections have one or more bus names associated with them.
A connection has exactly one bus name that is a <firstterm>unique
connection name</firstterm>. The unique connection name remains
with the connection for its entire lifetime.
A bus name is of type <literal>STRING</literal>,
meaning that it must be valid UTF-8. However, there are also
some additional restrictions that apply to bus names
specifically:
<itemizedlist>
<listitem><para>Bus names that start with a colon (':')
character are unique connection names. Other bus names
are called <firstterm>well-known bus names</firstterm>.
</para>
</listitem>
<listitem><para>Bus names are composed of 1 or more elements separated by
a period ('.') character. All elements must contain at least
one character.
</para>
</listitem>
<listitem><para>Each element must only contain the ASCII characters
"[A-Z][a-z][0-9]_-", with "-" discouraged in new bus
names. Only elements that are part of a unique
connection name may begin with a digit, elements in
other bus names must not begin with a digit.
</para>
</listitem>
<listitem><para>Bus names must contain at least one '.' (period)
character (and thus at least two elements).
</para></listitem>
<listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
<listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
</itemizedlist>
</para>
<para>
Note that the hyphen ('-') character is allowed in bus names
but not in interface names. It is also problematic or not
allowed in various specifications and APIs that refer to
D-Bus, such as <ulink
url="http://docs.flatpak.org/en/latest/introduction.html#identifiers"
>Flatpak application IDs</ulink>,
<ulink
url="https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html#dbus"
>the <literal>DBusActivatable</literal> interface in the
Desktop Entry Specification</ulink>, and the convention
that an application's "main" interface and object path
resemble its bus name. To avoid situations that require
special-case handling, it is recommended that new D-Bus names
consistently replace hyphens with underscores.
</para>
<para>
Like <link linkend="message-protocol-names-interface">interface
names</link>, well-known bus names should start with the
reversed DNS domain name of the author of the interface (in
lower-case), and it is conventional for the rest of the well-known
bus name to consist of words run together, with initial
capital letters. As with interface names, including a version
number in well-known bus names is a good idea; it's possible to
have the well-known bus name for more than one version
simultaneously if backwards compatibility is required.
</para>
<para>
As with <link linkend="message-protocol-names-interface">interface
names</link>, if the author's DNS domain name
contains hyphen/minus characters they should be replaced by
underscores, if it contains leading digits they should be
escaped by prepending an underscore, and internationalized
domain names (IDN) need to be encoded in their ASCII form
(ACE-encoding, Punycode) before replacing dashes with underscores.
For example, if the owner
of 7-zip.org used a D-Bus name for an archiving application,
it might be named <literal>org._7_zip.Archiver</literal>,
while the owner of <literal>δοκιμή.example</literal> might
use the name <literal>example.xn__jxalpdlp.ExampleService1</literal>.
</para>
<para>
If a well-known bus name implies the presence of a "main" interface,
that "main" interface is often given the same name as
the well-known bus name, and situated at the corresponding object
path. For instance, if the owner of <literal>example.com</literal>
is developing a D-Bus API for a music player, they might define
that any application that takes the well-known name
<literal>com.example.MusicPlayer1</literal> should have an object
at the object path <literal>/com/example/MusicPlayer1</literal>
which implements the interface
<literal>com.example.MusicPlayer1</literal>.
</para>
</sect3>
<sect3 id="message-protocol-names-member">
<title>Member names</title>
<para>
Member (i.e. method or signal) names:
<itemizedlist>
<listitem><para>Must only contain the ASCII characters
"[A-Z][a-z][0-9]_" and may not begin with a
digit.</para></listitem>
<listitem><para>Must not contain the '.' (period) character.</para></listitem>
<listitem><para>Must not exceed the maximum name length.</para></listitem>
<listitem><para>Must be at least 1 byte in length.</para></listitem>
</itemizedlist>
</para>
<para>
It is conventional for member names on D-Bus to consist of
capitalized words with no punctuation ("camel-case").
Method names should usually be verbs, such as
<literal>GetItems</literal>, and signal names should usually be
a description of an event, such as <literal>ItemsChanged</literal>.
</para>
</sect3>
<sect3 id="message-protocol-names-error">
<title>Error names</title>
<para>
Error names have the same restrictions as interface names.
</para>
<para>
Error names have the same naming conventions as interface
names, and often contain <literal>.Error.</literal>; for instance,
the owner of <literal>example.com</literal> might define the
errors <literal>com.example.MusicPlayer1.Error.FileNotFound</literal>
and <literal>com.example.MusicPlayer1.Error.OutOfMemory</literal>.
The errors defined by D-Bus itself, such as
<literal>org.freedesktop.DBus.Error.Failed</literal>, follow a
similar pattern.
</para>
</sect3>
</sect2>
<sect2 id="message-protocol-types">
<title>Message Types</title>
<para>
Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
<literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
This section describes these conventions.
</para>
<sect3 id="message-protocol-types-method">
<title>Method Calls</title>
<para>
Some messages invoke an operation on a remote object. These are
called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
messages map naturally to methods on objects in a typical program.
</para>
<para>
A method call message is required to have a <literal>MEMBER</literal> header field
indicating the name of the method. Optionally, the message has an
<literal>INTERFACE</literal> field giving the interface the method is a part of.
Including the <literal>INTERFACE</literal> in all method call
messages is strongly recommended.
</para>
<para>
In the absence of an <literal>INTERFACE</literal> field, if two
or more interfaces on the same object have a method with the same
name, it is undefined which of those methods will be invoked.
Implementations may choose to either return an error, or deliver the
message as though it had an arbitrary one of those interfaces.
</para>
<para>
In some situations (such as the well-known system bus), messages
are filtered through an access-control list external to the
remote object implementation. If that filter rejects certain
messages by matching their interface, or accepts only messages
to specific interfaces, it must also reject messages that have no
<literal>INTERFACE</literal>: otherwise, malicious
applications could use this to bypass the filter.
</para>
<para>
Method call messages also include a <literal>PATH</literal> field
indicating the object to invoke the method on. If the call is passing
through a message bus, the message will also have a
<literal>DESTINATION</literal> field giving the name of the connection
to receive the message.
</para>
<para>
When an application handles a method call message, it is required to
return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
</para>
<para>
If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
are the return value(s) or "out parameters" of the method call.
If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
and the call fails; no return value will be provided. It makes
no sense to send multiple replies to the same method call.
</para>
<para>
Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
reply is required, so the caller will know the method
was successfully processed.
</para>
<para>
The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
header field.
</para>
<para>
If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
then the application receiving the method should not send the reply message (regardless of
whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
</para>
<para>
Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
destination name does not exist then a program to own the destination
name will be started (activated) before the message is delivered. See
<xref linkend="message-bus-starting-services"/>.
The message
will be held until the new program is successfully started or has
failed to start; in case of failure, an error will be returned. This
flag is only relevant in the context of a message bus, it is ignored
during one-to-one communication with no intermediate bus.
</para>
<sect4 id="message-protocol-types-method-apis">
<title>Mapping method calls to native APIs</title>
<para>
APIs for D-Bus may map method calls to a method call in a specific
programming language, such as C++, or may map a method call written
in an IDL to a D-Bus message.
</para>
<para>
In APIs of this nature, arguments to a method are often termed "in"
(which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
"inout" arguments, which are both sent and received, i.e. the caller
passes in a value which is modified. Mapped to D-Bus, an "inout"
argument is equivalent to an "in" argument, followed by an "out"
argument. You can't pass things "by reference" over the wire, so
"inout" is purely an illusion of the in-process API.
</para>
<para>
Given a method with zero or one return values, followed by zero or more
arguments, where each argument may be "in", "out", or "inout", the
caller constructs a message by appending each "in" or "inout" argument,
in order. "out" arguments are not represented in the caller's message.
</para>
<para>
The recipient constructs a reply by appending first the return value
if any, then each "out" or "inout" argument, in order.
"in" arguments are not represented in the reply message.
</para>
<para>
Error replies are normally mapped to exceptions in languages that have
exceptions.
</para>
<para>
In converting from native APIs to D-Bus, it is perhaps nice to
map D-Bus naming conventions ("FooBar") to native conventions
such as "fooBar" or "foo_bar" automatically. This is OK
as long as you can say that the native API is one that
was specifically written for D-Bus. It makes the most sense
when writing object implementations that will be exported
over the bus. Object proxies used to invoke remote D-Bus
objects probably need the ability to call any D-Bus method,
and thus a magic name mapping like this could be a problem.
</para>
<para>
This specification doesn't require anything of native API bindings;
the preceding is only a suggested convention for consistency
among bindings.
</para>
</sect4>
</sect3>
<sect3 id="message-protocol-types-signal">
<title>Signal Emission</title>
<para>
Unlike method calls, signal emissions have no replies.
A signal emission is simply a single message of type <literal>SIGNAL</literal>.
It must have three header fields: <literal>PATH</literal> giving the object
the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
for signals, though it is optional for method calls.
</para>
</sect3>
<sect3 id="message-protocol-types-errors">
<title>Errors</title>
<para>
Messages of type <literal>ERROR</literal> are most commonly replies
to a <literal>METHOD_CALL</literal>, but may be returned in reply
to any kind of message. The message bus for example
will return an <literal>ERROR</literal> in reply to a signal emission if
the bus does not have enough memory to send the signal.
</para>
<para>
An <literal>ERROR</literal> may have any arguments, but if the first
argument is a <literal>STRING</literal>, it must be an error message.
The error message may be logged or shown to the user
in some way.
</para>
</sect3>
<sect3 id="message-protocol-types-notation">
<title>Notation in this document</title>
<para>
This document uses a simple pseudo-IDL to describe particular method
calls and signals. Here is an example of a method call:
<programlisting>
org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
out UINT32 resultcode)
</programlisting>
This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
<literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
characters so it's known that the last part of the name in
the "IDL" is the member name.
</para>
<para>
In C++ that might end up looking like this:
<programlisting>
unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
unsigned int flags);
</programlisting>
or equally valid, the return value could be done as an argument:
<programlisting>
void org::freedesktop::DBus::StartServiceByName (const char *name,
unsigned int flags,
unsigned int *resultcode);
</programlisting>
It's really up to the API designer how they want to make
this look. You could design an API where the namespace wasn't used
in C++, using STL or Qt, using varargs, or whatever you wanted.
</para>
<para>
Signals are written as follows:
<programlisting>
org.freedesktop.DBus.NameLost (STRING name)
</programlisting>
Signals don't specify "in" vs. "out" because only
a single direction is possible.
</para>
<para>
It isn't especially encouraged to use this lame pseudo-IDL in actual
API implementations; you might use the native notation for the
language you're using, or you might use COM or CORBA IDL, for example.
</para>
</sect3>
</sect2>
<sect2 id="message-protocol-handling-invalid">
<title>Invalid Protocol and Spec Extensions</title>
<para>
For security reasons, the D-Bus protocol should be strictly parsed and
validated, with the exception of defined extension points. Any invalid
protocol or spec violations should result in immediately dropping the
connection without notice to the other end. Exceptions should be
carefully considered, e.g. an exception may be warranted for a
well-understood idiosyncrasy of a widely-deployed implementation. In
cases where the other end of a connection is 100% trusted and known to
be friendly, skipping validation for performance reasons could also make
sense in certain cases.
</para>
<para>
Generally speaking violations of the "must" requirements in this spec
should be considered possible attempts to exploit security, and violations
of the "should" suggestions should be considered legitimate (though perhaps
they should generate an error in some cases).
</para>
<para>
The following extension points are built in to D-Bus on purpose and must
not be treated as invalid protocol. The extension points are intended
for use by future versions of this spec, they are not intended for third
parties. At the moment, the only way a third party could extend D-Bus
without breaking interoperability would be to introduce a way to negotiate new
feature support as part of the auth protocol, using EXTENSION_-prefixed
commands. There is not yet a standard way to negotiate features.
<itemizedlist>
<listitem>
<para>
In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
commands result in an ERROR rather than a disconnect. This enables
future extensions to the protocol. Commands starting with EXTENSION_ are
reserved for third parties.
</para>
</listitem>
<listitem>
<para>
The authentication protocol supports pluggable auth mechanisms.
</para>
</listitem>
<listitem>
<para>
The address format (see <xref linkend="addresses"/>) supports new
kinds of transport.
</para>
</listitem>
<listitem>
<para>
Messages with an unknown type (something other than
<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
<literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
Unknown-type messages must still be well-formed in the same way
as the known messages, however. They still have the normal
header and body.
</para>
</listitem>
<listitem>
<para>
Header fields with an unknown or unexpected field code must be ignored,
though again they must still be well-formed.
</para>
</listitem>
<listitem>
<para>
New standard interfaces (with new methods and signals) can of course be added.
</para>
</listitem>
</itemizedlist>
</para>
</sect2>
</sect1>
<sect1 id="auth-protocol">
<title>Authentication Protocol</title>
<para>
Before the flow of messages begins, two applications must
authenticate. A simple plain-text protocol is used for
authentication; this protocol is a SASL profile, and maps fairly
directly from the SASL specification. The message encoding is
NOT used here, only plain text messages.
</para>
<para>
Using SASL in D-Bus requires that we define the meaning of non-empty
authorization identity strings. When D-Bus is used on Unix platforms,
a non-empty SASL authorization identity represents a Unix user. An
authorization identity consisting entirely of ASCII decimal digits
represents a numeric user ID as defined by POSIX, for example
<literal>0</literal> for the root user or <literal>1000</literal>
for the first user created on many systems. Non-numeric authorization
identities are not required to be accepted or supported, but if used,
they must be interpreted as a login name as found in the
<literal>pw_name</literal> field of POSIX
<literal>struct passwd</literal>, for example
<literal>root</literal>, and normalized to the corresponding
numeric user ID. For best interoperability, clients and servers
should use numeric user IDs.
</para>
<para>
When D-Bus is used on Windows platforms, a non-empty SASL
authorization identity represents a Windows security identifier
(SID) in its string form, for example
<literal>S-1-5-21-3623811015-3361044348-30300820-1013</literal> for
a domain or local computer user or <literal>S-1-5-18</literal> for
the LOCAL_SYSTEM user. The user-facing usernames such as
<literal>Administrator</literal> or <literal>LOCAL_SYSTEM</literal>
are not used in the D-Bus protocol.
</para>
<para>
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively. The client sends the first line, and the
server must respond to each line from the client with a single-line
reply, with one exception: there is no reply to the BEGIN command.
</para>
<sect2 id="auth-protocol-overview">
<title>Protocol Overview</title>
<para>
The protocol is a line-based protocol, where each line ends with
\r\n. Each line begins with an all-caps ASCII command name containing
only the character range [A-Z_], a space, then any arguments for the
command, then the \r\n ending the line. The protocol is
case-sensitive. All bytes must be in the ASCII character set.
Commands from the client to the server are as follows:
<itemizedlist>
<listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
<listitem><para>CANCEL</para></listitem>
<listitem><para>BEGIN</para></listitem>
<listitem><para>DATA &lt;data in hex encoding&gt;</para></listitem>
<listitem><para>ERROR [human-readable error explanation]</para></listitem>
<listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
</itemizedlist>
From server to client are as follows:
<itemizedlist>
<listitem><para>REJECTED &lt;space-separated list of mechanism names&gt;</para></listitem>
<listitem><para>OK &lt;GUID in hex&gt;</para></listitem>
<listitem><para>DATA &lt;data in hex encoding&gt;</para></listitem>
<listitem><para>ERROR [human-readable error explanation]</para></listitem>
<listitem><para>AGREE_UNIX_FD</para></listitem>
</itemizedlist>
</para>
<para>
Unofficial extensions to the command set must begin with the letters
"EXTENSION_", to avoid conflicts with future official commands.
For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
</para>
</sect2>
<sect2 id="auth-nul-byte">
<title>Special credentials-passing nul byte</title>
<para>
Immediately after connecting to the server, the client must send a
single nul byte. This byte may be accompanied by credentials
information on some operating systems that use sendmsg() with
SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
sockets. However, the nul byte must be sent even on other kinds of
socket, and even on operating systems that do not require a byte to be
sent in order to transmit credentials. The text protocol described in
this document begins after the single nul byte. If the first byte
received from the client is not a nul byte, the server may disconnect
that client.
</para>
<para>
A nul byte in any context other than the initial byte is an error;
the protocol is ASCII-only.
</para>
<para>
The credentials sent along with the nul byte may be used with the
SASL mechanism EXTERNAL.
</para>
</sect2>
<sect2 id="auth-command-auth">
<title>AUTH command</title>
<para>
The AUTH command is sent by the client to the server.
The server replies with DATA, OK or REJECTED.
</para>
<para>
If an AUTH command has no arguments, it is a request to list
available mechanisms. The server must respond with a REJECTED
command listing the mechanisms it understands, or with an error.
</para>
<para>
If an AUTH command specifies a mechanism, and the server supports
said mechanism, the server should begin exchanging SASL
challenge-response data with the client using DATA commands.
</para>
<para>
If the server does not support the mechanism given in the AUTH
command, it must send either a REJECTED command listing the mechanisms
it does support, or an error.
</para>
<para>
If the [initial-response] argument is provided, it is intended for use
with mechanisms that have no initial challenge (or an empty initial
challenge), as if it were the argument to an initial DATA command. If
the selected mechanism has an initial challenge and [initial-response]
was provided, the server should reject authentication by sending
REJECTED.
</para>
<para>
If authentication succeeds after exchanging DATA commands,
an OK command must be sent to the client.
</para>
</sect2>
<sect2 id="auth-command-cancel">
<title>CANCEL Command</title>
<para>
The CANCEL command is sent by the client to the server.
The server replies with REJECTED.
