# D-Bus API Design Guidelines

## Summary

The most common use for D-Bus is in implementing a service which will be consumed by multiple client programs, and hence all interfaces exported on the bus form a public API. Designing a D-Bus API is like designing any other API: there is a lot of flexibility, but there are design patterns to follow and anti-patterns to avoid.

This guide aims to explain the best practices for writing D-Bus APIs. These have been refined over several years of use of D-Bus in many projects. Pointers will be given for implementing APIs using common D-Bus libraries like GDBus, but detailed implementation instructions are left to the libraries’ documentation. Note that you should not use dbus-glib to implement D-Bus services as it is deprecated and unmaintained. Most services should also avoid libdbus (dbus-1), which is a low-level library and is awkward to use correctly: it is designed to be used via a language binding such as QtDBus.

For documentation on D-Bus itself, see the D-Bus specification.

## APIs

A D-Bus API is a specification of one or more interfaces, which will be implemented by objects exposed by a service on the bus. Typically an API is designed as a set of interface files, and the implementation of the service follows those files. Some projects, however, choose to define the API in the code for the service, and to export XML interface files from the running service using D-Bus introspection. Both are valid approaches.

For simplicity, this document uses the XML descriptions of D-Bus interfaces as the canonical representation.

## Interface files

A D-Bus interface file is an XML file which describes one or more D-Bus interfaces, and is the best way of describing a D-Bus API in a machine readable way. The format is described in the D-Bus specification, and is supported by tools such as gdbus-codegen.

Interface files for public API should be installed to $(datadir)/dbus-1/interfaces so that other services can load them. Private APIs should not be installed. There should be one file installed per D-Bus interface, named after the interface, containing a single top-level <node> element with a single <interface> beneath it. For example, interface com.example.MyService1.Manager would be described by file $(datadir)/dbus-1/interfaces/com.example.MyService1.Manager.xml:

### Example D-Bus Interface XML

A brief example interface XML document.

    <!DOCTYPE node PUBLIC
        "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
        "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd" >
    <node xmlns:doc="http://www.freedesktop.org/dbus/1.0/doc.dtd">
      <interface name="com.example.MyService1.InterestingInterface">
        <method name="AddContact">
          <arg name="name" direction="in" type="s">
            <doc:doc><doc:summary>Name of new contact</doc:summary></doc:doc>
          </arg>
          <arg name="email" direction="in" type="s">
            <doc:doc><doc:summary>E-mail address of new contact</doc:summary></doc:doc>
          </arg>
          <arg name="id" direction="out" type="u">
            <doc:doc><doc:summary>ID of newly added contact</doc:summary></doc:doc>
          </arg>
          <doc:doc>
            <doc:description>
              <doc:para>
                Adds a new contact to the address book with their name and
                e-mail address.
              </doc:para>
            </doc:description>
          </doc:doc>
        </method>
      </interface>
    </node>

If an interface defined by service A needs to be used by client B, client B should declare a build time dependency on service A, and use the installed copy of the interface file for any code generation it has to do. It should not have a local copy of the interface, as that could then go out of sync with the canonical copy in service A’s git repository.

## API versioning

Just like C APIs, D-Bus interfaces should be designed to be usable in parallel with API-incompatible versions. This is achieved by including a version number in each interface name, service name and object path which is incremented on every backwards-incompatible change.

Version numbers should be included in all APIs from the first release, as that means moving to a new version is as simple as incrementing the number, rather than inserting a number everywhere, which takes more effort.

New API can be added to a D-Bus interface without incrementing the version number, as such additions are still backwards-compatible. However, clients should gracefully handle the org.freedesktop.DBus.Error.UnknownMethod error reply from all D-Bus method calls if they want to run against older versions of the service which don’t implement new methods. (This also prevents use of generated bindings; any method which a client wants to gracefully fall back from should be called using a generic D-Bus method invocation rather than a specific generated binding.)

When API is broken, changed or removed, the service’s version number must be bumped; for example, from com.example.MyService1 to com.example.MyService2. If backwards compatibility is maintained in the service by implementing both the old and new interfaces, the service can own both well-known names and clients can use whichever they support.

