1 <?xml version="1.0" standalone="no" ?>
2 <!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
3 "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd"
8 <title>D-Bus Specification</title>
9 <releaseinfo>Version 0.19</releaseinfo>
10 <date>2012-02-21</date>
13 <firstname>Havoc</firstname>
14 <surname>Pennington</surname>
16 <orgname>Red Hat, Inc.</orgname>
18 <email>hp@pobox.com</email>
23 <firstname>Anders</firstname>
24 <surname>Carlsson</surname>
26 <orgname>CodeFactory AB</orgname>
28 <email>andersca@codefactory.se</email>
33 <firstname>Alexander</firstname>
34 <surname>Larsson</surname>
36 <orgname>Red Hat, Inc.</orgname>
38 <email>alexl@redhat.com</email>
43 <firstname>Sven</firstname>
44 <surname>Herzberg</surname>
46 <orgname>Imendio AB</orgname>
48 <email>sven@imendio.com</email>
53 <firstname>Simon</firstname>
54 <surname>McVittie</surname>
56 <orgname>Collabora Ltd.</orgname>
58 <email>simon.mcvittie@collabora.co.uk</email>
63 <firstname>David</firstname>
64 <surname>Zeuthen</surname>
66 <orgname>Red Hat, Inc.</orgname>
68 <email>davidz@redhat.com</email>
75 <revnumber>current</revnumber>
76 <date><ulink url='http://cgit.freedesktop.org/dbus/dbus/log/doc/dbus-specification.xml'>commit log</ulink></date>
77 <authorinitials></authorinitials>
78 <revremark></revremark>
81 <revnumber>0.19</revnumber>
82 <date>20 February 2012</date>
83 <authorinitials>smcv/lp</authorinitials>
84 <revremark>formally define unique connection names and well-known
85 bus names; document best practices for interface, bus, member and
86 error names, and object paths; document the search path for session
87 and system services on Unix; document the systemd transport</revremark>
90 <revnumber>0.18</revnumber>
91 <date>29 July 2011</date>
92 <authorinitials>smcv</authorinitials>
93 <revremark>define eavesdropping, unicast, broadcast; add eavesdrop
94 match keyword; promote type system to a top-level section</revremark>
97 <revnumber>0.17</revnumber>
98 <date>1 June 2011</date>
99 <authorinitials>smcv/davidz</authorinitials>
100 <revremark>define ObjectManager; reserve extra pseudo-type-codes used
101 by GVariant</revremark>
104 <revnumber>0.16</revnumber>
105 <date>11 April 2011</date>
106 <authorinitials></authorinitials>
107 <revremark>add path_namespace, arg0namespace; argNpath matches object
111 <revnumber>0.15</revnumber>
112 <date>3 November 2010</date>
113 <authorinitials></authorinitials>
114 <revremark></revremark>
117 <revnumber>0.14</revnumber>
118 <date>12 May 2010</date>
119 <authorinitials></authorinitials>
120 <revremark></revremark>
123 <revnumber>0.13</revnumber>
124 <date>23 Dezember 2009</date>
125 <authorinitials></authorinitials>
126 <revremark></revremark>
129 <revnumber>0.12</revnumber>
130 <date>7 November, 2006</date>
131 <authorinitials></authorinitials>
132 <revremark></revremark>
135 <revnumber>0.11</revnumber>
136 <date>6 February 2005</date>
137 <authorinitials></authorinitials>
138 <revremark></revremark>
141 <revnumber>0.10</revnumber>
142 <date>28 January 2005</date>
143 <authorinitials></authorinitials>
144 <revremark></revremark>
147 <revnumber>0.9</revnumber>
148 <date>7 Januar 2005</date>
149 <authorinitials></authorinitials>
150 <revremark></revremark>
153 <revnumber>0.8</revnumber>
154 <date>06 September 2003</date>
155 <authorinitials></authorinitials>
156 <revremark>First released document.</revremark>
161 <sect1 id="introduction">
162 <title>Introduction</title>
164 D-Bus is a system for low-latency, low-overhead, easy to use
165 interprocess communication (IPC). In more detail:
169 D-Bus is <emphasis>low-latency</emphasis> because it is designed
170 to avoid round trips and allow asynchronous operation, much like
176 D-Bus is <emphasis>low-overhead</emphasis> because it uses a
177 binary protocol, and does not have to convert to and from a text
178 format such as XML. Because D-Bus is intended for potentially
179 high-resolution same-machine IPC, not primarily for Internet IPC,
180 this is an interesting optimization.
185 D-Bus is <emphasis>easy to use</emphasis> because it works in terms
186 of <firstterm>messages</firstterm> rather than byte streams, and
187 automatically handles a lot of the hard IPC issues. Also, the D-Bus
188 library is designed to be wrapped in a way that lets developers use
189 their framework's existing object/type system, rather than learning
190 a new one specifically for IPC.
197 The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
198 protocol, specified in <xref linkend="message-protocol"/>. That is, it is
199 a system for one application to talk to a single other
200 application. However, the primary intended application of the protocol is the
201 D-Bus <firstterm>message bus</firstterm>, specified in <xref
202 linkend="message-bus"/>. The message bus is a special application that
203 accepts connections from multiple other applications, and forwards
208 Uses of D-Bus include notification of system changes (notification of when
209 a camera is plugged in to a computer, or a new version of some software
210 has been installed), or desktop interoperability, for example a file
211 monitoring service or a configuration service.
215 D-Bus is designed for two specific use cases:
219 A "system bus" for notifications from the system to user sessions,
220 and to allow the system to request input from user sessions.
225 A "session bus" used to implement desktop environments such as
230 D-Bus is not intended to be a generic IPC system for any possible
231 application, and intentionally omits many features found in other
232 IPC systems for this reason.
236 At the same time, the bus daemons offer a number of features not found in
237 other IPC systems, such as single-owner "bus names" (similar to X
238 selections), on-demand startup of services, and security policies.
239 In many ways, these features are the primary motivation for developing
240 D-Bus; other systems would have sufficed if IPC were the only goal.
244 D-Bus may turn out to be useful in unanticipated applications, but future
245 versions of this spec and the reference implementation probably will not
246 incorporate features that interfere with the core use cases.
250 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
251 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
252 document are to be interpreted as described in RFC 2119. However, the
253 document could use a serious audit to be sure it makes sense to do
254 so. Also, they are not capitalized.
257 <sect2 id="stability">
258 <title>Protocol and Specification Stability</title>
260 The D-Bus protocol is frozen (only compatible extensions are allowed) as
261 of November 8, 2006. However, this specification could still use a fair
262 bit of work to make interoperable reimplementation possible without
263 reference to the D-Bus reference implementation. Thus, this
264 specification is not marked 1.0. To mark it 1.0, we'd like to see
265 someone invest significant effort in clarifying the specification
266 language, and growing the specification to cover more aspects of the
267 reference implementation's behavior.
270 Until this work is complete, any attempt to reimplement D-Bus will
271 probably require looking at the reference implementation and/or asking
272 questions on the D-Bus mailing list about intended behavior.
273 Questions on the list are very welcome.
276 Nonetheless, this document should be a useful starting point and is
277 to our knowledge accurate, though incomplete.
283 <sect1 id="type-system">
284 <title>Type System</title>
287 D-Bus has a type system, in which values of various types can be
288 serialized into a sequence of bytes referred to as the
289 <firstterm>wire format</firstterm> in a standard way.
290 Converting a value from some other representation into the wire
291 format is called <firstterm>marshaling</firstterm> and converting
292 it back from the wire format is <firstterm>unmarshaling</firstterm>.
296 The D-Bus protocol does not include type tags in the marshaled data; a
297 block of marshaled values must have a known <firstterm>type
298 signature</firstterm>. The type signature is made up of zero or more
299 <firstterm id="term-single-complete-type">single complete
300 types</firstterm>, each made up of one or more
301 <firstterm>type codes</firstterm>.
305 A type code is an ASCII character representing the
306 type of a value. Because ASCII characters are used, the type signature
307 will always form a valid ASCII string. A simple string compare
308 determines whether two type signatures are equivalent.
312 A single complete type is a sequence of type codes that fully describes
313 one type: either a basic type, or a single fully-described container type.
314 A single complete type is a basic type code, a variant type code,
315 an array with its element type, or a struct with its fields (all of which
316 are defined below). So the following signatures are not single complete
327 And the following signatures contain multiple complete types:
337 Note however that a single complete type may <emphasis>contain</emphasis>
338 multiple other single complete types, by containing a struct or dict
342 <sect2 id="basic-types">
343 <title>Basic types</title>
346 As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
347 the ASCII character 'i'. So the signature for a block of values
348 containing a single <literal>INT32</literal> would be:
352 A block of values containing two <literal>INT32</literal> would have this signature:
359 All fixed types work like
360 <literal>INT32</literal> in this example: to marshal and unmarshal
361 fixed types, you simply read one value from the data
362 block corresponding to each type code in the signature.
366 <sect2 id="container-types">
367 <title>Container types</title>
370 In addition to basic types, there are four <firstterm>container</firstterm>
371 types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
372 and <literal>DICT_ENTRY</literal>.
376 <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
377 code does not appear in signatures. Instead, ASCII characters
378 '(' and ')' are used to mark the beginning and end of the struct.
379 So for example, a struct containing two integers would have this
384 Structs can be nested, so for example a struct containing
385 an integer and another struct:
389 The value block storing that struct would contain three integers; the
390 type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
395 The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
396 but is useful in code that implements the protocol. This type code
397 is specified to allow such code to interoperate in non-protocol contexts.
401 Empty structures are not allowed; there must be at least one
402 type code between the parentheses.
406 <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
407 followed by a <firstterm>single complete type</firstterm>. The single
408 complete type following the array is the type of each array element. So
409 the simple example is:
413 which is an array of 32-bit integers. But an array can be of any type,
414 such as this array-of-struct-with-two-int32-fields:
418 Or this array of array of integer:
425 <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
426 type <literal>VARIANT</literal> will have the signature of a single complete type as part
427 of the <emphasis>value</emphasis>. This signature will be followed by a
428 marshaled value of that type.
432 A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
433 than parentheses it uses curly braces, and it has more restrictions.
434 The restrictions are: it occurs only as an array element type; it has
435 exactly two single complete types inside the curly braces; the first
436 single complete type (the "key") must be a basic type rather than a
437 container type. Implementations must not accept dict entries outside of
438 arrays, must not accept dict entries with zero, one, or more than two
439 fields, and must not accept dict entries with non-basic-typed keys. A
440 dict entry is always a key-value pair.
444 The first field in the <literal>DICT_ENTRY</literal> is always the key.
445 A message is considered corrupt if the same key occurs twice in the same
446 array of <literal>DICT_ENTRY</literal>. However, for performance reasons
447 implementations are not required to reject dicts with duplicate keys.
451 In most languages, an array of dict entry would be represented as a
452 map, hash table, or dict object.
457 <title>Summary of types</title>
460 The following table summarizes the D-Bus types.
465 <entry>Conventional Name</entry>
467 <entry>Description</entry>
472 <entry><literal>INVALID</literal></entry>
473 <entry>0 (ASCII NUL)</entry>
474 <entry>Not a valid type code, used to terminate signatures</entry>
476 <entry><literal>BYTE</literal></entry>
477 <entry>121 (ASCII 'y')</entry>
478 <entry>8-bit unsigned integer</entry>
480 <entry><literal>BOOLEAN</literal></entry>
481 <entry>98 (ASCII 'b')</entry>
482 <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
484 <entry><literal>INT16</literal></entry>
485 <entry>110 (ASCII 'n')</entry>
486 <entry>16-bit signed integer</entry>
488 <entry><literal>UINT16</literal></entry>
489 <entry>113 (ASCII 'q')</entry>
490 <entry>16-bit unsigned integer</entry>
492 <entry><literal>INT32</literal></entry>
493 <entry>105 (ASCII 'i')</entry>
494 <entry>32-bit signed integer</entry>
496 <entry><literal>UINT32</literal></entry>
497 <entry>117 (ASCII 'u')</entry>
498 <entry>32-bit unsigned integer</entry>
500 <entry><literal>INT64</literal></entry>
501 <entry>120 (ASCII 'x')</entry>
502 <entry>64-bit signed integer</entry>
504 <entry><literal>UINT64</literal></entry>
505 <entry>116 (ASCII 't')</entry>
506 <entry>64-bit unsigned integer</entry>
508 <entry><literal>DOUBLE</literal></entry>
509 <entry>100 (ASCII 'd')</entry>
510 <entry>IEEE 754 double</entry>
512 <entry><literal>STRING</literal></entry>
513 <entry>115 (ASCII 's')</entry>
514 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
516 <entry><literal>OBJECT_PATH</literal></entry>
517 <entry>111 (ASCII 'o')</entry>
518 <entry>Name of an object instance</entry>
520 <entry><literal>SIGNATURE</literal></entry>
521 <entry>103 (ASCII 'g')</entry>
522 <entry>A type signature</entry>
524 <entry><literal>ARRAY</literal></entry>
525 <entry>97 (ASCII 'a')</entry>
528 <entry><literal>STRUCT</literal></entry>
529 <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
530 <entry>Struct; type code 114 'r' is reserved for use in
531 bindings and implementations to represent the general
532 concept of a struct, and must not appear in signatures
533 used on D-Bus.</entry>
535 <entry><literal>VARIANT</literal></entry>
536 <entry>118 (ASCII 'v') </entry>
537 <entry>Variant type (the type of the value is part of the value itself)</entry>
539 <entry><literal>DICT_ENTRY</literal></entry>
540 <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
541 <entry>Entry in a dict or map (array of key-value pairs).
542 Type code 101 'e' is reserved for use in bindings and
543 implementations to represent the general concept of a
544 dict or dict-entry, and must not appear in signatures
545 used on D-Bus.</entry>
547 <entry><literal>UNIX_FD</literal></entry>
548 <entry>104 (ASCII 'h')</entry>
549 <entry>Unix file descriptor</entry>
552 <entry>(reserved)</entry>
553 <entry>109 (ASCII 'm')</entry>
554 <entry>Reserved for <ulink
555 url="https://bugs.freedesktop.org/show_bug.cgi?id=27857">a
556 'maybe' type compatible with the one in GVariant</ulink>,
557 and must not appear in signatures used on D-Bus until
558 specified here</entry>
561 <entry>(reserved)</entry>
562 <entry>42 (ASCII '*')</entry>
563 <entry>Reserved for use in bindings/implementations to
564 represent any <firstterm>single complete type</firstterm>,
565 and must not appear in signatures used on D-Bus.</entry>
568 <entry>(reserved)</entry>
569 <entry>63 (ASCII '?')</entry>
570 <entry>Reserved for use in bindings/implementations to
571 represent any <firstterm>basic type</firstterm>, and must
572 not appear in signatures used on D-Bus.</entry>
575 <entry>(reserved)</entry>
576 <entry>64 (ASCII '@'), 38 (ASCII '&'),
577 94 (ASCII '^')</entry>
578 <entry>Reserved for internal use by bindings/implementations,
579 and must not appear in signatures used on D-Bus.
580 GVariant uses these type-codes to encode calling
590 <sect2 id="message-protocol-marshaling">
591 <title>Marshaling (Wire Format)</title>
594 Given a type signature, a block of bytes can be converted into typed
595 values. This section describes the format of the block of bytes. Byte
596 order and alignment issues are handled uniformly for all D-Bus types.
600 A block of bytes has an associated byte order. The byte order
601 has to be discovered in some way; for D-Bus messages, the
602 byte order is part of the message header as described in
603 <xref linkend="message-protocol-messages"/>. For now, assume
604 that the byte order is known to be either little endian or big
609 Each value in a block of bytes is aligned "naturally," for example
610 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
611 8-byte boundary. To properly align a value, <firstterm>alignment
612 padding</firstterm> may be necessary. The alignment padding must always
613 be the minimum required padding to properly align the following value;
614 and it must always be made up of nul bytes. The alignment padding must
615 not be left uninitialized (it can't contain garbage), and more padding
616 than required must not be used.