</para>
<para>
At any time up to sending the BEGIN command, the client may send a
CANCEL command. On receiving the CANCEL command, the server must
send a REJECTED command and abort the current authentication
exchange.
</para>
</sect2>
<sect2 id="auth-command-data">
<title>DATA Command</title>
<para>
The DATA command may come from either client or server, and simply
contains a hex-encoded block of data to be interpreted
according to the SASL mechanism in use.
If sent by the client, the server replies with DATA, OK or REJECTED.
</para>
<para>
Some SASL mechanisms support sending an "empty string";
FIXME we need some way to do this.
</para>
</sect2>
<sect2 id="auth-command-begin">
<title>BEGIN Command</title>
<para>
The BEGIN command is sent by the client to the server.
The server does not reply.
</para>
<para>
The BEGIN command acknowledges that the client has received an
OK command from the server and completed any feature negotiation
that it wishes to do, and declares that the stream of messages
is about to begin.
</para>
<para>
The first octet received by the server after the \r\n of the BEGIN
command from the client must be the first octet of the
authenticated/encrypted stream of D-Bus messages.
</para>
<para>
Unlike all other commands, the server does not reply to
the BEGIN command with an authentication command of its own.
After the \r\n of the reply to the command before BEGIN,
the next octet received by the client must be the first octet
of the authenticated/encrypted stream of D-Bus messages.
</para>
</sect2>
<sect2 id="auth-command-rejected">
<title>REJECTED Command</title>
<para>
The REJECTED command is sent by the server to the client.
</para>
<para>
The REJECTED command indicates that the current authentication
exchange has failed, and further exchange of DATA is inappropriate.
The client would normally try another mechanism, or try providing
different responses to challenges.
</para><para>
Optionally, the REJECTED command has a space-separated list of
available auth mechanisms as arguments. If a server ever provides
a list of supported mechanisms, it must provide the same list
each time it sends a REJECTED message. Clients are free to
ignore all lists received after the first.
</para>
</sect2>
<sect2 id="auth-command-ok">
<title>OK Command</title>
<para>
The OK command is sent by the server to the client.
</para>
<para>
The OK command indicates that the client has been
authenticated. The client may now proceed with negotiating
Unix file descriptor passing. To do that it shall send
NEGOTIATE_UNIX_FD to the server.
</para>
<para>
Otherwise, the client must respond to the OK command by
sending a BEGIN command, followed by its stream of messages,
or by disconnecting. The server must not accept additional
commands using this protocol after the BEGIN command has been
received. Further communication will be a stream of D-Bus
messages (optionally encrypted, as negotiated) rather than
this protocol.
</para>
<para>
If there is no negotiation, the first octet received by the client
after the \r\n of the OK command must be the first octet of
the authenticated/encrypted stream of D-Bus messages. If the
client negotiates Unix file descriptor passing, the first
octet received by the client after the \r\n of the AGREE_UNIX_FD
or ERROR reply must be the first octet of the
authenticated/encrypted stream.
</para>
<para>
The OK command has one argument, which is the GUID of the server.
See <xref linkend="addresses"/> for more on server GUIDs.
</para>
</sect2>
<sect2 id="auth-command-error">
<title>ERROR Command</title>
<para>
The ERROR command can be sent in either direction.
If sent by the client, the server replies with REJECTED.
</para>
<para>
The ERROR command indicates that either server or client did not
know a command, does not accept the given command in the current
context, or did not understand the arguments to the command. This
allows the protocol to be extended; a client or server can send a
command present or permitted only in new protocol versions, and if
an ERROR is received instead of an appropriate response, fall back
to using some other technique.
</para>
<para>
If an ERROR is sent, the server or client that sent the
error must continue as if the command causing the ERROR had never been
received. However, the the server or client receiving the error
should try something other than whatever caused the error;
if only canceling/rejecting the authentication.
</para>
<para>
If the D-Bus protocol changes incompatibly at some future time,
applications implementing the new protocol would probably be able to
check for support of the new protocol by sending a new command and
receiving an ERROR from applications that don't understand it. Thus the
ERROR feature of the auth protocol is an escape hatch that lets us
negotiate extensions or changes to the D-Bus protocol in the future.
</para>
</sect2>
<sect2 id="auth-command-negotiate-unix-fd">
<title>NEGOTIATE_UNIX_FD Command</title>
<para>
The NEGOTIATE_UNIX_FD command is sent by the client to the server.
The server replies with AGREE_UNIX_FD or ERROR.
</para>
<para>
The NEGOTIATE_UNIX_FD command indicates that the client
supports Unix file descriptor passing. This command may only
be sent after the connection is authenticated, i.e. after OK
was received by the client. This command may only be sent on
transports that support Unix file descriptor passing.
</para>
<para>
On receiving NEGOTIATE_UNIX_FD the server must respond with
either AGREE_UNIX_FD or ERROR. It shall respond the former if
the transport chosen supports Unix file descriptor passing and
the server supports this feature. It shall respond the latter
if the transport does not support Unix file descriptor
passing, the server does not support this feature, or the
server decides not to enable file descriptor passing due to
security or other reasons.
</para>
</sect2>
<sect2 id="auth-command-agree-unix-fd">
<title>AGREE_UNIX_FD Command</title>
<para>
The AGREE_UNIX_FD command is sent by the server to the client.
</para>
<para>
The AGREE_UNIX_FD command indicates that the server supports
Unix file descriptor passing. This command may only be sent
after the connection is authenticated, and the client sent
NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
command may only be sent on transports that support Unix file
descriptor passing.
</para>
<para>
On receiving AGREE_UNIX_FD the client must respond with BEGIN,
followed by its stream of messages, or by disconnecting. The
server must not accept additional commands using this protocol
after the BEGIN command has been received. Further
communication will be a stream of D-Bus messages (optionally
encrypted, as negotiated) rather than this protocol.
</para>
</sect2>
<sect2 id="auth-command-future">
<title>Future Extensions</title>
<para>
Future extensions to the authentication and negotiation
protocol are possible. For that new commands may be
introduced. If a client or server receives an unknown command
it shall respond with ERROR and not consider this fatal. New
commands may be introduced both before, and after
authentication, i.e. both before and after the OK command.
</para>
</sect2>
<sect2 id="auth-examples">
<title>Authentication examples</title>
<para>
<figure>
<title>Example of successful EXTERNAL authentication</title>
<programlisting>
31303030 is ASCII decimal "1000" represented in hex, so
the client is authenticating as Unix uid 1000 in this example.
C: AUTH EXTERNAL 31303030
S: OK 1234deadbeef
C: BEGIN
</programlisting>
</figure>
<figure>
<title>Example of finding out mechanisms then picking one</title>
<programlisting>
C: AUTH
S: REJECTED KERBEROS_V4 SKEY
C: AUTH SKEY 7ab83f32ee
S: DATA 8799cabb2ea93e
C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
S: OK 1234deadbeef
C: BEGIN
</programlisting>
</figure>
<figure>
<title>Example of client sends unknown command then falls back to regular auth</title>
<programlisting>
532d312d352d3138 is the Windows SID "S-1-5-18" in hex,
so the client is authenticating as Windows SID S-1-5-18
in this example.
C: FOOBAR
S: ERROR
C: AUTH EXTERNAL 532d312d352d3138
S: OK 1234deadbeef
C: BEGIN
</programlisting>
</figure>
<figure>
<title>Example of server doesn't support initial auth mechanism</title>
<programlisting>
C: AUTH EXTERNAL
S: REJECTED KERBEROS_V4 SKEY
C: AUTH SKEY 7ab83f32ee
S: DATA 8799cabb2ea93e
C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
S: OK 1234deadbeef
C: BEGIN
</programlisting>
</figure>
<figure>
<title>Example of wrong password or the like followed by successful retry</title>
<programlisting>
C: AUTH EXTERNAL 736d6376
S: REJECTED KERBEROS_V4 SKEY
C: AUTH SKEY 7ab83f32ee
S: DATA 8799cabb2ea93e
C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
S: REJECTED
C: AUTH SKEY 7ab83f32ee
S: DATA 8799cabb2ea93e
C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
S: OK 1234deadbeef
C: BEGIN
</programlisting>
</figure>
<figure>
<title>Example of skey cancelled and restarted</title>
<programlisting>
C: AUTH EXTERNAL 32303438
S: REJECTED KERBEROS_V4 SKEY
C: AUTH SKEY 7ab83f32ee
S: DATA 8799cabb2ea93e
C: CANCEL
S: REJECTED
C: AUTH SKEY 7ab83f32ee
S: DATA 8799cabb2ea93e
C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
S: OK 1234deadbeef
C: BEGIN
</programlisting>
</figure>
<figure>
<title>Example of successful EXTERNAL authentication with successful negotiation of Unix FD passing</title>
<programlisting>
C: AUTH EXTERNAL 31303030
S: OK 1234deadbeef
C: NEGOTIATE_UNIX_FD
S: AGREE_UNIX_FD
C: BEGIN
</programlisting>
</figure>
<figure>
<title>Example of successful EXTERNAL authentication with unsuccessful negotiation of Unix FD passing</title>
<programlisting>
C: AUTH EXTERNAL 31303030
S: OK 1234deadbeef
C: NEGOTIATE_UNIX_FD
S: ERROR Not supported on this OS
C: BEGIN
</programlisting>
</figure>
</para>
</sect2>
<sect2 id="auth-states">
<title>Authentication state diagrams</title>
<para>
This section documents the auth protocol in terms of
a state machine for the client and the server. This is
probably the most robust way to implement the protocol.
</para>
<sect3 id="auth-states-client">
<title>Client states</title>
<para>
To more precisely describe the interaction between the
protocol state machine and the authentication mechanisms the
following notation is used: MECH(CHALL) means that the
server challenge CHALL was fed to the mechanism MECH, which
returns one of
<itemizedlist>
<listitem>
<para>
CONTINUE(RESP) means continue the auth conversation
and send RESP as the response to the server;
</para>
</listitem>
<listitem>
<para>
OK(RESP) means that after sending RESP to the server
the client side of the auth conversation is finished
and the server should return "OK";
</para>
</listitem>
<listitem>
<para>
ERROR means that CHALL was invalid and could not be
processed.
</para>
</listitem>
</itemizedlist>
Both RESP and CHALL may be empty.
</para>
<para>
The Client starts by getting an initial response from the
default mechanism and sends AUTH MECH RESP, or AUTH MECH if
the mechanism did not provide an initial response. If the
mechanism returns CONTINUE, the client starts in state
<emphasis>WaitingForData</emphasis>, if the mechanism
returns OK the client starts in state
<emphasis>WaitingForOK</emphasis>.
</para>
<para>
The client should keep track of available mechanisms and
which it mechanisms it has already attempted. This list is
used to decide which AUTH command to send. When the list is
exhausted, the client should give up and close the
connection.
</para>
<formalpara>
<title><emphasis>WaitingForData</emphasis></title>
<para>
<itemizedlist>
<listitem>
<para>
Receive DATA CHALL
<simplelist>
<member>
MECH(CHALL) returns CONTINUE(RESP) &rarr; send
DATA RESP, goto
<emphasis>WaitingForData</emphasis>
</member>
<member>
MECH(CHALL) returns OK(RESP) &rarr; send DATA
RESP, goto <emphasis>WaitingForOK</emphasis>
</member>
<member>
MECH(CHALL) returns ERROR &rarr; send ERROR
[msg], goto <emphasis>WaitingForData</emphasis>
</member>
</simplelist>
</para>
</listitem>
<listitem>
<para>
Receive REJECTED [mechs] &rarr;
send AUTH [next mech], goto
WaitingForData or <emphasis>WaitingForOK</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive ERROR &rarr; send
CANCEL, goto
<emphasis>WaitingForReject</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive OK &rarr; <emphasis>authenticated</emphasis>,
choose one:
<simplelist>
<member>
send NEGOTIATE_UNIX_FD, goto
<emphasis>WaitingForAgreeUnixFD</emphasis>
</member>
<member>
send BEGIN, terminate auth conversation (successfully)
</member>
</simplelist>
</para>
</listitem>
<listitem>
<para>
Receive anything else &rarr; send
ERROR, goto
<emphasis>WaitingForData</emphasis>
</para>
</listitem>
</itemizedlist>
</para>
</formalpara>
<formalpara>
<title><emphasis>WaitingForOK</emphasis></title>
<para>
<itemizedlist>
<listitem>
<para>
Receive OK &rarr; <emphasis>authenticated</emphasis>,
choose one:
<simplelist>
<member>
send NEGOTIATE_UNIX_FD, goto
<emphasis>WaitingForAgreeUnixFD</emphasis>
</member>
<member>
send BEGIN, terminate auth conversation (successfully)
</member>
</simplelist>
</para>
</listitem>
<listitem>
<para>
Receive REJECTED [mechs] &rarr; send AUTH [next mech],
goto <emphasis>WaitingForData</emphasis> or
<emphasis>WaitingForOK</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive DATA &rarr; send CANCEL, goto
<emphasis>WaitingForReject</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive ERROR &rarr; send CANCEL, goto
<emphasis>WaitingForReject</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive anything else &rarr; send ERROR, goto
<emphasis>WaitingForOK</emphasis>
</para>
</listitem>
</itemizedlist>
</para>
</formalpara>
<formalpara>
<title><emphasis>WaitingForReject</emphasis></title>
<para>
<itemizedlist>
<listitem>
<para>
Receive REJECTED [mechs] &rarr; send AUTH [next mech],
goto <emphasis>WaitingForData</emphasis> or
<emphasis>WaitingForOK</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive anything else &rarr; terminate auth
conversation, disconnect
</para>
</listitem>
</itemizedlist>
</para>
</formalpara>
<formalpara>
<title><emphasis>WaitingForAgreeUnixFD</emphasis></title>
<para>
By the time this state is reached, the client has already
been authenticated.
<itemizedlist>
<listitem>
<para>
Receive AGREE_UNIX_FD &rarr; enable Unix fd passing,
send BEGIN, terminate auth conversation (successfully)
</para>
</listitem>
<listitem>
<para>
Receive ERROR &rarr; disable Unix fd passing,
send BEGIN, terminate auth conversation (successfully)
</para>
</listitem>
<listitem>
<para>
Receive anything else &rarr; terminate auth
conversation, disconnect
</para>
</listitem>
</itemizedlist>
</para>
</formalpara>
</sect3>
<sect3 id="auth-states-server">
<title>Server states</title>
<para>
For the server MECH(RESP) means that the client response
RESP was fed to the the mechanism MECH, which returns one of
<itemizedlist>
<listitem>
<para>
CONTINUE(CHALL) means continue the auth conversation and
send CHALL as the challenge to the client;
</para>
</listitem>
<listitem>
<para>
OK means that the client has been successfully
authenticated;
</para>
</listitem>
<listitem>
<para>
REJECTED means that the client failed to authenticate or
there was an error in RESP.
</para>
</listitem>
</itemizedlist>
The server starts out in state
<emphasis>WaitingForAuth</emphasis>. If the client is
rejected too many times the server must disconnect the
client.
</para>
<formalpara>
<title><emphasis>WaitingForAuth</emphasis></title>
<para>
<itemizedlist>
<listitem>
<para>
Receive AUTH &rarr; send REJECTED [mechs], goto
<emphasis>WaitingForAuth</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive AUTH MECH RESP
<simplelist>
<member>
MECH not valid mechanism &rarr; send REJECTED
[mechs], goto
<emphasis>WaitingForAuth</emphasis>
</member>
<member>
MECH(RESP) returns CONTINUE(CHALL) &rarr; send
DATA CHALL, goto
<emphasis>WaitingForData</emphasis>
</member>
<member>
MECH(RESP) returns OK &rarr; send OK, goto
<emphasis>WaitingForBegin</emphasis>
</member>
<member>
MECH(RESP) returns REJECTED &rarr; send REJECTED
[mechs], goto
<emphasis>WaitingForAuth</emphasis>
</member>
</simplelist>
</para>
</listitem>
<listitem>
<para>
Receive BEGIN &rarr; terminate
auth conversation, disconnect
</para>
</listitem>
<listitem>
<para>
Receive ERROR &rarr; send REJECTED [mechs], goto
<emphasis>WaitingForAuth</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive anything else &rarr; send
ERROR, goto
<emphasis>WaitingForAuth</emphasis>
</para>
</listitem>
</itemizedlist>
</para>
</formalpara>
<formalpara>
<title><emphasis>WaitingForData</emphasis></title>
<para>
<itemizedlist>
<listitem>
<para>
Receive DATA RESP
<simplelist>
<member>
MECH(RESP) returns CONTINUE(CHALL) &rarr; send
DATA CHALL, goto
<emphasis>WaitingForData</emphasis>
</member>
<member>
MECH(RESP) returns OK &rarr; send OK, goto
<emphasis>WaitingForBegin</emphasis>
</member>
<member>
MECH(RESP) returns REJECTED &rarr; send REJECTED
[mechs], goto
<emphasis>WaitingForAuth</emphasis>
</member>
</simplelist>
</para>
</listitem>
<listitem>
<para>
Receive BEGIN &rarr; terminate auth conversation,
disconnect
</para>
</listitem>
<listitem>
<para>
Receive CANCEL &rarr; send REJECTED [mechs], goto
<emphasis>WaitingForAuth</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive ERROR &rarr; send REJECTED [mechs], goto
<emphasis>WaitingForAuth</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive anything else &rarr; send ERROR, goto
<emphasis>WaitingForData</emphasis>
</para>
</listitem>
</itemizedlist>
</para>
</formalpara>
<formalpara>
<title><emphasis>WaitingForBegin</emphasis></title>
<para>
<itemizedlist>
<listitem>
<para>
Receive BEGIN &rarr; terminate auth conversation,
client authenticated
</para>
</listitem>
<listitem>
<para>
Receive NEGOTIATE_UNIX_FD &rarr; send AGREE_UNIX_FD
or ERROR, goto <emphasis>WaitingForBegin</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive CANCEL &rarr; send REJECTED [mechs], goto
<emphasis>WaitingForAuth</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive ERROR &rarr; send REJECTED [mechs], goto
<emphasis>WaitingForAuth</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive anything else &rarr; send ERROR, goto
<emphasis>WaitingForBegin</emphasis>
</para>
</listitem>
</itemizedlist>
</para>
</formalpara>
</sect3>
</sect2>
<sect2 id="auth-mechanisms">
<title>Authentication mechanisms</title>
<para>
This section describes some authentication mechanisms that are
often supported by practical D-Bus implementations. The D-Bus
protocol also allows any other standard SASL mechanism, although
implementations of D-Bus often do not.