As discussed in Annotations, new or deprecated APIs should be marked in the interface XML using annotations.

Note, however, that supporting multiple interface versions simultaneously requires that object paths are versioned as well, so objects must not be put on the bus at the root path (‘/’). This is for technical reasons: signals sent from a D-Bus service have the originating service name overwritten by its unique name (e.g. com.example.MyService2 is overwritten by :1:15). If object paths are shared between objects implementing two versions of the service’s interface, client programs cannot tell which object a signal has come from. The solution is to include the version number in all object paths, for example /com/example/MyService1/Manager instead of / or /com/example/MyService/Manager.

In summary, version numbers should be included in all service names, interface names and object paths:

- com.example.MyService1

- com.example.MyService1.InterestingInterface

- com.example.MyService1.OtherInterface

- /com/example/MyService1/Manager

- /com/example/MyService1/OtherObject

## API design

D-Bus API design is broadly the same as C API design, but there are a few additional points to bear in mind which arise both from D-Bus’ features (explicit errors, signals and properties), and from its implementation as an IPC system.

D-Bus method calls are much more expensive than C function calls, typically taking on the order of milliseconds to complete a round trip. Therefore, the design should minimize the number of method calls needed to perform an operation.

The type system is very expressive, especially compared to C’s, and APIs should take full advantage of it.

Similarly, its support for signals and properties differentiates it from normal C APIs, and well written D-Bus APIs make full use of these features where appropriate. They can be coupled with standard interfaces defined in the D-Bus specification to allow for consistent access to properties and objects in a hierarchy.

### Minimizing Round Trips

Each D-Bus method call is a round trip from a client program to a service and back again, which is expensive — taking on the order of a millisecond. One of the top priorities in D-Bus API design is to minimize the number of round trips needed by clients. This can be achieved by a combination of providing specific convenience APIs and designing APIs which operate on large data sets in a single call, rather than requiring as many calls as elements in the data set.

Consider an address book API, com.example.MyService1.AddressBook. It might have an AddContact(ss) → (u) method to add a contact (taking name and e-mail address parameters and returning a unique contact ID), and a RemoveContact(u) method to remove one by ID. In the normal case of operating on single contacts in the address book, these calls are optimal. However, if the user wants to import a list of contacts, or delete multiple contacts simultaneously, one call has to be made per contact — this could take a long time for large contact lists.

Instead of the AddContact and RemoveContact methods, the interface could have an UpdateContacts(a(ss)au) → (au) method, which takes an array of structs containing the new contacts’ details, and an array of IDs of the contacts to delete, and returns an array of IDs for the new contacts. This reduces the number of round trips needed to one.

Adding convenience APIs to replace a series of common method calls with a single method call specifically for that task is best done after the API has been profiled and bottlenecks identified, otherwise it could lead to premature optimization. One area where convenience methods can typically be added is during initialization of a client, where multiple method calls are needed to establish its state with the service.

### Taking Advantage of the Type System

D-Bus’ type system is similar to Python’s, but with a terser syntax which may be unfamiliar to C developers. The key to using the type system effectively is to expose as much structure in types as possible. In particular, sending structured strings over D-Bus should be avoided, as they need to be built and parsed; both are complex operations which are prone to bugs.

For APIs being used in constrained situations, enumerated values should be transmitted as unsigned integers. For APIs which need to be extended by third parties or which are used in more loosely coupled systems, enumerated values should be strings in some defined format.

Transmitting values as integers means string parsing and matching can be avoided, the messages are more compact, and typos can be more easily avoided by developers (if, for example, C enums are used in the implementation).

Transmitting values as strings means additional values can be defined by third parties without fear of conflicting over integer values; for instance by using the same reverse-domain-name namespacing as D-Bus interfaces.

In both cases, the interface documentation should describe the meaning of each value. It should state whether the type can be extended in future and, if so, how the service and client should handle unrecognized values — typically by considering them equal to an ‘unknown’ or ‘failure’ value. Conventionally, zero is used as the ‘unknown’ value.