620 Given all this, the types are marshaled on the wire as follows:
625 <entry>Conventional Name</entry>
626 <entry>Encoding</entry>
627 <entry>Alignment</entry>
632 <entry><literal>INVALID</literal></entry>
633 <entry>Not applicable; cannot be marshaled.</entry>
636 <entry><literal>BYTE</literal></entry>
637 <entry>A single 8-bit byte.</entry>
640 <entry><literal>BOOLEAN</literal></entry>
641 <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
644 <entry><literal>INT16</literal></entry>
645 <entry>16-bit signed integer in the message's byte order.</entry>
648 <entry><literal>UINT16</literal></entry>
649 <entry>16-bit unsigned integer in the message's byte order.</entry>
652 <entry><literal>INT32</literal></entry>
653 <entry>32-bit signed integer in the message's byte order.</entry>
656 <entry><literal>UINT32</literal></entry>
657 <entry>32-bit unsigned integer in the message's byte order.</entry>
660 <entry><literal>INT64</literal></entry>
661 <entry>64-bit signed integer in the message's byte order.</entry>
664 <entry><literal>UINT64</literal></entry>
665 <entry>64-bit unsigned integer in the message's byte order.</entry>
668 <entry><literal>DOUBLE</literal></entry>
669 <entry>64-bit IEEE 754 double in the message's byte order.</entry>
672 <entry><literal>STRING</literal></entry>
673 <entry>A <literal>UINT32</literal> indicating the string's
674 length in bytes excluding its terminating nul, followed by
675 non-nul string data of the given length, followed by a terminating nul
682 <entry><literal>OBJECT_PATH</literal></entry>
683 <entry>Exactly the same as <literal>STRING</literal> except the
684 content must be a valid object path (see below).
690 <entry><literal>SIGNATURE</literal></entry>
691 <entry>The same as <literal>STRING</literal> except the length is a single
692 byte (thus signatures have a maximum length of 255)
693 and the content must be a valid signature (see below).
699 <entry><literal>ARRAY</literal></entry>
701 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
702 alignment padding to the alignment boundary of the array element type,
703 followed by each array element. The array length is from the
704 end of the alignment padding to the end of the last element,
705 i.e. it does not include the padding after the length,
706 or any padding after the last element.
707 Arrays have a maximum length defined to be 2 to the 26th power or
708 67108864. Implementations must not send or accept arrays exceeding this
715 <entry><literal>STRUCT</literal></entry>
717 A struct must start on an 8-byte boundary regardless of the
718 type of the struct fields. The struct value consists of each
719 field marshaled in sequence starting from that 8-byte
726 <entry><literal>VARIANT</literal></entry>
728 A variant type has a marshaled
729 <literal>SIGNATURE</literal> followed by a marshaled
730 value with the type given in the signature. Unlike
731 a message signature, the variant signature can
732 contain only a single complete type. So "i", "ai"
733 or "(ii)" is OK, but "ii" is not. Use of variants may not
734 cause a total message depth to be larger than 64, including
735 other container types such as structures.
738 1 (alignment of the signature)
741 <entry><literal>DICT_ENTRY</literal></entry>
749 <entry><literal>UNIX_FD</literal></entry>
750 <entry>32-bit unsigned integer in the message's byte
751 order. The actual file descriptors need to be
752 transferred out-of-band via some platform specific
753 mechanism. On the wire, values of this type store the index to the
754 file descriptor in the array of file descriptors that
755 accompany the message.</entry>
763 <sect3 id="message-protocol-marshaling-object-path">
764 <title>Valid Object Paths</title>
767 An object path is a name used to refer to an object instance.
768 Conceptually, each participant in a D-Bus message exchange may have
769 any number of object instances (think of C++ or Java objects) and each
770 such instance will have a path. Like a filesystem, the object
771 instances in an application form a hierarchical tree.
775 The following rules define a valid object path. Implementations must
776 not send or accept messages with invalid object paths.
780 The path may be of any length.
785 The path must begin with an ASCII '/' (integer 47) character,
786 and must consist of elements separated by slash characters.
791 Each element must only contain the ASCII characters
797 No element may be the empty string.
802 Multiple '/' characters cannot occur in sequence.
807 A trailing '/' character is not allowed unless the
808 path is the root path (a single '/' character).
815 Object paths are often namespaced by starting with a reversed
816 domain name and containing an interface version number, in the
818 <link linkend="message-protocol-names-interface">interface
820 <link linkend="message-protocol-names-bus">well-known
822 This makes it possible to implement more than one service, or
823 more than one version of a service, in the same process,
824 even if the services share a connection but cannot otherwise
825 co-operate (for instance, if they are implemented by different
830 For instance, if the owner of <literal>example.com</literal> is
831 developing a D-Bus API for a music player, they might use the
832 hierarchy of object paths that start with
833 <literal>/com/example/MusicPlayer1</literal> for its objects.
837 <sect3 id="message-protocol-marshaling-signature">
838 <title>Valid Signatures</title>
840 An implementation must not send or accept invalid signatures.
841 Valid signatures will conform to the following rules:
845 The signature ends with a nul byte.
850 The signature is a list of single complete types.
851 Arrays must have element types, and structs must
852 have both open and close parentheses.
857 Only type codes and open and close parentheses are
858 allowed in the signature. The <literal>STRUCT</literal> type code
859 is not allowed in signatures, because parentheses
865 The maximum depth of container type nesting is 32 array type
866 codes and 32 open parentheses. This implies that the maximum
867 total depth of recursion is 64, for an "array of array of array
868 of ... struct of struct of struct of ..." where there are 32
874 The maximum length of a signature is 255.
879 Signatures must be nul-terminated.
890 <sect1 id="message-protocol">
891 <title>Message Protocol</title>
894 A <firstterm>message</firstterm> consists of a
895 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
896 think of a message as a package, the header is the address, and the body
897 contains the package contents. The message delivery system uses the header
898 information to figure out where to send the message and how to interpret
899 it; the recipient interprets the body of the message.
903 The body of the message is made up of zero or more
904 <firstterm>arguments</firstterm>, which are typed values, such as an
905 integer or a byte array.
909 Both header and body use the D-Bus <link linkend="type-system">type
910 system</link> and format for serializing data.
913 <sect2 id="message-protocol-messages">
914 <title>Message Format</title>
917 A message consists of a header and a body. The header is a block of
918 values with a fixed signature and meaning. The body is a separate block
919 of values, with a signature specified in the header.
923 The length of the header must be a multiple of 8, allowing the body to
924 begin on an 8-byte boundary when storing the entire message in a single
925 buffer. If the header does not naturally end on an 8-byte boundary
926 up to 7 bytes of nul-initialized alignment padding must be added.
930 The message body need not end on an 8-byte boundary.
934 The maximum length of a message, including header, header alignment padding,
935 and body is 2 to the 27th power or 134217728. Implementations must not
936 send or accept messages exceeding this size.
940 The signature of the header is:
944 Written out more readably, this is:
946 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
951 These values have the following meanings:
957 <entry>Description</entry>
962 <entry>1st <literal>BYTE</literal></entry>
963 <entry>Endianness flag; ASCII 'l' for little-endian
964 or ASCII 'B' for big-endian. Both header and body are
965 in this endianness.</entry>
968 <entry>2nd <literal>BYTE</literal></entry>
969 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
970 Currently-defined types are described below.
974 <entry>3rd <literal>BYTE</literal></entry>
975 <entry>Bitwise OR of flags. Unknown flags
976 must be ignored. Currently-defined flags are described below.
980 <entry>4th <literal>BYTE</literal></entry>
981 <entry>Major protocol version of the sending application. If
982 the major protocol version of the receiving application does not
983 match, the applications will not be able to communicate and the
984 D-Bus connection must be disconnected. The major protocol
985 version for this version of the specification is 1.
989 <entry>1st <literal>UINT32</literal></entry>
990 <entry>Length in bytes of the message body, starting
991 from the end of the header. The header ends after
992 its alignment padding to an 8-boundary.
996 <entry>2nd <literal>UINT32</literal></entry>
997 <entry>The serial of this message, used as a cookie
998 by the sender to identify the reply corresponding
999 to this request. This must not be zero.
1003 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
1004 <entry>An array of zero or more <firstterm>header
1005 fields</firstterm> where the byte is the field code, and the
1006 variant is the field value. The message type determines
1007 which fields are required.
1015 <firstterm>Message types</firstterm> that can appear in the second byte
1021 <entry>Conventional name</entry>
1022 <entry>Decimal value</entry>
1023 <entry>Description</entry>
1028 <entry><literal>INVALID</literal></entry>
1030 <entry>This is an invalid type.</entry>
1033 <entry><literal>METHOD_CALL</literal></entry>
1035 <entry>Method call.</entry>
1038 <entry><literal>METHOD_RETURN</literal></entry>
1040 <entry>Method reply with returned data.</entry>
1043 <entry><literal>ERROR</literal></entry>
1045 <entry>Error reply. If the first argument exists and is a
1046 string, it is an error message.</entry>
1049 <entry><literal>SIGNAL</literal></entry>
1051 <entry>Signal emission.</entry>
1058 Flags that can appear in the third byte of the header:
1063 <entry>Conventional name</entry>
1064 <entry>Hex value</entry>
1065 <entry>Description</entry>
1070 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
1072 <entry>This message does not expect method return replies or
1073 error replies; the reply can be omitted as an
1074 optimization. However, it is compliant with this specification
1075 to return the reply despite this flag and the only harm
1076 from doing so is extra network traffic.
1080 <entry><literal>NO_AUTO_START</literal></entry>
1082 <entry>The bus must not launch an owner
1083 for the destination name in response to this message.
1091 <sect3 id="message-protocol-header-fields">
1092 <title>Header Fields</title>
1095 The array at the end of the header contains <firstterm>header
1096 fields</firstterm>, where each field is a 1-byte field code followed
1097 by a field value. A header must contain the required header fields for
1098 its message type, and zero or more of any optional header
1099 fields. Future versions of this protocol specification may add new
1100 fields. Implementations must ignore fields they do not
1101 understand. Implementations must not invent their own header fields;
1102 only changes to this specification may introduce new header fields.
1106 Again, if an implementation sees a header field code that it does not
1107 expect, it must ignore that field, as it will be part of a new
1108 (but compatible) version of this specification. This also applies
1109 to known header fields appearing in unexpected messages, for
1110 example: if a signal has a reply serial it must be ignored
1111 even though it has no meaning as of this version of the spec.
1115 However, implementations must not send or accept known header fields
1116 with the wrong type stored in the field value. So for example a
1117 message with an <literal>INTERFACE</literal> field of type
1118 <literal>UINT32</literal> would be considered corrupt.
1122 Here are the currently-defined header fields:
1127 <entry>Conventional Name</entry>
1128 <entry>Decimal Code</entry>
1130 <entry>Required In</entry>
1131 <entry>Description</entry>
1136 <entry><literal>INVALID</literal></entry>
1139 <entry>not allowed</entry>
1140 <entry>Not a valid field name (error if it appears in a message)</entry>
1143 <entry><literal>PATH</literal></entry>
1145 <entry><literal>OBJECT_PATH</literal></entry>
1146 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1147 <entry>The object to send a call to,
1148 or the object a signal is emitted from.
1150 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
1151 implementations should not send messages with this path,
1152 and the reference implementation of the bus daemon will
1153 disconnect any application that attempts to do so.
1157 <entry><literal>INTERFACE</literal></entry>
1159 <entry><literal>STRING</literal></entry>
1160 <entry><literal>SIGNAL</literal></entry>
1162 The interface to invoke a method call on, or
1163 that a signal is emitted from. Optional for
1164 method calls, required for signals.
1165 The special interface
1166 <literal>org.freedesktop.DBus.Local</literal> is reserved;
1167 implementations should not send messages with this
1168 interface, and the reference implementation of the bus
1169 daemon will disconnect any application that attempts to
1174 <entry><literal>MEMBER</literal></entry>
1176 <entry><literal>STRING</literal></entry>
1177 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1178 <entry>The member, either the method name or signal name.</entry>
1181 <entry><literal>ERROR_NAME</literal></entry>
1183 <entry><literal>STRING</literal></entry>
1184 <entry><literal>ERROR</literal></entry>
1185 <entry>The name of the error that occurred, for errors</entry>
1188 <entry><literal>REPLY_SERIAL</literal></entry>
1190 <entry><literal>UINT32</literal></entry>
1191 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
1192 <entry>The serial number of the message this message is a reply
1193 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
1196 <entry><literal>DESTINATION</literal></entry>
1198 <entry><literal>STRING</literal></entry>
1199 <entry>optional</entry>
1200 <entry>The name of the connection this message is intended for.
1201 Only used in combination with the message bus, see
1202 <xref linkend="message-bus"/>.</entry>
1205 <entry><literal>SENDER</literal></entry>
1207 <entry><literal>STRING</literal></entry>
1208 <entry>optional</entry>
1209 <entry>Unique name of the sending connection.
1210 The message bus fills in this field so it is reliable; the field is
1211 only meaningful in combination with the message bus.</entry>
1214 <entry><literal>SIGNATURE</literal></entry>
1216 <entry><literal>SIGNATURE</literal></entry>
1217 <entry>optional</entry>
1218 <entry>The signature of the message body.
1219 If omitted, it is assumed to be the
1220 empty signature "" (i.e. the body must be 0-length).</entry>
1223 <entry><literal>UNIX_FDS</literal></entry>
1225 <entry><literal>UINT32</literal></entry>
1226 <entry>optional</entry>
1227 <entry>The number of Unix file descriptors that
1228 accompany the message. If omitted, it is assumed
1229 that no Unix file descriptors accompany the
1230 message. The actual file descriptors need to be
1231 transferred via platform specific mechanism
1232 out-of-band. They must be sent at the same time as
1233 part of the message itself. They may not be sent
1234 before the first byte of the message itself is
1235 transferred or after the last byte of the message
1245 <sect2 id="message-protocol-names">
1246 <title>Valid Names</title>
1248 The various names in D-Bus messages have some restrictions.
1251 There is a <firstterm>maximum name length</firstterm>
1252 of 255 which applies to bus names, interfaces, and members.
1254 <sect3 id="message-protocol-names-interface">
1255 <title>Interface names</title>
1257 Interfaces have names with type <literal>STRING</literal>, meaning that
1258 they must be valid UTF-8. However, there are also some
1259 additional restrictions that apply to interface names
1262 <listitem><para>Interface names are composed of 1 or more elements separated by
1263 a period ('.') character. All elements must contain at least
1267 <listitem><para>Each element must only contain the ASCII characters
1268 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1272 <listitem><para>Interface names must contain at least one '.' (period)
1273 character (and thus at least two elements).
1276 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1277 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1282 Interface names should start with the reversed DNS domain name of
1283 the author of the interface (in lower-case), like interface names
1284 in Java. It is conventional for the rest of the interface name
1285 to consist of words run together, with initial capital letters
1286 on all words ("CamelCase"). Several levels of hierarchy can be used.
1287 It is also a good idea to include the major version of the interface
1288 in the name, and increment it if incompatible changes are made;
1289 this way, a single object can implement several versions of an
1290 interface in parallel, if necessary.
1294 For instance, if the owner of <literal>example.com</literal> is
1295 developing a D-Bus API for a music player, they might define
1296 interfaces called <literal>com.example.MusicPlayer1</literal>,
1297 <literal>com.example.MusicPlayer1.Track</literal> and
1298 <literal>com.example.MusicPlayer1.Seekable</literal>.
1302 D-Bus does not distinguish between the concepts that would be
1303 called classes and interfaces in Java: either can be identified on
1304 D-Bus by an interface name.
1307 <sect3 id="message-protocol-names-bus">
1308 <title>Bus names</title>
1310 Connections have one or more bus names associated with them.
1311 A connection has exactly one bus name that is a <firstterm>unique
1312 connection name</firstterm>. The unique connection name remains
1313 with the connection for its entire lifetime.
1314 A bus name is of type <literal>STRING</literal>,
1315 meaning that it must be valid UTF-8. However, there are also
1316 some additional restrictions that apply to bus names
1319 <listitem><para>Bus names that start with a colon (':')
1320 character are unique connection names. Other bus names
1321 are called <firstterm>well-known bus names</firstterm>.
1324 <listitem><para>Bus names are composed of 1 or more elements separated by
1325 a period ('.') character. All elements must contain at least
1329 <listitem><para>Each element must only contain the ASCII characters
1330 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1331 connection name may begin with a digit, elements in
1332 other bus names must not begin with a digit.
1336 <listitem><para>Bus names must contain at least one '.' (period)
1337 character (and thus at least two elements).