</para>
<sect3 id="auth-mechanisms-external">
<title>EXTERNAL</title>
<para>
The EXTERNAL mechanism is defined in <ulink
url="https://tools.ietf.org/html/rfc4422#appendix-A">RFC 4422
"Simple Authentication and Security Layer (SASL)", appendix A
"The SASL EXTERNAL Mechanism"</ulink>. This is the recommended
authentication mechanism on platforms where credentials can be
transferred out-of-band, in particular Unix platforms that can
perform credentials-passing over the
<link linkend="transports-unix-domain-sockets-addresses"
>unix: transport</link>.
</para>
<para>
On Unix platforms, interoperable clients should prefer to send
the ASCII decimal string form of the integer Unix user ID as
the authorization identity, for example 1000. When encoded in
hex by the authentication protocol, this will typically result
in a line like <literal>AUTH EXTERNAL 31303030</literal> followed
by \r\n.
</para>
<para>
On Windows platforms, clients that use the EXTERNAL mechanism
should use the Windows security identifier in its string form
as the authorization identity, for example
<literal>S-1-5-21-3623811015-3361044348-30300820-1013</literal> for
a domain or local computer user or <literal>S-1-5-18</literal> for
the LOCAL_SYSTEM user. When encoded in hex by the authentication
protocol, this will typically result in a line like
<literal>AUTH EXTERNAL 532d312d352d3138</literal> followed by
\r\n.
</para>
</sect3>
<sect3 id="auth-mechanisms-sha">
<title>DBUS_COOKIE_SHA1</title>
<para>
DBUS_COOKIE_SHA1 is a D-Bus-specific SASL mechanism. Its
reference implementation is part of the reference implementation
of D-Bus.
</para>
<para>
This mechanism is designed to establish that a client
has the ability to read a private file owned by the user being
authenticated. If the client can prove that it has access to a secret
cookie stored in this file, then the client is authenticated.
Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
directory. This is the recommended authentication mechanism
for platforms and configurations where EXTERNAL cannot be used.
</para>
<para>
Throughout this description, "hex encoding" must output the digits
from a to f in lower-case; the digits A to F must not be used
in the DBUS_COOKIE_SHA1 mechanism.
</para>
<para>
Authentication proceeds as follows:
<itemizedlist>
<listitem>
<para>
The client sends the username it would like to authenticate
as, hex-encoded.
</para>
</listitem>
<listitem>
<para>
The server sends the name of its "cookie context" (see below); a
space character; the integer ID of the secret cookie the client
must demonstrate knowledge of; a space character; then a
randomly-generated challenge string, all of this hex-encoded into
one, single string.
</para>
</listitem>
<listitem>
<para>
The client locates the cookie and generates its own
randomly-generated challenge string. The client then concatenates
the server's decoded challenge, a ":" character, its own challenge,
another ":" character, and the cookie. It computes the SHA-1 hash
of this composite string as a hex digest. It concatenates the
client's challenge string, a space character, and the SHA-1 hex
digest, hex-encodes the result and sends it back to the server.
</para>
</listitem>
<listitem>
<para>
The server generates the same concatenated string used by the
client and computes its SHA-1 hash. It compares the hash with
the hash received from the client; if the two hashes match, the
client is authenticated.
</para>
</listitem>
</itemizedlist>
</para>
<para>
Each server has a "cookie context," which is a name that identifies a
set of cookies that apply to that server. A sample context might be
"org_freedesktop_session_bus". Context names must be valid ASCII,
nonzero length, and may not contain the characters slash ("/"),
backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
tab ("\t"), or period ("."). There is a default context,
"org_freedesktop_general" that's used by servers that do not specify
otherwise.
</para>
<para>
Cookies are stored in a user's home directory, in the directory
<filename>~/.dbus-keyrings/</filename>. This directory must
not be readable or writable by other users. If it is,
clients and servers must ignore it. The directory
contains cookie files named after the cookie context.
</para>
<para>
A cookie file contains one cookie per line. Each line
has three space-separated fields:
<itemizedlist>
<listitem>
<para>
The cookie ID number, which must be a non-negative integer and
may not be used twice in the same file.
</para>
</listitem>
<listitem>
<para>
The cookie's creation time, in UNIX seconds-since-the-epoch
format.
</para>
</listitem>
<listitem>
<para>
The cookie itself, a hex-encoded random block of bytes. The cookie
may be of any length, though obviously security increases
as the length increases.
</para>
</listitem>
</itemizedlist>
</para>
<para>
Only server processes modify the cookie file.
They must do so with this procedure:
<itemizedlist>
<listitem>
<para>
Create a lockfile name by appending ".lock" to the name of the
cookie file. The server should attempt to create this file
using <literal>O_CREAT | O_EXCL</literal>. If file creation
fails, the lock fails. Servers should retry for a reasonable
period of time, then they may choose to delete an existing lock
to keep users from having to manually delete a stale
lock. <footnote><para>Lockfiles are used instead of real file
locking <literal>fcntl()</literal> because real locking
implementations are still flaky on network
filesystems.</para></footnote>
</para>
</listitem>
<listitem>
<para>
Once the lockfile has been created, the server loads the cookie
file. It should then delete any cookies that are old (the
timeout can be fairly short), or more than a reasonable
time in the future (so that cookies never accidentally
become permanent, if the clock was set far into the future
at some point). The reference implementation deletes cookies
that are more than 5 minutes into the future, or more than
7 minutes in the past. For interoperability, using the
same arbitrary times in other implementations is suggested.
</para>
</listitem>
<listitem>
<para>
If no sufficiently recent cookies remain, the server
generates a new cookie. To avoid spurious authentication
failures, cookies that are close to their deletion time
should not be used for new authentication operations.
For example, this avoids a client starting to use a cookie
whose age is 6m59s, and having authentication subsequently
fail because it takes 2 seconds, during which time the
cookie's age became 7m01s, greater than 7 minutes, causing
the server to delete it. The reference implementation
generates a new cookie whenever the most recent cookie is
older than 5 minutes, giving clients at least 2 minutes
to finish authentication. For interoperability, using the
same arbitrary time in other implementations is suggested.
</para>
</listitem>
<listitem>
<para>
The pruned and possibly added-to cookie file
must be resaved atomically (using a temporary
file which is rename()'d).
</para>
</listitem>
<listitem>
<para>
The lock must be dropped by deleting the lockfile.
</para>
</listitem>
</itemizedlist>
</para>
<para>
Clients need not lock the file in order to load it,
because servers are required to save the file atomically.
</para>
</sect3>
<sect3 id="auth-mechanisms-anonymous">
<title>ANONYMOUS</title>
<para>
The ANONYMOUS mechanism is defined in <ulink
url="https://tools.ietf.org/html/rfc4505">RFC 4505 "Anonymous
Simple Authentication and Security Layer (SASL)
Mechanism"</ulink>.
It does not perform any authentication at all, and should not
be accepted by message buses. However, it might sometimes be
useful for non-message-bus uses of D-Bus.
</para>
</sect3>
</sect2>
</sect1>
<sect1 id="addresses">
<title>Server Addresses</title>
<para>
Server addresses consist of a transport name followed by a colon, and
then an optional, comma-separated list of keys and values in the form key=value.
Each value is escaped.
</para>
<para>
For example:
<programlisting>unix:path=/tmp/dbus-test</programlisting>
Which is the address to a unix socket with the path /tmp/dbus-test.
</para>
<para>
Value escaping is similar to URI escaping but simpler.
<itemizedlist>
<listitem>
<para>
The set of optionally-escaped bytes is:
<literal>[-0-9A-Za-z_/.\*]</literal>. To escape, each
<emphasis>byte</emphasis> (note, not character) which is not in the
set of optionally-escaped bytes must be replaced with an ASCII
percent (<literal>%</literal>) and the value of the byte in hex.
The hex value must always be two digits, even if the first digit is
zero. The optionally-escaped bytes may be escaped if desired.
</para>
</listitem>
<listitem>
<para>
To unescape, append each byte in the value; if a byte is an ASCII
percent (<literal>%</literal>) character then append the following
hex value instead. It is an error if a <literal>%</literal> byte
does not have two hex digits following. It is an error if a
non-optionally-escaped byte is seen unescaped.
</para>
</listitem>
</itemizedlist>
The set of optionally-escaped bytes is intended to preserve address
readability and convenience.
</para>
<para>
A server may specify a key-value pair with the key <literal>guid</literal>
and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
describes the format of the <literal>guid</literal> field. If present,
this UUID may be used to distinguish one server address from another. A
server should use a different UUID for each address it listens on. For
example, if a message bus daemon offers both UNIX domain socket and TCP
connections, but treats clients the same regardless of how they connect,
those two connections are equivalent post-connection but should have
distinct UUIDs to distinguish the kinds of connection.
</para>
<para>
The intent of the address UUID feature is to allow a client to avoid
opening multiple identical connections to the same server, by allowing the
client to check whether an address corresponds to an already-existing
connection. Comparing two addresses is insufficient, because addresses
can be recycled by distinct servers, and equivalent addresses may look
different if simply compared as strings (for example, the host in a TCP
address can be given as an IP address or as a hostname).
</para>
<para>
Note that the address key is <literal>guid</literal> even though the
rest of the API and documentation says "UUID," for historical reasons.
</para>
<para>
[FIXME clarify if attempting to connect to each is a requirement
or just a suggestion]
When connecting to a server, multiple server addresses can be
separated by a semi-colon. The library will then try to connect
to the first address and if that fails, it'll try to connect to
the next one specified, and so forth. For example
<programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
</para>
<para>
Some addresses are <firstterm>connectable</firstterm>. A connectable
address is one containing enough information for a client to connect
to it. For instance, <literal>tcp:host=127.0.0.1,port=4242</literal>
is a connectable address. It is not necessarily possible to listen
on every connectable address: for instance, it is not possible to
listen on a <literal>unixexec:</literal> address.
</para>
<para>
Some addresses are <firstterm>listenable</firstterm>. A listenable
address is one containing enough information for a server to listen on
it, producing a connectable address (which may differ from the
original address). Many listenable addresses are not connectable:
for instance, <literal>tcp:host=127.0.0.1</literal>
is listenable, but not connectable (because it does not specify
a port number).
</para>
<para>
Listening on an address that is not connectable will result in a
connectable address that is not the same as the listenable address.
For instance, listening on <literal>tcp:host=127.0.0.1</literal>
might result in the connectable address
<literal>tcp:host=127.0.0.1,port=30958</literal>,
listening on <literal>unix:tmpdir=/tmp</literal>
might result in the connectable address
<literal>unix:abstract=/tmp/dbus-U8OSdmf7</literal>, or
listening on <literal>unix:runtime=yes</literal>
might result in the connectable address
<literal>unix:path=/run/user/1234/bus</literal>.
</para>
</sect1>
<sect1 id="transports">
<title>Transports</title>
<para>
[FIXME we need to specify in detail each transport and its possible arguments]
Current transports include: unix domain sockets (including
abstract namespace on linux), launchd, systemd, TCP/IP, an executed subprocess and a debug/testing transport
using in-process pipes. Future possible transports include one that
tunnels over X11 protocol.
</para>
<sect2 id="transports-unix-domain-sockets">
<title>Unix Domain Sockets</title>
<para>
Unix domain sockets can be either paths in the file system or on Linux
kernels, they can be abstract which are similar to paths but
do not show up in the file system.
</para>
<para>
When a socket is opened by the D-Bus library it truncates the path
name right before the first trailing Nul byte. This is true for both
normal paths and abstract paths. Note that this is a departure from
previous versions of D-Bus that would create sockets with a fixed
length path name. Names which were shorter than the fixed length
would be padded by Nul bytes.
</para>
<para>
Unix domain sockets are not available on Windows.
On all other platforms, they are the recommended transport for
D-Bus, either used alone or in conjunction with
<link linkend="transports-systemd">systemd</link> or
<link linkend="transports-launchd">launchd</link> addresses.
</para>
<para>
Unix addresses that specify <literal>path</literal> or
<literal>abstract</literal> are both listenable and connectable.
Unix addresses that specify <literal>tmpdir</literal>
or <literal>dir</literal> are only
listenable: the corresponding connectable address will specify
either <literal>path</literal> or <literal>abstract</literal>.
Similarly, Unix addresses that specify <literal>runtime</literal>
are only listenable, and the corresponding connectable address
will specify <literal>path</literal>.
</para>
<sect3 id="transports-unix-domain-sockets-addresses">
<title>Server Address Format</title>
<para>
Unix domain socket addresses are identified by the "unix:" prefix
and support the following key/value pairs:
</para>
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Name</entry>
<entry>Values</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>path</entry>
<entry>(path)</entry>
<entry>
Path of the unix domain socket.
</entry>
</row>
<row>
<entry>dir</entry>
<entry>(path)</entry>
<entry>
Directory in which a socket file with a random file name
starting with 'dbus-' will be created by the server. This key
can only be used in server addresses, not in client addresses;
the resulting client address will have the "path" key instead.
be set.
</entry>
</row>
<row>
<entry>tmpdir</entry>
<entry>(path)</entry>
<entry>
The same as "dir", except that on platforms with
abstract sockets, the server may attempt to create an
abstract socket whose name starts with this directory instead
of a path-based socket. This key can only be used in server
addresses, not in client addresses; the resulting client address
will have the "abstract" or "path" key instead.
</entry>
</row>
<row>
<entry>abstract</entry>
<entry>(string)</entry>
<entry>
Unique string in the abstract namespace, often syntactically
resembling a path but unconnected to the filesystem namespace.
This key is only supported on platforms with abstract Unix
sockets, of which Linux is the only known example.
</entry>
</row>
<row>
<entry>runtime</entry>
<entry><literal>yes</literal></entry>
<entry>If given, This key can only be used in server addresses, not in client addresses. If set, its value must be <literal>yes</literal>. This is typically used in an address string like <literal>unix:runtime=yes;unix:tmpdir=/tmp</literal> so that there can be a fallback if <literal>XDG_RUNTIME_DIR</literal> is not set.</entry>
</row>
</tbody>
</tgroup>
</informaltable>
<para>
Exactly one of the keys <literal>path</literal>,
<literal>abstract</literal>, <literal>runtime</literal>,
<literal>dir</literal> or <literal>tmpdir</literal> must be provided.
</para>
</sect3>
</sect2>
<sect2 id="transports-launchd">
<title>launchd</title>
<para>
launchd is an open-source server management system that replaces init, inetd
and cron on Apple Mac OS X versions 10.4 and above. It provides a common session
bus address for each user and deprecates the X11-enabled D-Bus launcher on OSX.
</para>
<para>
launchd allocates a socket and provides it with the unix path through the
DBUS_LAUNCHD_SESSION_BUS_SOCKET variable in launchd's environment. Every process
spawned by launchd (or dbus-daemon, if it was started by launchd) can access
it through its environment.
Other processes can query for the launchd socket by executing:
$ launchctl getenv DBUS_LAUNCHD_SESSION_BUS_SOCKET
This is normally done by the D-Bus client library so doesn't have to be done
manually.
</para>
<para>
launchd is not available on Microsoft Windows.
</para>
<para>
launchd addresses are listenable and connectable.
</para>
<sect3 id="transports-launchd-addresses">
<title>Server Address Format</title>
<para>
launchd addresses are identified by the "launchd:" prefix
and support the following key/value pairs:
</para>
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Name</entry>
<entry>Values</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>env</entry>
<entry>(environment variable)</entry>
<entry>path of the unix domain socket for the launchd created dbus-daemon.</entry>
</row>
</tbody>
</tgroup>
</informaltable>
<para>
The <literal>env</literal> key is required.
</para>
</sect3>
</sect2>
<sect2 id="transports-systemd">
<title>systemd</title>
<para>
systemd is an open-source server management system that
replaces init and inetd on newer Linux systems. It supports
socket activation. The D-Bus systemd transport is used to acquire
socket activation file descriptors from systemd and use them
as D-Bus transport when the current process is spawned by
socket activation from it.
</para>
<para>
The systemd transport accepts only one or more Unix domain or
TCP streams sockets passed in via socket activation. Using
Unix domain sockets is strongly recommended.
</para>
<para>
The systemd transport is not available on non-Linux operating systems.
</para>
<para>
The systemd transport defines no parameter keys.
</para>
<para>
systemd addresses are listenable, but not connectable. The
corresponding connectable address is the <literal>unix</literal>
or <literal>tcp</literal> address of the socket.
</para>
</sect2>
<sect2 id="transports-tcp-sockets">
<title>TCP Sockets</title>
<para>
The tcp transport provides TCP/IP based connections between clients
located on the same or different hosts.
</para>
<para>
Similar to remote X11, the TCP transport has no integrity or
confidentiality protection, so it should normally only be
used across the local loopback interface, for example using an
address like <literal>tcp:host=127.0.0.1</literal> or
<literal>tcp:host=localhost</literal>. In particular,
configuring the well-known system bus or the well-known session
bus to listen on a non-loopback TCP address is insecure.
