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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.

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

 <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.

 <listen>
  address="name"    mandatory attribute

    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 address="unix:path=/tmp/foo"/>

    If there are multiple <listen> elements, then the bus listens 
    on multiple addresses.

 <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).

 <policy>
  context="(default|required)"   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
       - all user="connection's auth user" policies are applied
       - all context="required" 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"

    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"/>

    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.

    For "servicename" or "messagename" 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.

    FIXME should we allow send/send_to and receive/receive_from 
    to both be specified, in which case they would be ANDed together?
    e.g. <deny send="foo.bar" send_to="foo.blah"/> would deny 
    messages of the given name AND to the given service.

    Probably need to see how hard/slow all this will be to implement.

 <allow>
  send="messagename"
  receive="messagename"
  own="servicename"
  send_to="servicename"
  receive_from="servicename"

    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.