1340 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1341 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1345 Note that the hyphen ('-') character is allowed in bus names but
1346 not in interface names.
1350 Like <link linkend="message-protocol-names-interface">interface
1351 names</link>, well-known bus names should start with the
1352 reversed DNS domain name of the author of the interface (in
1353 lower-case), and it is conventional for the rest of the well-known
1354 bus name to consist of words run together, with initial
1355 capital letters. As with interface names, including a version
1356 number in well-known bus names is a good idea; it's possible to
1357 have the well-known bus name for more than one version
1358 simultaneously if backwards compatibility is required.
1362 If a well-known bus name implies the presence of a "main" interface,
1363 that "main" interface is often given the same name as
1364 the well-known bus name, and situated at the corresponding object
1365 path. For instance, if the owner of <literal>example.com</literal>
1366 is developing a D-Bus API for a music player, they might define
1367 that any application that takes the well-known name
1368 <literal>com.example.MusicPlayer1</literal> should have an object
1369 at the object path <literal>/com/example/MusicPlayer1</literal>
1370 which implements the interface
1371 <literal>com.example.MusicPlayer1</literal>.
1374 <sect3 id="message-protocol-names-member">
1375 <title>Member names</title>
1377 Member (i.e. method or signal) names:
1379 <listitem><para>Must only contain the ASCII characters
1380 "[A-Z][a-z][0-9]_" and may not begin with a
1381 digit.</para></listitem>
1382 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1383 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1384 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1389 It is conventional for member names on D-Bus to consist of
1390 capitalized words with no punctuation ("camel-case").
1391 Method names should usually be verbs, such as
1392 <literal>GetItems</literal>, and signal names should usually be
1393 a description of an event, such as <literal>ItemsChanged</literal>.
1396 <sect3 id="message-protocol-names-error">
1397 <title>Error names</title>
1399 Error names have the same restrictions as interface names.
1403 Error names have the same naming conventions as interface
1404 names, and often contain <literal>.Error.</literal>; for instance,
1405 the owner of <literal>example.com</literal> might define the
1406 errors <literal>com.example.MusicPlayer.Error.FileNotFound</literal>
1407 and <literal>com.example.MusicPlayer.Error.OutOfMemory</literal>.
1408 The errors defined by D-Bus itself, such as
1409 <literal>org.freedesktop.DBus.Error.Failed</literal>, follow a
1415 <sect2 id="message-protocol-types">
1416 <title>Message Types</title>
1418 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1419 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1420 This section describes these conventions.
1422 <sect3 id="message-protocol-types-method">
1423 <title>Method Calls</title>
1425 Some messages invoke an operation on a remote object. These are
1426 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1427 messages map naturally to methods on objects in a typical program.
1430 A method call message is required to have a <literal>MEMBER</literal> header field
1431 indicating the name of the method. Optionally, the message has an
1432 <literal>INTERFACE</literal> field giving the interface the method is a part of. In the
1433 absence of an <literal>INTERFACE</literal> field, if two interfaces on the same object have
1434 a method with the same name, it is undefined which of the two methods
1435 will be invoked. Implementations may also choose to return an error in
1436 this ambiguous case. However, if a method name is unique
1437 implementations must not require an interface field.
1440 Method call messages also include a <literal>PATH</literal> field
1441 indicating the object to invoke the method on. If the call is passing
1442 through a message bus, the message will also have a
1443 <literal>DESTINATION</literal> field giving the name of the connection
1444 to receive the message.
1447 When an application handles a method call message, it is required to
1448 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1449 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1450 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1453 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1454 are the return value(s) or "out parameters" of the method call.
1455 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1456 and the call fails; no return value will be provided. It makes
1457 no sense to send multiple replies to the same method call.
1460 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1461 reply is required, so the caller will know the method
1462 was successfully processed.
1465 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1469 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1470 then as an optimization the application receiving the method
1471 call may choose to omit the reply message (regardless of
1472 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1473 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
1474 flag and reply anyway.
1477 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1478 destination name does not exist then a program to own the destination
1479 name will be started before the message is delivered. The message
1480 will be held until the new program is successfully started or has
1481 failed to start; in case of failure, an error will be returned. This
1482 flag is only relevant in the context of a message bus, it is ignored
1483 during one-to-one communication with no intermediate bus.
1485 <sect4 id="message-protocol-types-method-apis">
1486 <title>Mapping method calls to native APIs</title>
1488 APIs for D-Bus may map method calls to a method call in a specific
1489 programming language, such as C++, or may map a method call written
1490 in an IDL to a D-Bus message.
1493 In APIs of this nature, arguments to a method are often termed "in"
1494 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1495 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1496 "inout" arguments, which are both sent and received, i.e. the caller
1497 passes in a value which is modified. Mapped to D-Bus, an "inout"
1498 argument is equivalent to an "in" argument, followed by an "out"
1499 argument. You can't pass things "by reference" over the wire, so
1500 "inout" is purely an illusion of the in-process API.
1503 Given a method with zero or one return values, followed by zero or more
1504 arguments, where each argument may be "in", "out", or "inout", the
1505 caller constructs a message by appending each "in" or "inout" argument,
1506 in order. "out" arguments are not represented in the caller's message.
1509 The recipient constructs a reply by appending first the return value
1510 if any, then each "out" or "inout" argument, in order.
1511 "in" arguments are not represented in the reply message.
1514 Error replies are normally mapped to exceptions in languages that have
1518 In converting from native APIs to D-Bus, it is perhaps nice to
1519 map D-Bus naming conventions ("FooBar") to native conventions
1520 such as "fooBar" or "foo_bar" automatically. This is OK
1521 as long as you can say that the native API is one that
1522 was specifically written for D-Bus. It makes the most sense
1523 when writing object implementations that will be exported
1524 over the bus. Object proxies used to invoke remote D-Bus
1525 objects probably need the ability to call any D-Bus method,
1526 and thus a magic name mapping like this could be a problem.
1529 This specification doesn't require anything of native API bindings;
1530 the preceding is only a suggested convention for consistency
1536 <sect3 id="message-protocol-types-signal">
1537 <title>Signal Emission</title>
1539 Unlike method calls, signal emissions have no replies.
1540 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1541 It must have three header fields: <literal>PATH</literal> giving the object
1542 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1543 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1544 for signals, though it is optional for method calls.
1548 <sect3 id="message-protocol-types-errors">
1549 <title>Errors</title>
1551 Messages of type <literal>ERROR</literal> are most commonly replies
1552 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1553 to any kind of message. The message bus for example
1554 will return an <literal>ERROR</literal> in reply to a signal emission if
1555 the bus does not have enough memory to send the signal.
1558 An <literal>ERROR</literal> may have any arguments, but if the first
1559 argument is a <literal>STRING</literal>, it must be an error message.
1560 The error message may be logged or shown to the user
1565 <sect3 id="message-protocol-types-notation">
1566 <title>Notation in this document</title>
1568 This document uses a simple pseudo-IDL to describe particular method
1569 calls and signals. Here is an example of a method call:
1571 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1572 out UINT32 resultcode)
1574 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1575 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1576 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1577 characters so it's known that the last part of the name in
1578 the "IDL" is the member name.
1581 In C++ that might end up looking like this:
1583 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1584 unsigned int flags);
1586 or equally valid, the return value could be done as an argument:
1588 void org::freedesktop::DBus::StartServiceByName (const char *name,
1590 unsigned int *resultcode);
1592 It's really up to the API designer how they want to make
1593 this look. You could design an API where the namespace wasn't used
1594 in C++, using STL or Qt, using varargs, or whatever you wanted.
1597 Signals are written as follows:
1599 org.freedesktop.DBus.NameLost (STRING name)
1601 Signals don't specify "in" vs. "out" because only
1602 a single direction is possible.
1605 It isn't especially encouraged to use this lame pseudo-IDL in actual
1606 API implementations; you might use the native notation for the
1607 language you're using, or you might use COM or CORBA IDL, for example.
1612 <sect2 id="message-protocol-handling-invalid">
1613 <title>Invalid Protocol and Spec Extensions</title>
1616 For security reasons, the D-Bus protocol should be strictly parsed and
1617 validated, with the exception of defined extension points. Any invalid
1618 protocol or spec violations should result in immediately dropping the
1619 connection without notice to the other end. Exceptions should be
1620 carefully considered, e.g. an exception may be warranted for a
1621 well-understood idiosyncrasy of a widely-deployed implementation. In
1622 cases where the other end of a connection is 100% trusted and known to
1623 be friendly, skipping validation for performance reasons could also make
1624 sense in certain cases.
1628 Generally speaking violations of the "must" requirements in this spec
1629 should be considered possible attempts to exploit security, and violations
1630 of the "should" suggestions should be considered legitimate (though perhaps
1631 they should generate an error in some cases).
1635 The following extension points are built in to D-Bus on purpose and must
1636 not be treated as invalid protocol. The extension points are intended
1637 for use by future versions of this spec, they are not intended for third
1638 parties. At the moment, the only way a third party could extend D-Bus
1639 without breaking interoperability would be to introduce a way to negotiate new
1640 feature support as part of the auth protocol, using EXTENSION_-prefixed
1641 commands. There is not yet a standard way to negotiate features.
1645 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
1646 commands result in an ERROR rather than a disconnect. This enables
1647 future extensions to the protocol. Commands starting with EXTENSION_ are
1648 reserved for third parties.
1653 The authentication protocol supports pluggable auth mechanisms.
1658 The address format (see <xref linkend="addresses"/>) supports new
1664 Messages with an unknown type (something other than
1665 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
1666 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
1667 Unknown-type messages must still be well-formed in the same way
1668 as the known messages, however. They still have the normal
1674 Header fields with an unknown or unexpected field code must be ignored,
1675 though again they must still be well-formed.
1680 New standard interfaces (with new methods and signals) can of course be added.
1690 <sect1 id="auth-protocol">
1691 <title>Authentication Protocol</title>
1693 Before the flow of messages begins, two applications must
1694 authenticate. A simple plain-text protocol is used for
1695 authentication; this protocol is a SASL profile, and maps fairly
1696 directly from the SASL specification. The message encoding is
1697 NOT used here, only plain text messages.
1700 In examples, "C:" and "S:" indicate lines sent by the client and
1701 server respectively.
1703 <sect2 id="auth-protocol-overview">
1704 <title>Protocol Overview</title>
1706 The protocol is a line-based protocol, where each line ends with
1707 \r\n. Each line begins with an all-caps ASCII command name containing
1708 only the character range [A-Z_], a space, then any arguments for the
1709 command, then the \r\n ending the line. The protocol is
1710 case-sensitive. All bytes must be in the ASCII character set.
1712 Commands from the client to the server are as follows:
1715 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
1716 <listitem><para>CANCEL</para></listitem>
1717 <listitem><para>BEGIN</para></listitem>
1718 <listitem><para>DATA <data in hex encoding></para></listitem>
1719 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
1720 <listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
1723 From server to client are as follows:
1726 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
1727 <listitem><para>OK <GUID in hex></para></listitem>
1728 <listitem><para>DATA <data in hex encoding></para></listitem>
1729 <listitem><para>ERROR</para></listitem>
1730 <listitem><para>AGREE_UNIX_FD</para></listitem>
1734 Unofficial extensions to the command set must begin with the letters
1735 "EXTENSION_", to avoid conflicts with future official commands.
1736 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
1739 <sect2 id="auth-nul-byte">
1740 <title>Special credentials-passing nul byte</title>
1742 Immediately after connecting to the server, the client must send a
1743 single nul byte. This byte may be accompanied by credentials
1744 information on some operating systems that use sendmsg() with
1745 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
1746 sockets. However, the nul byte must be sent even on other kinds of
1747 socket, and even on operating systems that do not require a byte to be
1748 sent in order to transmit credentials. The text protocol described in
1749 this document begins after the single nul byte. If the first byte
1750 received from the client is not a nul byte, the server may disconnect
1754 A nul byte in any context other than the initial byte is an error;
1755 the protocol is ASCII-only.
1758 The credentials sent along with the nul byte may be used with the
1759 SASL mechanism EXTERNAL.
1762 <sect2 id="auth-command-auth">
1763 <title>AUTH command</title>
1765 If an AUTH command has no arguments, it is a request to list
1766 available mechanisms. The server must respond with a REJECTED
1767 command listing the mechanisms it understands, or with an error.
1770 If an AUTH command specifies a mechanism, and the server supports
1771 said mechanism, the server should begin exchanging SASL
1772 challenge-response data with the client using DATA commands.
1775 If the server does not support the mechanism given in the AUTH
1776 command, it must send either a REJECTED command listing the mechanisms
1777 it does support, or an error.
1780 If the [initial-response] argument is provided, it is intended for use
1781 with mechanisms that have no initial challenge (or an empty initial
1782 challenge), as if it were the argument to an initial DATA command. If
1783 the selected mechanism has an initial challenge and [initial-response]
1784 was provided, the server should reject authentication by sending
1788 If authentication succeeds after exchanging DATA commands,
1789 an OK command must be sent to the client.
1792 The first octet received by the server after the \r\n of the BEGIN
1793 command from the client must be the first octet of the
1794 authenticated/encrypted stream of D-Bus messages.
1797 If BEGIN is received by the server, the first octet received
1798 by the client after the \r\n of the OK command must be the
1799 first octet of the authenticated/encrypted stream of D-Bus
1803 <sect2 id="auth-command-cancel">
1804 <title>CANCEL Command</title>
1806 At any time up to sending the BEGIN command, the client may send a
1807 CANCEL command. On receiving the CANCEL command, the server must
1808 send a REJECTED command and abort the current authentication
1812 <sect2 id="auth-command-data">
1813 <title>DATA Command</title>
1815 The DATA command may come from either client or server, and simply
1816 contains a hex-encoded block of data to be interpreted
1817 according to the SASL mechanism in use.
1820 Some SASL mechanisms support sending an "empty string";
1821 FIXME we need some way to do this.
1824 <sect2 id="auth-command-begin">
1825 <title>BEGIN Command</title>
1827 The BEGIN command acknowledges that the client has received an
1828 OK command from the server, and that the stream of messages
1832 The first octet received by the server after the \r\n of the BEGIN
1833 command from the client must be the first octet of the
1834 authenticated/encrypted stream of D-Bus messages.
1837 <sect2 id="auth-command-rejected">
1838 <title>REJECTED Command</title>
1840 The REJECTED command indicates that the current authentication
1841 exchange has failed, and further exchange of DATA is inappropriate.
1842 The client would normally try another mechanism, or try providing
1843 different responses to challenges.
1845 Optionally, the REJECTED command has a space-separated list of
1846 available auth mechanisms as arguments. If a server ever provides
1847 a list of supported mechanisms, it must provide the same list
1848 each time it sends a REJECTED message. Clients are free to
1849 ignore all lists received after the first.
1852 <sect2 id="auth-command-ok">
1853 <title>OK Command</title>
1855 The OK command indicates that the client has been
1856 authenticated. The client may now proceed with negotiating
1857 Unix file descriptor passing. To do that it shall send
1858 NEGOTIATE_UNIX_FD to the server.
1861 Otherwise, the client must respond to the OK command by
1862 sending a BEGIN command, followed by its stream of messages,
1863 or by disconnecting. The server must not accept additional
1864 commands using this protocol after the BEGIN command has been
1865 received. Further communication will be a stream of D-Bus
1866 messages (optionally encrypted, as negotiated) rather than
1870 If a client sends BEGIN the first octet received by the client
1871 after the \r\n of the OK command must be the first octet of
1872 the authenticated/encrypted stream of D-Bus messages.
1875 The OK command has one argument, which is the GUID of the server.
1876 See <xref linkend="addresses"/> for more on server GUIDs.
1879 <sect2 id="auth-command-error">
1880 <title>ERROR Command</title>
1882 The ERROR command indicates that either server or client did not
1883 know a command, does not accept the given command in the current
1884 context, or did not understand the arguments to the command. This
1885 allows the protocol to be extended; a client or server can send a
1886 command present or permitted only in new protocol versions, and if
1887 an ERROR is received instead of an appropriate response, fall back
1888 to using some other technique.
1891 If an ERROR is sent, the server or client that sent the
1892 error must continue as if the command causing the ERROR had never been
1893 received. However, the the server or client receiving the error
1894 should try something other than whatever caused the error;
1895 if only canceling/rejecting the authentication.
