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DBus responder

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Problem Statement

The contemporary centralized user databases such as IPA or Active Directory store many attributes that describe the user. Apart from attributes that are related to a “computer user” entry such as user name, login shell or an ID, the databases often store data about the physical user represented by the entry, such as telephone number. Since the SSSD already has means of connecting to the remote directory, including advanced features like offline support or fail over, it would appear as a natural choice for retrieving these attributes. However, the only interface the SSSD provides towards the system at the moment is the standard POSIX interface and a couple of ad-hoc application specific responders (sudo, ssh, autofs). The purpose of this document is to describe a design of a new responder, that would listen on the system bus and allow third party consumers to retrieve custom attributes stored in a centralized database via a DBus call.

The DBus interface design

This section gathers feedback expressed in mailing lists, private e-mail conversations and IRC discussions and summarizes feature requests and areas that need improvement into a design proposal of both the DBus API and several required changes in the core SSSD daemon.

Cached objects

D-Bus Interface: Cached Objects

Object exposed on the bus

Instead of a single interface returning object attributes in an LDAP-like way, the interface would be built in an object-oriented fashion. Each object (i.e. a user or a group) would be identified with an object path and methods would be available to the interface user to make it possible to retrieve either a single object or a set of object.

The interface will support users, groups and domains.

Representing users and groups on the bus

D-Bus Interface: Users and Groups

Representing SSSD processes on the bus

  • object path: /org/freedesktop/sssd/infopipe/Components/monitor
  • object path: /org/freedesktop/sssd/infopipe/Components/Responders/$responder_name
  • object path: /org/freedesktop/sssd/infopipe/Components/Backends/$sssd_domain_name
  • method org.freedesktop.sssd.infopipe.ListComponents()
    • returns: Array of object paths representing component objects
  • method org.freedesktop.sssd.infopipe.ListResponders()
    • returns: Array of object paths representing component objects
  • method org.freedesktop.sssd.infopipe.ListBackends()
    • returns: Array of object paths representing component objects
  • method org.freedesktop.sssd.infopipe.FindMonitor()
    • returns: Object path representing the monitor object
  • method org.freedesktop.sssd.infopipe.FindResponderByName(String name)
    • name: The name of the responder to retrieve
    • returns: Object path representing the responder object
  • method org.freedesktop.sssd.infopipe.FindBackendByName(String name)
    • name: The name of the backend to retrieve
    • returns: Object path representing the backend object

The name “Components” is chosen to not imply any particular implementation on SSSD side.

The component objects implements org.freedesktop.sssd.infopipe.Components interface, which is define as:

  • method org.freedesktop.sssd.infopipe.Components.Enable()
    • returns: nothing
    • note: changes will be visible after SSSD is restarted
  • method org.freedesktop.sssd.infopipe.Components.Disable()
    • returns: nothing
    • note: changes will be visible after SSSD is restarted
  • method org.freedesktop.sssd.infopipe.Components.ChangeDebugLevel(Uint32 debug_level)
    • debug_level: Debug level to set
    • returns: nothing
    • note: changes will be permanent but do not require restart of the daemon
  • property String name
    • The name of this service.
  • property Uint32 debug_level
    • The name of this service.
  • property Boolean enabled
    • Whether the service is enabled or not
  • property string type
    • Type of the component. One of “monitor”, “responder”, “backend”.

This approach will completely distinguish SSSD processes from services and domains, which are logical units that should not contain any information about SSSD architecture.

Representing service objects on the bus

This API should include methods to represent service object(s) and provide basic information and configuration abilities.

  • object path: /org/freedesktop/sssd/infopipe/Services/$service
  • method org.freedesktop.sssd.infopipe.ListServices()
    • returns: Array of object paths representing Service objects
  • method org.freedesktop.sssd.infopipe.FindServiceByName(String name)
    • name: The name of the service to retrieve
    • returns: Object path representing the service object

The service object will in the first iteration include several properties describing the domain. As this iteration doesn’t allow any modification, only properties and not methods are considered:

  • property String name
    • The name of this service.
  • service dependent properties

Other properties might be added upon request.

Representing domain objects on the bus

For some consumers (such as realmd), it’s important to also know the properties of a domain. The API should include methods to retrieve a active domain object(s) and represent the domains as objects on the bus as well.

