D-BUS message bus daemon configuration
===

The message bus daemon has a configuration file that specializes it
for a particular application. For example, one configuration 
file might set up the message bus to be a systemwide message bus, 
while another might set it up to be a per-user login session bus.

The configuration file also establishes resource limits, security
parameters, and so forth.

The configuration file is not part of any interoperability
specification and its backward compatibility is not guaranteed; this
document is documentation, not specification.

A DTD should be written here eventually, but for now I suck.

Doctype declaration:

   <!DOCTYPE busconfig PUBLIC "-//freedesktop//DTD D-BUS Bus Configuration 1.0//EN"
    "http://www.freedesktop.org/standards/dbus/1.0/busconfig.dtd">

Elements:

 <busconfig>
 
    Root element.

 <type>

    The well-known type of the message bus. Currently known values
    are "system" and "session"; if other values are set, they should 
    be either added to the D-BUS specification, or namespaced.
    The last <type> element "wins"

    Example: <type>session</type>

 <include>
  ignore_missing="(yes|no)"   optional attribute, defaults to no
   
    Include a file <include>filename.conf</include> at this point.

 <includedir>

    Include all files in <includedir>foo.d</includedir> at this
    point. Files in the directory are included in undefined order.
    Only files ending in ".conf" are included.

    This is intended to allow extension of the system bus by
    particular packages. For example, if CUPS wants to be able to send
    out notification of printer queue changes, it could install a file
    to /etc/dbus/system.d that allowed all apps to receive this
    message and allowed the printer daemon user to send it.

 <user>

    The user account the daemon should run as, as either a username or
    a UID. If the daemon doesn't have and cannot change to this UID on
    startup, it will exit.  If this element is not present, the daemon 
    will not change or care about its UID.
 
    The last <user> entry in the file "wins", the others are ignored.

    The user is changed after the bus has completed initialization. 
    So sockets etc. will be created before changing user, but no 
    data will be read from clients before changing user.

 <fork>
    
    If present, the bus daemon becomes a real daemon (forks 
    into the background, etc.)

 <listen>

    Add an address that the bus should listen on. The 
    address is in the standard D-BUS format that contains 
    a transport name plus possible parameters/options.

    Example: <listen>unix:path=/tmp/foo</listen>

    If there are multiple <listen> elements, then the bus listens 
    on multiple addresses. The bus will pass its address to 
    activated services or other interested parties with 
    the last address given in <listen> first. That is, 
    apps will try to connect to the last <listen> address first.

 <auth>

    Lists permitted authorization mechanisms. If this element doesn't
    exist, then all known mechanisms are allowed.  If there are
    multiple <auth> elements, the last one wins (they are not merged).
    The order in which mechanisms are listed is not meaningful.
    
    Example: <auth>EXTERNAL</auth>
    Example: <auth>DBUS_COOKIE_SHA1</auth>

 <servicedir>

    Adds a directory to scan for .service files. Directories are
    scanned starting with the last to appear in the config file 
    (the first .service file found that provides a particular 
    service will be used).

 <policy>
  context="(default|mandatory)"  one of the context/user/group
                                 attributes is mandatory
  user="username or userid"
  group="group name or gid"

    Encloses a policy to be applied to a particular set of 
    connections to the bus. A policy is made up of <limit>, 
    <allow>, <deny> elements.
  
    Policies are applied to a connection as follows:
       - all context="default" policies are applied
       - all group="connection's user's group" policies are applied
         in undefined order
       - all user="connection's auth user" policies are applied
         in undefined order
       - all context="mandatory" policies are applied

    Policies applied later will override those applied earlier, 
    when the policies overlap. Multiple policies with the same 
    user/group/context are applied in the order they appear 
    in the config file.

 <limit>
  name="resource name"  mandatory
  
    Appears below a <policy> element and establishes a resource
    limit. For example:
      <limit name="max_message_size">64</limit>
      <limit name="max_connections">512</limit>

 <deny>
  send="messagename"
  receive="messagename"
  own="servicename"
  send_to="servicename"
  receive_from="servicename"
  user="username"
  group="groupname"

    Examples:
       <deny send="org.freedesktop.System.Reboot"/> 
       <deny receive="org.freedesktop.System.Reboot"/>
       <deny own="org.freedesktop.System"/>
       <deny send_to="org.freedesktop.System"/>
       <deny receive_from="org.freedesktop.System"/>
       <deny user="john"/>
       <deny group="enemies"/>

    send_to and receive_from mean that messages may not be sent to 
    or received from the *owner* of the given service, not that 
    they may not be sent *to that service name*. That is, if 
    a connection owns services A, B, C, and sending to A is denied, 
    sending to B or C will not work either.

    user and group denials mean that the given user or group may 
    not connect to the message bus.

    For "servicename" or "messagename" or "username" or "groupname"
    the character "*" can be substituted, meaning "any." Complex globs
    like "foo.bar.*" aren't allowed for now because they'd be work to
    implement and maybe encourage sloppy security anyway.

    It does not make sense to deny a user or group inside a <policy>
    for a user or group; user/group denials can only be inside
    context="default" or context="required" policies.

    A single <deny> rule may specify both send and send_to, OR both
    receive and receive_from. In this case, the denial applies only if
    both attributes match the message being denied.
    e.g. <deny send="foo.bar" send_to="foo.blah"/> would deny 
    messages of the given name AND to the given service.

 <allow>
  send="messagename"
  receive="messagename"
  own="servicename"
  send_to="servicename"
  receive_from="servicename"
  user="username"
  group="groupname"

    Makes an exception to previous <deny> statements. Works 
    just like <deny> but with the inverse meaning.

    An <allow> only punches holes in the equivalent <deny>, it does
    not unconditionally allow the message. For example:

      <deny send="*"/>
      <deny send_to="*"/>
      <allow send="org.foo.Bar"/>

    Here the policy still doesn't allow sending any messages, because 
    no recipients have been allowed. You have to add 
    <allow send_to="something"/> to make the policy useful.