1898 If the D-Bus protocol changes incompatibly at some future time,
1899 applications implementing the new protocol would probably be able to
1900 check for support of the new protocol by sending a new command and
1901 receiving an ERROR from applications that don't understand it. Thus the
1902 ERROR feature of the auth protocol is an escape hatch that lets us
1903 negotiate extensions or changes to the D-Bus protocol in the future.
1906 <sect2 id="auth-command-negotiate-unix-fd">
1907 <title>NEGOTIATE_UNIX_FD Command</title>
1909 The NEGOTIATE_UNIX_FD command indicates that the client
1910 supports Unix file descriptor passing. This command may only
1911 be sent after the connection is authenticated, i.e. after OK
1912 was received by the client. This command may only be sent on
1913 transports that support Unix file descriptor passing.
1916 On receiving NEGOTIATE_UNIX_FD the server must respond with
1917 either AGREE_UNIX_FD or ERROR. It shall respond the former if
1918 the transport chosen supports Unix file descriptor passing and
1919 the server supports this feature. It shall respond the latter
1920 if the transport does not support Unix file descriptor
1921 passing, the server does not support this feature, or the
1922 server decides not to enable file descriptor passing due to
1923 security or other reasons.
1926 <sect2 id="auth-command-agree-unix-fd">
1927 <title>AGREE_UNIX_FD Command</title>
1929 The AGREE_UNIX_FD command indicates that the server supports
1930 Unix file descriptor passing. This command may only be sent
1931 after the connection is authenticated, and the client sent
1932 NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
1933 command may only be sent on transports that support Unix file
1937 On receiving AGREE_UNIX_FD the client must respond with BEGIN,
1938 followed by its stream of messages, or by disconnecting. The
1939 server must not accept additional commands using this protocol
1940 after the BEGIN command has been received. Further
1941 communication will be a stream of D-Bus messages (optionally
1942 encrypted, as negotiated) rather than this protocol.
1945 <sect2 id="auth-command-future">
1946 <title>Future Extensions</title>
1948 Future extensions to the authentication and negotiation
1949 protocol are possible. For that new commands may be
1950 introduced. If a client or server receives an unknown command
1951 it shall respond with ERROR and not consider this fatal. New
1952 commands may be introduced both before, and after
1953 authentication, i.e. both before and after the OK command.
1956 <sect2 id="auth-examples">
1957 <title>Authentication examples</title>
1961 <title>Example of successful magic cookie authentication</title>
1963 (MAGIC_COOKIE is a made up mechanism)
1965 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
1971 <title>Example of finding out mechanisms then picking one</title>
1974 S: REJECTED KERBEROS_V4 SKEY
1975 C: AUTH SKEY 7ab83f32ee
1976 S: DATA 8799cabb2ea93e
1977 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1983 <title>Example of client sends unknown command then falls back to regular auth</title>
1987 C: AUTH MAGIC_COOKIE 3736343435313230333039
1993 <title>Example of server doesn't support initial auth mechanism</title>
1995 C: AUTH MAGIC_COOKIE 3736343435313230333039
1996 S: REJECTED KERBEROS_V4 SKEY
1997 C: AUTH SKEY 7ab83f32ee
1998 S: DATA 8799cabb2ea93e
1999 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2005 <title>Example of wrong password or the like followed by successful retry</title>
2007 C: AUTH MAGIC_COOKIE 3736343435313230333039
2008 S: REJECTED KERBEROS_V4 SKEY
2009 C: AUTH SKEY 7ab83f32ee
2010 S: DATA 8799cabb2ea93e
2011 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2013 C: AUTH SKEY 7ab83f32ee
2014 S: DATA 8799cabb2ea93e
2015 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2021 <title>Example of skey cancelled and restarted</title>
2023 C: AUTH MAGIC_COOKIE 3736343435313230333039
2024 S: REJECTED KERBEROS_V4 SKEY
2025 C: AUTH SKEY 7ab83f32ee
2026 S: DATA 8799cabb2ea93e
2029 C: AUTH SKEY 7ab83f32ee
2030 S: DATA 8799cabb2ea93e
2031 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2037 <title>Example of successful magic cookie authentication with successful negotiation of Unix FD passing</title>
2039 (MAGIC_COOKIE is a made up mechanism)
2041 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2043 C: NEGOTIATE_UNIX_FD
2049 <title>Example of successful magic cookie authentication with unsuccessful negotiation of Unix FD passing</title>
2051 (MAGIC_COOKIE is a made up mechanism)
2053 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2055 C: NEGOTIATE_UNIX_FD
2062 <sect2 id="auth-states">
2063 <title>Authentication state diagrams</title>
2066 This section documents the auth protocol in terms of
2067 a state machine for the client and the server. This is
2068 probably the most robust way to implement the protocol.
2071 <sect3 id="auth-states-client">
2072 <title>Client states</title>
2075 To more precisely describe the interaction between the
2076 protocol state machine and the authentication mechanisms the
2077 following notation is used: MECH(CHALL) means that the
2078 server challenge CHALL was fed to the mechanism MECH, which
2084 CONTINUE(RESP) means continue the auth conversation
2085 and send RESP as the response to the server;
2091 OK(RESP) means that after sending RESP to the server
2092 the client side of the auth conversation is finished
2093 and the server should return "OK";
2099 ERROR means that CHALL was invalid and could not be
2105 Both RESP and CHALL may be empty.
2109 The Client starts by getting an initial response from the
2110 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
2111 the mechanism did not provide an initial response. If the
2112 mechanism returns CONTINUE, the client starts in state
2113 <emphasis>WaitingForData</emphasis>, if the mechanism
2114 returns OK the client starts in state
2115 <emphasis>WaitingForOK</emphasis>.
2119 The client should keep track of available mechanisms and
2120 which it mechanisms it has already attempted. This list is
2121 used to decide which AUTH command to send. When the list is
2122 exhausted, the client should give up and close the
2127 <title><emphasis>WaitingForData</emphasis></title>
2135 MECH(CHALL) returns CONTINUE(RESP) → send
2137 <emphasis>WaitingForData</emphasis>
2141 MECH(CHALL) returns OK(RESP) → send DATA
2142 RESP, goto <emphasis>WaitingForOK</emphasis>
2146 MECH(CHALL) returns ERROR → send ERROR
2147 [msg], goto <emphasis>WaitingForData</emphasis>
2155 Receive REJECTED [mechs] →
2156 send AUTH [next mech], goto
2157 WaitingForData or <emphasis>WaitingForOK</emphasis>
2162 Receive ERROR → send
2164 <emphasis>WaitingForReject</emphasis>
2169 Receive OK → send
2170 BEGIN, terminate auth
2171 conversation, authenticated
2176 Receive anything else → send
2178 <emphasis>WaitingForData</emphasis>
2186 <title><emphasis>WaitingForOK</emphasis></title>
2191 Receive OK → send BEGIN, terminate auth
2192 conversation, <emphasis>authenticated</emphasis>
2197 Receive REJECT [mechs] → send AUTH [next mech],
2198 goto <emphasis>WaitingForData</emphasis> or
2199 <emphasis>WaitingForOK</emphasis>
2205 Receive DATA → send CANCEL, goto
2206 <emphasis>WaitingForReject</emphasis>
2212 Receive ERROR → send CANCEL, goto
2213 <emphasis>WaitingForReject</emphasis>
2219 Receive anything else → send ERROR, goto
2220 <emphasis>WaitingForOK</emphasis>
2228 <title><emphasis>WaitingForReject</emphasis></title>
2233 Receive REJECT [mechs] → send AUTH [next mech],
2234 goto <emphasis>WaitingForData</emphasis> or
2235 <emphasis>WaitingForOK</emphasis>
2241 Receive anything else → terminate auth
2242 conversation, disconnect
2251 <sect3 id="auth-states-server">
2252 <title>Server states</title>
2255 For the server MECH(RESP) means that the client response
2256 RESP was fed to the the mechanism MECH, which returns one of
2261 CONTINUE(CHALL) means continue the auth conversation and
2262 send CHALL as the challenge to the client;
2268 OK means that the client has been successfully
2275 REJECT means that the client failed to authenticate or
2276 there was an error in RESP.
2281 The server starts out in state
2282 <emphasis>WaitingForAuth</emphasis>. If the client is
2283 rejected too many times the server must disconnect the
2288 <title><emphasis>WaitingForAuth</emphasis></title>
2294 Receive AUTH → send REJECTED [mechs], goto
2295 <emphasis>WaitingForAuth</emphasis>
2301 Receive AUTH MECH RESP
2305 MECH not valid mechanism → send REJECTED
2307 <emphasis>WaitingForAuth</emphasis>
2311 MECH(RESP) returns CONTINUE(CHALL) → send
2313 <emphasis>WaitingForData</emphasis>
2317 MECH(RESP) returns OK → send OK, goto
2318 <emphasis>WaitingForBegin</emphasis>
2322 MECH(RESP) returns REJECT → send REJECTED
2324 <emphasis>WaitingForAuth</emphasis>
2332 Receive BEGIN → terminate
2333 auth conversation, disconnect
2339 Receive ERROR → send REJECTED [mechs], goto
2340 <emphasis>WaitingForAuth</emphasis>
2346 Receive anything else → send
2348 <emphasis>WaitingForAuth</emphasis>
2357 <title><emphasis>WaitingForData</emphasis></title>
2365 MECH(RESP) returns CONTINUE(CHALL) → send
2367 <emphasis>WaitingForData</emphasis>
2371 MECH(RESP) returns OK → send OK, goto
2372 <emphasis>WaitingForBegin</emphasis>
2376 MECH(RESP) returns REJECT → send REJECTED
2378 <emphasis>WaitingForAuth</emphasis>
2386 Receive BEGIN → terminate auth conversation,
2393 Receive CANCEL → send REJECTED [mechs], goto
2394 <emphasis>WaitingForAuth</emphasis>
2400 Receive ERROR → send REJECTED [mechs], goto
2401 <emphasis>WaitingForAuth</emphasis>
2407 Receive anything else → send ERROR, goto
2408 <emphasis>WaitingForData</emphasis>
2416 <title><emphasis>WaitingForBegin</emphasis></title>
2421 Receive BEGIN → terminate auth conversation,
2422 client authenticated
2428 Receive CANCEL → send REJECTED [mechs], goto
2429 <emphasis>WaitingForAuth</emphasis>
2435 Receive ERROR → send REJECTED [mechs], goto
2436 <emphasis>WaitingForAuth</emphasis>
2442 Receive anything else → send ERROR, goto
2443 <emphasis>WaitingForBegin</emphasis>
2453 <sect2 id="auth-mechanisms">
2454 <title>Authentication mechanisms</title>
2456 This section describes some new authentication mechanisms.
2457 D-Bus also allows any standard SASL mechanism of course.
2459 <sect3 id="auth-mechanisms-sha">
2460 <title>DBUS_COOKIE_SHA1</title>
2462 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2463 has the ability to read a private file owned by the user being
2464 authenticated. If the client can prove that it has access to a secret
2465 cookie stored in this file, then the client is authenticated.
2466 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2470 Throughout this description, "hex encoding" must output the digits
2471 from a to f in lower-case; the digits A to F must not be used
2472 in the DBUS_COOKIE_SHA1 mechanism.
2475 Authentication proceeds as follows:
2479 The client sends the username it would like to authenticate
2485 The server sends the name of its "cookie context" (see below); a
2486 space character; the integer ID of the secret cookie the client
2487 must demonstrate knowledge of; a space character; then a
2488 randomly-generated challenge string, all of this hex-encoded into
2494 The client locates the cookie and generates its own
2495 randomly-generated challenge string. The client then concatenates
2496 the server's decoded challenge, a ":" character, its own challenge,
2497 another ":" character, and the cookie. It computes the SHA-1 hash
2498 of this composite string as a hex digest. It concatenates the
2499 client's challenge string, a space character, and the SHA-1 hex
2500 digest, hex-encodes the result and sends it back to the server.
2505 The server generates the same concatenated string used by the
2506 client and computes its SHA-1 hash. It compares the hash with
2507 the hash received from the client; if the two hashes match, the
2508 client is authenticated.
2514 Each server has a "cookie context," which is a name that identifies a
2515 set of cookies that apply to that server. A sample context might be
2516 "org_freedesktop_session_bus". Context names must be valid ASCII,
2517 nonzero length, and may not contain the characters slash ("/"),
2518 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2519 tab ("\t"), or period ("."). There is a default context,
2520 "org_freedesktop_general" that's used by servers that do not specify
2524 Cookies are stored in a user's home directory, in the directory
2525 <filename>~/.dbus-keyrings/</filename>. This directory must
2526 not be readable or writable by other users. If it is,
2527 clients and servers must ignore it. The directory
2528 contains cookie files named after the cookie context.
2531 A cookie file contains one cookie per line. Each line
2532 has three space-separated fields:
2536 The cookie ID number, which must be a non-negative integer and
2537 may not be used twice in the same file.
2542 The cookie's creation time, in UNIX seconds-since-the-epoch
2548 The cookie itself, a hex-encoded random block of bytes. The cookie
2549 may be of any length, though obviously security increases
2550 as the length increases.
2556 Only server processes modify the cookie file.
2557 They must do so with this procedure:
2561 Create a lockfile name by appending ".lock" to the name of the
2562 cookie file. The server should attempt to create this file
2563 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2564 fails, the lock fails. Servers should retry for a reasonable
2565 period of time, then they may choose to delete an existing lock
2566 to keep users from having to manually delete a stale
2567 lock. <footnote><para>Lockfiles are used instead of real file
2568 locking <literal>fcntl()</literal> because real locking
2569 implementations are still flaky on network
2570 filesystems.</para></footnote>
2575 Once the lockfile has been created, the server loads the cookie
2576 file. It should then delete any cookies that are old (the
2577 timeout can be fairly short), or more than a reasonable
2578 time in the future (so that cookies never accidentally
2579 become permanent, if the clock was set far into the future
2580 at some point). If no recent keys remain, the
2581 server may generate a new key.
2586 The pruned and possibly added-to cookie file
2587 must be resaved atomically (using a temporary
2588 file which is rename()'d).
2593 The lock must be dropped by deleting the lockfile.
2599 Clients need not lock the file in order to load it,
2600 because servers are required to save the file atomically.
2605 <sect1 id="addresses">
2606 <title>Server Addresses</title>
2608 Server addresses consist of a transport name followed by a colon, and
2609 then an optional, comma-separated list of keys and values in the form key=value.
2610 Each value is escaped.
2614 <programlisting>unix:path=/tmp/dbus-test</programlisting>
2615 Which is the address to a unix socket with the path /tmp/dbus-test.
2618 Value escaping is similar to URI escaping but simpler.
2622 The set of optionally-escaped bytes is:
2623 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
2624 <emphasis>byte</emphasis> (note, not character) which is not in the
2625 set of optionally-escaped bytes must be replaced with an ASCII
2626 percent (<literal>%</literal>) and the value of the byte in hex.
2627 The hex value must always be two digits, even if the first digit is
2628 zero. The optionally-escaped bytes may be escaped if desired.
2633 To unescape, append each byte in the value; if a byte is an ASCII
2634 percent (<literal>%</literal>) character then append the following
2635 hex value instead. It is an error if a <literal>%</literal> byte
2636 does not have two hex digits following. It is an error if a
2637 non-optionally-escaped byte is seen unescaped.
2641 The set of optionally-escaped bytes is intended to preserve address
2642 readability and convenience.
2646 A server may specify a key-value pair with the key <literal>guid</literal>
2647 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
2648 describes the format of the <literal>guid</literal> field. If present,
2649 this UUID may be used to distinguish one server address from another. A
2650 server should use a different UUID for each address it listens on. For
2651 example, if a message bus daemon offers both UNIX domain socket and TCP
2652 connections, but treats clients the same regardless of how they connect,
2653 those two connections are equivalent post-connection but should have
2654 distinct UUIDs to distinguish the kinds of connection.
2658 The intent of the address UUID feature is to allow a client to avoid
2659 opening multiple identical connections to the same server, by allowing the
2660 client to check whether an address corresponds to an already-existing
2661 connection. Comparing two addresses is insufficient, because addresses
2662 can be recycled by distinct servers, and equivalent addresses may look
2663 different if simply compared as strings (for example, the host in a TCP
2664 address can be given as an IP address or as a hostname).
2668 Note that the address key is <literal>guid</literal> even though the
2669 rest of the API and documentation says "UUID," for historical reasons.