</para>
<para>
On Windows and most Unix platforms, the TCP stack is unable to transfer
credentials over a TCP connection, so the
<link linkend="auth-mechanisms-external">EXTERNAL</link>
authentication mechanism does not normally work for this transport
(although the reference implementation of D-Bus is able to identify
loopback TCPv4 connections on Windows by their port number,
partially enabling the EXTERNAL mechanism).
The <link linkend="auth-mechanisms-sha">DBUS_COOKIE_SHA1</link>
mechanism is normally used instead.
</para>
<para>
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 <ulink
url="https://lists.freedesktop.org/archives/dbus/2018-April/017447.html"
>relay connections via Secure Shell or a similar protocol</ulink>.
<!-- TODO: Ideally someone would write a more formal guide to
remote D-Bus debugging, and we could link to that instead -->
</para>
<para>
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.
</para>
<para>
All <literal>tcp</literal> addresses are listenable.
<literal>tcp</literal> addresses in which both
<literal>host</literal> and <literal>port</literal> are
specified, and <literal>port</literal> is non-zero,
are also connectable.
</para>
<sect3 id="transports-tcp-sockets-addresses">
<title>Server Address Format</title>
<para>
TCP/IP socket addresses are identified by the "tcp:" prefix
and support the following key/value pairs:
</para>
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Name</entry>
<entry>Values</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>host</entry>
<entry>(string)</entry>
<entry>DNS name or IP address</entry>
</row>
<row>
<entry>bind</entry>
<entry>(string)</entry>
<entry>Used in a listenable address to configure the interface
on which the server will listen: either the IP address of one of
the local machine's interfaces (most commonly <literal>127.0.0.1
</literal>), or a DNS name that resolves to one of those IP
addresses, or '*' to listen on all interfaces simultaneously.
If not specified, the default is the same value as "host".
</entry>
</row>
<row>
<entry>port</entry>
<entry>(number)</entry>
<entry>The tcp port the server will open. A zero value let the server
choose a free port provided from the underlaying operating system.
libdbus is able to retrieve the real used port from the server.
</entry>
</row>
<row>
<entry>family</entry>
<entry>(string)</entry>
<entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</sect3>
</sect2>
<sect2 id="transports-nonce-tcp-sockets">
<title>Nonce-authenticated TCP Sockets</title>
<para>
The nonce-tcp transport provides a modified TCP transport using a
simple authentication mechanism, to ensure that only clients with read
access to a certain location in the filesystem can connect to the server.
The server writes a secret, the nonce, to a file and an incoming client
connection is only accepted if the client sends the nonce right after
the connect. The nonce mechanism requires no setup and is orthogonal to
the higher-level authentication mechanisms described in the
Authentication section.
</para>
<para>
The nonce-tcp transport is conceptually similar to a combination
of the <link linkend="auth-mechanisms-sha">DBUS_COOKIE_SHA1</link>
authentication mechanism and the
<link linkend="transports-tcp-sockets">tcp</link> transport,
and appears to have originally been implemented as a result of
a misunderstanding of the SASL authentication mechanisms.
</para>
<para>
Like the ordinary tcp transport, the nonce-tcp transport has no
integrity or confidentiality protection, so it should normally
only be used across the local loopback interface, for example
using an address like <literal>tcp:host=127.0.0.1</literal> or
<literal>tcp:host=localhost</literal>. Other uses are insecure.
See <xref linkend="transports-tcp-sockets"/> for more
information on situations where these transports have been used,
and alternatives to these transports.
</para>
<para>
Implementations of D-Bus on Windows operating systems normally
use a nonce-tcp transport via the local loopback interface.
This is because the
<link linkend="transports-unix-domain-sockets">unix</link>
transport, which would otherwise be recommended, is not
available on these operating systems.
</para>
<para>
On start, the server generates a random 16 byte nonce and writes it
to a file in the user's temporary directory. The nonce file location
is published as part of the server's D-Bus address using the
"noncefile" key-value pair.
After an accept, the server reads 16 bytes from the socket. If the
read bytes do not match the nonce stored in the nonce file, the
server MUST immediately drop the connection.
If the nonce match the received byte sequence, the client is accepted
and the transport behaves like an ordinary tcp transport.
</para>
<para>
After a successful connect to the server socket, the client MUST read
the nonce from the file published by the server via the noncefile=
key-value pair and send it over the socket. After that, the
transport behaves like an ordinary tcp transport.
</para>
<para>
All nonce-tcp addresses are listenable. nonce-tcp addresses in which
<literal>host</literal>, <literal>port</literal> and
<literal>noncefile</literal> are all specified,
and <literal>port</literal> is nonzero, are also connectable.
</para>
<sect3 id="transports-nonce-tcp-sockets-addresses">
<title>Server Address Format</title>
<para>
Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix
and support the following key/value pairs:
</para>
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Name</entry>
<entry>Values</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>host</entry>
<entry>(string)</entry>
<entry>DNS name or IP address</entry>
</row>
<row>
<entry>bind</entry>
<entry>(string)</entry>
<entry>The same as for tcp: addresses
</entry>
</row>
<row>
<entry>port</entry>
<entry>(number)</entry>
<entry>The tcp port the server will open. A zero value let the server
choose a free port provided from the underlaying operating system.
libdbus is able to retrieve the real used port from the server.
</entry>
</row>
<row>
<entry>family</entry>
<entry>(string)</entry>
<entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
</row>
<row>
<entry>noncefile</entry>
<entry>(path)</entry>
<entry>File location containing the secret.
This is only meaningful in connectable addresses:
a listening D-Bus server that offers this transport
will always create a new nonce file.</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</sect3>
</sect2>
<sect2 id="transports-exec">
<title>Executed Subprocesses on Unix</title>
<para>
This transport forks off a process and connects its standard
input and standard output with an anonymous Unix domain
socket. This socket is then used for communication by the
transport. This transport may be used to use out-of-process
forwarder programs as basis for the D-Bus protocol.
</para>
<para>
The forked process will inherit the standard error output and
process group from the parent process.
</para>
<para>
Executed subprocesses are not available on Windows.
</para>
<para>
<literal>unixexec</literal> addresses are connectable, but are not
listenable.
</para>
<sect3 id="transports-exec-addresses">
<title>Server Address Format</title>
<para>
Executed subprocess addresses are identified by the "unixexec:" prefix
and support the following key/value pairs:
</para>
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Name</entry>
<entry>Values</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>path</entry>
<entry>(path)</entry>
<entry>Path of the binary to execute, either an absolute
path or a binary name that is searched for in the default
search path of the OS. This corresponds to the first
argument of execlp(). This key is mandatory.</entry>
</row>
<row>
<entry>argv0</entry>
<entry>(string)</entry>
<entry>The program name to use when executing the
binary. If omitted the same value as specified for path=
will be used. This corresponds to the second argument of
execlp().</entry>
</row>
<row>
<entry>argv1, argv2, ...</entry>
<entry>(string)</entry>
<entry>Arguments to pass to the binary. This corresponds
to the third and later arguments of execlp(). If a
specific argvX is not specified no further argvY for Y > X
are taken into account.</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</sect3>
</sect2>
</sect1>
<sect1 id="meta-transports">
<title>Meta Transports</title>
<para>
Meta transports are a kind of transport with special enhancements or
behavior. Currently available meta transports include: autolaunch
</para>
<sect2 id="meta-transports-autolaunch">
<title>Autolaunch</title>
<para>The autolaunch transport provides a way for dbus clients to autodetect
a running dbus session bus and to autolaunch a session bus if not present.
</para>
<para>
On Unix, <literal>autolaunch</literal> addresses are connectable,
but not listenable.
</para>
<para>
On Windows, <literal>autolaunch</literal> addresses are both
connectable and listenable.
</para>
<sect3 id="meta-transports-autolaunch-addresses">
<title>Server Address Format</title>
<para>
Autolaunch addresses uses the "autolaunch:" prefix and support the
following key/value pairs:
</para>
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Name</entry>
<entry>Values</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>scope</entry>
<entry>(string)</entry>
<entry>scope of autolaunch (Windows only)
<itemizedlist>
<listitem>
<para>
"*install-path" - limit session bus to dbus installation path.
The dbus installation path is determined from the location of
the shared dbus library. If the library is located in a 'bin'
subdirectory the installation root is the directory above,
otherwise the directory where the library lives is taken as
installation root.
<programlisting>
&lt;install-root&gt;/bin/[lib]dbus-1.dll
&lt;install-root&gt;/[lib]dbus-1.dll
</programlisting>
</para>
</listitem>
<listitem>
<para>
"*user" - limit session bus to the recent user.
</para>
</listitem>
<listitem>
<para>
other values - specify dedicated session bus like "release",
"debug" or other
</para>
</listitem>
</itemizedlist>
</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</sect3>
<sect3 id="meta-transports-autolaunch-windows-implementation">
<title>Windows implementation</title>
<para>
On start, the server opens a platform specific transport, creates a mutex
and a shared memory section containing the related session bus address.
This mutex will be inspected by the dbus client library to detect a
running dbus session bus. The access to the mutex and the shared memory
section are protected by global locks.
</para>
<para>
In the recent implementation the autolaunch transport uses a tcp transport
on localhost with a port choosen from the operating system. This detail may
change in the future.
</para>
<para>
Disclaimer: The recent implementation is in an early state and may not
work in all cirumstances and/or may have security issues. Because of this
the implementation is not documentated yet.
</para>
</sect3>
</sect2>
</sect1>
<sect1 id="uuids">
<title>UUIDs</title>
<para>
A working D-Bus implementation uses universally-unique IDs in two places.
First, each server address has a UUID identifying the address,
as described in <xref linkend="addresses"/>. Second, each operating
system kernel instance running a D-Bus client or server has a UUID
identifying that kernel, retrieved by invoking the method
org.freedesktop.DBus.Peer.GetMachineId() (see <xref
linkend="standard-interfaces-peer"/>).
</para>
<para>
The term "UUID" in this document is intended literally, i.e. an
identifier that is universally unique. It is not intended to refer to
RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
</para>
<para>
The UUID must contain 128 bits of data and be hex-encoded. The
hex-encoded string may not contain hyphens or other non-hex-digit
characters, and it must be exactly 32 characters long. To generate a
UUID, the current reference implementation concatenates 96 bits of random
data followed by the 32-bit time in seconds since the UNIX epoch (in big
endian byte order).
</para>
<para>
It would also be acceptable and probably better to simply generate 128
bits of random data, as long as the random number generator is of high
quality. The timestamp could conceivably help if the random bits are not
very random. With a quality random number generator, collisions are
extremely unlikely even with only 96 bits, so it's somewhat academic.
</para>
<para>
Implementations should, however, stick to random data for the first 96 bits
of the UUID.
</para>
</sect1>
<sect1 id="standard-interfaces">
<title>Standard Interfaces</title>
<para>
See <xref linkend="message-protocol-types-notation"/> for details on
the notation used in this section. There are some standard interfaces
that may be useful across various D-Bus applications.
</para>
<sect2 id="standard-interfaces-peer">
<title><literal>org.freedesktop.DBus.Peer</literal></title>
<para>
The <literal>org.freedesktop.DBus.Peer</literal> interface
has two methods:
<programlisting>
org.freedesktop.DBus.Peer.Ping ()
org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
</programlisting>
</para>
<para>
On receipt of the <literal>METHOD_CALL</literal> message
<literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
nothing other than reply with a <literal>METHOD_RETURN</literal> as
usual. It does not matter which object path a ping is sent to. The
reference implementation handles this method automatically.
</para>
<para>
On receipt of the <literal>METHOD_CALL</literal> message
<literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
UUID representing the identity of the machine the process is running on.
This UUID must be the same for all processes on a single system at least
until that system next reboots. It should be the same across reboots
if possible, but this is not always possible to implement and is not
guaranteed.
It does not matter which object path a GetMachineId is sent to. The
reference implementation handles this method automatically.
</para>
<para>
On Unix, implementations should try to read the machine ID from
<literal>/var/lib/dbus/machine-id</literal> and
<literal>/etc/machine-id</literal>. The latter is
<ulink url='https://www.freedesktop.org/software/systemd/man/machine-id.html'>
defined by systemd</ulink>, but systems not using systemd may provide
an equivalent file.
If both exist, they are expected to have the same contents, and if they
differ, the spec does not define which takes precedence (the reference
implementation prefers <literal>/var/lib/dbus/machine-id</literal>,
but sd-bus does not).
</para>
<para>
On Windows, the hardware profile GUID is used as the machine ID,
with the punctuation removed. This can be obtained with the
<ulink url='https://docs.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-getcurrenthwprofilea'>
<function>GetCurrentHwProfile</function></ulink> function.
</para>
<para>
The UUID is intended to be per-instance-of-the-operating-system, so may represent
a virtual machine running on a hypervisor, rather than a physical machine.
Basically if two processes see the same UUID, they should also see the same
shared memory, UNIX domain sockets, process IDs, and other features that require
a running OS kernel in common between the processes.
</para>
<para>
The UUID is often used where other programs might use a hostname. Hostnames
can change without rebooting, however, or just be "localhost" - so the UUID
is more robust.
</para>
<para>
<xref linkend="uuids"/> explains the format of the UUID.
</para>
</sect2>
<sect2 id="standard-interfaces-introspectable">
<title><literal>org.freedesktop.DBus.Introspectable</literal></title>
<para>
This interface has one method:
<programlisting>
org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
</programlisting>
</para>
<para>
Objects instances may implement
<literal>Introspect</literal> which returns an XML description of
the object, including its interfaces (with signals and methods), objects
below it in the object path tree, and its properties.
</para>
<para>
<xref linkend="introspection-format"/> describes the format of this XML string.
</para>
</sect2>
<sect2 id="standard-interfaces-properties">
<title><literal>org.freedesktop.DBus.Properties</literal></title>
<para>
Many native APIs will have a concept of object <firstterm>properties</firstterm>
or <firstterm>attributes</firstterm>. These can be exposed via the
<literal>org.freedesktop.DBus.Properties</literal> interface.
</para>
<para>
<programlisting>
org.freedesktop.DBus.Properties.Get (in STRING interface_name,
in STRING property_name,
out VARIANT value);
org.freedesktop.DBus.Properties.Set (in STRING interface_name,
in STRING property_name,
in VARIANT value);
org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
out ARRAY of DICT_ENTRY&lt;STRING,VARIANT&gt; props);
</programlisting>
</para>
<para>
It is conventional to give D-Bus properties names consisting of
capitalized words without punctuation ("CamelCase"), like
<link linkend="message-protocol-names-member">member names</link>.
For instance, the GObject property
<literal>connection-status</literal> or the Qt property
<literal>connectionStatus</literal> could be represented on D-Bus
as <literal>ConnectionStatus</literal>.
</para>
<para>
Strictly speaking, D-Bus property names are not required to follow
the same naming restrictions as member names, but D-Bus property
names that would not be valid member names (in particular,
GObject-style dash-separated property names) can cause interoperability
problems and should be avoided.
</para>
<para>
The available properties and whether they are writable can be determined
by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
see <xref linkend="standard-interfaces-introspectable"/>.
</para>
<para>
An empty string may be provided for the interface name; in this case,
if there are multiple properties on an object with the same name,
the results are undefined (picking one by according to an arbitrary
deterministic rule, or returning an error, are the reasonable
possibilities).
</para>
<para>
If <literal>org.freedesktop.DBus.Properties.GetAll</literal> is called
with a valid interface name which contains no properties, an empty array
should be returned. If it is called with a valid interface name for
which some properties are not accessible to the caller (for example, due
to per-property access control implemented in the service), those
properties should be silently omitted from the result array.
If <literal>org.freedesktop.DBus.Properties.Get</literal> is called for
any such properties, an appropriate access control error should be
returned.
</para>
<para>
If one or more properties change on an object, the
<literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
signal may be emitted (this signal was added in 0.14):
</para>
<para>
<programlisting>
org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
ARRAY of DICT_ENTRY&lt;STRING,VARIANT&gt; changed_properties,
ARRAY&lt;STRING&gt; invalidated_properties);
</programlisting>
</para>
<para>
where <literal>changed_properties</literal> is a dictionary
containing the changed properties with the new values and
<literal>invalidated_properties</literal> is an array of
properties that changed but the value is not conveyed.
</para>
<para>
Whether the <literal>PropertiesChanged</literal> signal is
supported can be determined by calling
<literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
that the signal may be supported for an object but it may
differ how whether and how it is used on a per-property basis
(for e.g. performance or security reasons). Each property (or
the parent interface) must be annotated with the
<literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
annotation to convey this (usually the default value
<literal>true</literal> is sufficient meaning that the
annotation does not need to be used). See <xref
linkend="introspection-format"/> for details on this
annotation.
</para>
</sect2>
<sect2 id="standard-interfaces-objectmanager">
<title><literal>org.freedesktop.DBus.ObjectManager</literal></title>
<para>
An API can optionally make use of this interface for one or
more sub-trees of objects. The root of each sub-tree implements
this interface so other applications can get all objects,
interfaces and properties in a single method call. It is
appropriate to use this interface if users of the tree of
objects are expected to be interested in all interfaces of all
objects in the tree; a more granular API should be used if
users of the objects are expected to be interested in a small
subset of the objects, a small subset of their interfaces, or
both.