For example, instead of:

    <!--
        Status:

        Status of the object.
        Valid statuses: ‘unknown’, ‘ready’, ‘complete’.
    -->
    <property name="Status" type="s" access="read" />

Use:

    <!--
        Status:

        Status of the object.
        Valid statuses: 0 = Unknown, 1 = Ready, 2 = Complete.
        Unrecognized statuses should be considered equal to Unknown.
    -->
    <property name="Status" type="u" access="read" />

Similarly, enumerated values should be used instead of booleans, as they allow extra values to be added in future, and there is no ambiguity about the sense of the boolean value.

For example, instead of:

    <!--
       MoveAddressBook:
       @direction: %TRUE to move it up in the list, %FALSE to move it down

       Move this address book up or down in the user’s list of address books.
       Higher address books have their contacts displayed first in search
       results.
    -->
    <method name="MoveAddressBook">
      <arg name="direction" type="b" direction="in" />
    </method>

Be more explicit than a boolean:

    <!--
       MoveAddressBook:
       @direction: 0 = Move it up in the list, 1 = Move it down

       Move this address book up or down in the user’s list of address books.
       Higher address books have their contacts displayed first in search
       results.

       Unrecognized enum values for @direction will result in the address book
       not moving.
    -->
    <method name="MoveAddressBook">
      <arg name="direction" type="u" direction="in" />
    </method>

Enumerated values should also be used instead of human readable strings, which should not be sent over the bus if possible. The service and client could be running in different locales, and hence interpret any human readable strings differently, or present them to the user in the wrong language. Transmit an enumerated value and convert it to a human readable string in the client.

In situations where a service has received a human readable string from somewhere else, it should pass it on unmodified to the client, ideally with its locale alongside. Passing human readable information to a client is better than passing nothing.

For example, instead of:

    <!--
       ProgressNotification:
       @progress_message: Human readable progress message string.

       Emitted whenever significant progress is made with some example
       operation. The @progress_message can be displayed in a UI dialogue to
       please the user.
    -->
    <signal name="ProgressNotification">
      <arg name="progress_message" type="s" />
    </signal>

The progress should be reported as an enumerated value:

    <!--
       ProgressNotification:
       @progress_state: 0 = Preparing, 1 = In progress, 2 = Finished

       Emitted whenever significant progress is made with some example
       operation. The @progress_state is typically converted to a human readable
       string and presented to the user. Unrecognized @progress_state values
       should be treated as state 1, in progress.
    -->
    <signal name="ProgressNotification">
      <arg name="progress_state" type="u" />
    </signal>

D-Bus has none of the problems of signed versus unsigned integers which C has (specifically, it does not do implicit sign conversion), so integer types should always be chosen to be an appropriate size and signedness for the data they could possibly contain. Typically, unsigned values are more frequently needed than signed values.

Structures can be used almost anywhere in a D-Bus type, and arrays of structures are particularly useful. Structures should be used wherever data fields are related. Note, however, that structures are not extensible in future, so always consider Extensibility.

For example, instead of several identically-indexed arrays containing different properties of the same set of items:

    <!--
       AddContacts:
       @names: Array of contact names to add.
       @emails: Corresponding array of contact e-mail addresses.
       @ids: Returned array of the IDs of the new contacts. This will be the
         same length as @names.

       Add zero or more contacts to the address book, using their names and
       e-mail addresses. @names and @emails must be the same length.
    -->
    <method name="AddContacts">
      <arg name="names" type="as" direction="in" />
      <arg name="emails" type="as" direction="in" />
      <arg name="ids" type="au" direction="out" />
    </method>

The arrays can be combined into a single array of structures:

    <!--
       AddContacts:
       @details: Array of (contact name, contact e-mail address) to add.
       @ids: Returned array of the IDs of the new contacts. This will be the
         same length as @details.

       Add zero or more contacts to the address book, using their names and
       e-mail addresses.
    -->
    <method name="AddContacts">
      <arg name="details" type="a(ss)" direction="in" />
      <arg name="ids" type="au" direction="out" />
    </method>

Note that D-Bus arrays are automatically transmitted with their length, so there is no need to null-terminate them or encode their length separately.