2673 [FIXME clarify if attempting to connect to each is a requirement
2674 or just a suggestion]
2675 When connecting to a server, multiple server addresses can be
2676 separated by a semi-colon. The library will then try to connect
2677 to the first address and if that fails, it'll try to connect to
2678 the next one specified, and so forth. For example
2679 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
2684 <sect1 id="transports">
2685 <title>Transports</title>
2687 [FIXME we need to specify in detail each transport and its possible arguments]
2689 Current transports include: unix domain sockets (including
2690 abstract namespace on linux), launchd, systemd, TCP/IP, an executed subprocess and a debug/testing transport
2691 using in-process pipes. Future possible transports include one that
2692 tunnels over X11 protocol.
2695 <sect2 id="transports-unix-domain-sockets">
2696 <title>Unix Domain Sockets</title>
2698 Unix domain sockets can be either paths in the file system or on Linux
2699 kernels, they can be abstract which are similar to paths but
2700 do not show up in the file system.
2704 When a socket is opened by the D-Bus library it truncates the path
2705 name right before the first trailing Nul byte. This is true for both
2706 normal paths and abstract paths. Note that this is a departure from
2707 previous versions of D-Bus that would create sockets with a fixed
2708 length path name. Names which were shorter than the fixed length
2709 would be padded by Nul bytes.
2712 Unix domain sockets are not available on Windows.
2714 <sect3 id="transports-unix-domain-sockets-addresses">
2715 <title>Server Address Format</title>
2717 Unix domain socket addresses are identified by the "unix:" prefix
2718 and support the following key/value pairs:
2725 <entry>Values</entry>
2726 <entry>Description</entry>
2732 <entry>(path)</entry>
2733 <entry>path of the unix domain socket. If set, the "tmpdir" and "abstract" key must not be set.</entry>
2736 <entry>tmpdir</entry>
2737 <entry>(path)</entry>
2738 <entry>temporary 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. If set, the "path" and "abstract" key must not be set.</entry>
2741 <entry>abstract</entry>
2742 <entry>(string)</entry>
2743 <entry>unique string (path) in the abstract namespace. If set, the "path" or "tempdir" key must not be set.</entry>
2750 <sect2 id="transports-launchd">
2751 <title>launchd</title>
2753 launchd is an open-source server management system that replaces init, inetd
2754 and cron on Apple Mac OS X versions 10.4 and above. It provides a common session
2755 bus address for each user and deprecates the X11-enabled D-Bus launcher on OSX.
2759 launchd allocates a socket and provides it with the unix path through the
2760 DBUS_LAUNCHD_SESSION_BUS_SOCKET variable in launchd's environment. Every process
2761 spawned by launchd (or dbus-daemon, if it was started by launchd) can access
2762 it through its environment.
2763 Other processes can query for the launchd socket by executing:
2764 $ launchctl getenv DBUS_LAUNCHD_SESSION_BUS_SOCKET
2765 This is normally done by the D-Bus client library so doesn't have to be done
2769 launchd is not available on Microsoft Windows.
2771 <sect3 id="transports-launchd-addresses">
2772 <title>Server Address Format</title>
2774 launchd addresses are identified by the "launchd:" prefix
2775 and support the following key/value pairs:
2782 <entry>Values</entry>
2783 <entry>Description</entry>
2789 <entry>(environment variable)</entry>
2790 <entry>path of the unix domain socket for the launchd created dbus-daemon.</entry>
2797 <sect2 id="transports-systemd">
2798 <title>systemd</title>
2800 systemd is an open-source server management system that
2801 replaces init and inetd on newer Linux systems. It supports
2802 socket activation. The D-Bus systemd transport is used to acquire
2803 socket activation file descriptors from systemd and use them
2804 as D-Bus transport when the current process is spawned by
2805 socket activation from it.
2808 The systemd transport accepts only one or more Unix domain or
2809 TCP streams sockets passed in via socket activation.
2812 The systemd transport is not available on non-Linux operating systems.
2815 The systemd transport defines no parameter keys.
2818 <sect2 id="transports-tcp-sockets">
2819 <title>TCP Sockets</title>
2821 The tcp transport provides TCP/IP based connections between clients
2822 located on the same or different hosts.
2825 Using tcp transport without any additional secure authentification mechanismus
2826 over a network is unsecure.
2829 Windows notes: Because of the tcp stack on Windows does not provide sending
2830 credentials over a tcp connection, the EXTERNAL authentification
2831 mechanismus does not work.
2833 <sect3 id="transports-tcp-sockets-addresses">
2834 <title>Server Address Format</title>
2836 TCP/IP socket addresses are identified by the "tcp:" prefix
2837 and support the following key/value pairs:
2844 <entry>Values</entry>
2845 <entry>Description</entry>
2851 <entry>(string)</entry>
2852 <entry>dns name or ip address</entry>
2856 <entry>(number)</entry>
2857 <entry>The tcp port the server will open. A zero value let the server
2858 choose a free port provided from the underlaying operating system.
2859 libdbus is able to retrieve the real used port from the server.
2863 <entry>family</entry>
2864 <entry>(string)</entry>
2865 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
2872 <sect2 id="transports-nonce-tcp-sockets">
2873 <title>Nonce-secured TCP Sockets</title>
2875 The nonce-tcp transport provides a secured TCP transport, using a
2876 simple authentication mechanism to ensure that only clients with read
2877 access to a certain location in the filesystem can connect to the server.
2878 The server writes a secret, the nonce, to a file and an incoming client
2879 connection is only accepted if the client sends the nonce right after
2880 the connect. The nonce mechanism requires no setup and is orthogonal to
2881 the higher-level authentication mechanisms described in the
2882 Authentication section.
2886 On start, the server generates a random 16 byte nonce and writes it
2887 to a file in the user's temporary directory. The nonce file location
2888 is published as part of the server's D-Bus address using the
2889 "noncefile" key-value pair.
2891 After an accept, the server reads 16 bytes from the socket. If the
2892 read bytes do not match the nonce stored in the nonce file, the
2893 server MUST immediately drop the connection.
2894 If the nonce match the received byte sequence, the client is accepted
2895 and the transport behaves like an unsecured tcp transport.
2898 After a successful connect to the server socket, the client MUST read
2899 the nonce from the file published by the server via the noncefile=
2900 key-value pair and send it over the socket. After that, the
2901 transport behaves like an unsecured tcp transport.
2903 <sect3 id="transports-nonce-tcp-sockets-addresses">
2904 <title>Server Address Format</title>
2906 Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix
2907 and support the following key/value pairs:
2914 <entry>Values</entry>
2915 <entry>Description</entry>
2921 <entry>(string)</entry>
2922 <entry>dns name or ip address</entry>
2926 <entry>(number)</entry>
2927 <entry>The tcp port the server will open. A zero value let the server
2928 choose a free port provided from the underlaying operating system.
2929 libdbus is able to retrieve the real used port from the server.
2933 <entry>family</entry>
2934 <entry>(string)</entry>
2935 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
2938 <entry>noncefile</entry>
2939 <entry>(path)</entry>
2940 <entry>file location containing the secret</entry>
2947 <sect2 id="transports-exec">
2948 <title>Executed Subprocesses on Unix</title>
2950 This transport forks off a process and connects its standard
2951 input and standard output with an anonymous Unix domain
2952 socket. This socket is then used for communication by the
2953 transport. This transport may be used to use out-of-process
2954 forwarder programs as basis for the D-Bus protocol.
2957 The forked process will inherit the standard error output and
2958 process group from the parent process.
2961 Executed subprocesses are not available on Windows.
2963 <sect3 id="transports-exec-addresses">
2964 <title>Server Address Format</title>
2966 Executed subprocess addresses are identified by the "unixexec:" prefix
2967 and support the following key/value pairs:
2974 <entry>Values</entry>
2975 <entry>Description</entry>
2981 <entry>(path)</entry>
2982 <entry>Path of the binary to execute, either an absolute
2983 path or a binary name that is searched for in the default
2984 search path of the OS. This corresponds to the first
2985 argument of execlp(). This key is mandatory.</entry>
2988 <entry>argv0</entry>
2989 <entry>(string)</entry>
2990 <entry>The program name to use when executing the
2991 binary. If omitted the same value as specified for path=
2992 will be used. This corresponds to the second argument of
2996 <entry>argv1, argv2, ...</entry>
2997 <entry>(string)</entry>
2998 <entry>Arguments to pass to the binary. This corresponds
2999 to the third and later arguments of execlp(). If a
3000 specific argvX is not specified no further argvY for Y > X
3001 are taken into account.</entry>
3009 <sect1 id="meta-transports">
3010 <title>Meta Transports</title>
3012 Meta transports are a kind of transport with special enhancements or
3013 behavior. Currently available meta transports include: autolaunch
3016 <sect2 id="meta-transports-autolaunch">
3017 <title>Autolaunch</title>
3018 <para>The autolaunch transport provides a way for dbus clients to autodetect
3019 a running dbus session bus and to autolaunch a session bus if not present.
3021 <sect3 id="meta-transports-autolaunch-addresses">
3022 <title>Server Address Format</title>
3024 Autolaunch addresses uses the "autolaunch:" prefix and support the
3025 following key/value pairs:
3032 <entry>Values</entry>
3033 <entry>Description</entry>
3038 <entry>scope</entry>
3039 <entry>(string)</entry>
3040 <entry>scope of autolaunch (Windows only)
3044 "*install-path" - limit session bus to dbus installation path.
3045 The dbus installation path is determined from the location of
3046 the shared dbus library. If the library is located in a 'bin'
3047 subdirectory the installation root is the directory above,
3048 otherwise the directory where the library lives is taken as
3051 <install-root>/bin/[lib]dbus-1.dll
3052 <install-root>/[lib]dbus-1.dll
3058 "*user" - limit session bus to the recent user.
3063 other values - specify dedicated session bus like "release",
3075 <sect3 id="meta-transports-autolaunch-windows-implementation">
3076 <title>Windows implementation</title>
3078 On start, the server opens a platform specific transport, creates a mutex
3079 and a shared memory section containing the related session bus address.
3080 This mutex will be inspected by the dbus client library to detect a
3081 running dbus session bus. The access to the mutex and the shared memory
3082 section are protected by global locks.
3085 In the recent implementation the autolaunch transport uses a tcp transport
3086 on localhost with a port choosen from the operating system. This detail may
3087 change in the future.
3090 Disclaimer: The recent implementation is in an early state and may not
3091 work in all cirumstances and/or may have security issues. Because of this
3092 the implementation is not documentated yet.
3099 <title>UUIDs</title>
3101 A working D-Bus implementation uses universally-unique IDs in two places.
3102 First, each server address has a UUID identifying the address,
3103 as described in <xref linkend="addresses"/>. Second, each operating
3104 system kernel instance running a D-Bus client or server has a UUID
3105 identifying that kernel, retrieved by invoking the method
3106 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
3107 linkend="standard-interfaces-peer"/>).
3110 The term "UUID" in this document is intended literally, i.e. an
3111 identifier that is universally unique. It is not intended to refer to
3112 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
3115 The UUID must contain 128 bits of data and be hex-encoded. The
3116 hex-encoded string may not contain hyphens or other non-hex-digit
3117 characters, and it must be exactly 32 characters long. To generate a
3118 UUID, the current reference implementation concatenates 96 bits of random
3119 data followed by the 32-bit time in seconds since the UNIX epoch (in big
3123 It would also be acceptable and probably better to simply generate 128
3124 bits of random data, as long as the random number generator is of high
3125 quality. The timestamp could conceivably help if the random bits are not
3126 very random. With a quality random number generator, collisions are
3127 extremely unlikely even with only 96 bits, so it's somewhat academic.
3130 Implementations should, however, stick to random data for the first 96 bits
3135 <sect1 id="standard-interfaces">
3136 <title>Standard Interfaces</title>
3138 See <xref linkend="message-protocol-types-notation"/> for details on
3139 the notation used in this section. There are some standard interfaces
3140 that may be useful across various D-Bus applications.
3142 <sect2 id="standard-interfaces-peer">
3143 <title><literal>org.freedesktop.DBus.Peer</literal></title>
3145 The <literal>org.freedesktop.DBus.Peer</literal> interface
3148 org.freedesktop.DBus.Peer.Ping ()
3149 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
3153 On receipt of the <literal>METHOD_CALL</literal> message
3154 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
3155 nothing other than reply with a <literal>METHOD_RETURN</literal> as
3156 usual. It does not matter which object path a ping is sent to. The
3157 reference implementation handles this method automatically.
3160 On receipt of the <literal>METHOD_CALL</literal> message
3161 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
3162 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
3163 UUID representing the identity of the machine the process is running on.
3164 This UUID must be the same for all processes on a single system at least
3165 until that system next reboots. It should be the same across reboots
3166 if possible, but this is not always possible to implement and is not
3168 It does not matter which object path a GetMachineId is sent to. The
3169 reference implementation handles this method automatically.
3172 The UUID is intended to be per-instance-of-the-operating-system, so may represent
3173 a virtual machine running on a hypervisor, rather than a physical machine.
3174 Basically if two processes see the same UUID, they should also see the same
3175 shared memory, UNIX domain sockets, process IDs, and other features that require
3176 a running OS kernel in common between the processes.
3179 The UUID is often used where other programs might use a hostname. Hostnames
3180 can change without rebooting, however, or just be "localhost" - so the UUID
3184 <xref linkend="uuids"/> explains the format of the UUID.
3188 <sect2 id="standard-interfaces-introspectable">
3189 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
3191 This interface has one method:
3193 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
3197 Objects instances may implement
3198 <literal>Introspect</literal> which returns an XML description of
3199 the object, including its interfaces (with signals and methods), objects
3200 below it in the object path tree, and its properties.
3203 <xref linkend="introspection-format"/> describes the format of this XML string.
3206 <sect2 id="standard-interfaces-properties">
3207 <title><literal>org.freedesktop.DBus.Properties</literal></title>
3209 Many native APIs will have a concept of object <firstterm>properties</firstterm>
3210 or <firstterm>attributes</firstterm>. These can be exposed via the
3211 <literal>org.freedesktop.DBus.Properties</literal> interface.
3215 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
3216 in STRING property_name,
3218 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
3219 in STRING property_name,
3221 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
3222 out DICT<STRING,VARIANT> props);
3226 It is conventional to give D-Bus properties names consisting of
3227 capitalized words without punctuation ("CamelCase"), like
3228 <link linkend="message-protocol-names-member">member names</link>.
3229 For instance, the GObject property
3230 <literal>connection-status</literal> or the Qt property
3231 <literal>connectionStatus</literal> could be represented on D-Bus
3232 as <literal>ConnectionStatus</literal>.
3235 Strictly speaking, D-Bus property names are not required to follow
3236 the same naming restrictions as member names, but D-Bus property
3237 names that would not be valid member names (in particular,
3238 GObject-style dash-separated property names) can cause interoperability
3239 problems and should be avoided.
3242 The available properties and whether they are writable can be determined
3243 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
3244 see <xref linkend="standard-interfaces-introspectable"/>.
3247 An empty string may be provided for the interface name; in this case,
3248 if there are multiple properties on an object with the same name,
3249 the results are undefined (picking one by according to an arbitrary
3250 deterministic rule, or returning an error, are the reasonable
3254 If one or more properties change on an object, the
3255 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3256 signal may be emitted (this signal was added in 0.14):
3260 org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
3261 DICT<STRING,VARIANT> changed_properties,
3262 ARRAY<STRING> invalidated_properties);
3266 where <literal>changed_properties</literal> is a dictionary
3267 containing the changed properties with the new values and
3268 <literal>invalidated_properties</literal> is an array of
3269 properties that changed but the value is not conveyed.