</para>
<para>
The method that applications can use to get all objects and
properties is <literal>GetManagedObjects</literal>:
</para>
<para>
<programlisting>
org.freedesktop.DBus.ObjectManager.GetManagedObjects (out ARRAY of DICT_ENTRY&lt;OBJPATH,ARRAY of DICT_ENTRY&lt;STRING,ARRAY of DICT_ENTRY&lt;STRING,VARIANT&gt;&gt;&gt; objpath_interfaces_and_properties);
</programlisting>
</para>
<para>
The return value of this method is a dict whose keys are
object paths. All returned object paths are children of the
object path implementing this interface, i.e. their object
paths start with the ObjectManager's object path plus '/'.
</para>
<para>
Each value is a dict whose keys are interfaces names. Each
value in this inner dict is the same dict that would be
returned by the <link
linkend="standard-interfaces-properties">org.freedesktop.DBus.Properties.GetAll()</link>
method for that combination of object path and interface. If
an interface has no properties, the empty dict is returned.
</para>
<para>
Changes are emitted using the following two signals:
</para>
<para>
<programlisting>
org.freedesktop.DBus.ObjectManager.InterfacesAdded (OBJPATH object_path,
ARRAY of DICT_ENTRY&lt;STRING,ARRAY of DICT_ENTRY&lt;STRING,VARIANT&gt;&gt; interfaces_and_properties);
org.freedesktop.DBus.ObjectManager.InterfacesRemoved (OBJPATH object_path,
ARRAY&lt;STRING&gt; interfaces);
</programlisting>
</para>
<para>
The <literal>InterfacesAdded</literal> signal is emitted when
either a new object is added or when an existing object gains
one or more interfaces. The
<literal>InterfacesRemoved</literal> signal is emitted
whenever an object is removed or it loses one or more
interfaces. The second parameter of the
<literal>InterfacesAdded</literal> signal contains a dict with
the interfaces and properties (if any) that have been added to
the given object path. Similarly, the second parameter of the
<literal>InterfacesRemoved</literal> signal contains an array
of the interfaces that were removed. Note that changes on
properties on existing interfaces are not reported using this
interface - an application should also monitor the existing <link
linkend="standard-interfaces-properties">PropertiesChanged</link>
signal on each object.
</para>
<para>
Applications SHOULD NOT export objects that are children of an
object (directly or otherwise) implementing this interface but
which are not returned in the reply from the
<literal>GetManagedObjects()</literal> method of this
interface on the given object.
</para>
<para>
The intent of the <literal>ObjectManager</literal> interface
is to make it easy to write a robust client
implementation. The trivial client implementation only needs
to make two method calls:
</para>
<para>
<programlisting>
org.freedesktop.DBus.AddMatch (bus_proxy,
"type='signal',sender='org.example.App2',path_namespace='/org/example/App2'");
objects = org.freedesktop.DBus.ObjectManager.GetManagedObjects (app_proxy);
</programlisting>
</para>
<para>
on the message bus and the remote application's
<literal>ObjectManager</literal>, respectively. Whenever a new
remote object is created (or an existing object gains a new
interface), the <literal>InterfacesAdded</literal> signal is
emitted, and since this signal contains all properties for the
interfaces, no calls to the
<literal>org.freedesktop.Properties</literal> interface on the
remote object are needed. Additionally, since the initial
<literal>AddMatch()</literal> rule already includes signal
messages from the newly created child object, no new
<literal>AddMatch()</literal> call is needed.
</para>
<para>
<emphasis>
The <literal>org.freedesktop.DBus.ObjectManager</literal>
interface was added in version 0.17 of the D-Bus
specification.
</emphasis>
</para>
</sect2>
</sect1>
<sect1 id="introspection-format">
<title>Introspection Data Format</title>
<para>
As described in <xref linkend="standard-interfaces-introspectable"/>,
objects may be introspected at runtime, returning an XML string
that describes the object. The same XML format may be used in
other contexts as well, for example as an "IDL" for generating
static language bindings.
</para>
<para>
Here is an example of introspection data:
<programlisting>
&lt;!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
"http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd"&gt;
&lt;node name="/com/example/sample_object0"&gt;
&lt;interface name="com.example.SampleInterface0"&gt;
&lt;method name="Frobate"&gt;
&lt;arg name="foo" type="i" direction="in"/&gt;
&lt;arg name="bar" type="s" direction="out"/&gt;
&lt;arg name="baz" type="a{us}" direction="out"/&gt;
&lt;annotation name="org.freedesktop.DBus.Deprecated" value="true"/&gt;
&lt;/method&gt;
&lt;method name="Bazify"&gt;
&lt;arg name="bar" type="(iiu)" direction="in"/&gt;
&lt;arg name="bar" type="v" direction="out"/&gt;
&lt;/method&gt;
&lt;method name="Mogrify"&gt;
&lt;arg name="bar" type="(iiav)" direction="in"/&gt;
&lt;/method&gt;
&lt;signal name="Changed"&gt;
&lt;arg name="new_value" type="b"/&gt;
&lt;/signal&gt;
&lt;property name="Bar" type="y" access="readwrite"/&gt;
&lt;/interface&gt;
&lt;node name="child_of_sample_object"/&gt;
&lt;node name="another_child_of_sample_object"/&gt;
&lt;/node&gt;
</programlisting>
</para>
<para>
A more formal DTD and spec needs writing, but here are some quick notes.
<itemizedlist>
<listitem>
<para>
Only the root &lt;node&gt; element can omit the node name, as it's
known to be the object that was introspected. If the root
&lt;node&gt; does have a name attribute, it must be an absolute
object path. If child &lt;node&gt; have object paths, they must be
relative.
</para>
</listitem>
<listitem>
<para>
If a child &lt;node&gt; has any sub-elements, then they
must represent a complete introspection of the child.
If a child &lt;node&gt; is empty, then it may or may
not have sub-elements; the child must be introspected
in order to find out. The intent is that if an object
knows that its children are "fast" to introspect
it can go ahead and return their information, but
otherwise it can omit it.
</para>
</listitem>
<listitem>
<para>
The direction element on &lt;arg&gt; may be omitted,
in which case it defaults to "in" for method calls
and "out" for signals. Signals only allow "out"
so while direction may be specified, it's pointless.
</para>
</listitem>
<listitem>
<para>
The possible directions are "in" and "out",
unlike CORBA there is no "inout"
</para>
</listitem>
<listitem>
<para>
The possible property access flags are
"readwrite", "read", and "write"
</para>
</listitem>
<listitem>
<para>
Multiple interfaces can of course be listed for
one &lt;node&gt;.
</para>
</listitem>
<listitem>
<para>
The "name" attribute on arguments is optional.
</para>
</listitem>
</itemizedlist>
</para>
<para>
Method, interface, property, signal, and argument elements may have
"annotations", which are generic key/value pairs of metadata.
They are similar conceptually to Java's annotations and C# attributes.
Well-known annotations:
</para>
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Name</entry>
<entry>Values (separated by ,)</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>org.freedesktop.DBus.Deprecated</entry>
<entry>true,false</entry>
<entry>Whether or not the entity is deprecated; defaults to false</entry>
</row>
<row>
<entry>org.freedesktop.DBus.GLib.CSymbol</entry>
<entry>(string)</entry>
<entry>The C symbol; may be used for methods and interfaces</entry>
</row>
<row>
<entry>org.freedesktop.DBus.Method.NoReply</entry>
<entry>true,false</entry>
<entry>If set, don't expect a reply to the method call; defaults to false.</entry>
</row>
<row>
<entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
<entry>true,invalidates,const,false</entry>
<entry>
<para>
If set to <literal>false</literal>, the
<literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
signal, see <xref
linkend="standard-interfaces-properties"/> is not
guaranteed to be emitted if the property changes.
</para>
<para>
If set to <literal>const</literal> the property never
changes value during the lifetime of the object it
belongs to, and hence the signal is never emitted for
it.
</para>
<para>
If set to <literal>invalidates</literal> the signal
is emitted but the value is not included in the
signal.
</para>
<para>
If set to <literal>true</literal> the signal is
emitted with the value included.
</para>
<para>
The value for the annotation defaults to
<literal>true</literal> if the enclosing interface
element does not specify the annotation. Otherwise it
defaults to the value specified in the enclosing
interface element.
</para>
<para>
This annotation is intended to be used by code
generators to implement client-side caching of
property values. For all properties for which the
annotation is set to <literal>const</literal>,
<literal>invalidates</literal> or
<literal>true</literal> the client may
unconditionally cache the values as the properties
don't change or notifications are generated for them
if they do.
</para>
</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</sect1>
<sect1 id="message-bus">
<title>Message Bus Specification</title>
<sect2 id="message-bus-overview">
<title>Message Bus Overview</title>
<para>
The message bus accepts connections from one or more applications.
Once connected, applications can exchange messages with other
applications that are also connected to the bus.
</para>
<para>
In order to route messages among connections, the message bus keeps a
mapping from names to connections. Each connection has one
unique-for-the-lifetime-of-the-bus name automatically assigned.
Applications may request additional names for a connection. Additional
names are usually "well-known names" such as
"com.example.TextEditor1". When a name is bound to a connection,
that connection is said to <firstterm>own</firstterm> the name.
</para>
<para>
The bus itself owns a special name,
<literal>org.freedesktop.DBus</literal>, with an object
located at <literal>/org/freedesktop/DBus</literal> that
implements the <literal>org.freedesktop.DBus</literal>
interface. This service allows applications to make
administrative requests of the bus itself. For example,
applications can ask the bus to assign a name to a connection.
</para>
<para>
Each name may have <firstterm>queued owners</firstterm>. When an
application requests a name for a connection and the name is already in
use, the bus will optionally add the connection to a queue waiting for
the name. If the current owner of the name disconnects or releases
the name, the next connection in the queue will become the new owner.
</para>
<para>
This feature causes the right thing to happen if you start two text
editors for example; the first one may request "com.example.TextEditor1",
and the second will be queued as a possible owner of that name. When
the first exits, the second will take over.
</para>
<para>
Applications may send <firstterm>unicast messages</firstterm> to
a specific recipient or to the message bus itself, or
<firstterm>broadcast messages</firstterm> to all interested recipients.
See <xref linkend="message-bus-routing"/> for details.
</para>
</sect2>
<sect2 id="message-bus-names">
<title>Message Bus Names</title>
<para>
Each connection has at least one name, assigned at connection time and
returned in response to the
<literal>org.freedesktop.DBus.Hello</literal> method call. This
automatically-assigned name is called the connection's <firstterm>unique
name</firstterm>. Unique names are never reused for two different
connections to the same bus.
</para>
<para>
Ownership of a unique name is a prerequisite for interaction with
the message bus. It logically follows that the unique name is always
the first name that an application comes to own, and the last
one that it loses ownership of.
</para>
<para>
Unique connection names must begin with the character ':' (ASCII colon
character); bus names that are not unique names must not begin
with this character. (The bus must reject any attempt by an application
to manually request a name beginning with ':'.) This restriction
categorically prevents "spoofing"; messages sent to a unique name
will always go to the expected connection.
</para>
<para>
When a connection is closed, all the names that it owns are deleted (or
transferred to the next connection in the queue if any).
</para>
<para>
A connection can request additional names to be associated with it using
the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
linkend="message-protocol-names-bus"/> describes the format of a valid
name. These names can be released again using the
<literal>org.freedesktop.DBus.ReleaseName</literal> message.
</para>
</sect2>
<sect2 id="message-bus-routing">
<title>Message Bus Message Routing</title>
<para>
Messages may have a <literal>DESTINATION</literal> field (see <xref
linkend="message-protocol-header-fields"/>), resulting in a
<firstterm>unicast message</firstterm>. If the
<literal>DESTINATION</literal> field is present, it specifies a message
recipient by name. Method calls and replies normally specify this field.
The message bus must send messages (of any type) with the
<literal>DESTINATION</literal> field set to the specified recipient,
regardless of whether the recipient has set up a match rule matching
the message.
</para>
<para>
When the message bus receives a signal, if the
<literal>DESTINATION</literal> field is absent, it is considered to
be a <firstterm>broadcast signal</firstterm>, and is sent to all
applications with <firstterm>message matching rules</firstterm> that
match the message. Most signal messages are broadcasts, and
no other message types currently defined in this specification
may be broadcast.
</para>
<para>
Unicast signal messages (those with a <literal>DESTINATION</literal>
field) are not commonly used, but they are treated like any unicast
message: they are delivered to the specified receipient,
regardless of its match rules. One use for unicast signals is to
avoid a race condition in which a signal is emitted before the intended
recipient can call <xref linkend="bus-messages-add-match"/> to
receive that signal: if the signal is sent directly to that recipient
using a unicast message, it does not need to add a match rule at all,
and there is no race condition. Another use for unicast signals,
on message buses whose security policy prevents eavesdropping, is to
send sensitive information which should only be visible to one
recipient.
</para>
<para>
When the message bus receives a method call, if the
<literal>DESTINATION</literal> field is absent, the call is taken to be
a standard one-to-one message and interpreted by the message bus
itself. For example, sending an
<literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
<literal>DESTINATION</literal> will cause the message bus itself to
reply to the ping immediately; the message bus will not make this
message visible to other applications.
</para>
<para>
Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
the ping message were sent with a <literal>DESTINATION</literal> name of
<literal>com.yoyodyne.Screensaver</literal>, then the ping would be
forwarded, and the Yoyodyne Corporation screensaver application would be
expected to reply to the ping.
</para>
<para>
Message bus implementations may impose a security policy which
prevents certain messages from being sent or received.
When a method call message cannot be sent or received due to a security
policy, the message bus should send an error reply, unless the
original message had the <literal>NO_REPLY</literal> flag.
</para>
<sect3 id="message-bus-routing-eavesdropping">
<title>Eavesdropping</title>
<para>
Receiving a unicast message whose <literal>DESTINATION</literal>
indicates a different recipient is called
<firstterm>eavesdropping</firstterm>. On a message bus which acts as
a security boundary (like the standard system bus), the security
policy should usually prevent eavesdropping, since unicast messages
are normally kept private and may contain security-sensitive
information.
</para>
<para>
Eavesdropping interacts poorly with buses with non-trivial
access control restrictions, and is deprecated. The
<literal>BecomeMonitor</literal> method (see
<xref linkend="bus-messages-become-monitor"/>) provides
a preferable way to monitor buses.
</para>
<para>
Eavesdropping is mainly useful for debugging tools, such as
the <literal>dbus-monitor</literal> tool in the reference
implementation of D-Bus. Tools which eavesdrop on the message bus
should be careful to avoid sending a reply or error in response to
messages intended for a different client.
</para>
<para>
Clients may attempt to eavesdrop by adding match rules
(see <xref linkend="message-bus-routing-match-rules"/>) containing
the <literal>eavesdrop='true'</literal> match. For
compatibility with older message bus implementations, if adding such
a match rule results in an error reply, the client may fall back to
adding the same rule with the <literal>eavesdrop</literal> match
omitted.
</para>
</sect3>
<sect3 id="message-bus-routing-match-rules">
<title>Match Rules</title>
<para>
An important part of the message bus routing protocol is match
rules. Match rules describe the messages that should be sent to a
client, based on the contents of the message. Broadcast signals
are only sent to clients which have a suitable match rule: this
avoids waking up client processes to deal with signals that are
not relevant to that client.
</para>
<para>
Messages that list a client as their <literal>DESTINATION</literal>
do not need to match the client's match rules, and are sent to that
client regardless. As a result, match rules are mainly used to
receive a subset of broadcast signals.
</para>
<para>
Match rules can also be used for eavesdropping
(see <xref linkend="message-bus-routing-eavesdropping"/>),
if the security policy of the message bus allows it, but this
usage is deprecated in favour of the <literal>BecomeMonitor</literal>
method (see <xref linkend="bus-messages-become-monitor"/>).
</para>
<para>
Match rules are added using the AddMatch bus method
(see <xref linkend="bus-messages-add-match"/>). Rules are
specified as a string of comma separated key/value pairs.
Excluding a key from the rule indicates a wildcard match.
For instance excluding the the member from a match rule but
adding a sender would let all messages from that sender through.
An example of a complete rule would be
"type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
</para>
<para>
Within single quotes (ASCII apostrophe, U+0027), a backslash
(U+005C) represents itself, and an apostrophe ends the quoted
section. Outside single quotes, \' (backslash, apostrophe)
represents an apostrophe, and any backslash not followed by
an apostrophe represents itself. For instance, the match rules
<literal>arg0=''\''',arg1='\',arg2=',',arg3='\\'</literal> and
<literal>arg0=\',arg1=\,arg2=',',arg3=\\</literal>
both match messages where the arguments are a 1-character string
containing an apostrophe, a 1-character string containing a
backslash, a 1-character string containing a comma, and a
2-character string containing two backslashes<footnote>
<para>
This idiosyncratic quoting style is based on the rules for
escaping items to appear inside single-quoted strings
in POSIX <literal>/bin/sh</literal>, but please
note that backslashes that are not inside single quotes have
different behaviour. This syntax does not offer any way to
represent an apostrophe inside single quotes (it is necessary
to leave the single-quoted section, backslash-escape the
apostrophe and re-enter single quotes), or to represent a
comma outside single quotes (it is necessary to wrap it in
a single-quoted section).
</para>
</footnote>.
</para>
<para>
The following table describes the keys that can be used to create
a match rule.
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Key</entry>
<entry>Possible Values</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>type</literal></entry>
<entry>'signal', 'method_call', 'method_return', 'error'</entry>
<entry>Match on the message type. An example of a type match is type='signal'</entry>
</row>
<row>
<entry><literal>sender</literal></entry>
<entry>A bus or unique name (see <xref linkend="term-bus-name"/>
and <xref linkend="term-unique-name"/> respectively)
</entry>
<entry>Match messages sent by a particular sender. An example of a sender match
is sender='org.freedesktop.Hal'</entry>
</row>
<row>
<entry><literal>interface</literal></entry>
<entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
<entry>Match messages sent over or to a particular interface. An example of an
interface match is interface='org.freedesktop.Hal.Manager'.