### Extensibility

Some D-Bus APIs have very well-defined use cases, and will never need extension. Others are used in more loosely coupled systems which may change over time, and hence should be designed to be extensible from the beginning without the need to break API in future. This is a trade off between having a more complex API, and being able to easily extend it in future.

The key tool for extensibility in D-Bus is a{sv}, the dictionary mapping strings to variants. If well-defined namespaced strings are used as the dictionary keys, arbitrary D-Bus peers can add whatever information they need into the dictionary. Any other peer which understands it can query and retrieve the information; other peers will ignore it.

The canonical example of an extensible API using a{sv} is Telepathy. It uses a{sv} values as the final element in structures to allow them to be extended in future.

A secondary tool is the use of flag fields in method calls. The set of accepted flags is entirely under the control of the interface designer and, as with enumerated types, can be extended in future without breaking API. Adding more flags allows the functionality of the method call to be tweaked.

### Using Signals, Properties and Errors

D-Bus method calls are explicitly asynchronous due to the latency inherent in IPC. This means that peers should not block on receiving a reply from a method call; they should schedule other work (in a main loop) and handle the reply when it is received. Even though method replies may take a while, the caller is guaranteed to receive exactly one reply eventually. This reply could be the return value from the method, an error from the method, or an error from the D-Bus daemon indicating the service failed in some way (e.g. due to crashing).

Because of this, service implementations should be careful to always reply exactly once to each method call. Replying at the end of a long-running operation is correct — the client will patiently wait until the operation has finished and the reply is received.

Note that D-Bus client bindings may implement synthetic timeouts of several tens of seconds, unless explicitly disabled for a call. For very long-running operations, you should disable the client bindings’ timeout and make it clear in the client’s UI that the application has not frozen and is simply running a long operation.

An anti-pattern to avoid in this situation is to start a long-running operation when a method call is received, then to never reply to the method call and instead notify the client of completion of the operation via a signal. This approach is incorrect as signal emissions do not have a one-to-one relationship with method calls — the signal could theoretically be emitted multiple times, or never, which the client would not be able to handle.

Similarly, use a D-Bus error reply to signify failure of an operation triggered by a method call, rather than using a custom error code in the method’s reply. This means that a reply always indicates success, and an error always indicates failure — rather than a reply indicating either depending on its parameters, and having to return dummy values in the other parameters. Using D-Bus error replies also means such failures can be highlighted in debugging tools, simplifying debugging.

Clients should handle all possible standard and documented D-Bus errors for each method call. IPC inherently has more potential failures than normal C function calls, and clients should be prepared to handle all of them gracefully.

### Using Standard Interfaces

Use standard D-Bus interfaces where possible.

#### Properties

The D-Bus specification defines the org.freedesktop.DBus.Properties interface, which should be used by all objects to notify clients of changes to their property values, with the PropertiesChanged signal. This signal eliminates the need for individual PropertyNameChanged signals, and allows multiple properties to be notified in a single signal emission, reducing IPC round trips. Similarly, the Get and Set methods can be used to manipulate properties on an object, eliminating redundant GetPropertyName and SetPropertyName methods.

For example, consider an object implementing an interface com.example.MyService1.SomeInterface with methods:

- GetName() → (s)

- SetName(s) → ()

- GetStatus() → (u)

- RunOperation(ss) → (u)

and signals:

- NameChanged(u)

- StatusChanged(u)

The interface could be cut down to a single method:

- RunOperation(ss) → (u)

The object could then implement the org.freedesktop.DBus.Properties interface and define properties:

- Name of type s, read–write

- Status of type u, read-only

The NameChanged and StatusChanged signals would be replaced by org.freedesktop.DBus.Properties.PropertiesChanged.