  • object path: /org/freedesktop/sssd/infopipe/Domains/$domain

The synopsis of these calls would look like:

  • method org.freedesktop.sssd.infopipe.ListDomains()
    • returns: Array of object paths representing Domain objects
  • method org.freedesktop.sssd.infopipe.ListSubdomainsByDomain(String name)
    • returns: Array of object paths representing Domain objects associated with domain $name
  • method org.freedesktop.sssd.infopipe.FindDomainByName(String name)
    • name: The name of the domain to retrieve
    • returns: Object path representing the domain object

The domain object will in the first iteration include several properties describing the domain. As this iteration doesn’t allow any modification, only properties and not methods are considered:

  • property String name
    • The name of this domain. Same as the domain stanza in the sssd.conf
  • property String[] primary_servers
    • Array of primary servers associated with this domain
  • property String[] backup_servers
    • Array of backup servers associated with this domain
  • property Uint32 min_id
    • Minimum UID and GID value for this domain
  • property Uint32 max_id
    • Maximum UID and GID value for this domain
  • property String realm
    • The Kerberos realm this domain is configured with
  • property String forest
    • The domain forest this domain belongs to
  • property String login_format
    • The login format this domain expects.
  • property String fully_qualified_name_format
    • The format of fully qualified names this domain uses
  • property Boolean enumerable
    • Whether this domain can be enumerated or not
  • property Boolean use_fully_qualified_names
    • Whether this domain requires fully qualified names
  • property Boolean subdomain
    • Whether the domain is an autodiscovered subdomain or a user-defined domain
  • property ObjectPath parent_domain
    • Object path of a parent domain or empty string if this is a root domain

Other properties such as provider type or case sensitivity might be added upon request. Right now, we need something other developers can experiment with.

Synchronous getter behaviour

Retrieving a property with a getter will always be synchronous and return the value currently cached. The getter might schedule an out-of-band update depending on the state of the cache object. The primary reason for the getter being synchronous is to be able to be composable, in other words being able to call N getters in a loop and construct a reply message containing N properties without resorting to asynchronous updates of the properties.

Callers that with to have an up-to-date view of the properties should update the object by calling a special update (not included ATM) method or subscribe to the PropertiesChanged interface.

SSSD daemon features

Apart from features that will directly benefit the new interface, the SSSD itself must adapt to some requirements as well.

Access control

The DBus responder needs to limit who can request information at all and what attributes can be returned.

Limiting access to the responder

The DBus responder will re-use the same mechanism the PAC responder uses where UIDs of clients that can contact the responder will be enumerated in the “allowed_uids” parameter of the responder configuration.

In a future enhancement, we might add a “self” mechanism, where client will be allowed to read its own attributes. As limiting attribute access might be different for this use-case, the first iteration of the responder will not include the “self” mechanism.

Limiting access to attributes

The responder will have a whitelist of attributes that the client can query. No other attributes will be returned. Requesting an attribute that is not permitted will yield an empty response, same as if the attribute didn’t exist. The whitelist will include the standard set of POSIX attributes as returned by i.e. getpwnam by default.

The administrator will be allowed to extend the whitelist in sssd.conf using a configuration directive either in the [ifp] section itself or per-domain. The configuration directive shall allow either explicitly adding attributes to the whitelist (using +attrname) or explicitly remove them using -attrname.

The following example illustrates explicitly allowing the telephoneNumber attribute to be queried and removing the gecos attribute from the whitelist. :

[ifp]
user_attributes = +telephoneNumer, -gecos

Support for non-POSIX users and groups

Currently the SSSD supports looking up POSIX users and groups, mostly due to the fact that primary consumers are POSIX interfaces such as the Name Service Switch. For instance, the search filters in back ends require the presence of attributes ID.

In contrast, users and groups that consumers of this new interface require often lack the POSIX attributes. The SSSD must be extended so that even non-POSIX users and groups are handled well.

Do not require enumeration to be enabled to retrieve set of users

At the moment, the SSSD can either fetch a single user (using getpwnam for example) or all available users (using getpwent). As an effect, all proposed DBus calls require enumeration to be switched on in order to be able to retrieve sets of users. The SSSD needs to either grow a way to retrieve several entries at once without enumerating or needs to make enumeration much faster.

Authors

  • Jakub Hrozek <jhrozek@redhat.com>
  • Pavel Březina <pbrezina@redhat.com>
  • Stef Walter <stefw@redhat.com>