If a message omits the interface header, it must not match any rule
that specifies this key.</entry>
</row>
<row>
<entry><literal>member</literal></entry>
<entry>Any valid method or signal name</entry>
<entry>Matches messages which have the give method or signal name. An example of
a member match is member='NameOwnerChanged'</entry>
</row>
<row>
<entry><literal>path</literal></entry>
<entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
<entry>Matches messages which are sent from or to the given object. An example of a
path match is path='/org/freedesktop/Hal/Manager'</entry>
</row>
<row>
<entry><literal>path_namespace</literal></entry>
<entry>An object path</entry>
<entry>
<para>
Matches messages which are sent from or to an
object for which the object path is either the
given value, or that value followed by one or
more path components.
</para>
<para>
For example,
<literal>path_namespace='/com/example/foo'</literal>
would match signals sent by
<literal>/com/example/foo</literal>
or by
<literal>/com/example/foo/bar</literal>,
but not by
<literal>/com/example/foobar</literal>.
</para>
<para>
Using both <literal>path</literal> and
<literal>path_namespace</literal> in the same match
rule is not allowed.
</para>
<para>
<emphasis>
This match key was added in version 0.16 of the
D-Bus specification and implemented by the bus
daemon in dbus 1.5.0 and later.
</emphasis>
</para>
</entry>
</row>
<row>
<entry><literal>destination</literal></entry>
<entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
<entry>Matches messages which are being sent to the given unique name. An
example of a destination match is destination=':1.0'</entry>
</row>
<row>
<entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
<entry>Any string</entry>
<entry>Arg matches are special and are used for further restricting the
match based on the arguments in the body of a message. Only arguments of type
STRING can be matched in this way. An example of an argument match
would be arg3='Foo'. Only argument indexes from 0 to 63 should be
accepted.</entry>
</row>
<row>
<entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
<entry>Any string</entry>
<entry>
<para>Argument path matches provide a specialised form of wildcard matching for
path-like namespaces. They can match arguments whose type is either STRING or
OBJECT_PATH. As with normal argument matches,
if the argument is exactly equal to the string given in the match
rule then the rule is satisfied. Additionally, there is also a
match when either the string given in the match rule or the
appropriate message argument ends with '/' and is a prefix of the
other. An example argument path match is arg0path='/aa/bb/'. This
would match messages with first arguments of '/', '/aa/',
'/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>
<para>This is intended for monitoring “directories” in file system-like
hierarchies, as used in the <citetitle>dconf</citetitle> configuration
system. An application interested in all nodes in a particular hierarchy would
monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
represent a modification to the “bar” property, or a signal with zeroth
argument <literal>"/ca/example/"</literal> to represent atomic modification of
many properties within that directory, and the interested application would be
notified in both cases.</para>
<para>
<emphasis>
This match key was added in version 0.12 of the
D-Bus specification, implemented for STRING
arguments by the bus daemon in dbus 1.2.0 and later,
and implemented for OBJECT_PATH arguments in dbus 1.5.0
and later.
</emphasis>
</para>
</entry>
</row>
<row>
<entry><literal>arg0namespace</literal></entry>
<entry>Like a bus name, except that the string is not
required to contain a '.' (period)</entry>
<entry>
<para>Match messages whose first argument is of type STRING, and is a bus name
or interface name within the specified namespace. This is primarily intended
for watching name owner changes for a group of related bus names, rather than
for a single name or all name changes.</para>
<para>Because every valid interface name is also a valid
bus name, this can also be used for messages whose
first argument is an interface name.</para>
<para>For example, the match rule
<literal>member='NameOwnerChanged',arg0namespace='com.example.backend1'</literal>
matches name owner changes for bus names such as
<literal>com.example.backend1.foo</literal>,
<literal>com.example.backend1.foo.bar</literal>, and
<literal>com.example.backend1</literal> itself.</para>
<para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para>
<para>
<emphasis>
This match key was added in version 0.16 of the
D-Bus specification and implemented by the bus
daemon in dbus 1.5.0 and later.
</emphasis>
</para>
</entry>
</row>
<row>
<entry><literal>eavesdrop</literal></entry>
<entry><literal>'true'</literal>, <literal>'false'</literal></entry>
<entry>
<para>
Since D-Bus 1.5.6, match rules do not
match messages which have a <literal>DESTINATION</literal>
field unless the match rule specifically
requests this
(see <xref linkend="message-bus-routing-eavesdropping"/>)
by specifying <literal>eavesdrop='true'</literal>
in the match rule. <literal>eavesdrop='false'</literal>
restores the default behaviour. Messages are
delivered to their <literal>DESTINATION</literal>
regardless of match rules, so this match does not
affect normal delivery of unicast messages.
In older versions of D-Bus, this match was not allowed
in match rules, and all match rules behaved as if
<literal>eavesdrop='true'</literal> had been used.
</para>
<para>
Use of <literal>eavesdrop='true'</literal> is
deprecated. Monitors should prefer to use the
<literal>BecomeMonitor</literal> method (see
<xref linkend="bus-messages-become-monitor"/>),
which was introduced in version 0.26 of the D-Bus
specification and version 1.9.10 of the reference
dbus-daemon.
</para>
<para>
Message bus implementations may restrict match rules
with <literal>eavesdrop='true'</literal> so that they
can only be added by privileged connections.
</para>
<para>
<emphasis>
This match key was added in version 0.18 of the
D-Bus specification and implemented by the bus
daemon in dbus 1.5.6 and later.
</emphasis>
</para>
</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
</sect3>
</sect2>
<sect2 id="message-bus-starting-services">
<title>Message Bus Starting Services (Activation)</title>
<para>
The message bus can start applications on behalf of other applications.
This is referred to as <firstterm>service activation</firstterm> or
<firstterm>activation</firstterm>.
An application that can be started in this way is called a
<firstterm>service</firstterm> or an
<firstterm>activatable service</firstterm>.
</para>
<para>
<firstterm>Starting a service</firstterm> should be read as synonymous
with service activation.
</para>
<para>
In D-Bus, service activation is normally done by
<firstterm>auto-starting</firstterm>.
In auto-starting, applications send a
message to a particular well-known name, such as
<literal>com.example.TextEditor1</literal>, without specifying the
<literal>NO_AUTO_START</literal> flag in the message header.
If no application on the bus owns the requested name, but the bus
daemon does know how to start an activatable service for that name,
then the bus daemon will start that service, wait for it to request
that name, and deliver the message to it.
</para>
<para>
It is also possible for applications to send an explicit request to
start a service: this is another form of activation, distinct from
auto-starting. See
<xref linkend="bus-messages-start-service-by-name"/> for details.
</para>
<para>
In either case, this implies a contract documented along with the name
<literal>com.example.TextEditor1</literal> for which object
the owner of that name will provide, and what interfaces those
objects will have.
</para>
<para>
To find an executable corresponding to a particular name, the bus daemon
looks for <firstterm>service description files</firstterm>. Service
description files define a mapping from names to executables. Different
kinds of message bus will look for these files in different places, see
<xref linkend="message-bus-types"/>.
</para>
<para>
Service description files have the ".service" file
extension. The message bus will only load service description files
ending with .service; all other files will be ignored. The file format
is similar to that of <ulink
url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
entries</ulink>. All service description files must be in UTF-8
encoding. To ensure that there will be no name collisions, service files
must be namespaced using the same mechanism as messages and service
names.
</para>
<para>
On the well-known system bus, the name of a service description file
must be its well-known name plus <literal>.service</literal>,
for instance
<literal>com.example.ConfigurationDatabase1.service</literal>.
</para>
<para>
On the well-known session bus, services should follow the same
service description file naming convention as on the system bus,
but for backwards compatibility they are not required to do so.
</para>
<para>
[FIXME the file format should be much better specified than "similar to
.desktop entries" esp. since desktop entries are already
badly-specified. ;-)]
These sections from the specification apply to service files as well:
<itemizedlist>
<listitem><para>General syntax</para></listitem>
<listitem><para>Comment format</para></listitem>
</itemizedlist>
Service description files must contain a
<literal>D-BUS Service</literal> group with at least the keys
<literal>Name</literal> (the well-known name of the service)
and <literal>Exec</literal> (the command to be executed).
<figure>
<title>Example service description file</title>
<programlisting>
# Sample service description file
[D-BUS Service]
Name=com.example.ConfigurationDatabase1
Exec=/usr/bin/sample-configd
</programlisting>
</figure>
</para>
<para>
Additionally, service description files for the well-known system
bus on Unix must contain a <literal>User</literal> key, whose value
is the name of a user account (e.g. <literal>root</literal>).
The system service will be run as that user.
</para>
<para>
When an application asks to start a service by name, the bus daemon tries to
find a service that will own that name. It then tries to spawn the
executable associated with it. If this fails, it will report an
error.
</para>
<para>
On the well-known system bus, it is not possible for two .service files
in the same directory to offer the same service, because they are
constrained to have names that match the service name.
</para>
<para>
On the well-known session bus, if two .service files in the same
directory offer the same service name, the result is undefined.
Distributors should avoid this situation, for instance by naming
session services' .service files according to their service name.
</para>
<para>
If two .service files in different directories offer the same
service name, the one in the higher-priority directory is used:
for instance, on the system bus, .service files in
/usr/local/share/dbus-1/system-services take precedence over those
in /usr/share/dbus-1/system-services.
</para>
<para>
The executable launched will have the environment variable
<literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
message bus so it can connect and request the appropriate names.
</para>
<para>
The executable being launched may want to know whether the message bus
starting it is one of the well-known message buses (see <xref
linkend="message-bus-types"/>). To facilitate this, the bus must also set
the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
of the well-known buses. The currently-defined values for this variable
are <literal>system</literal> for the systemwide message bus,
and <literal>session</literal> for the per-login-session message
bus. The new executable must still connect to the address given
in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
resulting connection is to the well-known bus.
</para>
<para>
[FIXME there should be a timeout somewhere, either specified
in the .service file, by the client, or just a global value
and if the client being activated fails to connect within that
timeout, an error should be sent back.]
</para>
<sect3 id="message-bus-starting-services-scope">
<title>Message Bus Service Scope</title>
<para>
The "scope" of a service is its "per-", such as per-session,
per-machine, per-home-directory, or per-display. The reference
implementation doesn't yet support starting services in a different
scope from the message bus itself. So e.g. if you start a service
on the session bus its scope is per-session.
</para>
<para>
We could add an optional scope to a bus name. For example, for
per-(display,session pair), we could have a unique ID for each display
generated automatically at login and set on screen 0 by executing a
special "set display ID" binary. The ID would be stored in a
<literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
random bytes. This ID would then be used to scope names.
Starting/locating a service could be done by ID-name pair rather than
only by name.
</para>
<para>
Contrast this with a per-display scope. To achieve that, we would
want a single bus spanning all sessions using a given display.
So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
property on screen 0 of the display, pointing to this bus.
</para>
</sect3>
<sect3 id="message-bus-starting-services-systemd">
<title>systemd Activation</title>
<para>
Service description files may contain a
<literal>SystemdService</literal> key. Its value is the name of a
<ulink
url="https://www.freedesktop.org/wiki/Software/systemd/">systemd</ulink>
service, for example
<literal>dbus-com.example.MyDaemon.service</literal>.
</para>
<para>
If this key is present, the bus daemon may carry out activation for
this D-Bus service by sending a request to systemd asking it to
start the systemd service whose name is the value of
<literal>SystemdService</literal>. For example, the reference
<literal>dbus-daemon</literal> has a
<literal>--systemd-activation</literal> option that enables this
feature, and that option is given when it is started by systemd.
</para>
<para>
On the well-known system bus, it is a common practice to set
<literal>SystemdService</literal> to <literal>dbus-</literal>,
followed by the well-known bus name, followed by
<literal>.service</literal>, then register that name as an alias
for the real systemd service. This allows D-Bus activation of a
service to be enabled or disabled independently of whether the
service is started by systemd during boot.
</para>
</sect3>
<sect3 id="message-bus-starting-services-apparmor">
<title>Mediating Activation with AppArmor</title>
<para>
Please refer to
<ulink url="http://wiki.apparmor.net/index.php/Documentation">AppArmor documentation</ulink>
for general information on AppArmor, and how it mediates D-Bus
messages when used in conjunction with a kernel and
<literal>dbus-daemon</literal> that support this.
</para>
<para>
In recent versions of the reference <literal>dbus-daemon</literal>,
AppArmor policy rules of type <literal>dbus send</literal>
are also used to control auto-starting: if a message is sent to
the well-known name of an activatable service, the
<literal>dbus-daemon</literal> will attempt to determine whether
it would deliver the message to that service
<emphasis>before</emphasis>auto-starting it, by making some
assumptions about the resulting process's credentials.
</para>
<para>
If it does proceed with auto-starting, when the service appears, the
<literal>dbus-daemon</literal> repeats the policy check (with
the service's true credentials, which might not be identical)
before delivering the message. In practice, this second check will
usually be more strict than the first; the first check would only
be more strict if there are "blacklist"-style rules like
<literal>deny dbus send peer=(label=/usr/bin/protected)</literal>
that match on the peer's specific credentials, but AppArmor is
normally used in a "whitelist" style where this does not apply.
</para>
<para>
To support this process, service description files may contain a
<literal>AssumedAppArmorLabel</literal> key. Its value is the name
of an AppArmor label, for example
<literal>/usr/sbin/mydaemon</literal>.
If present, AppArmor mediation of messages that auto-start a
service will decide whether to allow auto-starting to occur based
on the assumption that the activated service will be confined
under the specified label; in particular, rules of the form
<literal>dbus send peer=(label=/usr/sbin/mydaemon)</literal> or
<literal>deny dbus send peer=(label=/usr/sbin/mydaemon)</literal>
will match it, allowing or denying as appropriate
(even if there is in fact no profile of that name loaded).
</para>
<para>
Otherwise, AppArmor mediation of messages that auto-start a
service will decide whether to allow auto-starting to occur
without specifying any particular label. In particular, any rule of
the form <literal>dbus send peer=(label=X)</literal> or
<literal>deny dbus send peer=(label=X)</literal>
(for any value of X, including the special label
<literal>unconfined</literal>) will not influence whether the
auto-start is allowed.
</para>
<para>
Rules of type <literal>dbus receive</literal> are not checked
when deciding whether to allow auto-starting; they are only checked
against the service's profile after the service has started, when
deciding whether to deliver the message that caused the auto-starting
operation.
</para>
<para>
Explicit activation via
<xref linkend="bus-messages-start-service-by-name"/> is not currently
affected by this mediation: if a confined process is to be prevented
from starting arbitrary services, then it must not be allowed to call
that method.
</para>
</sect3>
</sect2>
<sect2 id="message-bus-types">
<title>Well-known Message Bus Instances</title>
<para>
Two standard message bus instances are defined here, along with how
to locate them and where their service files live.
</para>
<sect3 id="message-bus-types-login">
<title>Login session message bus</title>
<para>
Each time a user logs in, a <firstterm>login session message
bus</firstterm> may be started. All applications in the user's login
session may interact with one another using this message bus.
</para>
<para>
The address of the login session message bus is given
in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
variable. If that variable is not set, applications may
also try to read the address from the X Window System root
window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
The root window property must have type <literal>STRING</literal>.
The environment variable should have precedence over the
root window property.
</para>
<para>The address of the login session message bus is given in the
<literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
"autolaunch:", the system should use platform-specific methods of
locating a running D-Bus session server, or starting one if a running
instance cannot be found. Note that this mechanism is not recommended
for attempting to determine if a daemon is running. It is inherently
racy to attempt to make this determination, since the bus daemon may
be started just before or just after the determination is made.
Therefore, it is recommended that applications do not try to make this
determination for their functionality purposes, and instead they
should attempt to start the server.</para>
<sect4 id="message-bus-types-login-x-windows">
<title>X Windowing System</title>
<para>
For the X Windowing System, the application must locate the
window owner of the selection represented by the atom formed by
concatenating:
<itemizedlist>
<listitem>
<para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
</listitem>
<listitem>
<para>the current user's username</para>
</listitem>
<listitem>
<para>the literal character '_' (underscore)</para>
</listitem>
<listitem>
<para>the machine's ID</para>
</listitem>
</itemizedlist>
</para>
<para>
The following properties are defined for the window that owns
this X selection:
<informaltable frame="all">
<tgroup cols="2">
<tbody>
<row>
<entry>
<para>Atom</para>
</entry>
<entry>
<para>meaning</para>
</entry>
</row>
<row>
<entry>
<para>_DBUS_SESSION_BUS_ADDRESS</para>
</entry>
<entry>
<para>the actual address of the server socket</para>
</entry>
</row>
<row>
<entry>
<para>_DBUS_SESSION_BUS_PID</para>
</entry>
<entry>
<para>the PID of the server process</para>
</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
present in this window.
</para>
<para>
If the X selection cannot be located or if reading the
properties from the window fails, the implementation MUST conclude
that there is no D-Bus server running and proceed to start a new
server. (See below on concurrency issues)
</para>
<para>
Failure to connect to the D-Bus server address thus obtained
MUST be treated as a fatal connection error and should be reported
to the application.