#### Object Manager

The specification also defines the org.freedesktop.DBus.ObjectManager interface, which should be used whenever a service needs to expose a variable number of objects of the same class in a flat or tree-like structure, and clients are expected to be interested in most or all of the objects. For example, this could be used by an address book service which exposes multiple address books, each as a separate object. The GetManagedObjects method allows the full object tree to be queried, returning all the objects’ properties too, eliminating the need for further IPC round trips to query the properties.

If clients are not expected to be interested in most of the exposed objects, ObjectManager should not be used, as it will send all of the objects to each client anyway, wasting bus bandwidth. A file manager, therefore, should not expose the entire file system hierarchy using ObjectManager.

For example, consider an object implementing an interface com.example.MyService1.AddressBookManager with methods:

- GetAddressBooks() → (ao)

and signals:

- AddressBookAdded(o)

- AddressBookRemoved(o)

If the manager object is at path /com/example/MyService1/AddressBookManager, each address book is a child object, e.g. /com/example/MyService1/AddressBookManager/SomeAddressBook.

The interface could be eliminated, and the org.freedesktop.DBus.ObjectManager interface implemented on the manager object instead.

Calls to GetAddressBooks would become calls to GetManagedObjects and emissions of the AddressBookAdded and AddressBookRemoved signals would become emissions of InterfacesAdded and InterfacesRemoved.

### Naming Conventions

All D-Bus names, from service names through to method parameters, follow a set of conventions. Following these conventions makes APIs more natural to use and consistent with all other services on the system.

Services use reverse-domain-name notation, based on the domain name of the project providing the service (all in lower case), plus a unique name for the service (in camel case).

For example, version 2 of an address book application called ContactDex provided by a software vendor whose website is chocolateteapot.com would use service name com.chocolateteapot.ContactDex2.

Almost all names use camel case with no underscores, as in the examples below. Method and signal parameters are an exception, using lowercase_with_underscores. Type information is never encoded in the parameter name (i.e. not Hungarian notation).

Service Name

  com.example.MyService1

Interface Name

  com.example.MyService1.SomeInterface

Object Path (Root Object)

  /com/example/MyService1

Object Path (Child Object)

  /com/example/MyService1/SomeChild

Object Path (Grandchild Object)

  /com/example/MyService1/AnotherChild/Grandchild1

Method Name

  com.example.MyService1.SomeInterface.MethodName

Signal Name

  com.example.MyService1.SomeInterface.SignalName

Property Name

  com.example.MyService1.SomeInterface.PropertyName

See also: API versioning.

## Code generation

Rather than manually implementing both the server and client sides of a D-Bus interface, it is often easier to write the interface XML description and use a tool such as gdbus-codegen to generate type-safe C APIs, then build the implementation using those. This avoids the tedious and error-prone process of writing code to build and read D-Bus parameter variants for each method call.

Use of code generators is beyond the scope of this guide; for more information, see the gdbus-codegen manual.

## Annotations

Annotations may be added to the interface XML to expose metadata on the API which can be used by documentation or code generation tools to modify their output. Some standard annotations are given in the D-Bus specification, but further annotations may be defined by specific tools.

For example, gdbus-codegen defines several useful annotations, listed on its man page.

The following annotations are the most useful:

org.freedesktop.DBus.Deprecated

  Mark a symbol as deprecated. This should be used whenever the API is changed, specifying the version introducing the deprecation, the reason for deprecation, and a replacement symbol to use.

org.gtk.GDBus.Since

  Mark a symbol as having been added to the API after the initial release. This should include the version the symbol was first added in.

org.gtk.GDBus.C.Name and org.freedesktop.DBus.GLib.CSymbol

  Both used interchangeably to hint at a C function name to use when generating code for a symbol. Use of this annotation can make generated bindings a lot more pleasant to use.

org.freedesktop.DBus.Property.EmitsChangedSignal

  Indicate whether a property is expected to emit change signals. This can affect code generation, but is also useful documentation, as client programs then know when to expect property change notifications and when they have to requery.

## Documentation

Also just like C APIs, D-Bus APIs must be documented. There are several methods for documenting the interfaces, methods, properties and signals in a D-Bus interface XML file, each unfortunately with their own downsides. You should choose the method which best matches the tooling and workflow you are using.