3272 Whether the <literal>PropertiesChanged</literal> signal is
3273 supported can be determined by calling
3274 <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
3275 that the signal may be supported for an object but it may
3276 differ how whether and how it is used on a per-property basis
3277 (for e.g. performance or security reasons). Each property (or
3278 the parent interface) must be annotated with the
3279 <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
3280 annotation to convey this (usually the default value
3281 <literal>true</literal> is sufficient meaning that the
3282 annotation does not need to be used). See <xref
3283 linkend="introspection-format"/> for details on this
3288 <sect2 id="standard-interfaces-objectmanager">
3289 <title><literal>org.freedesktop.DBus.ObjectManager</literal></title>
3291 An API can optionally make use of this interface for one or
3292 more sub-trees of objects. The root of each sub-tree implements
3293 this interface so other applications can get all objects,
3294 interfaces and properties in a single method call. It is
3295 appropriate to use this interface if users of the tree of
3296 objects are expected to be interested in all interfaces of all
3297 objects in the tree; a more granular API should be used if
3298 users of the objects are expected to be interested in a small
3299 subset of the objects, a small subset of their interfaces, or
3303 The method that applications can use to get all objects and
3304 properties is <literal>GetManagedObjects</literal>:
3308 org.freedesktop.DBus.ObjectManager.GetManagedObjects (out DICT<OBJPATH,DICT<STRING,DICT<STRING,VARIANT>>> objpath_interfaces_and_properties);
3312 The return value of this method is a dict whose keys are
3313 object paths. All returned object paths are children of the
3314 object path implementing this interface, i.e. their object
3315 paths start with the ObjectManager's object path plus '/'.
3318 Each value is a dict whose keys are interfaces names. Each
3319 value in this inner dict is the same dict that would be
3320 returned by the <link
3321 linkend="standard-interfaces-properties">org.freedesktop.DBus.Properties.GetAll()</link>
3322 method for that combination of object path and interface. If
3323 an interface has no properties, the empty dict is returned.
3326 Changes are emitted using the following two signals:
3330 org.freedesktop.DBus.ObjectManager.InterfacesAdded (OBJPATH object_path,
3331 DICT<STRING,DICT<STRING,VARIANT>> interfaces_and_properties);
3332 org.freedesktop.DBus.ObjectManager.InterfacesRemoved (OBJPATH object_path,
3333 ARRAY<STRING> interfaces);
3337 The <literal>InterfacesAdded</literal> signal is emitted when
3338 either a new object is added or when an existing object gains
3339 one or more interfaces. The
3340 <literal>InterfacesRemoved</literal> signal is emitted
3341 whenever an object is removed or it loses one or more
3342 interfaces. The second parameter of the
3343 <literal>InterfacesAdded</literal> signal contains a dict with
3344 the interfaces and properties (if any) that have been added to
3345 the given object path. Similarly, the second parameter of the
3346 <literal>InterfacesRemoved</literal> signal contains an array
3347 of the interfaces that were removed. Note that changes on
3348 properties on existing interfaces are not reported using this
3349 interface - an application should also monitor the existing <link
3350 linkend="standard-interfaces-properties">PropertiesChanged</link>
3351 signal on each object.
3354 Applications SHOULD NOT export objects that are children of an
3355 object (directly or otherwise) implementing this interface but
3356 which are not returned in the reply from the
3357 <literal>GetManagedObjects()</literal> method of this
3358 interface on the given object.
3361 The intent of the <literal>ObjectManager</literal> interface
3362 is to make it easy to write a robust client
3363 implementation. The trivial client implementation only needs
3364 to make two method calls:
3368 org.freedesktop.DBus.AddMatch (bus_proxy,
3369 "type='signal',name='org.example.App',path_namespace='/org/example/App'");
3370 objects = org.freedesktop.DBus.ObjectManager.GetManagedObjects (app_proxy);
3374 on the message bus and the remote application's
3375 <literal>ObjectManager</literal>, respectively. Whenever a new
3376 remote object is created (or an existing object gains a new
3377 interface), the <literal>InterfacesAdded</literal> signal is
3378 emitted, and since this signal contains all properties for the
3379 interfaces, no calls to the
3380 <literal>org.freedesktop.Properties</literal> interface on the
3381 remote object are needed. Additionally, since the initial
3382 <literal>AddMatch()</literal> rule already includes signal
3383 messages from the newly created child object, no new
3384 <literal>AddMatch()</literal> call is needed.
3389 The <literal>org.freedesktop.DBus.ObjectManager</literal>
3390 interface was added in version 0.17 of the D-Bus
3397 <sect1 id="introspection-format">
3398 <title>Introspection Data Format</title>
3400 As described in <xref linkend="standard-interfaces-introspectable"/>,
3401 objects may be introspected at runtime, returning an XML string
3402 that describes the object. The same XML format may be used in
3403 other contexts as well, for example as an "IDL" for generating
3404 static language bindings.
3407 Here is an example of introspection data:
3409 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
3410 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
3411 <node name="/org/freedesktop/sample_object">
3412 <interface name="org.freedesktop.SampleInterface">
3413 <method name="Frobate">
3414 <arg name="foo" type="i" direction="in"/>
3415 <arg name="bar" type="s" direction="out"/>
3416 <arg name="baz" type="a{us}" direction="out"/>
3417 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
3419 <method name="Bazify">
3420 <arg name="bar" type="(iiu)" direction="in"/>
3421 <arg name="bar" type="v" direction="out"/>
3423 <method name="Mogrify">
3424 <arg name="bar" type="(iiav)" direction="in"/>
3426 <signal name="Changed">
3427 <arg name="new_value" type="b"/>
3429 <property name="Bar" type="y" access="readwrite"/>
3431 <node name="child_of_sample_object"/>
3432 <node name="another_child_of_sample_object"/>
3437 A more formal DTD and spec needs writing, but here are some quick notes.
3441 Only the root <node> element can omit the node name, as it's
3442 known to be the object that was introspected. If the root
3443 <node> does have a name attribute, it must be an absolute
3444 object path. If child <node> have object paths, they must be
3450 If a child <node> has any sub-elements, then they
3451 must represent a complete introspection of the child.
3452 If a child <node> is empty, then it may or may
3453 not have sub-elements; the child must be introspected
3454 in order to find out. The intent is that if an object
3455 knows that its children are "fast" to introspect
3456 it can go ahead and return their information, but
3457 otherwise it can omit it.
3462 The direction element on <arg> may be omitted,
3463 in which case it defaults to "in" for method calls
3464 and "out" for signals. Signals only allow "out"
3465 so while direction may be specified, it's pointless.
3470 The possible directions are "in" and "out",
3471 unlike CORBA there is no "inout"
3476 The possible property access flags are
3477 "readwrite", "read", and "write"
3482 Multiple interfaces can of course be listed for
3488 The "name" attribute on arguments is optional.
3494 Method, interface, property, and signal elements may have
3495 "annotations", which are generic key/value pairs of metadata.
3496 They are similar conceptually to Java's annotations and C# attributes.
3497 Well-known annotations:
3504 <entry>Values (separated by ,)</entry>
3505 <entry>Description</entry>
3510 <entry>org.freedesktop.DBus.Deprecated</entry>
3511 <entry>true,false</entry>
3512 <entry>Whether or not the entity is deprecated; defaults to false</entry>
3515 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
3516 <entry>(string)</entry>
3517 <entry>The C symbol; may be used for methods and interfaces</entry>
3520 <entry>org.freedesktop.DBus.Method.NoReply</entry>
3521 <entry>true,false</entry>
3522 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
3525 <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
3526 <entry>true,invalidates,false</entry>
3529 If set to <literal>false</literal>, the
3530 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3532 linkend="standard-interfaces-properties"/> is not
3533 guaranteed to be emitted if the property changes.
3536 If set to <literal>invalidates</literal> the signal
3537 is emitted but the value is not included in the
3541 If set to <literal>true</literal> the signal is
3542 emitted with the value included.
3545 The value for the annotation defaults to
3546 <literal>true</literal> if the enclosing interface
3547 element does not specify the annotation. Otherwise it
3548 defaults to the value specified in the enclosing
3557 <sect1 id="message-bus">
3558 <title>Message Bus Specification</title>
3559 <sect2 id="message-bus-overview">
3560 <title>Message Bus Overview</title>
3562 The message bus accepts connections from one or more applications.
3563 Once connected, applications can exchange messages with other
3564 applications that are also connected to the bus.
3567 In order to route messages among connections, the message bus keeps a
3568 mapping from names to connections. Each connection has one
3569 unique-for-the-lifetime-of-the-bus name automatically assigned.
3570 Applications may request additional names for a connection. Additional
3571 names are usually "well-known names" such as
3572 "org.freedesktop.TextEditor". When a name is bound to a connection,
3573 that connection is said to <firstterm>own</firstterm> the name.
3576 The bus itself owns a special name, <literal>org.freedesktop.DBus</literal>.
3577 This name routes messages to the bus, allowing applications to make
3578 administrative requests. For example, applications can ask the bus
3579 to assign a name to a connection.
3582 Each name may have <firstterm>queued owners</firstterm>. When an
3583 application requests a name for a connection and the name is already in
3584 use, the bus will optionally add the connection to a queue waiting for
3585 the name. If the current owner of the name disconnects or releases
3586 the name, the next connection in the queue will become the new owner.
3590 This feature causes the right thing to happen if you start two text
3591 editors for example; the first one may request "org.freedesktop.TextEditor",
3592 and the second will be queued as a possible owner of that name. When
3593 the first exits, the second will take over.
3597 Applications may send <firstterm>unicast messages</firstterm> to
3598 a specific recipient or to the message bus itself, or
3599 <firstterm>broadcast messages</firstterm> to all interested recipients.
3600 See <xref linkend="message-bus-routing"/> for details.
3604 <sect2 id="message-bus-names">
3605 <title>Message Bus Names</title>
3607 Each connection has at least one name, assigned at connection time and
3608 returned in response to the
3609 <literal>org.freedesktop.DBus.Hello</literal> method call. This
3610 automatically-assigned name is called the connection's <firstterm>unique
3611 name</firstterm>. Unique names are never reused for two different
3612 connections to the same bus.
3615 Ownership of a unique name is a prerequisite for interaction with
3616 the message bus. It logically follows that the unique name is always
3617 the first name that an application comes to own, and the last
3618 one that it loses ownership of.
3621 Unique connection names must begin with the character ':' (ASCII colon
3622 character); bus names that are not unique names must not begin
3623 with this character. (The bus must reject any attempt by an application
3624 to manually request a name beginning with ':'.) This restriction
3625 categorically prevents "spoofing"; messages sent to a unique name
3626 will always go to the expected connection.
3629 When a connection is closed, all the names that it owns are deleted (or
3630 transferred to the next connection in the queue if any).
3633 A connection can request additional names to be associated with it using
3634 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
3635 linkend="message-protocol-names-bus"/> describes the format of a valid
3636 name. These names can be released again using the
3637 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
3640 <sect3 id="bus-messages-request-name">
3641 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
3645 UINT32 RequestName (in STRING name, in UINT32 flags)
3652 <entry>Argument</entry>
3654 <entry>Description</entry>
3660 <entry>STRING</entry>
3661 <entry>Name to request</entry>
3665 <entry>UINT32</entry>
3666 <entry>Flags</entry>
3676 <entry>Argument</entry>
3678 <entry>Description</entry>
3684 <entry>UINT32</entry>
3685 <entry>Return value</entry>
3692 This method call should be sent to
3693 <literal>org.freedesktop.DBus</literal> and asks the message bus to
3694 assign the given name to the method caller. Each name maintains a
3695 queue of possible owners, where the head of the queue is the primary
3696 or current owner of the name. Each potential owner in the queue
3697 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
3698 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
3699 call. When RequestName is invoked the following occurs:
3703 If the method caller is currently the primary owner of the name,
3704 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
3705 values are updated with the values from the new RequestName call,
3706 and nothing further happens.
3712 If the current primary owner (head of the queue) has
3713 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
3714 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
3715 the caller of RequestName replaces the current primary owner at
3716 the head of the queue and the current primary owner moves to the
3717 second position in the queue. If the caller of RequestName was
3718 in the queue previously its flags are updated with the values from
3719 the new RequestName in addition to moving it to the head of the queue.
3725 If replacement is not possible, and the method caller is
3726 currently in the queue but not the primary owner, its flags are
3727 updated with the values from the new RequestName call.
3733 If replacement is not possible, and the method caller is
3734 currently not in the queue, the method caller is appended to the
3741 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
3742 set and is not the primary owner, it is removed from the
3743 queue. This can apply to the previous primary owner (if it
3744 was replaced) or the method caller (if it updated the
3745 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
3746 queue, or if it was just added to the queue with that flag set).
3752 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
3753 queue," even if another application already in the queue had specified
3754 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
3755 that does not allow replacement goes away, and the next primary owner
3756 does allow replacement. In this case, queued items that specified
3757 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
3758 automatically replace the new primary owner. In other words,
3759 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
3760 time RequestName is called. This is deliberate to avoid an infinite loop
3761 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
3762 and DBUS_NAME_FLAG_REPLACE_EXISTING.
3765 The flags argument contains any of the following values logically ORed
3772 <entry>Conventional Name</entry>
3773 <entry>Value</entry>
3774 <entry>Description</entry>
3779 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
3783 If an application A specifies this flag and succeeds in
3784 becoming the owner of the name, and another application B
3785 later calls RequestName with the
3786 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
3787 will lose ownership and receive a
3788 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
3789 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
3790 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
3791 is not specified by application B, then application B will not replace
3792 application A as the owner.
3797 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
3801 Try to replace the current owner if there is one. If this
3802 flag is not set the application will only become the owner of
3803 the name if there is no current owner. If this flag is set,
3804 the application will replace the current owner if
3805 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
3810 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
3814 Without this flag, if an application requests a name that is
3815 already owned, the application will be placed in a queue to
3816 own the name when the current owner gives it up. If this
3817 flag is given, the application will not be placed in the
3818 queue, the request for the name will simply fail. This flag
3819 also affects behavior when an application is replaced as
3820 name owner; by default the application moves back into the
3821 waiting queue, unless this flag was provided when the application
3822 became the name owner.
3830 The return code can be one of the following values:
3836 <entry>Conventional Name</entry>
3837 <entry>Value</entry>
3838 <entry>Description</entry>
3843 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
3844 <entry>1</entry> <entry>The caller is now the primary owner of
3845 the name, replacing any previous owner. Either the name had no
3846 owner before, or the caller specified
3847 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
3848 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
3851 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
3854 <entry>The name already had an owner,
3855 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
3856 the current owner did not specify
3857 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
3858 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
3862 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
3863 <entry>The name already has an owner,
3864 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
3865 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
3866 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
3867 specified by the requesting application.</entry>
3870 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
3872 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
3880 <sect3 id="bus-messages-release-name">
3881 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
3885 UINT32 ReleaseName (in STRING name)
3892 <entry>Argument</entry>
3894 <entry>Description</entry>
3900 <entry>STRING</entry>
3901 <entry>Name to release</entry>
3911 <entry>Argument</entry>
3913 <entry>Description</entry>
3919 <entry>UINT32</entry>
3920 <entry>Return value</entry>
3927 This method call should be sent to
3928 <literal>org.freedesktop.DBus</literal> and asks the message bus to
3929 release the method caller's claim to the given name. If the caller is
3930 the primary owner, a new primary owner will be selected from the
3931 queue if any other owners are waiting. If the caller is waiting in
3932 the queue for the name, the caller will removed from the queue and
3933 will not be made an owner of the name if it later becomes available.
3934 If there are no other owners in the queue for the name, it will be
3935 removed from the bus entirely.
3937 The return code can be one of the following values:
3943 <entry>Conventional Name</entry>
3944 <entry>Value</entry>
3945 <entry>Description</entry>
3950 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
3951 <entry>1</entry> <entry>The caller has released his claim on
3952 the given name. Either the caller was the primary owner of
3953 the name, and the name is now unused or taken by somebody
3954 waiting in the queue for the name, or the caller was waiting
3955 in the queue for the name and has now been removed from the
3959 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
3961 <entry>The given name does not exist on this bus.</entry>
3964 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
3966 <entry>The caller was not the primary owner of this name,
3967 and was also not waiting in the queue to own this name.</entry>
3975 <sect3 id="bus-messages-list-queued-owners">
3976 <title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
3980 ARRAY of STRING ListQueuedOwners (in STRING name)
3987 <entry>Argument</entry>
3989 <entry>Description</entry>
3995 <entry>STRING</entry>
3996 <entry>The well-known bus name to query, such as
3997 <literal>com.example.cappuccino</literal></entry>
4007 <entry>Argument</entry>
4009 <entry>Description</entry>
4015 <entry>ARRAY of STRING</entry>
4016 <entry>The unique bus names of connections currently queued
4017 for the name</entry>
4024 This method call should be sent to
4025 <literal>org.freedesktop.DBus</literal> and lists the connections
4026 currently queued for a bus name (see
4027 <xref linkend="term-queued-owner"/>).