</para>
<para>
As an alternative, an implementation MAY find the information
in the following file located in the current user's home directory,
in subdirectory .dbus/session-bus/:
<itemizedlist>
<listitem>
<para>the machine's ID</para>
</listitem>
<listitem>
<para>the literal character '-' (dash)</para>
</listitem>
<listitem>
<para>the X display without the screen number, with the
following prefixes removed, if present: ":", "localhost:"
."localhost.localdomain:". That is, a display of
"localhost:10.0" produces just the number "10"</para>
</listitem>
</itemizedlist>
</para>
<para>
The contents of this file NAME=value assignment pairs and
lines starting with # are comments (no comments are allowed
otherwise). The following variable names are defined:
<informaltable
frame="all">
<tgroup cols="2">
<tbody>
<row>
<entry>
<para>Variable</para>
</entry>
<entry>
<para>meaning</para>
</entry>
</row>
<row>
<entry>
<para>DBUS_SESSION_BUS_ADDRESS</para>
</entry>
<entry>
<para>the actual address of the server socket</para>
</entry>
</row>
<row>
<entry>
<para>DBUS_SESSION_BUS_PID</para>
</entry>
<entry>
<para>the PID of the server process</para>
</entry>
</row>
<row>
<entry>
<para>DBUS_SESSION_BUS_WINDOWID</para>
</entry>
<entry>
<para>the window ID</para>
</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
in this file.
</para>
<para>
Failure to open this file MUST be interpreted as absence of a
running server. Therefore, the implementation MUST proceed to
attempting to launch a new bus server if the file cannot be
opened.
</para>
<para>
However, success in opening this file MUST NOT lead to the
conclusion that the server is running. Thus, a failure to connect to
the bus address obtained by the alternative method MUST NOT be
considered a fatal error. If the connection cannot be established,
the implementation MUST proceed to check the X selection settings or
to start the server on its own.
</para>
<para>
If the implementation concludes that the D-Bus server is not
running it MUST attempt to start a new server and it MUST also
ensure that the daemon started as an effect of the "autolaunch"
mechanism provides the lookup mechanisms described above, so
subsequent calls can locate the newly started server. The
implementation MUST also ensure that if two or more concurrent
initiations happen, only one server remains running and all other
initiations are able to obtain the address of this server and
connect to it. In other words, the implementation MUST ensure that
the X selection is not present when it attempts to set it, without
allowing another process to set the selection between the
verification and the setting (e.g., by using XGrabServer /
XungrabServer).
</para>
</sect4>
<sect4>
<title>Finding session services</title>
<para>
On Unix systems, the session bus should search for .service files
in <literal>$XDG_DATA_DIRS/dbus-1/services</literal> as defined
by the
<ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG Base Directory Specification</ulink>.
Implementations may also search additional locations,
with a higher or lower priority than the XDG directories.
</para>
<para>
As described in the XDG Base Directory Specification, software
packages should install their session .service files to their
configured <literal>${datadir}/dbus-1/services</literal>,
where <literal>${datadir}</literal> is as defined by the GNU
coding standards. System administrators or users can arrange
for these service files to be read by setting XDG_DATA_DIRS or by
symlinking them into the default locations.
</para>
</sect4>
</sect3>
<sect3 id="message-bus-types-system">
<title>System message bus</title>
<para>
A computer may have a <firstterm>system message bus</firstterm>,
accessible to all applications on the system. This message bus may be
used to broadcast system events, such as adding new hardware devices,
changes in the printer queue, and so forth.
</para>
<para>
The address of the system message bus is given
in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
variable. If that variable is not set, applications should try
to connect to the well-known address
<literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
<footnote>
<para>
The D-Bus reference implementation actually honors the
<literal>$(localstatedir)</literal> configure option
for this address, on both client and server side.
</para>
</footnote>
</para>
<para>
On Unix systems, the system bus should default to searching
for .service files in
<literal>/usr/local/share/dbus-1/system-services</literal>,
<literal>/usr/share/dbus-1/system-services</literal> and
<literal>/lib/dbus-1/system-services</literal>, with that order
of precedence. It may also search other implementation-specific
locations, but should not vary these locations based on environment
variables.
<footnote>
<para>
The system bus is security-sensitive and is typically executed
by an init system with a clean environment. Its launch helper
process is particularly security-sensitive, and specifically
clears its own environment.
</para>
</footnote>
</para>
<para>
Software packages should install their system .service
files to their configured
<literal>${datadir}/dbus-1/system-services</literal>,
where <literal>${datadir}</literal> is as defined by the GNU
coding standards. System administrators can arrange
for these service files to be read by editing the system bus'
configuration file or by symlinking them into the default
locations.
</para>
</sect3>
</sect2>
<sect2 id="message-bus-messages">
<title>Message Bus Messages</title>
<para>
The special message bus name <literal>org.freedesktop.DBus</literal>
responds to a number of additional messages at the object path
<literal>/org/freedesktop/DBus</literal>.
That object path is also used when emitting the
<xref linkend='bus-messages-name-owner-changed'/> signal.
</para>
<para>
For historical reasons, some of the methods in the
<literal>org.freedesktop.DBus</literal> interface are available
on multiple object paths. Message bus implementations should
accept method calls that were added before specification version
0.26 on any object path. Message bus implementations should
not accept newer method calls on unexpected object paths,
and as a security hardening measure, older method calls
that are security-sensitive may be rejected with the error
<literal>org.freedesktop.DBus.Error.AccessDenied</literal> when
called on an unexpected object path. Client software should send
all method calls to <literal>/org/freedesktop/DBus</literal>
instead of relying on this.
</para>
<para>
In addition to the method calls listed below, the message bus
should implement the standard Introspectable, Properties and Peer
interfaces (see <xref linkend="standard-interfaces"/>).
Support for the Properties and Peer interfaces was added in version
1.11.x of the reference implementation of the message bus.
</para>
<sect3 id="bus-messages-hello">
<title><literal>org.freedesktop.DBus.Hello</literal></title>
<para>
As a method:
<programlisting>
STRING Hello ()
</programlisting>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Unique name assigned to the connection</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
Before an application is able to send messages to other applications
it must send the <literal>org.freedesktop.DBus.Hello</literal> message
to the message bus to obtain a unique name. If an application without
a unique name tries to send a message to another application, or a
message to the message bus itself that isn't the
<literal>org.freedesktop.DBus.Hello</literal> message, it will be
disconnected from the bus.
</para>
<para>
There is no corresponding "disconnect" request; if a client wishes to
disconnect from the bus, it simply closes the socket (or other
communication channel).
</para>
</sect3>
<sect3 id="bus-messages-request-name">
<title><literal>org.freedesktop.DBus.RequestName</literal></title>
<para>
As a method:
<programlisting>
UINT32 RequestName (in STRING name, in UINT32 flags)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Name to request</entry>
</row>
<row>
<entry>1</entry>
<entry>UINT32</entry>
<entry>Flags</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>UINT32</entry>
<entry>Return value</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
Ask the message bus to assign the given name to the method caller. Each
name maintains a queue of possible owners, where the head of the queue is
the primary or current owner of the name. Each potential owner in the
queue maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName call.
When RequestName is invoked the following occurs:
<itemizedlist>
<listitem>
<para>
If the method caller is currently the primary owner of the name,
the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
values are updated with the values from the new RequestName call,
and nothing further happens.
</para>
</listitem>
<listitem>
<para>
If the current primary owner (head of the queue) has
DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
the caller of RequestName replaces the current primary owner at
the head of the queue and the current primary owner moves to the
second position in the queue. If the caller of RequestName was
in the queue previously its flags are updated with the values from
the new RequestName in addition to moving it to the head of the queue.
</para>
</listitem>
<listitem>
<para>
If replacement is not possible, and the method caller is
currently in the queue but not the primary owner, its flags are
updated with the values from the new RequestName call.
</para>
</listitem>
<listitem>
<para>
If replacement is not possible, and the method caller is
currently not in the queue, the method caller is appended to the
queue.
</para>
</listitem>
<listitem>
<para>
If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
set and is not the primary owner, it is removed from the
queue. This can apply to the previous primary owner (if it
was replaced) or the method caller (if it updated the
DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
queue, or if it was just added to the queue with that flag set).
</para>
</listitem>
</itemizedlist>
</para>
<para>
Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
queue," even if another application already in the queue had specified
DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
that does not allow replacement goes away, and the next primary owner
does allow replacement. In this case, queued items that specified
DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
automatically replace the new primary owner. In other words,
DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
time RequestName is called. This is deliberate to avoid an infinite loop
anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
and DBUS_NAME_FLAG_REPLACE_EXISTING.
</para>
<para>
The flags argument contains any of the following values logically ORed
together:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Conventional Name</entry>
<entry>Value</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
<entry>0x1</entry>
<entry>
If an application A specifies this flag and succeeds in
becoming the owner of the name, and another application B
later calls RequestName with the
DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
will lose ownership and receive a
<literal>org.freedesktop.DBus.NameLost</literal> signal, and
application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
is not specified by application B, then application B will not replace
application A as the owner.
</entry>
</row>
<row>
<entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
<entry>0x2</entry>
<entry>
Try to replace the current owner if there is one. If this
flag is not set the application will only become the owner of
the name if there is no current owner. If this flag is set,
the application will replace the current owner if
the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
</entry>
</row>
<row>
<entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
<entry>0x4</entry>
<entry>
Without this flag, if an application requests a name that is
already owned, the application will be placed in a queue to
own the name when the current owner gives it up. If this
flag is given, the application will not be placed in the
queue, the request for the name will simply fail. This flag
also affects behavior when an application is replaced as
name owner; by default the application moves back into the
waiting queue, unless this flag was provided when the application
became the name owner.
</entry>
</row>
</tbody>
</tgroup>
</informaltable>
The return code can be one of the following values:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Conventional Name</entry>
<entry>Value</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
<entry>1</entry> <entry>The caller is now the primary owner of
the name, replacing any previous owner. Either the name had no
owner before, or the caller specified
DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
</row>
<row>
<entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
<entry>2</entry>
<entry>The name already had an owner,
DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
the current owner did not specify
DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
</entry>
</row>
<row>
<entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
<entry>The name already has an owner,
DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
specified by the requesting application.</entry>
</row>
<row>
<entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
<entry>4</entry>
<entry>The application trying to request ownership of a name is already the owner of it.</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
</sect3>
<sect3 id="bus-messages-release-name">
<title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
<para>
As a method:
<programlisting>
UINT32 ReleaseName (in STRING name)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Name to release</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>UINT32</entry>
<entry>Return value</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
Ask the message bus to release the method caller's claim to the given
name. If the caller is the primary owner, a new primary owner will be
selected from the queue if any other owners are waiting. If the
caller is waiting in the queue for the name, the caller will removed
from the queue and will not be made an owner of the name if it later
becomes available. If there are no other owners in the queue for the
name, it will be removed from the bus entirely.
The return code can be one of the following values:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Conventional Name</entry>
<entry>Value</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
<entry>1</entry> <entry>The caller has released his claim on
the given name. Either the caller was the primary owner of
the name, and the name is now unused or taken by somebody
waiting in the queue for the name, or the caller was waiting
in the queue for the name and has now been removed from the
queue.</entry>
</row>
<row>
<entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
<entry>2</entry>
<entry>The given name does not exist on this bus.</entry>
</row>
<row>
<entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
<entry>3</entry>
<entry>The caller was not the primary owner of this name,
and was also not waiting in the queue to own this name.</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
</sect3>
<sect3 id="bus-messages-list-queued-owners">
<title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
<para>
As a method:
<programlisting>
ARRAY of STRING ListQueuedOwners (in STRING name)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>The well-known bus name to query, such as
<literal>com.example.cappuccino</literal></entry>
</row>
</tbody>
</tgroup>
</informaltable>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>ARRAY of STRING</entry>
<entry>The unique bus names of connections currently queued
for the name</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
List the connections currently queued for a bus name (see
<xref linkend="term-queued-owner"/>).
</para>
</sect3>
<sect3 id="bus-messages-list-names">
<title><literal>org.freedesktop.DBus.ListNames</literal></title>
<para>
As a method:
<programlisting>
ARRAY of STRING ListNames ()
</programlisting>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>ARRAY of STRING</entry>
<entry>Array of strings where each string is a bus name</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
Returns a list of all currently-owned names on the bus.
</para>
</sect3>
<sect3 id="bus-messages-list-activatable-names">
<title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
<para>
As a method:
<programlisting>
ARRAY of STRING ListActivatableNames ()
</programlisting>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>ARRAY of STRING</entry>
<entry>Array of strings where each string is a bus name</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
Returns a list of all names that can be activated on the bus.
</para>
</sect3>
<sect3 id="bus-messages-name-exists">
<title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
<para>
As a method:
<programlisting>
BOOLEAN NameHasOwner (in STRING name)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Name to check</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>BOOLEAN</entry>
<entry>Return value, true if the name exists</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
Checks if the specified name exists (currently has an owner).
</para>
</sect3>
<sect3 id="bus-messages-name-owner-changed">
<title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
<para>
This is a signal:
<programlisting>
NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Name with a new owner</entry>
</row>
<row>
<entry>1</entry>
<entry>STRING</entry>
<entry>Old owner or empty string if none</entry>
</row>
<row>
<entry>2</entry>
<entry>STRING</entry>
<entry>New owner or empty string if none</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
This signal indicates that the owner of a name has changed.
It's also the signal to use to detect the appearance of
new names on the bus.
</para>
</sect3>
<sect3 id="bus-messages-name-lost">
<title><literal>org.freedesktop.DBus.NameLost</literal></title>
<para>
This is a signal:
<programlisting>
NameLost (STRING name)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Name which was lost</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
This signal is sent to a specific application when it loses
ownership of a name.
</para>
</sect3>
<sect3 id="bus-messages-name-acquired">
<title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
<para>
This is a signal:
<programlisting>
NameAcquired (STRING name)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Name which was acquired</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
This signal is sent to a specific application when it gains
ownership of a name.
</para>
</sect3>
<sect3 id="bus-messages-activatable-services-changed">
<title><literal>org.freedesktop.DBus.ActivatableServicesChanged</literal></title>
<para>
This is a signal:
<programlisting>
ActivatableServicesChanged ()
</programlisting>
</para>
<para>
This signal is sent when the list of activatable services,
as returned by ListActivatableNames(), might have changed
(see <xref linkend="bus-messages-list-activatable-names"/>).
Clients that have cached information about the activatable
services should call ListActivatableNames() again to update
their cache.
</para>
<para>
The presence of this signal is indicated by a
bus feature property (for details see
<xref linkend="message-bus-properties-features"/>).
In older implementations that do not have this feature,
there is no way to be informed when the list of activatable
names has changed.
</para>
</sect3>
<sect3 id="bus-messages-start-service-by-name">
<title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
<para>
As a method:
<programlisting>
UINT32 StartServiceByName (in STRING name, in UINT32 flags)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Name of the service to start</entry>
</row>
<row>
<entry>1</entry>
<entry>UINT32</entry>
<entry>Flags (currently not used)</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>UINT32</entry>
<entry>Return value</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Tries to launch the executable associated with a name (service
activation), as an explicit request. This is an alternative to
relying on auto-starting. For more information on how services
are activated and the difference between auto-starting and explicit
activation, see
<xref linkend="message-bus-starting-services"/>.
</para>
<para>
It is often preferable to carry out auto-starting
instead of calling this method. This is because calling this method
is subject to a
<ulink url="https://en.wikipedia.org/wiki/Time_of_check_to_time_of_use">time-of-check/time-of-use</ulink>
issue: if a caller asks the message bus to start a service so that
the same caller can make follow-up method calls to that service,
the fact that the message bus was able to start the required
service is no guarantee that it will not have crashed or otherwise
exited by the time the caller makes those follow-up method calls.
As a result, calling this method does not remove the need for
the caller to handle errors from method calls. Given that fact,
it is usually simpler to rely on auto-starting, in which the
required service starts as a side-effect of the first method call.
</para>
<para>
The return value can be one of the following values:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Identifier</entry>
<entry>Value</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>DBUS_START_REPLY_SUCCESS</entry>
<entry>1</entry>
<entry>The service was successfully started.</entry>
</row>
<row>
<entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
<entry>2</entry>
<entry>A connection already owns the given name.</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
</sect3>
<sect3 id="bus-messages-update-activation-environment">
<title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
<para>
As a method:
<programlisting>
UpdateActivationEnvironment (in ARRAY of DICT_ENTRY&lt;STRING,STRING&gt; environment)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>ARRAY of DICT_ENTRY&lt;STRING,STRING&gt;</entry>
<entry>Environment to add or update</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
</para>
<para>
Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
</para>
<para>
Note, both the environment variable names and values must be valid UTF-8. There's no way to update the activation environment with data that is invalid UTF-8.
</para>
</sect3>
<sect3 id="bus-messages-get-name-owner">
<title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
<para>
As a method:
<programlisting>
STRING GetNameOwner (in STRING name)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Name to get the owner of</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Return value, a unique connection name</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Returns the unique connection name of the primary owner of the name
given. If the requested name doesn't have an owner, returns a
<literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
</para>
</sect3>
<sect3 id="bus-messages-get-connection-unix-user">
<title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
<para>
As a method:
<programlisting>
UINT32 GetConnectionUnixUser (in STRING bus_name)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Unique or well-known bus name of the connection to
query, such as <literal>:12.34</literal> or
<literal>com.example.tea</literal></entry>
</row>
</tbody>
</tgroup>
</informaltable>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>UINT32</entry>
<entry>Unix user ID</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Returns the Unix user ID of the process connected to the server. If
unable to determine it (for instance, because the process is not on the
same machine as the bus daemon), an error is returned.
</para>
</sect3>
<sect3 id="bus-messages-get-connection-unix-process-id">
<title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
<para>
As a method:
<programlisting>
UINT32 GetConnectionUnixProcessID (in STRING bus_name)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Unique or well-known bus name of the connection to
query, such as <literal>:12.34</literal> or
<literal>com.example.tea</literal></entry>
</row>
</tbody>
</tgroup>
</informaltable>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>UINT32</entry>
<entry>Unix process id</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Returns the Unix process ID of the process connected to the server. If
unable to determine it (for instance, because the process is not on the
same machine as the bus daemon), an error is returned.