### XML Comments

XML comments containing documentation in the gtk-doc format is the recommended format for use with gdbus-codegen. Using gdbus-codegen, these comments can be extracted, converted to DocBook format and included in the project’s API manual. For example:

#### Documentation Comments in D-Bus Interface XML

Example gtk-doc–style documentation comments in the introspection XML for the org.freedesktop.DBus.Properties interface.

    <!--
        org.freedesktop.DBus.Properties:
        @short_description: Standard property getter/setter interface

        Interface for all objects which expose properties on the bus, allowing
        those properties to be got, set, and signals emitted to notify of changes
        to the property values.
    -->
    <interface name="org.freedesktop.DBus.Properties">
      <!--
          Get:
          @interface_name: Name of the interface the property is defined on.
          @property_name: Name of the property to get.
          @value: Property value, wrapped in a variant.

          Retrieves the value of the property at @property_name on
          @interface_name on this object. If @interface_name is an empty string,
          all interfaces will be searched for @property_name; if multiple
          properties match, the result is undefined.

          If @interface_name or @property_name do not exist, a
          #org.freedesktop.DBus.Error.InvalidArgs error is returned.
      -->
      <method name="Get">
        <arg type="s" name="interface_name" direction="in"/>
        <arg type="s" name="property_name" direction="in"/>
        <arg type="v" name="value" direction="out"/>
      </method>

      <!--
          PropertiesChanged:
          @interface_name: Name of the interface the properties changed on.
          @changed_properties: Map of property name to updated value for the
            changed properties.
          @invalidated_properties: List of names of other properties which have
            changed, but whose updated values are not notified.

          Emitted when one or more properties change values on @interface_name.
          A property may be listed in @changed_properties or
          @invalidated_properties depending on whether the service wants to
          broadcast the property’s new value. If a value is large or infrequently
          used, the service might not want to broadcast it, and will wait for
          clients to request it instead.
      -->
      <signal name="PropertiesChanged">
        <arg type="s" name="interface_name"/>
        <arg type="a{sv}" name="changed_properties"/>
        <arg type="as" name="invalidated_properties"/>
      </signal>
    </interface>

### XML Annotations

Documentation can also be added as annotation elements in the XML. This format is also supported by gdbus-codegen, but gtk-doc comments are preferred. For example:

#### Documentation Annotations in D-Bus Interface XML

Example GDBus documentation annotations in the introspection XML for the org.freedesktop.DBus.Properties interface.

    <interface name="org.freedesktop.DBus.Properties">
      <annotation name="org.gtk.GDBus.DocString" value="Interface for all
        objects which expose properties on the bus, allowing those properties to
        be got, set, and signals emitted to notify of changes to the property
        values."/>
      <annotation name="org.gtk.GDBus.DocString.Short"
        value="Standard property getter/setter interface"/>

      <method name="Get">
        <annotation name="org.gtk.GDBus.DocString" value="Retrieves the value of
          the property at @property_name on @interface_name on this object. If
          @interface_name is an empty string, all interfaces will be searched
          for @property_name; if multiple properties match, the result is
          undefined.

          If @interface_name or @property_name do not exist, a
          #org.freedesktop.DBus.Error.InvalidArgs error is returned."/>

        <arg type="s" name="interface_name" direction="in">
          <annotation name="org.gtk.GDBus.DocString"
            value="Name of the interface the property is defined on."/>
        </arg>

        <arg type="s" name="property_name" direction="in">
          <annotation name="org.gtk.GDBus.DocString"
            value="Name of the property to get."/>
        </arg>

        <arg type="v" name="value" direction="out">
          <annotation name="org.gtk.GDBus.DocString"
            value="Property value, wrapped in a variant."/>
        </arg>
      </method>

      <signal name="PropertiesChanged">
        <annotation name="org.gtk.GDBus.DocString" value="Emitted when one or
          more properties change values on @interface_name. A property may be
          listed in @changed_properties or @invalidated_properties depending on
          whether the service wants to broadcast the property’s new value. If a
          value is large or infrequently used, the service might not want to
          broadcast it, and will wait for clients to request it instead."/>