4032 <sect2 id="message-bus-routing">
4033 <title>Message Bus Message Routing</title>
4036 Messages may have a <literal>DESTINATION</literal> field (see <xref
4037 linkend="message-protocol-header-fields"/>), resulting in a
4038 <firstterm>unicast message</firstterm>. If the
4039 <literal>DESTINATION</literal> field is present, it specifies a message
4040 recipient by name. Method calls and replies normally specify this field.
4041 The message bus must send messages (of any type) with the
4042 <literal>DESTINATION</literal> field set to the specified recipient,
4043 regardless of whether the recipient has set up a match rule matching
4048 When the message bus receives a signal, if the
4049 <literal>DESTINATION</literal> field is absent, it is considered to
4050 be a <firstterm>broadcast signal</firstterm>, and is sent to all
4051 applications with <firstterm>message matching rules</firstterm> that
4052 match the message. Most signal messages are broadcasts.
4056 Unicast signal messages (those with a <literal>DESTINATION</literal>
4057 field) are not commonly used, but they are treated like any unicast
4058 message: they are delivered to the specified receipient,
4059 regardless of its match rules. One use for unicast signals is to
4060 avoid a race condition in which a signal is emitted before the intended
4061 recipient can call <xref linkend="bus-messages-add-match"/> to
4062 receive that signal: if the signal is sent directly to that recipient
4063 using a unicast message, it does not need to add a match rule at all,
4064 and there is no race condition. Another use for unicast signals,
4065 on message buses whose security policy prevents eavesdropping, is to
4066 send sensitive information which should only be visible to one
4071 When the message bus receives a method call, if the
4072 <literal>DESTINATION</literal> field is absent, the call is taken to be
4073 a standard one-to-one message and interpreted by the message bus
4074 itself. For example, sending an
4075 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
4076 <literal>DESTINATION</literal> will cause the message bus itself to
4077 reply to the ping immediately; the message bus will not make this
4078 message visible to other applications.
4082 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
4083 the ping message were sent with a <literal>DESTINATION</literal> name of
4084 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
4085 forwarded, and the Yoyodyne Corporation screensaver application would be
4086 expected to reply to the ping.
4090 Message bus implementations may impose a security policy which
4091 prevents certain messages from being sent or received.
4092 When a message cannot be sent or received due to a security
4093 policy, the message bus should send an error reply, unless the
4094 original message had the <literal>NO_REPLY</literal> flag.
4097 <sect3 id="message-bus-routing-eavesdropping">
4098 <title>Eavesdropping</title>
4100 Receiving a unicast message whose <literal>DESTINATION</literal>
4101 indicates a different recipient is called
4102 <firstterm>eavesdropping</firstterm>. On a message bus which acts as
4103 a security boundary (like the standard system bus), the security
4104 policy should usually prevent eavesdropping, since unicast messages
4105 are normally kept private and may contain security-sensitive
4110 Eavesdropping is mainly useful for debugging tools, such as
4111 the <literal>dbus-monitor</literal> tool in the reference
4112 implementation of D-Bus. Tools which eavesdrop on the message bus
4113 should be careful to avoid sending a reply or error in response to
4114 messages intended for a different client.
4118 Clients may attempt to eavesdrop by adding match rules
4119 (see <xref linkend="message-bus-routing-match-rules"/>) containing
4120 the <literal>eavesdrop='true'</literal> match. If the message bus'
4121 security policy does not allow eavesdropping, the match rule can
4122 still be added, but will not have any practical effect. For
4123 compatibility with older message bus implementations, if adding such
4124 a match rule results in an error reply, the client may fall back to
4125 adding the same rule with the <literal>eavesdrop</literal> match
4130 <sect3 id="message-bus-routing-match-rules">
4131 <title>Match Rules</title>
4133 An important part of the message bus routing protocol is match
4134 rules. Match rules describe the messages that should be sent to a
4135 client, based on the contents of the message. Broadcast signals
4136 are only sent to clients which have a suitable match rule: this
4137 avoids waking up client processes to deal with signals that are
4138 not relevant to that client.
4141 Messages that list a client as their <literal>DESTINATION</literal>
4142 do not need to match the client's match rules, and are sent to that
4143 client regardless. As a result, match rules are mainly used to
4144 receive a subset of broadcast signals.
4147 Match rules can also be used for eavesdropping
4148 (see <xref linkend="message-bus-routing-eavesdropping"/>),
4149 if the security policy of the message bus allows it.
4152 Match rules are added using the AddMatch bus method
4153 (see <xref linkend="bus-messages-add-match"/>). Rules are
4154 specified as a string of comma separated key/value pairs.
4155 Excluding a key from the rule indicates a wildcard match.
4156 For instance excluding the the member from a match rule but
4157 adding a sender would let all messages from that sender through.
4158 An example of a complete rule would be
4159 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
4162 The following table describes the keys that can be used to create
4164 The following table summarizes the D-Bus types.
4170 <entry>Possible Values</entry>
4171 <entry>Description</entry>
4176 <entry><literal>type</literal></entry>
4177 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
4178 <entry>Match on the message type. An example of a type match is type='signal'</entry>
4181 <entry><literal>sender</literal></entry>
4182 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
4183 and <xref linkend="term-unique-name"/> respectively)
4185 <entry>Match messages sent by a particular sender. An example of a sender match
4186 is sender='org.freedesktop.Hal'</entry>
4189 <entry><literal>interface</literal></entry>
4190 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
4191 <entry>Match messages sent over or to a particular interface. An example of an
4192 interface match is interface='org.freedesktop.Hal.Manager'.
4193 If a message omits the interface header, it must not match any rule
4194 that specifies this key.</entry>
4197 <entry><literal>member</literal></entry>
4198 <entry>Any valid method or signal name</entry>
4199 <entry>Matches messages which have the give method or signal name. An example of
4200 a member match is member='NameOwnerChanged'</entry>
4203 <entry><literal>path</literal></entry>
4204 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
4205 <entry>Matches messages which are sent from or to the given object. An example of a
4206 path match is path='/org/freedesktop/Hal/Manager'</entry>
4209 <entry><literal>path_namespace</literal></entry>
4210 <entry>An object path</entry>
4213 Matches messages which are sent from or to an
4214 object for which the object path is either the
4215 given value, or that value followed by one or
4216 more path components.
4221 <literal>path_namespace='/com/example/foo'</literal>
4222 would match signals sent by
4223 <literal>/com/example/foo</literal>
4225 <literal>/com/example/foo/bar</literal>,
4227 <literal>/com/example/foobar</literal>.
4231 Using both <literal>path</literal> and
4232 <literal>path_namespace</literal> in the same match
4233 rule is not allowed.
4238 This match key was added in version 0.16 of the
4239 D-Bus specification and implemented by the bus
4240 daemon in dbus 1.5.0 and later.
4246 <entry><literal>destination</literal></entry>
4247 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
4248 <entry>Matches messages which are being sent to the given unique name. An
4249 example of a destination match is destination=':1.0'</entry>
4252 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
4253 <entry>Any string</entry>
4254 <entry>Arg matches are special and are used for further restricting the
4255 match based on the arguments in the body of a message. Only arguments of type
4256 STRING can be matched in this way. An example of an argument match
4257 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
4261 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
4262 <entry>Any string</entry>
4264 <para>Argument path matches provide a specialised form of wildcard matching for
4265 path-like namespaces. They can match arguments whose type is either STRING or
4266 OBJECT_PATH. As with normal argument matches,
4267 if the argument is exactly equal to the string given in the match
4268 rule then the rule is satisfied. Additionally, there is also a
4269 match when either the string given in the match rule or the
4270 appropriate message argument ends with '/' and is a prefix of the
4271 other. An example argument path match is arg0path='/aa/bb/'. This
4272 would match messages with first arguments of '/', '/aa/',
4273 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
4274 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>
4276 <para>This is intended for monitoring “directories” in file system-like
4277 hierarchies, as used in the <citetitle>dconf</citetitle> configuration
4278 system. An application interested in all nodes in a particular hierarchy would
4279 monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
4280 emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
4281 represent a modification to the “bar” property, or a signal with zeroth
4282 argument <literal>"/ca/example/"</literal> to represent atomic modification of
4283 many properties within that directory, and the interested application would be
4284 notified in both cases.</para>
4287 This match key was added in version 0.12 of the
4288 D-Bus specification, implemented for STRING
4289 arguments by the bus daemon in dbus 1.2.0 and later,
4290 and implemented for OBJECT_PATH arguments in dbus 1.5.0
4297 <entry><literal>arg0namespace</literal></entry>
4298 <entry>Like a bus name, except that the string is not
4299 required to contain a '.' (period)</entry>
4301 <para>Match messages whose first argument is of type STRING, and is a bus name
4302 or interface name within the specified namespace. This is primarily intended
4303 for watching name owner changes for a group of related bus names, rather than
4304 for a single name or all name changes.</para>
4306 <para>Because every valid interface name is also a valid
4307 bus name, this can also be used for messages whose
4308 first argument is an interface name.</para>
4310 <para>For example, the match rule
4311 <literal>member='NameOwnerChanged',arg0namespace='com.example.backend'</literal>
4312 matches name owner changes for bus names such as
4313 <literal>com.example.backend.foo</literal>,
4314 <literal>com.example.backend.foo.bar</literal>, and
4315 <literal>com.example.backend</literal> itself.</para>
4317 <para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para>
4320 This match key was added in version 0.16 of the
4321 D-Bus specification and implemented by the bus
4322 daemon in dbus 1.5.0 and later.
4328 <entry><literal>eavesdrop</literal></entry>
4329 <entry><literal>'true'</literal>, <literal>'false'</literal></entry>
4330 <entry>Since D-Bus 1.5.6, match rules do not
4331 match messages which have a <literal>DESTINATION</literal>
4332 field unless the match rule specifically
4334 (see <xref linkend="message-bus-routing-eavesdropping"/>)
4335 by specifying <literal>eavesdrop='true'</literal>
4336 in the match rule. <literal>eavesdrop='false'</literal>
4337 restores the default behaviour. Messages are
4338 delivered to their <literal>DESTINATION</literal>
4339 regardless of match rules, so this match does not
4340 affect normal delivery of unicast messages.
4341 If the message bus has a security policy which forbids
4342 eavesdropping, this match may still be used without error,
4343 but will not have any practical effect.
4344 In older versions of D-Bus, this match was not allowed
4345 in match rules, and all match rules behaved as if
4346 <literal>eavesdrop='true'</literal> had been used.
4355 <sect2 id="message-bus-starting-services">
4356 <title>Message Bus Starting Services</title>
4358 The message bus can start applications on behalf of other applications.
4359 In CORBA terms, this would be called <firstterm>activation</firstterm>.
4360 An application that can be started in this way is called a
4361 <firstterm>service</firstterm>.
4364 With D-Bus, starting a service is normally done by name. That is,
4365 applications ask the message bus to start some program that will own a
4366 well-known name, such as <literal>org.freedesktop.TextEditor</literal>.
4367 This implies a contract documented along with the name
4368 <literal>org.freedesktop.TextEditor</literal> for which objects
4369 the owner of that name will provide, and what interfaces those
4373 To find an executable corresponding to a particular name, the bus daemon
4374 looks for <firstterm>service description files</firstterm>. Service
4375 description files define a mapping from names to executables. Different
4376 kinds of message bus will look for these files in different places, see
4377 <xref linkend="message-bus-types"/>.
4380 Service description files have the ".service" file
4381 extension. The message bus will only load service description files
4382 ending with .service; all other files will be ignored. The file format
4383 is similar to that of <ulink
4384 url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
4385 entries</ulink>. All service description files must be in UTF-8
4386 encoding. To ensure that there will be no name collisions, service files
4387 must be namespaced using the same mechanism as messages and service
4392 [FIXME the file format should be much better specified than "similar to
4393 .desktop entries" esp. since desktop entries are already
4394 badly-specified. ;-)]
4395 These sections from the specification apply to service files as well:
4398 <listitem><para>General syntax</para></listitem>
4399 <listitem><para>Comment format</para></listitem>
4403 <title>Example service description file</title>
4405 # Sample service description file
4407 Names=org.freedesktop.ConfigurationDatabase;org.gnome.GConf;
4408 Exec=/usr/libexec/gconfd-2
4413 When an application asks to start a service by name, the bus daemon tries to
4414 find a service that will own that name. It then tries to spawn the
4415 executable associated with it. If this fails, it will report an
4416 error. [FIXME what happens if two .service files offer the same service;
4417 what kind of error is reported, should we have a way for the client to
4421 The executable launched will have the environment variable
4422 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
4423 message bus so it can connect and request the appropriate names.
4426 The executable being launched may want to know whether the message bus
4427 starting it is one of the well-known message buses (see <xref
4428 linkend="message-bus-types"/>). To facilitate this, the bus must also set
4429 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
4430 of the well-known buses. The currently-defined values for this variable
4431 are <literal>system</literal> for the systemwide message bus,
4432 and <literal>session</literal> for the per-login-session message
4433 bus. The new executable must still connect to the address given
4434 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
4435 resulting connection is to the well-known bus.
4438 [FIXME there should be a timeout somewhere, either specified
4439 in the .service file, by the client, or just a global value
4440 and if the client being activated fails to connect within that
4441 timeout, an error should be sent back.]
4444 <sect3 id="message-bus-starting-services-scope">
4445 <title>Message Bus Service Scope</title>
4447 The "scope" of a service is its "per-", such as per-session,
4448 per-machine, per-home-directory, or per-display. The reference
4449 implementation doesn't yet support starting services in a different
4450 scope from the message bus itself. So e.g. if you start a service
4451 on the session bus its scope is per-session.
4454 We could add an optional scope to a bus name. For example, for
4455 per-(display,session pair), we could have a unique ID for each display
4456 generated automatically at login and set on screen 0 by executing a
4457 special "set display ID" binary. The ID would be stored in a
4458 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
4459 random bytes. This ID would then be used to scope names.
4460 Starting/locating a service could be done by ID-name pair rather than
4464 Contrast this with a per-display scope. To achieve that, we would
4465 want a single bus spanning all sessions using a given display.
4466 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
4467 property on screen 0 of the display, pointing to this bus.
4472 <sect2 id="message-bus-types">
4473 <title>Well-known Message Bus Instances</title>
4475 Two standard message bus instances are defined here, along with how
4476 to locate them and where their service files live.
4478 <sect3 id="message-bus-types-login">
4479 <title>Login session message bus</title>
4481 Each time a user logs in, a <firstterm>login session message
4482 bus</firstterm> may be started. All applications in the user's login
4483 session may interact with one another using this message bus.
4486 The address of the login session message bus is given
4487 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
4488 variable. If that variable is not set, applications may
4489 also try to read the address from the X Window System root
4490 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
4491 The root window property must have type <literal>STRING</literal>.
4492 The environment variable should have precedence over the
4493 root window property.
4495 <para>The address of the login session message bus is given in the
4496 <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
4497 DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
4498 "autolaunch:", the system should use platform-specific methods of
4499 locating a running D-Bus session server, or starting one if a running
4500 instance cannot be found. Note that this mechanism is not recommended
4501 for attempting to determine if a daemon is running. It is inherently
4502 racy to attempt to make this determination, since the bus daemon may
4503 be started just before or just after the determination is made.
4504 Therefore, it is recommended that applications do not try to make this
4505 determination for their functionality purposes, and instead they
4506 should attempt to start the server.</para>
4508 <sect4 id="message-bus-types-login-x-windows">
4509 <title>X Windowing System</title>
4511 For the X Windowing System, the application must locate the
4512 window owner of the selection represented by the atom formed by
4516 <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
4520 <para>the current user's username</para>
4524 <para>the literal character '_' (underscore)</para>
4528 <para>the machine's ID</para>
4534 The following properties are defined for the window that owns
4536 <informaltable frame="all">
4545 <para>meaning</para>
4551 <para>_DBUS_SESSION_BUS_ADDRESS</para>
4555 <para>the actual address of the server socket</para>
4561 <para>_DBUS_SESSION_BUS_PID</para>
4565 <para>the PID of the server process</para>
4574 At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
4575 present in this window.