</para>
</sect3>
<sect3 id="bus-messages-get-connection-credentials">
<title><literal>org.freedesktop.DBus.GetConnectionCredentials</literal></title>
<para>
As a method:
<programlisting>
ARRAY of DICT_ENTRY&lt;STRING,VARIANT&gt; GetConnectionCredentials (in STRING bus_name)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Unique or well-known bus name of the connection to
query, such as <literal>:12.34</literal> or
<literal>com.example.tea</literal></entry>
</row>
</tbody>
</tgroup>
</informaltable>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>ARRAY of DICT_ENTRY&lt;STRING,VARIANT&gt;</entry>
<entry>Credentials</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
Returns as many credentials as possible for the process connected to
the server. If unable to determine certain credentials (for instance,
because the process is not on the same machine as the bus daemon,
or because this version of the bus daemon does not support a
particular security framework), or if the values of those credentials
cannot be represented as documented here, then those credentials
are omitted.
</para>
<para>
Keys in the returned dictionary not containing "." are defined
by this specification. Bus daemon implementors supporting
credentials frameworks not mentioned in this document should either
contribute patches to this specification, or use keys containing
"." and starting with a reversed domain name.
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Key</entry>
<entry>Value type</entry>
<entry>Value</entry>
</row>
</thead>
<tbody>
<row>
<entry>UnixUserID</entry>
<entry>UINT32</entry>
<entry>The numeric Unix user ID, as defined by POSIX</entry>
</row>
<row>
<entry>UnixGroupIDs</entry>
<entry>ARRAY of UINT32</entry>
<entry>The numeric Unix group IDs (including both the primary
group and the supplementary groups), as defined by POSIX, in
numerically sorted order. This array is either complete or
absent: if the message bus is able to determine some but not
all of the caller's groups, or if one of the groups is not
representable in a UINT32, it must not add this credential
to the dictionary.
</entry>
</row>
<row>
<entry>ProcessID</entry>
<entry>UINT32</entry>
<entry>The numeric process ID, on platforms that have
this concept. On Unix, this is the process ID defined by
POSIX.</entry>
</row>
<row>
<entry>WindowsSID</entry>
<entry>STRING</entry>
<entry>The Windows security identifier in its string form,
e.g. "S-1-5-21-3623811015-3361044348-30300820-1013" for
a domain or local computer user or "S-1-5-18" for the
LOCAL_SYSTEM user</entry>
</row>
<row>
<entry>LinuxSecurityLabel</entry>
<entry>ARRAY of BYTE</entry>
<entry>
<para>On Linux systems, the security label that would result
from the SO_PEERSEC getsockopt call. The array contains
the non-zero bytes of the security label in an unspecified
ASCII-compatible encoding<footnote>
<para>It could be ASCII or UTF-8, but could also be
ISO Latin-1 or any other encoding.</para>
</footnote>, followed by a single zero byte.</para>
<para>
For example, the SELinux context
<literal>system_u:system_r:init_t:s0</literal>
(a string of length 27) would be encoded as 28 bytes
ending with ':', 's', '0', '\x00'.<footnote>
<para>Note that this is not the same as the older
GetConnectionSELinuxContext method, which does
not append the zero byte. Always appending the
zero byte allows callers to read the string
from the message payload without copying.</para>
</footnote>
</para>
<para>
On SELinux systems this is the SELinux context, as output
by <literal>ps -Z</literal> or <literal>ls -Z</literal>.
Typical values might include
<literal>system_u:system_r:init_t:s0</literal>,
<literal>unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023</literal>,
or
<literal>unconfined_u:unconfined_r:chrome_sandbox_t:s0-s0:c0.c1023</literal>.
</para>
<para>
On Smack systems, this is the Smack label.
Typical values might include
<literal>_</literal>, <literal>*</literal>,
<literal>User</literal>, <literal>System</literal>
or <literal>System::Shared</literal>.
</para>
<para>
On AppArmor systems, this is the AppArmor context,
a composite string encoding the AppArmor label (one or more
profiles) and the enforcement mode.
Typical values might include <literal>unconfined</literal>,
<literal>/usr/bin/firefox (enforce)</literal> or
<literal>user1 (complain)</literal>.
</para>
</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
This method was added in D-Bus 1.7 to reduce the round-trips
required to list a process's credentials. In older versions, calling
this method will fail: applications should recover by using the
separate methods such as
<xref linkend="bus-messages-get-connection-unix-user"/>
instead.
</para>
</sect3>
<sect3 id="bus-messages-get-adt-audit-session-data">
<title><literal>org.freedesktop.DBus.GetAdtAuditSessionData</literal></title>
<para>
As a method:
<programlisting>
ARRAY of BYTE GetAdtAuditSessionData (in STRING bus_name)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Unique or well-known bus name of the connection to
query, such as <literal>:12.34</literal> or
<literal>com.example.tea</literal></entry>
</row>
</tbody>
</tgroup>
</informaltable>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>ARRAY of BYTE</entry>
<entry>auditing data as returned by
adt_export_session_data()</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Returns auditing data used by Solaris ADT, in an unspecified
binary format. If you know what this means, please contribute
documentation via the D-Bus bug tracking system.
This method is on the core DBus interface for historical reasons;
the same information should be made available via
<xref linkend="bus-messages-get-connection-credentials"/>
in future.
</para>
</sect3>
<sect3 id="bus-messages-get-connection-selinux-security-context">
<title><literal>org.freedesktop.DBus.GetConnectionSELinuxSecurityContext</literal></title>
<para>
As a method:
<programlisting>
ARRAY of BYTE GetConnectionSELinuxSecurityContext (in STRING bus_name)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Unique or well-known bus name of the connection to
query, such as <literal>:12.34</literal> or
<literal>com.example.tea</literal></entry>
</row>
</tbody>
</tgroup>
</informaltable>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>ARRAY of BYTE</entry>
<entry>some sort of string of bytes, not necessarily UTF-8,
not including '\0'</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Returns the security context used by SELinux, in an unspecified
format. If you know what this means, please contribute
documentation via the D-Bus bug tracking system.
This method is on the core DBus interface for historical reasons;
the same information should be made available via
<xref linkend="bus-messages-get-connection-credentials"/>
in future.
</para>
</sect3>
<sect3 id="bus-messages-add-match">
<title><literal>org.freedesktop.DBus.AddMatch</literal></title>
<para>
As a method:
<programlisting>
AddMatch (in STRING rule)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Match rule to add to the connection</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
error is returned.
</para>
</sect3>
<sect3 id="bus-messages-remove-match">
<title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
<para>
As a method:
<programlisting>
RemoveMatch (in STRING rule)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Match rule to remove from the connection</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
error is returned.
</para>
</sect3>
<sect3 id="bus-messages-get-id">
<title><literal>org.freedesktop.DBus.GetId</literal></title>
<para>
As a method:
<programlisting>
GetId (out STRING id)
</programlisting>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Unique ID identifying the bus daemon</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Gets the unique ID of the bus. The unique ID here is shared among all addresses the
bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
<xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
by org.freedesktop.DBus.Peer.GetMachineId().
For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
</para>
</sect3>
<sect3 id="bus-messages-become-monitor">
<title><literal>org.freedesktop.DBus.Monitoring.BecomeMonitor</literal></title>
<para>
As a method:
<programlisting>
BecomeMonitor (in ARRAY of STRING rule, in UINT32 flags)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>ARRAY of STRING</entry>
<entry>Match rules to add to the connection</entry>
</row>
<row>
<entry>1</entry>
<entry>UINT32</entry>
<entry>Not used, must be 0</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
Converts the connection into a <emphasis>monitor
connection</emphasis> which can be used as a debugging/monitoring
tool. Only a user who is privileged on this
bus (by some implementation-specific definition) may create
monitor connections<footnote>
<para>
In the reference implementation,
the default configuration is that each user (identified by
numeric user ID) may monitor their own session bus,
and the root user (user ID zero) may monitor the
system bus.
</para>
</footnote>.
</para>
<para>
Monitor connections lose all their bus names, including the unique
connection name, and all their match rules. Sending messages on a
monitor connection is not allowed: applications should use a private
connection for monitoring.
</para>
<para>
Monitor connections may receive all messages, even messages that
should only have gone to some other connection ("eavesdropping").
The first argument is a list of match rules, which replace any
match rules that were previously active for this connection.
These match rules are always treated as if they contained the
special <literal>eavesdrop='true'</literal> member.
</para>
<para>
As a special case, an empty list of match rules (which would
otherwise match nothing, making the monitor useless) is treated
as a shorthand for matching all messages.
</para>
<para>
The second argument might be used for flags to influence the
behaviour of the monitor connection in future D-Bus versions.
</para>
<para>
Message bus implementations should attempt to minimize the
side-effects of monitoring — in particular, unlike ordinary
eavesdropping, monitoring the system bus does not require the
access control rules to be relaxed, which would change the set
of messages that can be delivered to their (non-monitor)
destinations. However, it is unavoidable that monitoring
will increase the message bus's resource consumption. In
edge cases where there was barely enough time or memory without
monitoring, this might result in message deliveries failing
when they would otherwise have succeeded.
</para>
</sect3>
</sect2>
<sect2 id="message-bus-properties">
<title>Message Bus Properties</title>
<para>
The special message bus name <literal>org.freedesktop.DBus</literal>
exports several properties (see
<xref linkend="standard-interfaces-properties"/>) on the object path
<literal>/org/freedesktop/DBus</literal>.
</para>
<sect3 id="message-bus-properties-features">
<title><literal>org.freedesktop.DBus.Features</literal></title>
<para>
As a property:
<programlisting>
Read-only constant ARRAY of STRING Features
</programlisting>
This property lists abstract “features” provided by the message
bus, and can be used by clients to detect the capabilities
of the message bus with which they are communicating.
This property was added in version 1.11.x of the reference
implementation of the message bus.
</para>
<para>
Items in the returned array not containing “.” are defined
by this specification. Bus daemon implementors wishing to advertise
features not mentioned in this document should either contribute
patches to this specification, or use keys containing “.” and
starting with their own reversed domain name, for example
<literal>com.example.MyBus.SubliminalMessages</literal>.
</para>
<para>
The features currently defined in this specification are as follows:
<variablelist>
<varlistentry>
<term><literal>ActivatableServicesChanged</literal></term>
<listitem>
<para>
This message bus emits the <literal>ActivatableServicesChanged</literal>
signal whenever its list of activatable services might have changed
(for details see <xref linkend="bus-messages-activatable-services-changed"/>).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>AppArmor</literal></term>
<listitem>
<para>
This message bus filters messages via the
<ulink url="http://wiki.apparmor.net/">AppArmor</ulink>
security framework. This feature should only be
advertised if AppArmor mediation is enabled and
active at runtime; merely compiling in support
for AppArmor should not result in this feature being
advertised on message bus instances where it is disabled by
message bus or operating system configuration.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>HeaderFiltering</literal></term>
<listitem>
<para>
This message bus guarantees that it will remove
header fields that it does not understand when it
relays messages, so that a client receiving a
recently-defined header field that is specified to be
controlled by the message bus can safely assume that
it was in fact set by the message bus. This check is
needed because older message bus implementations did
not guarantee to filter headers in this way, so a
malicious client could send any recently-defined
header field with a crafted value of its choice
through an older message bus that did not understand
that header field.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>SELinux</literal></term>
<listitem>
<para>
This message bus filters messages via the
<ulink url="https://selinuxproject.org/">SELinux</ulink>
security framework. Similar to <literal>apparmor</literal>,
this feature should only be advertised if SELinux mediation
is enabled and active at runtime (if SELinux is placed in
permissive mode, that is still considered to be active).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>SystemdActivation</literal></term>
<listitem>
<para>
When asked to activate a service that has the
<literal>SystemdService</literal> field in its
<filename>.service</filename> file, this message bus will
carry out systemd activation (for details see
<xref linkend="message-bus-starting-services-systemd"/>).
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</sect3>
<sect3 id="message-bus-properties-interfaces">
<title><literal>org.freedesktop.DBus.Interfaces</literal></title>
<para>
As a property:
<programlisting>
Read-only constant ARRAY of STRING Interfaces
</programlisting>
This property lists interfaces provided by the
<literal>/org/freedesktop/DBus</literal> object,
and can be used by clients to detect the capabilities
of the message bus with which they are communicating.
Unlike the standard Introspectable interface, querying this
property does not require parsing XML.
This property was added in version 1.11.x of the reference
implementation of the message bus.
</para>
<para>
The standard <literal>org.freedesktop.DBus</literal> and
<literal>org.freedesktop.DBus.Properties</literal> interfaces
are not included in the value of this property, because their
presence can be inferred from the fact that a method call on
<literal>org.freedesktop.DBus.Properties</literal> asking for
properties of <literal>org.freedesktop.DBus</literal> was
successful. The standard <literal>org.freedesktop.DBus.Peer</literal>
and <literal>org.freedesktop.DBus.Introspectable</literal>
interfaces are not included in the value of this property either,
because they do not indicate features of the message bus
implementation.
</para>
</sect3>
</sect2>
</sect1>
<!--
<appendix id="implementation-notes">
<title>Implementation notes</title>
<sect1 id="implementation-notes-subsection">
<title></title>
<para>
</para>
</sect1>
</appendix>
-->
<glossary><title>Glossary</title>
<para>
This glossary defines some of the terms used in this specification.
</para>
<glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
<glossdef>
<para>
The message bus maintains an association between names and
connections. (Normally, there's one connection per application.) A
bus name is simply an identifier used to locate connections. For
example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
name might be used to send a message to a screensaver from Yoyodyne
Corporation. An application is said to <firstterm>own</firstterm> a
name if the message bus has associated the application's connection
with the name. Names may also have <firstterm>queued
owners</firstterm> (see <xref linkend="term-queued-owner"/>).
The bus assigns a unique name to each connection,
see <xref linkend="term-unique-name"/>. Other names
can be thought of as "well-known names" and are
used to find applications that offer specific functionality.
</para>
<para>
See <xref linkend="message-protocol-names-bus"/> for details of
the syntax and naming conventions for bus names.
</para>
</glossdef>
</glossentry>
<glossentry id="term-message"><glossterm>Message</glossterm>
<glossdef>
<para>
A message is the atomic unit of communication via the D-Bus
protocol. It consists of a <firstterm>header</firstterm> and a
<firstterm>body</firstterm>; the body is made up of
<firstterm>arguments</firstterm>.
</para>
</glossdef>
</glossentry>
<glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
<glossdef>
<para>
The message bus is a special application that forwards
or routes messages between a group of applications
connected to the message bus. It also manages
<firstterm>names</firstterm> used for routing
messages.
</para>
</glossdef>
</glossentry>
<glossentry id="term-name"><glossterm>Name</glossterm>
<glossdef>
<para>
See <xref linkend="term-bus-name"/>. "Name" may
also be used to refer to some of the other names
in D-Bus, such as interface names.
</para>
</glossdef>
</glossentry>
<glossentry id="namespace"><glossterm>Namespace</glossterm>
<glossdef>
<para>
Used to prevent collisions when defining new interfaces, bus names
etc. The convention used is the same one Java uses for defining
classes: a reversed domain name.
See <xref linkend="message-protocol-names-bus"/>,
<xref linkend="message-protocol-names-interface"/>,
<xref linkend="message-protocol-names-error"/>,
<xref linkend="message-protocol-marshaling-object-path"/>.
</para>
</glossdef>
</glossentry>
<glossentry id="term-object"><glossterm>Object</glossterm>
<glossdef>
<para>
Each application contains <firstterm>objects</firstterm>, which have
<firstterm>interfaces</firstterm> and
<firstterm>methods</firstterm>. Objects are referred to by a name,
called a <firstterm>path</firstterm>.
</para>
</glossdef>
</glossentry>
<glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
<glossdef>
<para>
An application talking directly to another application, without going
through a message bus. One-to-one connections may be "peer to peer" or
"client to server." The D-Bus protocol has no concept of client
vs. server after a connection has authenticated; the flow of messages
is symmetrical (full duplex).
</para>
</glossdef>
</glossentry>
<glossentry id="term-path"><glossterm>Path</glossterm>
<glossdef>
<para>
Object references (object names) in D-Bus are organized into a
filesystem-style hierarchy, so each object is named by a path. As in
LDAP, there's no difference between "files" and "directories"; a path
can refer to an object, while still having child objects below it.
</para>
</glossdef>
</glossentry>
<glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
<glossdef>
<para>
Each bus name has a primary owner; messages sent to the name go to the
primary owner. However, certain names also maintain a queue of
secondary owners "waiting in the wings." If the primary owner releases
the name, then the first secondary owner in the queue automatically
becomes the new owner of the name.
</para>
</glossdef>
</glossentry>
<glossentry id="term-service"><glossterm>Service</glossterm>
<glossdef>
<para>
A service is an executable that can be launched by the bus daemon.
Services normally guarantee some particular features, for example they
may guarantee that they will request a specific name such as
"com.example.Screensaver1", have a singleton object
"/com/example/Screensaver1", and that object will implement the
interface "com.example.Screensaver1.Control".
</para>
</glossdef>
</glossentry>
<glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
<glossdef>
<para>
".service files" tell the bus about service applications that can be
launched (see <xref linkend="term-service"/>). Most importantly they
provide a mapping from bus names to services that will request those
names when they start up.
</para>
</glossdef>
</glossentry>
<glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
<glossdef>
<para>
The special name automatically assigned to each connection by the
message bus. This name will never change owner, and will be unique
(never reused during the lifetime of the message bus).
It will begin with a ':' character.
</para>
</glossdef>
</glossentry>
</glossary>
</article>
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