        <arg type="s" name="interface_name">
          <annotation name="org.gtk.GDBus.DocString"
            value="Name of the interface the properties changed on."/>
        </arg>

        <arg type="a{sv}" name="changed_properties">
          <annotation name="org.gtk.GDBus.DocString"
            value="Map of property name to updated value for the changed
              properties."/>
        </arg>

        <arg type="as" name="invalidated_properties">
          <annotation name="org.gtk.GDBus.DocString"
            value="List of names of other properties which have changed, but
              whose updated values are not notified."/>
        </arg>
      </signal>
    </interface>

## Service files

Each D-Bus service must install a .service file describing its service name and the command to run to start the service. This allows for service activation (see the D-Bus specification).

Service files must be named after the service’s well-known name, for example file com.example.MyService1.service for well-known name com.example.MyService1. Files must be installed in $(datadir)/dbus-1/services for the session bus and $(datadir)/dbus-1/system-services for the system bus. Note, however, that services on the system bus almost always need a security policy as well.

## Security Policies

At a high level, the D-Bus security model is:

- There is a system bus, and zero or more session buses.

- Any process may connect to the system bus. The system bus limits which can own names or send method calls, and only processes running as privileged users can receive unicast messages not addressed to them. Every process may receive broadcasts.

- Each session bus has an owner (a user). Only its owner may connect; on general-purpose Linux, a session bus is not treated as a privilege boundary, so there is no further privilege separation between processes on it.

Full coverage of securing D-Bus services is beyond the scope of this guide, however there are some steps which you can take when designing an API to ease security policy implementation.

D-Bus security policies are written as XML files in $(datadir)/dbus-1/system.d, $(datadir)/dbus-1/session.d, $(sysconfdir)/dbus-1/system.d and $(sysconfdir)/dbus-1/session.d and use an allow/deny model, where each message (method call, signal emission, etc.) can be allowed or denied according to the sum of all policy rules which match it. Each <allow> or <deny> rule in the policy should have the own, send_destination or receive_sender attribute set.

When designing an API, bear in mind the need to write and install such a security policy, and consider splitting up methods or providing more restricted versions which accept constrained parameters, so that they can be exposed with less restrictive security policies if needed by less trusted clients. Since dbus-daemon 1.10, security policies should be installed to $(datadir) rather than (file($(sysconfdir)); the latter is intended for system administators.

Secondly, the default D-Bus security policy for the system bus is restrictive enough to allow sensitive data, such as passwords, to be safely sent over the bus in unicast messages (including unicast signals); so there is no need to complicate APIs by implementing extra security. Note, however, that sensitive data must not be sent in broadcast signals, as they can be seen by all peers on the bus. The default policy for the session bus is not restrictive, but it is typically not a security boundary.

## Debugging

Debugging services running on D-Bus can be tricky, especially if they are launched via service activation and hence in an environment out of your control.

Three main tools are available: D-Bus Monitor, Bustle and D-Feet.

### D-Bus Monitor

dbus-monitor is a core D-Bus tool, and allows eavesdropping on the session or system bus, printing all messages it sees. The messages may be filtered using a standard D-Bus match rule to make the stream more manageable.

Previous versions of D-Bus have required the security policy for the system bus to be manually relaxed to allow eavesdropping on all messages. This meant that debugging it was difficult and insecure. The latest versions of D-Bus add support for monitor-only connections for the root user, which means that dbus-monitor can be run as root to painlessly monitor all messages on the system bus without modifying its security policy.

### Bustle

Bustle is a graphical version of dbus-monitor, with a UI focused on profiling D-Bus performance by plotting messages on a timeline. It is ideal for finding bottlenecks in IPC performance between a service and client.

### D-Feet

D-Feet is an introspection tool for D-Bus, which displays all peers on the bus graphically, with their objects, interfaces, methods, signals and properties listed for examination. It is useful for debugging all kinds of issues related to presence of services on the bus and the objects they expose.