4579 If the X selection cannot be located or if reading the
4580 properties from the window fails, the implementation MUST conclude
4581 that there is no D-Bus server running and proceed to start a new
4582 server. (See below on concurrency issues)
4586 Failure to connect to the D-Bus server address thus obtained
4587 MUST be treated as a fatal connection error and should be reported
4592 As an alternative, an implementation MAY find the information
4593 in the following file located in the current user's home directory,
4594 in subdirectory .dbus/session-bus/:
4597 <para>the machine's ID</para>
4601 <para>the literal character '-' (dash)</para>
4605 <para>the X display without the screen number, with the
4606 following prefixes removed, if present: ":", "localhost:"
4607 ."localhost.localdomain:". That is, a display of
4608 "localhost:10.0" produces just the number "10"</para>
4614 The contents of this file NAME=value assignment pairs and
4615 lines starting with # are comments (no comments are allowed
4616 otherwise). The following variable names are defined:
4623 <para>Variable</para>
4627 <para>meaning</para>
4633 <para>DBUS_SESSION_BUS_ADDRESS</para>
4637 <para>the actual address of the server socket</para>
4643 <para>DBUS_SESSION_BUS_PID</para>
4647 <para>the PID of the server process</para>
4653 <para>DBUS_SESSION_BUS_WINDOWID</para>
4657 <para>the window ID</para>
4666 At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
4671 Failure to open this file MUST be interpreted as absence of a
4672 running server. Therefore, the implementation MUST proceed to
4673 attempting to launch a new bus server if the file cannot be
4678 However, success in opening this file MUST NOT lead to the
4679 conclusion that the server is running. Thus, a failure to connect to
4680 the bus address obtained by the alternative method MUST NOT be
4681 considered a fatal error. If the connection cannot be established,
4682 the implementation MUST proceed to check the X selection settings or
4683 to start the server on its own.
4687 If the implementation concludes that the D-Bus server is not
4688 running it MUST attempt to start a new server and it MUST also
4689 ensure that the daemon started as an effect of the "autolaunch"
4690 mechanism provides the lookup mechanisms described above, so
4691 subsequent calls can locate the newly started server. The
4692 implementation MUST also ensure that if two or more concurrent
4693 initiations happen, only one server remains running and all other
4694 initiations are able to obtain the address of this server and
4695 connect to it. In other words, the implementation MUST ensure that
4696 the X selection is not present when it attempts to set it, without
4697 allowing another process to set the selection between the
4698 verification and the setting (e.g., by using XGrabServer /
4705 On Unix systems, the session bus should search for .service files
4706 in <literal>$XDG_DATA_DIRS/dbus-1/services</literal> as defined
4708 <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG Base Directory Specification</ulink>.
4709 Implementations may also search additional locations, which
4710 should be searched with lower priority than anything in
4711 XDG_DATA_HOME, XDG_DATA_DIRS or their respective defaults;
4712 for example, the reference implementation also
4713 looks in <literal>${datadir}/dbus-1/services</literal> as
4714 set at compile time.
4717 As described in the XDG Base Directory Specification, software
4718 packages should install their session .service files to their
4719 configured <literal>${datadir}/dbus-1/services</literal>,
4720 where <literal>${datadir}</literal> is as defined by the GNU
4721 coding standards. System administrators or users can arrange
4722 for these service files to be read by setting XDG_DATA_DIRS or by
4723 symlinking them into the default locations.
4727 <sect3 id="message-bus-types-system">
4728 <title>System message bus</title>
4730 A computer may have a <firstterm>system message bus</firstterm>,
4731 accessible to all applications on the system. This message bus may be
4732 used to broadcast system events, such as adding new hardware devices,
4733 changes in the printer queue, and so forth.
4736 The address of the system message bus is given
4737 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
4738 variable. If that variable is not set, applications should try
4739 to connect to the well-known address
4740 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
4743 The D-Bus reference implementation actually honors the
4744 <literal>$(localstatedir)</literal> configure option
4745 for this address, on both client and server side.
4750 On Unix systems, the system bus should default to searching
4751 for .service files in
4752 <literal>/usr/local/share/dbus-1/system-services</literal>,
4753 <literal>/usr/share/dbus-1/system-services</literal> and
4754 <literal>/lib/dbus-1/system-services</literal>, with that order
4755 of precedence. It may also search other implementation-specific
4756 locations, but should not vary these locations based on environment
4760 The system bus is security-sensitive and is typically executed
4761 by an init system with a clean environment. Its launch helper
4762 process is particularly security-sensitive, and specifically
4763 clears its own environment.
4768 Software packages should install their system .service
4769 files to their configured
4770 <literal>${datadir}/dbus-1/system-services</literal>,
4771 where <literal>${datadir}</literal> is as defined by the GNU
4772 coding standards. System administrators can arrange
4773 for these service files to be read by editing the system bus'
4774 configuration file or by symlinking them into the default
4780 <sect2 id="message-bus-messages">
4781 <title>Message Bus Messages</title>
4783 The special message bus name <literal>org.freedesktop.DBus</literal>
4784 responds to a number of additional messages.
4787 <sect3 id="bus-messages-hello">
4788 <title><literal>org.freedesktop.DBus.Hello</literal></title>
4799 <entry>Argument</entry>
4801 <entry>Description</entry>
4807 <entry>STRING</entry>
4808 <entry>Unique name assigned to the connection</entry>
4815 Before an application is able to send messages to other applications
4816 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
4817 to the message bus to obtain a unique name. If an application without
4818 a unique name tries to send a message to another application, or a
4819 message to the message bus itself that isn't the
4820 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
4821 disconnected from the bus.
4824 There is no corresponding "disconnect" request; if a client wishes to
4825 disconnect from the bus, it simply closes the socket (or other
4826 communication channel).
4829 <sect3 id="bus-messages-list-names">
4830 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
4834 ARRAY of STRING ListNames ()
4841 <entry>Argument</entry>
4843 <entry>Description</entry>
4849 <entry>ARRAY of STRING</entry>
4850 <entry>Array of strings where each string is a bus name</entry>
4857 Returns a list of all currently-owned names on the bus.
4860 <sect3 id="bus-messages-list-activatable-names">
4861 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
4865 ARRAY of STRING ListActivatableNames ()
4872 <entry>Argument</entry>
4874 <entry>Description</entry>
4880 <entry>ARRAY of STRING</entry>
4881 <entry>Array of strings where each string is a bus name</entry>
4888 Returns a list of all names that can be activated on the bus.
4891 <sect3 id="bus-messages-name-exists">
4892 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
4896 BOOLEAN NameHasOwner (in STRING name)
4903 <entry>Argument</entry>
4905 <entry>Description</entry>
4911 <entry>STRING</entry>
4912 <entry>Name to check</entry>
4922 <entry>Argument</entry>
4924 <entry>Description</entry>
4930 <entry>BOOLEAN</entry>
4931 <entry>Return value, true if the name exists</entry>
4938 Checks if the specified name exists (currently has an owner).
4942 <sect3 id="bus-messages-name-owner-changed">
4943 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
4947 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
4954 <entry>Argument</entry>
4956 <entry>Description</entry>
4962 <entry>STRING</entry>
4963 <entry>Name with a new owner</entry>
4967 <entry>STRING</entry>
4968 <entry>Old owner or empty string if none</entry>
4972 <entry>STRING</entry>
4973 <entry>New owner or empty string if none</entry>
4980 This signal indicates that the owner of a name has changed.
4981 It's also the signal to use to detect the appearance of
4982 new names on the bus.
4985 <sect3 id="bus-messages-name-lost">
4986 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
4990 NameLost (STRING name)
4997 <entry>Argument</entry>
4999 <entry>Description</entry>
5005 <entry>STRING</entry>
5006 <entry>Name which was lost</entry>
5013 This signal is sent to a specific application when it loses
5014 ownership of a name.
5018 <sect3 id="bus-messages-name-acquired">
5019 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
5023 NameAcquired (STRING name)
5030 <entry>Argument</entry>
5032 <entry>Description</entry>
5038 <entry>STRING</entry>
5039 <entry>Name which was acquired</entry>
5046 This signal is sent to a specific application when it gains
5047 ownership of a name.
5051 <sect3 id="bus-messages-start-service-by-name">
5052 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
5056 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
5063 <entry>Argument</entry>
5065 <entry>Description</entry>
5071 <entry>STRING</entry>
5072 <entry>Name of the service to start</entry>
5076 <entry>UINT32</entry>
5077 <entry>Flags (currently not used)</entry>
5087 <entry>Argument</entry>
5089 <entry>Description</entry>
5095 <entry>UINT32</entry>
5096 <entry>Return value</entry>
5101 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
5105 The return value can be one of the following values:
5110 <entry>Identifier</entry>
5111 <entry>Value</entry>
5112 <entry>Description</entry>
5117 <entry>DBUS_START_REPLY_SUCCESS</entry>
5119 <entry>The service was successfully started.</entry>
5122 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
5124 <entry>A connection already owns the given name.</entry>
5133 <sect3 id="bus-messages-update-activation-environment">
5134 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
5138 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
5145 <entry>Argument</entry>
5147 <entry>Description</entry>
5153 <entry>ARRAY of DICT<STRING,STRING></entry>
5154 <entry>Environment to add or update</entry>
5159 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
5162 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
5165 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.
5170 <sect3 id="bus-messages-get-name-owner">
5171 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
5175 STRING GetNameOwner (in STRING name)
5182 <entry>Argument</entry>
5184 <entry>Description</entry>
5190 <entry>STRING</entry>
5191 <entry>Name to get the owner of</entry>
5201 <entry>Argument</entry>
5203 <entry>Description</entry>
5209 <entry>STRING</entry>
5210 <entry>Return value, a unique connection name</entry>
5215 Returns the unique connection name of the primary owner of the name
5216 given. If the requested name doesn't have an owner, returns a
5217 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
5221 <sect3 id="bus-messages-get-connection-unix-user">
5222 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
5226 UINT32 GetConnectionUnixUser (in STRING bus_name)
5233 <entry>Argument</entry>
5235 <entry>Description</entry>
5241 <entry>STRING</entry>
5242 <entry>Unique or well-known bus name of the connection to
5243 query, such as <literal>:12.34</literal> or
5244 <literal>com.example.tea</literal></entry>
5254 <entry>Argument</entry>
5256 <entry>Description</entry>
5262 <entry>UINT32</entry>
5263 <entry>Unix user ID</entry>
5268 Returns the Unix user ID of the process connected to the server. If
5269 unable to determine it (for instance, because the process is not on the
5270 same machine as the bus daemon), an error is returned.
5274 <sect3 id="bus-messages-get-connection-unix-process-id">
5275 <title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
5279 UINT32 GetConnectionUnixProcessID (in STRING bus_name)
5286 <entry>Argument</entry>
5288 <entry>Description</entry>
5294 <entry>STRING</entry>
5295 <entry>Unique or well-known bus name of the connection to
5296 query, such as <literal>:12.34</literal> or
5297 <literal>com.example.tea</literal></entry>
5307 <entry>Argument</entry>
5309 <entry>Description</entry>
5315 <entry>UINT32</entry>
5316 <entry>Unix process id</entry>
5321 Returns the Unix process ID of the process connected to the server. If
5322 unable to determine it (for instance, because the process is not on the
5323 same machine as the bus daemon), an error is returned.
5327 <sect3 id="bus-messages-add-match">
5328 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
5332 AddMatch (in STRING rule)
5339 <entry>Argument</entry>
5341 <entry>Description</entry>
5347 <entry>STRING</entry>
5348 <entry>Match rule to add to the connection</entry>
5353 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
5354 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
5358 <sect3 id="bus-messages-remove-match">
5359 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
5363 RemoveMatch (in STRING rule)
5370 <entry>Argument</entry>
5372 <entry>Description</entry>
5378 <entry>STRING</entry>
5379 <entry>Match rule to remove from the connection</entry>
5384 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
5385 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
5390 <sect3 id="bus-messages-get-id">
5391 <title><literal>org.freedesktop.DBus.GetId</literal></title>
5395 GetId (out STRING id)
5402 <entry>Argument</entry>
5404 <entry>Description</entry>
5410 <entry>STRING</entry>
5411 <entry>Unique ID identifying the bus daemon</entry>
5416 Gets the unique ID of the bus. The unique ID here is shared among all addresses the
5417 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
5418 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
5419 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
5420 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
5421 by org.freedesktop.DBus.Peer.GetMachineId().
5422 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
5430 <appendix id="implementation-notes">
5431 <title>Implementation notes</title>
5432 <sect1 id="implementation-notes-subsection">
5440 <glossary><title>Glossary</title>
5442 This glossary defines some of the terms used in this specification.
5445 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
5448 The message bus maintains an association between names and
5449 connections. (Normally, there's one connection per application.) A
5450 bus name is simply an identifier used to locate connections. For
5451 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
5452 name might be used to send a message to a screensaver from Yoyodyne
5453 Corporation. An application is said to <firstterm>own</firstterm> a
5454 name if the message bus has associated the application's connection
5455 with the name. Names may also have <firstterm>queued
5456 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
5457 The bus assigns a unique name to each connection,
5458 see <xref linkend="term-unique-name"/>. Other names
5459 can be thought of as "well-known names" and are
5460 used to find applications that offer specific functionality.
5464 See <xref linkend="message-protocol-names-bus"/> for details of
5465 the syntax and naming conventions for bus names.
5470 <glossentry id="term-message"><glossterm>Message</glossterm>
5473 A message is the atomic unit of communication via the D-Bus
5474 protocol. It consists of a <firstterm>header</firstterm> and a
5475 <firstterm>body</firstterm>; the body is made up of
5476 <firstterm>arguments</firstterm>.
5481 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
5484 The message bus is a special application that forwards
5485 or routes messages between a group of applications
5486 connected to the message bus. It also manages
5487 <firstterm>names</firstterm> used for routing
5493 <glossentry id="term-name"><glossterm>Name</glossterm>
5496 See <xref linkend="term-bus-name"/>. "Name" may
5497 also be used to refer to some of the other names
5498 in D-Bus, such as interface names.
5503 <glossentry id="namespace"><glossterm>Namespace</glossterm>
5506 Used to prevent collisions when defining new interfaces, bus names
5507 etc. The convention used is the same one Java uses for defining
5508 classes: a reversed domain name.
5509 See <xref linkend="message-protocol-names-bus"/>,
5510 <xref linkend="message-protocol-names-interface"/>,
5511 <xref linkend="message-protocol-names-error"/>,
5512 <xref linkend="message-protocol-marshaling-object-path"/>.
5517 <glossentry id="term-object"><glossterm>Object</glossterm>
5520 Each application contains <firstterm>objects</firstterm>, which have
5521 <firstterm>interfaces</firstterm> and
5522 <firstterm>methods</firstterm>. Objects are referred to by a name,
5523 called a <firstterm>path</firstterm>.
5528 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
5531 An application talking directly to another application, without going
5532 through a message bus. One-to-one connections may be "peer to peer" or
5533 "client to server." The D-Bus protocol has no concept of client
5534 vs. server after a connection has authenticated; the flow of messages
5535 is symmetrical (full duplex).
5540 <glossentry id="term-path"><glossterm>Path</glossterm>
5543 Object references (object names) in D-Bus are organized into a
5544 filesystem-style hierarchy, so each object is named by a path. As in
5545 LDAP, there's no difference between "files" and "directories"; a path
5546 can refer to an object, while still having child objects below it.
5551 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
5554 Each bus name has a primary owner; messages sent to the name go to the
5555 primary owner. However, certain names also maintain a queue of
5556 secondary owners "waiting in the wings." If the primary owner releases
5557 the name, then the first secondary owner in the queue automatically
5558 becomes the new owner of the name.
5563 <glossentry id="term-service"><glossterm>Service</glossterm>
5566 A service is an executable that can be launched by the bus daemon.
5567 Services normally guarantee some particular features, for example they
5568 may guarantee that they will request a specific name such as
5569 "org.freedesktop.Screensaver", have a singleton object
5570 "/org/freedesktop/Application", and that object will implement the
5571 interface "org.freedesktop.ScreensaverControl".
5576 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
5579 ".service files" tell the bus about service applications that can be
5580 launched (see <xref linkend="term-service"/>). Most importantly they
5581 provide a mapping from bus names to services that will request those
5582 names when they start up.
5587 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
5590 The special name automatically assigned to each connection by the
5591 message bus. This name will never change owner, and will be unique
5592 (never reused during the lifetime of the message bus).
5593 It will begin with a ':' character.